Sebastopol City Council
June 7, 2022

Chapter 4
Risk Assessment
City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2016 Page 4-22 The risk assessment for most hazards is built upon the frequency of past events and the assumption that historic occurrence rates are a good predictor of future event probability. With climate change; as previously noted, history is not an adequate predictor of the probability of future occurrences (SHMP 2018). Planning for climate change is based on understanding and integrating evolving climate change science and modeled projections that account for shifts in historic conditions due to climate change into hazard mitigation planning (SHMP 2018). For these reasons, the likelihood of future occurrences for climate change and sea level rise impacts (along the Laguna de Santa Rosa) is based on climate change science and modeled projections.
• Climate Change Considerations – Climate change refers to a long-term change in the earth’s temperature, precipitation, humidity, and seasons. This section addresses the probable effects of climate change qualitatively and as a secondary impact for each identified hazard. In other words, it describes the potential for climate change to affect the frequency and severity of natural hazards.

Impacts can include water supply shortages; changes in the frequency, intensity, and extent of drought and extreme heat events; more precipitation and flooding risk;, and increasing temperatures.

This section concludes whether climate change is anticipated to have a low, medium, or high influence on future hazard impacts.

The discussion relies on information from the Sixth Assessment Report from the Intergovernmental Panel on Climate Change (IPCC) Climate Change 2021: The Physical Science Basis Contribution of Working Group I to the Sixth Assessment Report of the IPCC (IPCC 2018). It also relies on numerous California publications on climate change and climate adaptation including:
• California’s Fifth Climate Change Assessment: Climate Change 2013: The Physical Science Basis Contribution of Working Group I to the Fifth Assessment Report of the IPCC (IPCC 2018) • California’s Fourth Climate Assessment (California Natural Resources Agency 2018a); • Safeguarding California Plan: 2018 Update – California’s Climate Adaptation Strategy (California Natural Resources Agency 2018b);
• 2014 Safeguarding California: Reducing Climate Risk (California Natural Resources Agency 2014); and • 2009 California Climate Adaptation Strategy (CAS) (California Natural Resources Agency 2009).

The discussion integrates climate information from Cal-Adapt, a website that gathers data on how climate change might affect California at the local level based on the state’s scientific and research community (CEC 2018). The climate change considerations subsections also summarizes climate change modelling and findings from the following two RCPA-prepared documents: Climate Ready Sonoma County: Climate Hazards and Vulnerabilities (2014) and Sonoma County’s Regional Climate Action Plan: Climate Action 2020 and Beyond (2016) (referred to as the County’s Climate Action Plan [CAP]). Climate change projections summarized in Sonoma County’s CAP are based on BCM projections, which as previously mentioned were developed by applying scaled-down models that identify watershed-level climate change impacts specific to Sonoma County (RCPA 2016). Climate change is addressed in the plan as a secondary impact for each hazard.

Vulnerability Assessment – The vulnerability of the Planning Area to a specific natural hazard is assessed through the study of potential impacts to specific sectors:
• Property
• People
• Economy
• Critical Facilities and Infrastructure
• Historic, Cultural, and Natural Resources
• Future Development
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2016 Page 4-23 Risk Summary – This is a summary of key findings and risk based on threat, vulnerability and consequences to the Planning Area from the specific hazard.

The significance of each hazard was determined based on the hazard profile, focusing on key criteria such as frequency and resulting damage, including deaths/injuries, and property and economic damage. This assessment was used by the HMPC to prioritize those hazards of greatest significance to the Planning Area thereby allowing the City to focus resources where they are most needed. The following sections provide profiles of the natural hazards, listed alphabetically that the HMPC identified in Section 4.1 Identifying Hazards. Human-caused hazards are addressed in Section 4.4.

4.3.1 Earthquake
Hazard Description
An earthquake is caused by a sudden slip on a fault. Stresses in the earth’s outer layer push the sides of the fault together. Stress builds up, and the rocks slip suddenly, releasing energy in waves that travel through the earth’s crust and cause the shaking that is felt during an earthquake. The amount of energy released during an earthquake is usually expressed as a magnitude and is measured directly from the earthquake as recorded on seismographs. Another measure of earthquake severity is intensity. Intensity is an expression of the amount of shaking at any given location on the ground surface (see discussion in the Extent section). Seismic shaking is typically the greatest cause of losses to structures during earthquakes.

Seismic Hazards
Earthquakes can cause structural losses, injury, and possibly death, as well as damage to infrastructure such as water, power, gas, communication, and transportation networks and systems. The degree of damage depends on many interrelated factors. Among these are the magnitude, focal depth, distance from the causative fault, source mechanism, duration of shaking, high rock accelerations, type of surface deposits or bedrock, degree of consolidation of surface deposits, presence of high groundwater, topography, and the design, type, and quality of building construction. Primary hazards associated with seismic activity include surface rupture along faults, ground shaking, and associated building failure.

Secondary hazards result from the interaction of ground shaking with existing ground instabilities or facilities and include liquefaction, settlement, debris flows, landslides, tsunamis and seiches, and flooding or wildfires from broken pipelines, gas, or electrical infrastructure.

Ground Shaking
When movement occurs along a fault, the energy generated is released as waves, which cause ground shaking. Ground shaking intensity varies with the magnitude of the earthquake, the distance from the epicenter, and the type of rock or sediment through which the seismic waves move. The geological characteristics of an area can be a greater hazard than the area’s distance to the earthquake epicenter.

The City is situated within an area of high potential seismic activity (the San Francisco Bay Region), and so the fault systems within and around the City have the potential to produce earthquakes that could impact the City significantly (e.g. the San Andreas Fault System which is currently active). A high-magnitude earthquake on one of these faults could cause moderate to high ground shaking in the City. Figure 4-3 below is an earthquake shaking map for the City that is based on the two percent probability of occurrence in 50 years, based on the USGS analyses of nearby faults (USGS 2003). Also known as the 2,500 year probabilistic scenario map, this scenario indicates that the City has the potential to experience strong ground shaking at a level that will be damaging to buildings and structures.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-24 Figure 4-3: Potential Ground Shaking Probability in the City of Sebastopol Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-25 Liquefaction Susceptibility
Liquefaction can be defined as the loss of soil strength or stiffness due to a buildup of pore-water pressure during a seismic event, and is associated primarily with relatively loose, saturated fine to medium-grained unconsolidated soils. Seismic ground shaking of relatively loose, granular soils that are saturated or submerged can cause the soils to liquefy and temporarily behave as a dense fluid. If this layer is at the surface, its effect is much like that of quicksand for any structure located on it. If the liquefied layer is in the subsurface, the material above it may slide laterally depending on the confinement of the unstable mass. Liquefaction is caused by a sudden temporary increase in pore-water pressure due to seismic densification or other displacement of submerged granular soils. Liquefiable soil conditions are not uncommon in alluvial deposits in moderate to large canyons and could also be present in other areas of alluvial soils where the groundwater level is shallow (i.e. 50 feet below the surface). Bedrock units, due to their dense nature, are unlikely to present a liquefaction hazard.

According to the USGS Earthquake Hazards Program data for liquefaction susceptibility, there are several areas of liquefaction susceptibility in the City Planning Area (see Figure 4-4). The majority of the City is in the very low liquefaction susceptibility zone, while parts of the City are in a very high zone on the east and west edges of town. Approximately 120 acres fall in the very high liquefaction susceptibility areas within Sebastopol and 1,277 acres in the very low liquefaction susceptibility areas within the City’s Planning Area.

Most of the highly susceptible category is situated east of town and follows the Laguna de Santa Rosa River that crosses the main bridge along Sebastopol Road or Highway 12 East.

Earthquakes can also lead to secondary hazards including flooding, building structure failure, debris flows, and fire (among others).

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-26 Figure 4-4: Liquefaction Susceptibility in the City of Sebastopol Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-27 Landslide Potential and Susceptibility
A landslide is a geologic hazard where the force of gravity combines with other factors to cause earth material to move or slide down an incline. Some landslides move slowly and cause damage gradually, whereas others move so rapidly that they can destroy property and take lives suddenly and unexpectedly.

Slopes with the greatest potential for sliding are between 34 degrees and 37 degrees. Although steep slopes are commonly present where landslides occur, it is not necessary for the slopes to be long.

There are predictable relationships between local geology and landslides. The down-slope movement of earth material as a landslide is part of the continuous, natural process of erosion. This process, however, can be influenced by a variety of causes that change the stability of the slope. Slope instability may result from natural processes, such as the erosion of the toe of a slope by a stream, or by ground shaking caused by an earthquake. Slopes can also be modified artificially by grading, or by the addition of water or structures to a slope. Landslide problems can be caused by land mismanagement, particularly in mountain, canyon, and coastal regions. In areas burned by forest and brush fires, a lower threshold of precipitation may initiate landslides and debris flows. As human populations expand over more of the land surface, these processes become an increasing concern. Development that occurs on a slope can substantially increase the frequency and extent of potential slope stability hazards. Knowledge of these relationships can improve planning and reduce vulnerability. Slope stability is dependent on many factors and their interrelationships, including rock type, moisture content, slope steepness, and natural or manmade undercutting.

The California Geological Survey (CGS) along with the California Department of Conservation have generated a landslide dataset that classifies susceptibility in California to various degrees, from very high (the most potentially dangerous) to a none or dry category (the least risk) (CGS 2019). Unknown or undetermined areas exist as well, as displayed in Figure 4-5 below. In Sebastopol the majority of the Planning Area is in the lower risk categories of landslide susceptibility, meaning that the local soils, slopes and geology are not very likely to lead to landslide activity.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-28 Figure 4-5: Landslide Susceptibility in the City of Sebastopol and Its Planning Area Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-29 Faults
California is a seismically active area with numerous faults throughout the region. An active fault is defined by CGS as a fault that has had surface rupture or displacement within the last 11,000 years (Holocene times). This does not mean, however, that faults having no evidence of surface displacement within the last 11,000 years are necessarily inactive. Potentially active faults are those that have shown displacement within the last 1.8 million years (Quaternary period) but have not moved within the Holocene times. Any fault older than Pleistocene (>1.8 million years) is considered inactive and dormant.

Although based on the history of fault movement and seismic activity in the area, it is known that the main faults posing risk to the City are the San Andreas Fault system and the Healdsburg-Rodgers Creek Fault (northern portion of the Hayward Fault), although none of these faults are within the City (see the Location section of this chapter for additional details).

As shown in Figure 4-6, there are several earthquake faults classified as Quaternary (those which are recognized at the surface and which have shown activity in the past 1.6 million years, or during the geologic Quaternary epoch) in the Planning Area. Additional faults are present, which are located farther from the City and could cause seismic activity in the future. The faults illustrated in Figure 4-6 are summarized below:
• Joy Woods fault is west of the City.
• San Andreas fault is west of the City.
• Part of Tolay fault is southeast of the City boundary but outside of the limits. • Bloomfield fault is south of the City but outside of its limits. • Healdsburg-Rodgers Creek fault, along Rodgers Creek to the east of the City but outside of the limits.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-30 Figure 4-6: Earthquake Faults near the Planning Area
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-31 Location
Extensive – There are no known faults within the City, but there are many near the Planning Area, which are discussed in more detail below. Additional faults nearby the Planning Area are illustrated in Figure 4-6.

There are two active faults: San Andreas Fault and active the Healdsburg-Rodgers Creek Fault (northern portion of the Hayward Fault) that fall outside the Planning Area but have historically been the source of earthquakes felt in Sebastopol. These local and regional faults are described in more detail below based on information summarized in the Sonoma County HMP.
• San Andreas Fault. The San Andreas Fault is located approximately 14 miles west of the Planning Area. It is a shallow fault and is considered the most active fault in California. Historically, the San Andreas Fault system is the main fault responsible for earthquakes felt in the City and is also expected to continue being the source of future earthquake activity. • Healdsburg-Rodgers Creek Fault. The Healdsburg-Rodgers Creek Fault is an active fault known as the northern extension of the Hayward Fault and it is associated with the Santa Rosa Plain, in Sonoma County. It marks the break between the Santa Rosa Plain and the uplifting Santa Rosa block to the east and the Sebastopol fault to the west marks the break between the Valley and the uplifting Sebastopol Block to the west (SFEI 2020). It is a strike slip fault, measuring around 117 kilometers in length. The most notable earthquake activity along the Healdsburg-Rodgers Creek fault took place in 1969 during the Santa Rosa Earthquakes. These were a pair of earthquakes, one a magnitude of 5.6 and the other a magnitude of 5.7 both that struck early October of that year in the nearby City of Santa Rosa northwest of the City of Sebastopol. The two earthquakes resulted in $8.35 million in damages related to building removal and repairs.
• Tolay Fault. The Tolay Fault is located south of the City of Sebastopol along the slope of the southwestern ridge in the lower portion of the Tolay Creek watershed. This fault, like the HealdsburgRodgers Creek fault, have produced sheared or weak zones in rock formations that weather and erode, and sometimes form conditions for hillslope wetlands. According to the USGS Earthquake Hazards Program, the fault lacks associated historic seismicity.

Extent (Magnitude/Severity)
Catastrophic – Extent (meaning the severity of an earthquake) refers to the amount of energy released during an earthquake and is usually expressed in terms of intensity or magnitude. These metrics are measured directly from the earthquake as recorded on seismographs.

Intensity represents the observed effects of ground-shaking at any specified location, and earthquake shaking decreases with distance from the earthquake epicenter. Intensity is an expression of the amount of shaking at any given location on the ground surface based on felt or observed effects. Seismic shaking is typically the greatest cause of losses to structures during earthquakes. Intensity is measured with the Modified Mercalli Intensity (MMI) scale (see Table 4-8).

Magnitude represents the amount of seismic energy released at the hypocenter of an earthquake and is based on the amplitude of the earthquake waves recorded. Seismologists have developed several magnitude scales; one of the first was the Richter Scale, developed in 1932 by Dr. Charles F. Richter of the California Institute of Technology. The Moment Magnitude Scale is the current scale used to quantify the magnitude or strength of the seismic energy released by an earthquake.

Table 4-8 below compares magnitude and the felt effects associated with the MMI scale. Damage typically occurs in MMI of VII or above and based on Figure 4-3. The majority of the City is found in areas where spectral acceleration is expected to surpass the 70 percent g (or gravitational velocity); this means that there is a high probability of the City experiencing strong seismic movements in the next few decades.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-32 Table 4-8: Magnitude and Mercalli Intensity Scale Measurements and Associated Characteristics Magnitude Mercalli
Intensity Effects Frequency
Less than 2.0 I Micro-earthquakes, not felt or rarely felt; recorded by seismographs. Continual

2.0-2.9 I to II Felt slightly by some people; damages to buildings. Over 1M per year

3.0-3.9 II to IV Often felt by people; rarely causes damage; shaking of indoor objects noticeable. Over 100,000 per year

4.0-4.9 IV to VI
Noticeable shaking of indoor objects and rattling noises; felt by most people in the affected area; slightly felt outside; generally, no to minimal damage.
10K to 15K per year

5.0-5.9 VI to VIII
Can cause damage of varying severity to poorly constructed buildings; at most, none to slight damage to all other
buildings. Felt by everyone.
1K to 1,500 per year

6.0-6.9 VII to X
Damage to a moderate number of well-built structures in populated areas; earthquake-resistant structures survive with slight to moderate damage; poorly designed structures
receive moderate to severe damage; felt in wider areas; up to hundreds of miles/kilometers from the epicenter; strong to violent shaking in epicentral area.
100 to 150 per year

7.0-7.9 VIII<
Causes damage to most buildings, some to partially or
completely collapse or receive severe damage; welldesigned structures are likely to receive damage; felt across great distances with major damage mostly limited to 250 km from epicenter.
10 to 20 per year

8.0-8.9 VIII<
Major damage to buildings, structures likely to be destroyed; will cause moderate to heavy damage to sturdy or
earthquake-resistant buildings; damaging in large areas; felt in extremely large regions.

One per year

9.0 and
Greater VIII<
At or near total destruction - severe damage or collapse to all buildings; heavy damage and shaking extends to distant locations; permanent changes in ground topography.

One per 10-50 years
Source: USGS
Previous Occurrences
Earthquakes have occurred nearby the Planning Area in
the past (within Sonoma County and adjacent areas).

According to the USGS, a recent earthquake event of a
magnitude of 6.0 took place near South Napa, about 30
miles to the southeast of the City Planning Area. This
event occurred the morning of August 24, 2014 and had
a reported intensity of VII in the Mercalli scale. The
earthquake was on the West Napa Fault, which was not
mapped under the Alquist-Priolo earthquake fault
hazard zone and was the largest event of this kind in the San Francisco Bay area since the 1989 Loma Prieta
earthquake (further summarized below). The seismic
activity of this event had an estimated 11.1 kilometers of depth. Thousands of structures across Sonoma County
were damaged, and hundreds of people were injured
during the earthquake across the affected areas in the
In 2014 a 6.0 magnitude earthquake occurred in the
southern portion of the City of Napa on the West
Napa Fault. The event was the largest earthquake in
the San Francisco Bay Area since the 1989 Loma Prieta
earthquake. Total damage in the southern Napa and
Vallejo areas ranged from $362 million to $1 billion.

Photo Credit: LA Times 2014
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-33 County. One person was reported as being killed during the earthquake. Because of the extensive damages, the California Governor issued an emergency proclamation on August 24, 2014, and the U.S.

President declared the incident a major disaster on September 11, 2014. Total economic losses were around $400 million, and state and federal assistance surpassed the $30 million mark. The SBA granted over $21 million in low-interest disaster loans to local businesses and agencies affected by the event.

Other recent earthquake events in the area include smaller magnitude earthquakes such as:
• A magnitude 2.6 earthquake with reported intensity of III occurred on September 8, 2020. The epicenter of the event was approximately 4 kilometers southeast of Larkfield-Wikiup at a depth of 6.8 kilometers.
• A magnitude 3.2 earthquake occurred on May 2, 2019. The event’s epicenter was approximately 4 kilometers south/southwest of Calistoga, CA in adjacent Napa County. The depth of the event was

10.9 kilometers.
• A magnitude 2.8 earthquake with reported intensity of III, on December 24, 2017. This event’s epicenter was about 6 kilometers west of Temelec, near Sonoma. The depth of the event was of 1 kilometer.
• A magnitude 2.7 earthquake with reported intensity of II, on November 17, 2013. The epicenter of this incident was about 5 kilometers east-southeast of Penngrove, southeast of Sebastopol. The depth of the event was of 4.4 kilometers.
• A magnitude 3.3 earthquake took place on July 25, 2011 and had a reported intensity of IV. Its depth was of 6.7 kilometers and the epicenter was located approximately 17 kilometers south-southeast from Sebastopol.

The Bay Area, including the City, has experienced several significant and other well-documented earthquakes and since 1855, and more than 140 earthquakes have been felt in the Santa Rosa near Sebastopol (Sonoma County 2017). The three major earthquakes included the 1906 San Francisco Earthquake, 1969 Rodgers Creek-Healdsburg Fault Earthquake, and 1989 Loma Prieta Earthquake. These three major earthquakes are summarized below:
• 1906 San Francisco Earthquake. This earthquake had an 8.3 magnitude and occurred on the northern segment of the San Andreas Fault and resulted in devastating damage to San Francisco and major damage in Santa Rosa, Sebastopol, Healdsburg, Petaluma, and other Sonoma County communities. Nearby Santa Rosa suffered more damage proportionally to its size than any other Bay Area city and the only casualties reported were in the City of Santa Rosa, where 65 people died and 12 went missing (Sonoma County 2017).
• 1969 Rodgers Creek - Healdsburg Fault Earthquake. Two earthquakes with Magnitudes of 5.6 and

5.7 originated near the juncture of the Rodgers Creek and Healdsburg Fault, approximately two miles north of Santa Rosa. Damage was concentrated in the City of Santa Rosa, and principally confined to the partial collapse or near collapse of unreinforced masonry (URM) buildings and wood frame buildings with substandard foundations or inadequate bracing. According to the 2016 Sonoma County Operational Area HMP, 99 structures were significantly damaged, approximately half in the business district and half in residential areas (Sonoma County 2017). Nearly half of all significantly damaged buildings were demolished. Total building damage was estimated at $6 million, with dwelling contents losses at $1.25 million.
• 1989 Loma Prieta Earthquake. This 6.9 magnitude earthquake was caused by a slip along the San Andreas Fault south of the Bay Area. Though the damage in Sonoma County from the earthquake was minor, the earthquake killed 63 people and injured 3,757 throughout northern California. It caused a total of over 16,700 housing units to be uninhabitable throughout the Monterey and San Francisco Bay Areas and left approximately 3,000-12,000 people homeless. The earthquake caused severe Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-34 damage in specific locations in the San Francisco Bay Area, most notably on unstable soil in San Francisco and Oakland, where approximately 12,000 homes and 2,600 businesses were damaged. In Santa Cruz, close to the epicenter, 40 buildings collapsed. The earthquake caused an estimated $6 billion ($11 billion in current value) in property damage, becoming one of the most expensive natural disasters in U.S. history at the time. It was the largest earthquake to occur on the San Andreas Fault since the 1906 San Francisco earthquake.

Probability of Future Occurrences
Likely – Given the information presented herein as well as recent earthquake activity history, earthquake hazards are expected to be a likely occurrence in the City as well as in Sonoma County. It is estimated that similar seismic activity events may occur every 20 to 30 years in the Planning Area and the overall San Francisco Bay region (State of California Seismic Safety Commission 2020).

The USGS noted in 2014 that there was a 72 percent probability of a strong earthquake (of magnitude 6.7 or greater) striking the San Francisco Bay Region (of which Sebastopol is part) by 2043. The probability of having a strong earthquake (of this magnitude 6.7 or greater) generated from the Healdsburg-Rodgers Creek Fault was then estimated at about 33 percent. However, more recent information released in 2015 by the USGS new Uniform California Earthquake Rupture Forecast 3, or UCERF3, considers additional parameters and data. With this new criteria and advanced technology, the updated results estimate that the actual chance of a strong earthquake of magnitude 6.7 or above is around 72 percent in the San Francisco Bay Region. The San Andreas fault now has a 33 percent chance of rupturing and causing earthquake activity, though the Rodgers Creek fault system’s probability has decreased to about 15 percent chance of rupture (UCERF3 2014).

Climate Change Considerations
While climate change is not expected to directly affect earthquake frequency or intensity it could exacerbate indirect or secondary impacts of earthquakes. For example, climate change could increase the frequency and intensity of extreme precipitation events, in turn increasing the probability of landslides and liquefaction events during an earthquake if the earthquake coincided with a wet cycle. Increased precipitation due to climate change will also result in increased frequency of landslide potential, as the added weight of rain-saturated soils on steeper hill slopes and the weakening of slopes caused by the pressure groundwater exerts on porous hillsides could trigger slope failure (SHMP 2018). These impacts are more likely to occur along the southern portion of the City’s Planning Area where there is low to moderate landslide potential; the majority of the City otherwise has a low landslide potential. For these reasons, climate change would have a “low” influence on earthquake hazards, specifically landslide susceptibility within the City.

Vulnerability Assessment
Ground shaking is the primary hazard related to earthquake activity. Many factors affect the survivability of structures and systems from earthquake-caused ground motions. These factors include proximity to the fault, direction of rupture, epicentral location and depth, magnitude, local geologic and soils conditions, types and quality of construction, building configurations and heights, and comparable factors that relate to utility, transportation, and other network systems. Ground motions become structurally damaging when average peak ground accelerations reach 10 to 15 percent of gravity, average peak velocities reach 8 to 12 centimeters per second, and when the MMI Scale is about VII, which is considered to be very strong (general alarm; walls crack; plaster falls).

Fault rupture itself contributes very little to damage unless the structure or system element crosses the active fault. In general, newer construction is more earthquake resistant than older construction because Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-35 of improved building codes and enforcement. Manufactured housing is very susceptible to damage because rarely are the foundation systems braced for earthquake motions. Locally generated earthquake motions, even from very moderate events, tend to be more damaging to smaller buildings, especially those constructed of URM.

URM buildings can pose significant loss of life, and social and economic risks in the City even when few buildings have this building composition or have been retrofitted. Risks to URM buildings are often located in the downtown areas of older cities. URM buildings without retrofits are also more vulnerable to life-threatening damage than other building types (Turner 2020). Numerous earthquakes over the past century have led to modifications to construction techniques in California and in the 1920’s to seismic provisions for new construction as part of the first edition of the Uniform Building Code (UBC). At the same time, local municipalities began adopting various editions to the UBC and establishing building departments to enforce the revised construction standards and ordinances.

The City was one of the first of two cities to enact an URM Ordinance following the two earthquakes of magnitudes 5.6 and 5.7 that occurred in 1969 and originated near the juncture of the Rodgers Creek and Healdsburg Fault (Turner 2020). These earthquakes resulted in damage that was concentrated within the City of Santa Rosa and related to partial collapse and near collapse of URM buildings and wood frame buildings with substandard foundations or inadequate bracing. Other common impacts from earthquakes include damage to infrastructure and buildings (e.g., crumbling of URM, failure of architectural facades, rupturing of underground utilities, and road closures). Earthquakes also frequently trigger secondary hazards, such as dam and levee failures, flooding, and fires that can become disasters themselves.

FEMA’s loss estimation software, Hazus-MH was used to analyze the City’s vulnerability to earthquakes, at the census tract level (for three tract units that cover the City of Sebastopol, displayed in Figure 4-7 below). Note that these census tract boundaries do not neatly line up with the City’s boundary, and as such a slightly larger area than that covered in this planning context was necessary to include Sebastopol.

Because of these boundary differences, the damage and loss estimates may be slightly exaggerated (given the larger coverage of structures and population).
2,500-Year Probabilistic Earthquake Scenario
A Level 1 Plus 2,500-year probabilistic Hazus earthquake scenario was performed for Sebastopol. This Level 1 Plus analysis is a standard Level 1 Hazus model that incorporates liquefaction susceptibility data and updated parameters that were imported into Hazus to enhance the accuracy of earthquake modelling for the City. Loss estimates and vulnerability assessment discussions are based on the following subsections: property; people; economy; critical facilities and infrastructure; historic, cultural, natural resources; and future development in the Planning Area.

The total losses by census tract are shown in Figure 4-7. Refer to Section 4.1.1 and FEMA’s Hazus 4.2 Loss Estimation Tool for more information on the Hazus tool and its analysis functions. This methodology was selected to support the vulnerability assessment, as it is a national standard for modelling earthquake loss.

To evaluate potential losses associated with earthquake activity in the Planning Area, the Hazus Level 1 Plus 2,500-year probabilistic scenario including liquefaction susceptibility was run for the City’s three census tracts, using a Magnitude of 7.0 as the driving magnitude parameter. Due to these inputs, this 2,500-year scenario is representative of a worst-case level of shaking that considers multiple faults in the region. Hazus estimates the number of people displaced, the number of buildings damaged and their type (e.g., construction material, occupancy class), the number of causalities, and the damage to transportation systems and utilities (e.g., critical facilities).

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-36 Figure 4-7: Hazus 2,500 Year Probabilistic Earthquake Scenario with Liquefaction Susceptibility – Total Losses by Census Tract Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-37 A summary of the key losses based on the Hazus earthquake analysis results included the following:
• Total economic loss estimated for the earthquake was $782.5 million, which includes building losses and lifeline related losses based on the Hazus inventory for the Planning Area. • Building-related losses, including direct building damages and business interruption losses, totaled $767.9 million.
• $108.2 million in losses came from income related losses from wage-related, capital-related, rental properties, and relocation costs, while almost $767.9 million came from capital stock losses related to structural, contents-based, and inventory property categories. • 14 percent of the estimated losses were related to business interruptions. • 3,243 buildings (51 percent of total in the region) would be at least moderately damaged; 413 of those buildings would be damaged beyond repair.
• Residential structures made up 57 percent of the total earthquake-induced losses. • $11.6 million in losses are associated with transportation system economic damages and losses (e.g. highways, buses, airport facilities and related infrastructure). • $2.9 million in losses are associated with utility and lifeline system economic damages and losses (e.g. potable water, wastewater, natural gas, oil systems, communications, and related infrastructure). • The mid-day earthquake (2 p.m.) caused the most injuries and casualties: 273 injuries and 19 fatalities. • The model estimates that a total of 119,000 tons of debris will be generated. Brick and wood structures will comprise 34 percent of the total, with the rest being reinforced concrete and steel materials.
• Around 5,619 households are expected to suffer from potable water or electric power losses, or both, in the first day of the earthquake event.
• Of the total 18 essential facilities considered by the Hazus earthquake scenario for the Planning Area (hospitals, schools, emergency operations centers, police stations, and fire stations), all 18 will be at least moderately damaged.
• Before the earthquake, the region had 82 hospital beds available for use. On the day of the earthquake, the model estimates that only 23 hospital beds (28 percent) would be available for use by patients already in the hospital and those injured by the earthquake.

Property
Significant earthquakes can cause damages to buildings, private and public property, and other infrastructure. The number of properties at risk is also based on when the majority of development was constructed in the City’s Planning Area and whether that development was developed after the City adopted the latest state seismic code. The CBC was modified several times since 1960, which resulted in code requirements that directly affected the structural integrity of development in California. According to the HMPC, the City adopted the 2019 CBC, which includes the building and seismic code improvements, and most redevelopment in the City’s Planning Area occurred during the past 40 years when the City enforced the new code requirements. The Hazus earthquake results also accounted for the improved seismic codes in the model, given the City requires new construction in the to comply with the 2019 CBC.

Hazus estimates that 3,243 buildings (51 percent of the total buildings in the region) would be at least moderately damaged, and of these 413 buildings would be damaged beyond repair by the earthquake scenario. A majority of the buildings experiencing damage are residential structures, and wood frame construction makes up the majority of building/structure material in the planning area’s inventory. Figure 4-8 shows a graph and table that summarize the specific estimated damages to buildings based on occupancy and damage category.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-38 Figure 4-8: Estimated Building Damage by General Occupancy Type and Damage Category Source: Hazus 4.2
With a majority of the buildings in the Planning Area being residential, the Hazus model estimates that over 57 percent of the total losses incurred by this earthquake scenario are single family homes and other residential categories. The building inventory in the region varies in terms of construction types. A large number of buildings are also constructed of wood materials, though the building inventory includes URM buildings and manufactured housing. These types of wood, masonry, and manufactured housing structures are particularly vulnerable to ground shaking in an earthquake event. Table 4-9 describes the Hazus results of expected building damage by building type. Most buildings/structures found are expected to sustain slight to moderate damages.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-39 Table 4-9: Expected Building Damage by Building Type (All Building Design Levels) Source: Hazus 4.2
People
Hazus estimates the number of people that would be injured or killed by the 2,500-probabilistic earthquake scenario, which includes liquefaction susceptibility. The causalities are broken down into four severity levels. Level 1 means that injuries occur but do not need hospitalization (i.e. the lowest level, causing the least damages or injuries) to Level 4, where victims are killed by the earthquake (i.e. the highest, or worst, of the levels). The estimates are provided for three times of the day which represent the periods of a standard working day when different sectors of the community are likely at their peak occupancy loads (e.g., in business/office settings versus residing at home).

As shown in Table 4-10 below, the highest number of injuries and casualties are estimated to occur in the early afternoon (2 p.m.) with the greatest impacts on the commercial and educational sectors when those sector loads are considered to be at their maximum. The 2 p.m. time has the greatest potential for fatalities, with an estimate of 19, followed by the 5 p.m. scenario which estimates 14 fatalities (more information below).

Some populations in the Planning Area may be more vulnerable to an earthquake event than others. For example, those with mobility issues as well as the elderly may have challenges with evacuating or traveling to a shelter without assistance if they cannot stay in their homes. Other vulnerable populations may be individuals whom English is not their native language. Of these socially vulnerable populations and according to the census tracts and block groups in the City, several of these populations are anticipated to reside within central Sebastopol and within older housing that may have been constructed prior to the seismic code improvements.

According to 2015-2019 American Community Survey (ACS), 9.8 percent of individuals in the City speak a language other than English in their home. These individuals may not receive or understand evacuation information including where shelters are located or where to receive resources to aid in the recovery process. These same individuals and households are designated as socially vulnerable populations, many which reside in the downtown area. Figure 4-9 shows a graph from the Hazus report estimates for the total number of households expected to be displaced as result of the earthquake. The report estimates 378 households to be displaced, and of those, 195 individuals will be seeking temporary shelter. This does not take into account future population growth or other variables, such as populations increases due to tourism.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-40 Figure 4-9: Displaced Households and Persons Seeking Shelter Estimates Source: Hazus 4.2
Table 4-10 shows the Hazus estimates for total casualties and injuries related to the earthquake scenario.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-41 Table 4-10: Casualty and Injury Estimates from Hazus Results Source: Hazus 4.2
Economy
Earthquakes can have a severe impact on local and regional economies. Impacts can be both direct, such as damages to commercial and residential structures, as well as indirect such as cascading effects involving business interruptions due to employees being displaced from their homes. Another secondary or cascading impact an earthquake could have on Sebastopol is causing damages to transportation infrastructure that is critical to employees and business activity in the region. Based on the Hazus results, Sebastopol could potentially experience approximately $782.5 million in economic losses. This amount includes both income losses (estimated to be $108.2 million) as well as capital stock losses ($659.7 Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-42 million). Another secondary impact of an earthquake is business disruption and the resulting economic loss as a result of that disruption. Hazus describes business interruption losses as those losses associated with the inability to operate a business because of the damage sustained by the earthquake and includes the temporary living expenses for individuals displaced from their homes.

Hazus also estimates the total building-related losses. This includes business interruption losses and direct building losses (the estimated costs to repair or replace the damage caused to buildings and its contents) at $767.9 million, 14 percent of which are related to business interruption in the region. As shown in the pie charge and graphs in Figure 4-10 and Table 4-11 below, the largest loss in this scenario consisted of non-structural losses, which are sustained by residential occupancies, making up 57 percent of total loss.

The following figures and tables report the estimate of losses by loss type, occupancy type, and buildingrelated loss.

Figure 4-10: Economic Loss Estimates by Loss Type and Occupancy Type Source: Hazus 4.2
Table 4-11: Building-Related Economic Loss Estimates (Millions of dollars) Source: Hazus 4.2
In addition to economic losses experienced by building-related losses, Hazus estimates the economic losses as a result of transportation and utility lifeline losses and the direct repair cost for each component.

As shown in Table 4-12 and Table 4-13 below it is estimated that $11.6 million will be lost as a result of Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-43 damages to transportation components and $2.9 million are expected to be lost as result of utility system damages.

Table 4-12: Transportation System Economic Losses (Millions of Dollars) Source: Hazus 4.2
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-44 Table 4-13: Utility System Economic Losses (Millions of Dollars) Source: Hazus 4.2
Critical Facilities and Infrastructure
Large seismic events could have catastrophic effects on the City and surrounding areas, possibly damaging transportation and utility lifelines, bridges, railroads, and other critical facilities and infrastructure, similar to the economic losses associated with these facilities. Hazus estimates impacts to essential facilities including hospitals, schools, Emergency Operations Centers (EOCs), police stations, and fire stations. The Hazus analysis also takes into account hazardous material sites; however, no hazardous material sites, nuclear power plants, or military installations fall within the City’s Planning Area.

According to the earthquake analysis, there is one hospital with a total capacity of 47 beds, 15 schools, Hazus data; current data lists 37 beds are available [American Hospital Directory 2020]). Hazus estimates that 18 of these essential facilities are expected to suffer moderate damage. With regards to Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-45 transportation systems, 11 bridges will suffer at least moderate damage, and one will suffer complete damage. However, no utility system facilities will suffer moderate damage.

Table 4-14, Table 4-15, and Table 4-16 summarize the expected damages generated by the Hazus scenario for each type of transportation system and utility system, including pipelines in the area.

Table 4-14: Expected Damage to the Transportation Systems Source: Hazus 4.2
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-46 Table 4-15: Expected Utility System Facility Damage
Source: Hazus 4.2
Table 4-16: Expected Utility System Pipeline Damage (Site Specific) Source: Hazus 4.2
Historic, Cultural, and Natural Resources
An earthquake in the City’s Planning Area or in the surrounding region could cause cascading (secondary) effects, including dam or pipeline failure that would impact the natural environment in different ways, depending on the extent of the cascading hazard. For example, earthquake-induced landslides or debris flows could significantly damage habitat and re-route streams and waterways, causing water quality impacts. Other types of ground deformation could also result. Earthquakes could also damage historic buildings in downtown Sebastopol.

Magnitude 7 Deterministic ShakeMap Earthquake Scenario
A second Hazus-based earthquake scenario was run for the Sebastopol Planning Area using census tract units that was based on a deterministic model. A deterministic scenario relies on seismic data to predict the outcome of a specific earthquake event. This deterministic scenario used USGS provided ShakeMap datasets to model what a Magnitude 7.2 earthquake of the Hayward-Rodgers Creek Faults (Rodgers Creek – Hayward North Fault [northern portion of the Hayward fault]) would generate in terms of damages and losses for the chosen area of interest (i.e. the Sebastopol Planning Area). These faults were selected because they are known to have caused seismic activity and pose a risk to Sebastopol and nearby communities. The M7.2 ShakeMap scenario datasets used to import into Hazus 4.2 include four USGSprovided key data layers in spatial format: peak ground velocity, peak ground acceleration, peak spectral acceleration for 0.3 seconds (0.3 percent g, or gravitational velocity), and peak ground acceleration for 1.0 seconds (1.0 percent g) (USGS 2016a). The epicenter of this USGS modeled scenario is located at latitude Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-47

38.43 North and 122.68 West and had a depth of 8.7 kilometers. A fifth layer of liquefaction susceptibility was also included in the Hazus model, which is the same utilized in the previous 2,500-year probabilistic scenario to enhance the model with more accurate ground and soil conditions. Figure 4-11 includes the general location of the scenario’s epicenter (marked with a start northeast of the San Francisco Bay area) as well as intensity information and reference to the USGS ShakeMap data.

Figure 4-11: USGS Generated ShakeMap Earthquake Scenario for Hayward-Rodgers Creek Faults Sources: USGS 2016b, Weld, et al 1999
For more information on the USGS generated ShakeMap scenarios, modeling criteria, manual information, and overall catalog of available data refer to the USGS Earthquake Hazards Program ShakeMap information page.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-48 This deterministic M7.2 scenario results included liquefaction susceptibility. Loss estimates and vulnerability assessment were completed based on the following subsections, similar to the previous scenario: property; people; the local economy; critical facilities and infrastructure; historic, cultural, natural resources; and future development of the Planning Area. The total losses by census tract from this M7.2 scenario are shown in Figure 4-11 and summarized in Table 4-17 below.

Refer to Section 4.1.1 and FEMA’s Hazus 4.2 Loss Estimation Tool for more information on the Hazus tool and its analysis functions. As stated in the previous section, Hazus is a loss estimation tool which derives totals on the number of people displaced, the number of buildings damaged and their type (e.g. construction material, occupancy class), the number of causalities, and the damage to transportation systems and utilities (e.g. critical facilities) given the input parameters, scenario type, and region/area of interest. In comparison to the Hazus Level 1 Plus 2,500 Probabilistic Scenario, the deterministic M7.2 scenario results in $66.7 million in total economic losses (compared to $782.5 million), 340 buildings with moderate to complete damage (compared to 3,243 buildings), 61 percent of damage related to residential structures (compared to 57 percent), $53,800 in transportation system economic losses (compared to $11.6 million), eight casualties and no fatalities (compared to 273 casualties and 19 fatalities), and 6,000 tons of debris generation (compared to 119,000 tons). This scenario also results in eight displaced households (compared to 378 households) compared to the 2,500 probabilistic scenario.

Table 4-17: Hazus 4.2 Deterministic M7.2 Earthquake Scenario Loss Estimations for Sebastopol’s Census Tracts
Type of Impact Impacts to Planning Area
Total Buildings Damaged
Slight: 1274
Moderate: 301
Extensive: 35
Complete: 4
Building and Income Related Losses
$ 66.5 million
61 % of damage related to residential structures
12 % of loss due to business interruption
Total Economic Losses (Includes building, income,
and lifeline/critical facility losses) $ 66.7 million
Casualties (Based on a 2 a.m. time of occurrence)
Without requiring hospitalization: 4
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Casualties (Based on a 2 p.m. time of occurrence)
Without requiring hospitalization: 7
Requiring hospitalization: 1
Life threatening: 0
Fatalities: 0
Casualties (Based on a 5 p.m. time of occurrence)
Without requiring hospitalization: 5
Requiring hospitalization: 1
Life threatening: 0
Fatalities: 0
Transportation System Economic Losses
$ 0 damages to highway
$ 53,800 in damages to bridges
$ 0 damages to airport facilities or runways
$ 0 damages to bus facilities
$ 0 damages to light rail
$ 0 damages to ferry and port facilities
Damage to Transportation Systems
0 damages to highway or bridges
0 damages to airport facilities or runways
0 damages to bus facilities
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-49 Type of Impact Impacts to Planning Area
0 damages to light rail
0 damages to ferry and port facilities
Damage to Essential Facilities 0 damages to schools, police stations, fire stations, emergency operations centers, or hospitals
Damage to Utility Systems
2 of the following facilities will suffer damages: potable water; wastewater; natural gas; oil systems; electrical power; and communications.

Potable water breaks: 5
Wastewater breaks: 3
Natural gas breaks: 0
Oil pipeline breaks: 0
Households without Power/Water Service (Based
on 26,824 total households)
Power loss, Day 1: 0
Power loss, Day 3: 0
Power loss, Day 7: 0
Power loss, Day 30: 0
Power loss, Day 90: 0
Water loss, Day 1: 0
Water loss, Day 3: 0
Water loss, Day 7: 0
Water loss, Day 30: 0
Water loss, Day 90: 0
Displaced Households 8
Persons Seeking Temporary Shelter 4
Debris Generation 6,000 tons
Sources: Hazus 4.2, USGS ShakeMap M7.2 Scenario for Hayward-Rodgers Creek Future Development
The Hazus scenario only estimates damage and causalities for existing building inventory and populations and does not consider future development plans. The City of Sebastopol has experienced growth over the 10 years since the last Census (2010-2020), after experiencing a drop in population of approximately 5 percent since the 2000 Census which was the City’s highest recorded population. (Refer to Chapter 2:

Community Profile for further discussion on demographics and population changes). The latest California Department of Finance (DOF) estimates show the population is 7,745 as of 2020, which reflects a growth rate of 4.7 percent since 2010 (DOF 2020).

According to the Sonoma County Economic Development Board’s 2018 City Profile and Projection’s Report for Sebastopol, the City is expected to grow in population to 7,955 by 2022 and according to the City’s General Plan Housing Element by an additional 455 residents by 2030, resulting in a 5.5 percent increase over the next decade. This is on the lower end of growth rates for Sonoma County, but still outpaces the state and national projected growth rates. With this in mind, the Sebastopol General Plan and its Safety Element establishes goals to protect the safety of life and property by ensuring emergency preparedness with efforts to increase community awareness of earthquakes and preparedness measures. New development projects will also be reviewed for conformance with applicable City building standards related to geologic and seismic safety.

Risk Summary
• The overall risk significance of earthquake hazards to the City of Sebastopol is High. • Earthquakes and seismic activity are expected to have a probability of occasional occurrence in the future, given the local seismic conditions, past history, and input from the City.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-50 • Two earthquake faults of concern can affect the City: the San Andreas Fault and the HealdsburgRodgers Creek Fault (modelled as the Hayward – Rodgers Creek Fault), both are considered to be active faults that may lead to damages or losses in the future. • The City of Sebastopol was one of the first cities to enact an URM Ordinance following two earthquakes that occurred in 1969 and originated near the juncture of the Rodgers Creek and Healdsburg Fault.
• The majority of the Planning Area is within very low liquefaction susceptibility zones, with some areas on the far west and far east of the City extending into very high liquefaction susceptibility zones. • Climate change is not expected to directly affect earthquake frequency or intensity but it could exacerbate secondary impacts by increasing the frequency and intensity of extreme precipitation events. This could increase the likelihood of landslides and liquefaction events during an earthquake, therefore climate change would have a “low” influence on earthquake hazards. • Based on the first Hazus earthquake analysis, it is expected that a 2,500-year probabilistic earthquake (worst-case scenario) with liquefaction susceptibility taken into account would cause $782.5 million in total economic losses. A M7 deterministic earthquake scenario would cause $66.7 million in total economic losses.
• Under the 2,500-year probabilistic scenario, it is estimated that 3,243 buildings (51 percent of total in the region) would be at least moderately damaged; 413 of those buildings would be damaged beyond repair. Residential structures made up 57 percent of the total earthquake-induced losses. In comparison, the M7 deterministic scenario would result in 340 buildings being moderate to complete damage.
• The 2,500-year probabilistic Hazus scenario resulted in $11.6 million in losses to the transportation systems, while $2.9 million would be incurred in damages and losses to the utility and lifeline systems.

Around 5,619 households would be affected by potable water or electric power losses, or both, on the first day of this earthquake scenario.
• The 2,500-year probabilistic Hazus scenario also estimates that around 119,000 tons of debris would be generated, while the M7 deterministic scenario resulted in 6,000 tons of debris, with wood, masonry, and manufactured housing structures suffering the most. • The potential for casualties during the worst-case 2,500-year probabilistic scenario for which time of day of the earthquake might hit (the 2 p.m. scenario) would lead to 19 fatalities and 273 injuries, while the M7 deterministic scenario resulted in only eight casualties and no fatalities.

4.3.2 Flooding
Hazard Description
Floods are among the most frequent and costly natural disasters in terms of human hardship and economic loss. Flooding is usually the result of, or often exacerbated by, weather events. Floods can cause substantial damage to structures, landscapes, and utilities as well as life safety issues. Certain health hazards are also common to flood events; standing water and wet materials in structures can become breeding grounds for microorganisms such as bacteria, mold, and viruses. Standing water or affected infrastructure can in turn cause disease, trigger allergic reactions, and damage materials long after the flood. When floodwaters contain sewage or decaying animal carcasses, infectious disease also becomes a concern. Direct impacts such as drowning can be limited with adequate warning and public education about what to do during floods. Where flooding occurs in populated areas, warning and evacuation will be of critical importance to reduce life and safety impacts.

Floodplains are defined as the areas immediately adjacent to a channel from a river, stream, or other waterway. Floodplains are illustrated on inundation maps, which show areas of potential flooding and Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-51 water depths. In its common usage and based on FEMA guidelines, the floodplain most often refers to the area that is inundated by the 100-year flood, or the flood that has a one percent chance in any given year of being equaled or exceeded. The 100-year flood is the national minimum standard to which communities regulate their floodplains through the FEMA National Flood Insurance Program (NFIP). The 500-year flood is the flood that has a 0.2 percent chance of being equaled or exceeded in any given year.

A 500-year flood event would be slightly deeper and cover a greater area than a 100-year flood event.

The potential for flooding can change and increase through various land use changes and changes to land surface, which then may result in a change to the floodplain. A change in environment can create localized flooding problems inside and outside of natural floodplains by altering or confining natural drainage channels. These changes are most often created by human activity.

The City of Sebastopol is susceptible to various types of flood events as described below. • Riverine Flooding - Riverine flooding, defined as the condition when a watercourse (e.g. river or channel) exceeds its “bank-full” capacity, generally occurs as a result of prolonged rainfall, or rainfall that is combined with already saturated soils from previous rain events. This type of flood occurs in river systems whose tributaries may drain large geographic areas and include one or more independent river basins. The onset and duration of riverine floods may vary from a few hours to many days. Factors that directly affect the amount of flood runoff include precipitation amount, intensity and distribution, the amount of soil moisture, seasonal variation in vegetation, snow depth, and waterresistance of the surface due to urbanization. In the Planning Area, flooding is largely caused by heavy and continued rains, increased outflows from upstream dams, and heavy flow from tributary streams.

Local intense storms can overwhelm nearby waterways as well as the integrity of flood control structures. The warning time associated with slow rise floods assists in life and property protection. • Localized Flooding - Flash flooding describes localized floods of great volume and short duration.

This type of flood usually results from a heavy rainfall on a relatively small drainage area. Precipitation of this sort usually occurs in the winter and spring. Flash floods often require immediate evacuation.

Related to this type of flooding is also localized flooding, which is often caused by flash flooding, severe weather, or an unusual amount of rainfall. Flooding from these intense weather events occurs in areas experiencing an increase in runoff from impervious surfaces associated with development and urbanization as well as inadequate storm drainage systems. • Dam Incident Flooding – Potential inundation caused by failure or mis-operation of one or more upstream dams is also a concern to the City. A catastrophic flood control or water retention structural failure could easily overwhelm local response capabilities and require evacuations. Impacts to life safety will depend on the warning time and the resources available to notify and evacuate the public, as well as the magnitude of the event. Loss of life could potentially result, however, and there could be associated health concerns as well as negative effects to local buildings and infrastructure. Dam failure and associated inundation is addressed in more detail under Section 4.3.5 Dam Incidents.

Location
Significant – Flooding of various types may occur anywhere within the City’s Planning Area, and the City has a history of flooding along the Laguna de Santa Rosa. Details on local water features, watersheds, and flood control structures and systems are provided below.

City of Sebastopol Watersheds and Waterways
The City of Sebastopol is primarily located within the Mark West Creek and Lower Russian River watersheds, under Hydrologic Unit Codes (HUCs): 1801011007 and 18011009. The main waterways in the City include the Laguna de Santa Rosa, Atascadero Creek, Zimpher Creek, Calder Creek, and Pleasant Hill Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-52 Creek. The Laguna de Santa Rosa is one of the largest freshwater marshes in Northern California and has a critical role in reducing flood levels in the Russian River. The Laguna de Santa Rosa drains on the eastern side of Sebastopol and is identified in the Sonoma County Flood Insurance Study (FIS) as being the principal flooding problem for the City due to ponding on the lower reaches caused by backwater from the Russian River. The City’s watersheds are described below and illustrated in Figure 4-12.

Mark West Creek Watershed (HUC 1801011007) – Mark West Creek is the second largest subwatershed in the Russian River basin (CDFW 2018). The Mark West Creek Watershed covers approximately 254 square miles (162,792 acres) and spans the cities of Sebastopol, Santa Rosa, and Cotati all within Sonoma County.

Lower Russian River Watershed (HUC 1801011009) – The Lower Russian River Watershed covers approximately 382 square miles (94,495 acres) and covers the western side of Sebastopol. The Lower Russian River is a sub-watershed to the larger Russian River Watershed.

Local and Regional Drainage Facilities
Major drainage features within the Planning Area or managed by the City include:
• Laguna De Santa Rosa
• Atascadero Creek
• Zimpher Creek
• Calder Creek
• Pleasant Hill Creek
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-53 Figure 4-12: Watersheds in the City of Sebastopol Planning Area Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-54 Floodplain Mapping and Studies
FEMA established standards for floodplain mapping studies as part of the NFIP (FEMA 2019). The NFIP makes flood insurance available to property owners in participating communities adopting FEMAapproved local floodplain studies, maps, and regulations. Floodplain studies that may be approved by FEMA include federally funded studies; studies developed by state, city, and regional public agencies; and technical studies generated by private interests as part of property annexation and land development efforts. Such studies may include entire stream reaches or limited stream sections depending on the nature and scope of a study. An overview of floodplain mapping is provided below.

Flood Insurance Study (FIS) - The FIS develops flood-risk data for various areas of a community that are used to establish flood insurance rates and assist the community in its efforts to promote sound floodplain management. The latest FIS applicable to the City of Sebastopol was included in a five-volume report along with other incorporated jurisdictions and unincorporated areas studied in Sonoma County; this recent report was last revised March 7, 2017 (FEMA 2017a).

Flood Insurance Rate Map (FIRM) - The FIRM is designed for flood insurance and floodplain management applications. For flood insurance, the FIRM designates flood insurance rate zones to assign premium rates for flood insurance policies. The designated flood zones are based on flood risk in the area.

For floodplain management, the FIRM delineates 100- and 500-year floodplains, floodways, and the locations of selected cross sections used in the hydrology and hydraulic analyses and local floodplain regulations.

Land areas that are high risk within the 100-year floodplain (meaning they have a one percent annual chance of flooding), are called Special Flood Hazard Areas (SFHAs) and are mapped as A or AE zones. The difference between A and AE zones are the level of detail in analysis and mapping, so that A zones are more general while AE contain additional detail and also display Base Flood Elevations, or BFEs. In communities that participate in the NFIP, mandatory flood insurance purchase requirements apply to Zones A and AE (i.e., those areas subject to a 100-year flood event).

The City of Sebastopol FIRMs, as with most portions of California and larger developments across the U.S., have been replaced by new digital flood insurance rate maps (or DFIRMS) as part of FEMA’s Risk Map and Map Modernization programs. DFIRMs and related datasets (e.g. cross sections used in floodplain studies and analyses, BFEs, etc.) are now delivered via National Flood Hazard Layer (NFHL) databases, accessible for free online at FEMA’s Flood Map Service Center site.

These digital DFIRMs achieve the following purposes:
• Incorporate the latest flood study updates (LOMRs and LOMAs); • Utilize community supplied data;
What are flood zones?
Flood zones are geographic areas on a flood map that indicate flood risk. Zones are determined by assessing the expected height of a flood that has a 1 percent chance of occurring in any given year (“100-year flood”), as well as potential wave heights, the distance from the nearest water body, and the ground elevation. While there is only a 1 percent chance of a flood of such magnitude to occur every year, there is a 26 percent chance of such a flood to occur over the lifecycle of a 30-year mortgage.

Source: Wetlands Watch 2019
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-55 • Verify the currency of the floodplains and refit them to community supplied base maps and base data • Upgrade the FIRMs to a GIS database format to set the stage for future updates and to enable manipulation, storage, and support for GIS analyses and other digital applications; and • Solicit community participation.

The most current DFIRMs for the City of Sebastopol and other jurisdictions or unincorporated areas within Sonoma County are included in the County’s NFHL database. The latest effective date for studies in the County is March 7, 2017 (FEMA 2017a). The spatial features available in this NFHL database, such as floodplains, cross sections and the discharge summary table were used for the analyses and mapping in this plan as they relate to flooding hazards.

Letter of Map Revision (LOMR) and Letter of Map Amendment (LOMA) - LOMRs and LOMAs represent separate floodplain studies dealing with individual properties or limited stream segments that update the FIS and FIRM data (as revisions or amendments) between periodic FEMA publications of the FIS and FIRM products.

Major Sources of Flooding
General rainfall floods, primarily associated with the rainy season, taking place from October to May, with the most severe flooding, historically, taking place between December and March (City of Sebastopol 2015). This type of flood results from prolonged heavy rainfall over tributary areas and is characterized by high peak flows of moderate duration. Flooding is more severe when antecedent rain has resulted in saturated ground conditions.

In the more urbanized areas of Sebastopol, flood problems intensify because the immediate areas are developed and contain mostly impervious surfaces such as roads and paved structures. Because of this, the nearby open land available to absorb rainfall and runoff is often limited or difficult to access naturally.

In other words, the decrease in the amount of open land that can absorb precipitation increases the volume of water that must be carried away by waterways and developed infrastructure, causing localized flash flooding and stormwater issues.

The latest FEMA NFHL data indicates that 100- and 500-year floodplains are predominantly located east and west of the City, along the Laguna de Santa Rosa and Atascadero Creek (see Figure 4-13) While the Laguna de Santa Rosa is critical to reducing flood levels in the Russian River, ponding due to backwater from the Russian River on the lower reaches of Laguna de Santa Rosa is identified in the Sonoma County FIS as the principal flooding problem for the City. The river levels and flooding in the City are not directly related, in addition to the ponding in the Laguna de Santa Rosa other factors including tidal influences affecting the Russian River, the intensity and duration of precipitation, soil saturation can also lead to flood impacts in Sebastopol (City of Sebastopol 2016). Based on input received from the public survey for the planning process, the following were identified by members of the public as being problem areas for flooding:
• Laguna floodplains along Highway 12 impact exit on main highway out of City, and • Heavy rains flowing on Edman Way into businesses on Main Street.

The Sonoma Water Authority was enabled, in 1958, to create several geographic zones encompassing major watersheds in the County to finance development and maintenance of flood protection projects as flood control zones. The City falls in Zone 1A1, named the Laguna de Santa Rosa – Mark West Creek Watershed. Zone advisory committees exist which are in charge of prioritizing, managing, and approving zone related capital improvement plans (CIPs) and projects such as flood protection and drainage facility works, natural waterway maintenance, plan development, erosion and sedimentation control activities, and others also pertaining to Sonoma Water Authority’s goals and objectives.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-56 Localized Flooding Problem Areas
Based on historical occurrence data, the eastern portion of the City along Sebastopol Avenue, and Morris Street, near the Laguna de Santa Rosa floodplain, as well as Pleasant Hill Road have been severely impacted by flooding from storms and flash floods compared to other locations in the City’s Planning Area. More information on potential flood-affected areas is summarized in the Previous Occurrences subsection and the Vulnerability Assessment (including the Insurance Coverage and NFIP Claims and Losses subsection, information on repetitive loss properties and Community Information System [CIS] records for Sebastopol).

Extent (Magnitude/Severity)
Limited – Flood maps can be used as an indicator of flood extent. Flood depth and velocity also affect the extent of flood hazards and resulting damage. The deeper and faster flood flows become, the more damage they can cause in a community. However, shallow flooding with high velocities (e.g., such as a flash flood event caused by precipitation) can cause as much damage as deep flooding with a slow velocity (e.g., from a riverine flood event). This typically happens when a channel migrates over a floodplain and redirects flows and transports debris and sediment.

While cities can implement measures to prevent or reduce the severity and magnitude of flood hazards, some level of risk often remains. These types of threats include upstream dam failure, infrastructure failure, and severe flood events that exceed flood design standards or drainage capacity, leading to flash flooding. Flood severity can be determined by logging peak discharge flows. This information is tracked by both FEMA and the USGS. FEMA’s BFE depth datasets can provide further insight as to how much of a community gets flooded and where exactly, enhancing the level of detail on the magnitude of flooding that can affect a particular community. Based on the most recent NFHL database from FEMA (which includes these BFEs), the City’s Planning Area is expected to experience the worst flooding conditions on the east with pockets of deep inundation across the central portions of the Laguna de Santa Rosa near the commercial areas, and along Sebastopol Avenue, SR 12, a major state highway into the City.

Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-57 Figure 4-13: FEMA Special Flood Hazard Areas for the 1% and 0.2% Annual Chance Flood Events in the City of Sebastopol Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-58 Other localized flooding from existing stormwater infrastructure, for example, is more difficult to estimate but could happen anywhere in the Planning Area and could be severe depending on the flood event itself and the conditions of the existing infrastructure. Table 4-18 below summarizes the general FEMAavailable flood zones for context.

Table 4-18: FEMA’s Special Flood Hazard Area Zone Descriptions Flood Zone Definition
FEMA Special Flood Hazard Areas (SFHA) Subject to Inundation by the 100- or 500-Year Floods Zone A 100-year floodplain, or areas with a 1% annual chance of flooding. Because detailed analyses are not performed these areas, no depths or base flood elevations are shown in Zone A areas.

Zone AE Detailed studies for the 100-year floodplain. The base floodplain where base flood elevations are provided.

AE Zones are now used on new format FIRMs instead of A1-A30 zones.

Zone AH Areas with a 1% chance of shallow flooding, usually in the form of a pong with an average depth ranging from 1 to 3 feet. These are flood elevations derived from detailed analyses.

Zone AO
River or stream flood hazard areas and areas with a 1% or greater chance of shallow flooding each year, usually in the form of sheet flow, with an average depth ranging from 1 to 3 feet. Average flood depths derived from detailed analyses.

Zone A99
100-year floodplain, areas with a 1% annual chance of flooding that will be protected by a federal flood control system where construction has reached specified legal requirements. No depths or base flood elevations are shown within these zones.

Other Flood Areas
Floodway
A regulatory floodway is the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated height.

Zone X (shaded)
Areas with a 0.2% annual chance flooding (1 in 500 chance), between the limits of the 100-year and 500-year floodplains. This zone is also used to designate base floodplains of lesser hazards, such as areas protected by levees from the 100-year flood, shallow flooding areas with average depths of less than one foot, or drainage areas less than 1 square mile.

Zone X (unshaded) 500-year floodplain (0.2% annual chance). Area of minimal flood hazard.

Source: FEMA Flood Map Service Center 2018
Previous Occurrences
The City has historically been impacted by flooding from sources such as general riverine and flash flooding from winter and early spring storms as well as atmospheric river events. Historical records are described below but may not represent all historical events. The records below are from several sources, including NOAA’s National Center for Environmental Information (NCEI) Storm Events database; 2016 Sonoma County Operational Area HMP; and the 2018 California SHMP.

January 1, 1997 – Heavy rain fell for several hours across Sonoma County. The Russian River at the Hacienda Bridge was above the flood stage and continued to rise during the event.

December 31, 2005 – Widespread flooding occurred across Sonoma County causing many of the mainstem river gages along the Russian River to remain above flood stage for several days. In Sebastopol, water covered Morris Street from Sebastopol Avenue to the Sebastopol Community Cultural Center. Flood waters approximately 18 inches to 2 feet deep engulfed the City’s sewer pump station and caused damages to buildings at the Barlow Company. Countywide property damages are estimated to be Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-59 $104,000 and $3,000,000 crop damages were reported.

This flood event led to a State and Federal Disaster
Declaration.

January 6, 2016 – A strong Pacific storm produced heavy rain flooding and damaging winds. In Sebastopol,
roadways were completely flooded including Pleasant Hill Road at Mitchell Court.

February 7, 2017 – An atmospheric river swept through
the Bay Area beginning on the night of February 6th. This system produced widespread roadway flooding, debris
flows, and strong winds. In Sebastopol roadway flooding was reported at Pleasant Hill Road and Grundel Drive.

February 13, 2019 – An atmospheric river with an
associated cold front moved through the region from
February 12th to the 15th bringing widespread flooding
and debris flows. Multiple mainstem rivers flooded
prompting evacuations from local officials. Strong wind gusts caused downed trees, power outages, and structural damage. Roadways in Sebastopol were reported to include flooding including SR 116 and Sparkes Road, Pleasant Hill Road and Bodega Avenue.

February 26, 2019 – An atmospheric river moved through the region near the end of the month with the most impacts occurring in the North Bay. Widespread flooding was reported from minor roadway flooding to major flooding of mainstem rivers. The Russian River reached major flood stage (45.5 feet highest level in almost a quarter century) flooding dozens of homes and businesses prompting mandatory evacuations and rescues. Flooding in the westbound lane of SR 116 at Sparkes Road was reported in Sebastopol. Low-lying areas within and close to the Laguna de Santa Rosa floodplain experienced significant flooding including, an estimated two-thirds of the Barlow shopping district, a neighborhood on Flynn Street, Park Village a City-owned low-income housing site, and the Community Cultural Center located on Morris Street. The Sebastopol Fire Department performed water rescues within these areas.

Countywide the flood events is estimated to have resulted in $150 million in private property damages and $77.9 million in public infrastructure, emergency response costs, and debris removal.

November 30, 2019 – A low pressure system moving in from the Gulf of Alaska and drawing in moisture from the tropics combined to bring the first atmospheric river event of the winter season to the Greater Bay Area. This system brought widespread heavy rainfall, roadway flooding, and strong winds to the region. Flooding from the Atascadero Creek was reported in the South Bound lanes of Watertrough Road and Blackney Road, causing vehicle hydroplaning due to large amount of water in the roadway.

Probability of Future Occurrences
Likely – The 100-year flood is the flood that has a one percent chance in any given year of being equaled or exceeded, while the 500-year flood is expected to have a 0.2 percent chance of occurring (or being exceeded) in any year, respectively. As such, it is likely that riverine flooding will occur in the future, though localized stormwater flooding and general flash flooding is also expected to take place especially during the wet months and heavy rain or storm events.

Climate Change Considerations
Emerging findings from California’s Fourth Climate Assessment show that costs associated with direct climate change impacts by 2050 will be dominated by human mortality, coastal damage, and the potential In February 2019, several businesses in The Barlow
shopping district suffered from flooding after an
atmospheric river stalled over the Russian River
Watershed. An estimated two-thirds of the shopping
district was flooded, including Park Village, a Cityowned low income housing site, and the Community Cultural Center located on Morris Street.

Photo Credit: John Burgess/The Press Democrat
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-60 for droughts and mega-floods (California Natural Resources Agency 2018a). Scientific studies outlined in the same assessment indicated shifts in California’s precipitation regime, which show more dry days, more dry years, a longer dry season, mixed with increases in occasional heavy precipitation events and floods (i.e., a shift towards potentially less frequent but more extreme precipitation events). Studies also project great storm intensity with climate change, resulting in more direct runoff and flooding due to the flash flooding or precipitation nature of these expected events. As a result of fewer but more violent precipitation events, high frequency flood events will increase with climate change. Also, with wildfires already being a problem in California, increasing periods of drought and lack of precipitation are expected to exacerbate conditions for wildfires to occur, and in turn worsen the potential for runoff and flooding associated with burned areas due to increased impermeability and damage terrain and soils.

This Fifth Climate Assessment indicates that climate change is expected to alter built water supply systems, so that current management practices for flood control and water supplies across the state of California may need to be revised. Future revisions should aim to account for subsidence-prone infrastructure (e.g. levees), which coupled with rising sea levels and worsening storm conditions can lead to overtopping or failure of these flood control structures (IPCC 2020).

Based on Sonoma County’s 2016 CAP and GHG emission modelling, climate change is projected to result in an increased risk of extreme flood, and an increased seasonal variability of precipitation, runoff, and stream flows for Sonoma County, along with increased likelihood of “extreme” precipitation and drought events. There may be more years with more frequent storm events and occasional events that are much stronger than historical ones and the length of season over which storm events occur is predicted to increase (SCTA 2016). Also, according to the CAP, more frequent coastal flooding and increased erosion is anticipated.

In addition to flooding, sea levels are projected to rise between 16.5 and 65.8 inches by 2100. Rising sea levels combined with increased storm surge is anticipated to lead to more frequent inundation of lowlying areas, and flooding of homes, infrastructure, agricultural land, and natural areas. Sea level rise is also expected to substantially influence the Russian River and the tidally-influenced Laguna de Santa Rosa and FEMA studies are underway to better understand the level of effect on these waterways. For example, while the Laguna de Santa Rosa functions as a huge reservoir for water storage that can lower the Russian River’s downstream flood levels by more than 10 feet, FEMA studies underway indicate flood elevation levels in the Laguna de Santa Rosa could rise by approximately one foot due to various factors such as erosion and sedimentation (decreasing the capacity of the reservoir) and sea level rise. The greatest impacts are anticipated during winter storms. For these reasons, climate change would have a “high” influence on flooding hazards.

Vulnerability Assessment
Historically, the Planning Area has been at risk to flooding primarily on the east and west portions of the City. Normally, storm floodwaters are kept within defined limits by a variety of storm drainage and flood control measures (e.g., levees), but there are no accredited or non-accredited levees within the City (National Levee Database 2020). Occasionally, extended heavy rains result in floodwaters that exceed local drainage infrastructure capacity and cause damage.

Flooding has occurred in the past: within the 100-year floodplain and in other localized areas. In addition to damage to City facilities and infrastructure, other problems associated with flooding include erosion, sedimentation, degradation of water quality, loss of environmental resources, health hazards, and the inconvenience or financial and accessibility issues that come with road closures and other such effects.

The City of has mapped flood hazard areas as illustrated in the figures contained throughout this section.

For the following vulnerability assessment, GIS was used to identify and quantify the possible impacts of Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-61 flooding within the City’s Planning Area. The methodologies described in the following subsections was followed to develop the flood vulnerability maps and determine values at risk to the 100- and 500-year flood events.

Insurance Coverage and NFIP Claims and Losses for Repetitive Loss Properties The City of Sebastopol joined the NFIP (regular entry) on June 18, 1980. The current effective map date is from October 16, 2012. NFIP CIS insurance data indicates that as of January 7, 2021 there were 36 policies in place in the City, resulting in $15,008,600 of insurance in effect. Since the City began participating in the NFIP there have been 63 total closed losses, amounting to $1,630,932 in payments.

NFIP Claims
The following table summarizes the NFIP claims by property type. Single-family homes represented the largest amount (37) of closed paid losses in the City, followed by non-residential properties (17). While non-residential had less paid losses, this property type had the largest amount of closed paid losses. Not shown in Table 4-19 is manufactured homes. This type of property represents one closed paid loss and $10,000 in closed paid losses.

Table 4-19: NFIP Claims in City of Sebastopol by Building Type (Residential versus Non-Residential) Property Type Policies in Force Insurance in Force Number of Closed Paid Losses
Amount of Closed Paid
Losses
Single Family 5 $1,480,000 37 $450,805.09
2-4 Family 1 $250,000 6 $78,938.48
All Other
Residential
0 $0 3 $11,213.24
Non-Residential 30 $13,278,600 17 $1,089,976.17
Total 36 $15,008,600 63 $1,630,932.98
Source: CIS Repetitive Loss report for the City of Sebastopol (FEMA) 2021 Repetitive Loss Properties
According to the 2018 California SHMP, in 2017, Sonoma County was the top-ranking county in state for repetitive losses, accounting for more than 48 percent of the total top 10 repetitive losses. According to the latest data available through CIS, the City shows a total of 23 Repetitive Loss buildings (3 of which are insured) as defined by FEMA. Repetitive loss properties have incurred 31 total losses, 4 of which were insured cases, and these accrued a total of $1,478,770 in payments from both building- and contentsrelated losses. The majority of these repetitive loss properties fell within AE, A, A1-A30, AO, AH flood hazard areas (FEMA 2021). Table 4-20 below summarizes the repetitive loss information detailed in the City’s latest CIS report for repetitive loss properties (FEMA 2021a).

Table 4-20: City of Sebastopol Repetitive Losses in Floodplains Repetitive Loss Component Zones AE, A, A1-30, AO, AH Zones B, C, X Total Total Buildings 18 5 23
Insured Buildings 2 1 3
Total Losses 23 8 31
Insured Losses 3 1 4
Total Payments $1,339,904.97 $138,865.38 $1,478,770.35
Building related losses $739,549.89 $72,922.36 $812,472.25 Contents related losses $600,355.08 $65,943.02 $666,298.10 Insured Payments $333,836.18 $22,356.39 $356,192.57
Building related payments $312,738.04 $21,542.56 $334,280.60 Contents related payments $21,098.14 $813.83 $21,911.97 Source: Sebastopol CIS Repetitive Loss Report (FEMA 2021a); no repetitive loss information was noted for Zones VE, V, or V1-30; column was excluded.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-62 Severe Repetitive Loss Properties
According to the City Floodplain Administrator and CIS Severe Repetitive Loss Report for the City of Sebastopol, of the 23 Repetitive Loss buildings, there is one Severe Repetitive Loss property within the City that has incurred two total losses for a total of $21,000 in building payments (FEMA 2021b).

Property
To determine vulnerability of people and property to
riverine flood, an enhanced flood risk analysis was
performed utilizing FEMA’s Hazus software. The analysis used for Sebastopol consisted of a Level 2 assessment,
with depth grids to enhance the accuracy of flood risk
modelling within Hazus and Parcels imported into Hazus
through the Comprehensive Data Management System
(CDMS) tool as a User Defined Facility (UDF).

Depth grids were created for the Laguna de Santa Rosa
River within Hazus utilizing FEMA-created cross sections from the effective NFHL and the Summary of Discharges
by flooding source from the FEMA FIS for Sebastopol. The Laguna de Santa Rosa is the only waterbody with a FEMA
detailed study within the City. However, there is a FEMA approximate flood zone west of the City on the Atascadero Creek that intersects the Bodega Highway on the edge of the Planning Area. Since there is not a detailed study for this area, depth grids could not be made for this area of flooding within Sebastopol. The properties that intersected with the Zone A floodplain were analyzed with the use of FEMA Depth Damage Curves outside of Hazus to estimate loss and combined to the final Hazus Level 2 results.

Default Hazus inventories for structures were replaced by using the parcel layer and additional structure attributes in the Assessor’s data. GIS was used to create a centroid or point representing the center of the parcel polygon in the parcel layer to show the location of the primary structure on each parcel. The structure inventory data set was formatted using the CDMS tool for use in Hazus. This tool syncs data and the attribute fields necessary for Hazus analysis and imports the enhanced data set into the Hazus study region. After the hazard and inventory data was imported into Hazus, analysis was completed to determine the potential losses as a result of a one percent annual chance flood.

According to the Level 2 assessment for flood hazards, Sebastopol has 72 parcels within the one percent annual chance floodplain with a total value of $35 million at risk. Hazus estimated total losses are $19.5 million. Table 4-21 summarizes the values at risk in the created depth grid and approximate floodplain.

Figure 4-14 displays the location of flooding within Sebastopol based on the depth grid information.

Table 4-21: City of Sebastopol Combined Hazus Level 2 & FEMA Flood Loss Estimates Property
Type
Improved
Parcel
Improved
Value
Estimated
Building
Loss
Content
Value
Estimated
Content
Loss
Total Loss
Estimate
Population
at Risk
Commercial 25 $25,557,464 $3,977,180 $25,557,464 $12,112,037 $16,089,217 0 Industrial 3 $197,649 $33,235 $296,474 $92,736 $125,971 0 Multi-Family 3 $1,083,094 $160,841 $541,547 $86,616 $247,456 9 Residential 41 $8,925,689 $2,300,383 $4,462,845 $774,960 $3,075,343 129 Total 72 $35,763,896 $6,471,638 $30,858,329 $13,066,349 $19,537,987 139 Sources: City of Sebastopol, Sonoma County Assessor’s Office, FEMA NFHL, Hazus 4.2, Wood Parcel Analysis The 2019 flood event inundated a large portion of the
eastern end of the City, including the City-owned Park
Village, a low-income and homeless housing site along
Sebastopol Avenue, and the Sebastopol Community
Cultural Center on Morris Street as shown above.

Photo Credit: Dante del Prete/2019
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-63 Figure 4-14: City of Sebastopol Parcels in the FEMA and Hazus One Percent Annual Chance Depth Grid Floodplain Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-64 The Level 2 assessment does not include five parcels that contain City-owned buildings that are within the one percent annual chance floodplain. These parcels included the City’s Police Department and Emergency Operations Center (EOC), Laguna Youth Park, Sebastopol Community Cultural Center, Public Works Department building (part of the EOC), and Wischeman Hall. The City-owned buildings do not have an assessed value associated with the parcels and they were therefore not accounted for in the parcel-level analysis. Parcels that lack improved values, but contain a building or structure commonly refer to an exempt property (e.g. public buildings). The HAZUS-generated depth grid model also does not match the FEMA 100-year flood hazard boundary since it takes into account varying ground level elevations and flood depths compared to a distinct boundary delineated in the FEMA effective maps.

While these five parcels were not accounted for in the Level 2 assessment, with the exception of Wischeman Hall, each City-owned building is considered a critical facility. Therefore, each structure was evaluated as part of the critical facility overlay analysis.

A second flood assessment was performed using the NFHL and the same parcel layer database that was imported into Hazus, because Hazus does not model the 0.2 percent annual flood risk. Based on this second flood assessment, there are 23 parcels at risk to the 0.2 percent annual chance floodplain totaling $29 million. A 25 percent damage factor was applied to the total value column of the tables below to estimate potential losses from flood related hazards to the City’s parcels. Twenty-five percent is the typical loss ratio associated with a 2-foot-deep flood, based on FEMA and U.S. Army Corp of Engineer depthdamage relationships. The total values were calculated by adding up the improved structure values of the parcels in the floodplains with the content values. These values were then estimated with the following formulas: a) residential and multi-family properties received contents valued at 50 percent of the parcel improved value; b) commercial contents were valued at 100 percent of the parcel improved value; and c) industrial parcels were valued at 150 percent of the parcel improved value. These valuation assignments are based on FEMA’s methodology for estimating contents within their loss estimation.

Table 4-22 and Table 4-23 summarize the values at risk based on the 0.2 percent annual flood risk, or the 500-year event floodplain. Figure 4-15 displays the location of these parcels in the 500-year flood areas.

Overall, Sebastopol has 23 parcels valued at roughly $29.3 million that are within the 500-year floodplain, $7.3 million of which is estimated to be a potential loss if a flood event of this magnitude were to take place.

Table 4-22: City of Sebastopol Parcels in 0.2 Percent Annual Chance Floodplains by Property Type Flood
Event
Property
Type
Improved
Parcel
Improved
Value
Content
Value
Total Value Loss Estimate
(25% of the
Total Value)
Population
at Risk

0.2%
Annual
Chance
Commercial 18 $14,288,020 $13,790,929 $28,078,949 $7,019,737 0 Multi-Family 1 $240,664 $120,332 $360,996 $90,249 3
Residential 4 $602,565 $301,283 $903,848 $225,962 13
Total 23 $15,131,249 $14,212,544 $29,343,793 $7,335,948 16 Sources: City of Sebastopol GIS, Sonoma County Assessor’s Office, FEMA NFHL, Wood Parcel Analysis Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-66 People
Of greatest concern in the event of a flooding event is the potential for injury or loss of life. Based on the U.S. Census Bureau’s 2018 populations estimates, the City’s average household size is 3.16. This metric was multiplied by the number of parcels of defined as residential at risk of flooding to determine the total potential affected population. Population at risk estimates are summarized in the last column of Table 4- 24 for the Hazus Level 2 assessment and Table 4-25 for the FEMA floodplain analysis for the 500-year flood event above, by flood event type and parcel type. Based on both assessments, there are nearly 139 people at risk to the 100-year floodplain based on the Hazus Level 2 analysis and 16 people are at risk within the 500-year floodplain.

Economy
Flood events have the potential to affect infrastructure (e.g. roads), homes, and populations (possibly displacing families), impacts to the local economy could include business interruptions, lost or reduced wages from potential relocation of populations, infrastructure and resource downtime costs, and reduced city revenues from lack of tourism or inability to run/maintain certain services (like potable water based utilities). Other secondary hazard impacts such as reduced water quality or resource availability, which could in turn raise costs of water processing and distribution are also possible results from a severe flooding event, whether from riverine flooding, flash flooding, or an event caused by local stormwater/drainage infrastructure failures. Based on the history of flooding in Sebastopol, the Barlow shopping district and nearby commercial areas on the eastern side of the City near the Laguna de Santa Rosa have historically been affected the most in terms of economic losses, which largely encompass damages to property (including disruption to business and commerce operations), as well as City infrastructure.

Critical Facilities and Infrastructure
Critical facilities are those community components that are most needed to withstand the impacts of disaster as previously described. GIS was used to determine what City facilities and infrastructure occur within Sebastopol’s mapped flood hazard areas. The NFHL flood layers were used to identify where the 100- and 500-year floodplains intersected with critical facilities. Figure 4-16 illustrates the locations of these critical facilities relative to the FEMA floodplains and Figure 4-17 illustrates facilities within the Hazus Depth Grid. Table 4-23 provides an inventory of the eight facilities that are located within the 100-year FEMA floodplain. None are in the 500-year floodplain.

The impact to the community could be substantial if these critical facilities were damaged or destroyed during a flood event, particularly those which provide lifeline utilities or health/medical services. Overall, the breakdown by FEMA Community Lifelines is there is a total of one Energy facility in the flooding areas, three facilities in the Food Water Shelter category (one shelter facility is the City Community Cultural Center), one Safety and Security facility, and three bridges in the Transportation Community Lifeline found at risk in FEMA SFHAs. A separate analysis was conducted by intersecting the critical facilities with the Hazus Depth Grid and flood depth values. This information is summarized in Table 4-23.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-69 Table 4-23: Sebastopol’s Critical Facilities in the 100-Year Floodplain and Hazus Level 2 Depth Grid FEMA Lifeline Critical Facility Type Total Facilities Flood Depth Energy Substation 1 n/a
Food, Water, Shelter Shelter 1 4.53
Shelter 1 7.24
Wastewater 1 9.13
Safety and Security Police 1 n/a
Transportation Bridge 1 22.01
Bridge 2 n/a
Total 8
Source: City of Sebastopol 2020; Santa Rosa GIS 2020; HIFLD 2020,; NBI, FEMA NFHL, Wood GIS Analysis Historic, Cultural, and Natural Resources
The City of Sebastopol has seven historic and cultural resources located in or nearby the Planning Area boundaries as previously described in Section 4.2 Asset Summary, under the Historic, Cultural, and Natural Resources subsection. Cultural or historical resources such as older buildings or districts may be more affected by flooding hazards, given their likely older construction methods, weaker materials, and potential failure to meet current building code standards. However, none of these seven historic resource sites would be impacted by flood hazards, and several of them are located outside the City’s Planning Area.

Climate change studies at the county and regional level indicate the likelihood that increasingly unpredictable flash flooding and uncertainty in storm occurrence will lead to a worsening in erosion and sedimentation conditions. However, natural areas within the floodplain around the Laguna de Santa Rosa for instance benefit from periodic flooding as a naturally recurring phenomenon, and these natural areas often reduce flood impacts by allowing absorption and infiltration of floodwaters.

Future Development
The potential for flooding may increase as stormwater runoff increases due to land development. Such changes can create localized flooding problems inside and outside of natural floodplains by altering or confining natural drainage channels. Floodplain modeling and master planning should be based on buildout land use to ensure that all new development is properly designed and remains safe from future flooding. While certain local floodplain management and water quality regulations and policies exist, as well as specific regulatory control of building codes, flood insurance requirements, and other such aspects at the federal or state level, the cumulative effects of flood related hazards can have a negative impact on the floodplain and the community into the future.

City floodplain management ordinances require that new construction be built at least one foot above the BFE (per Sebastopol Municipal Code Chapter 15.16 Flood Damage Protection Section 120) and for certain building structures up to two feet above the BFE (Chapter 15.16.200 Standards for Manufactured Homes).

New development that complies with the mandated elevation requirements in addition to other requirements for maintaining elevation certificates, implementing stormwater program elements, and complying with erosion or sediment controls for all new development in the floodplain may protect new constructions from 100-year and possibly other floods events (refer to Section 2.9: Mitigation Capabilities in Chapter 2: Community Profile for more information on the City’s existing floodplain standards and regulations).

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-70 Risk Summary
• Overall, the significance of flood hazards is Medium. • Floods impacts will vary by location and severity and will likely only affect certain areas of the City Planning Area at any one time.
• Based on the risk assessment, floods will continue to have economic impacts to certain areas of the City’s Planning Area, and the estimated losses for properties based on the Hazus Level 2 Assessment impacts 72 parcels and amounts to $19.5 million at risk (with a total of 167 potential people at risk), in addition to the 8 critical facilities which are located in the floodplains. • 23 properties are at risk of the 500-year (0.2 percent annual chance) floodplain, roughly valued at $7.3 million.
• Seven noteworthy flooding events have taken place in Sebastopol since 1986. These caused significant damages and several required water rescues, though other minor flooding cases have taken place in the City.
• Impacts that are not directly quantified but could be anticipated in large future events include: 1) injury and loss of life; 2) disruption of and damage to public infrastructure; 3) disruption to trade, commerce, commuting, mobility, and other activities that may rely on the road networks; 4) health hazards associated with mold and mildew; 5) significant direct and indirect economic impact (jobs, sales, tax revenue) upon the community; and 6) negative impact on commercial and residential property values.
• NFIP CIS insurance data indicates that as of January 7, 2021 there were 36 policies in place in the City, resulting in $15,008,600 of insurance in effect. Since the City began participating in the NFIP there have been 63 total closed losses, amounting to $1,630,932 in payments. • There are 23 Repetitive Loss buildings (2 of which are insured) in Sebastopol. Repetitive loss properties have incurred 31 total losses, 4 of which were insured cases, and these accrued to $1,478,770 in payments from both building- and contents-related losses.

4.3.3 Wildfire
Hazard Description
Wildfires are any uncontrolled fires that occur most often on undeveloped land and require fire suppression. They are caused by lightning or by human-activities such as smoking, arson, equipment misuse, and from electrical infrastructure. Wildfires are a significant concern throughout California. In recent years wildfires have occurred in vegetated areas in the vicinity of the City. Wildfires even a few counties away, can create significant impacts to the City such as those stemming from intense smoke, which can then lead to poor air quality, traffic visibility issues, and public health concerns. Generally, the fire season extends from June through October of each year during the hot, dry months. Wildfire conditions arise from a combination of high temperatures, intense sunlight, low rainfall, an accumulation of vegetation, and high winds.

Throughout California, communities are increasingly concerned about wildfire safety as increased development in the foothills and mountain areas and subsequent fire control practices have affected the natural cycle of the ecosystem. While wildfire risk is predominantly associated with wildland-urban interface (WUI) areas, significant wildfires can also occur in heavily populated areas. The WUI is a general term that applies to development adjacent to landscapes that support wildfire.

Location
Limited – Wildfires affect grass, forest, and brushlands, as well as any structures populations located within or surrounding them. Where there is human access to wildland areas the risk of fire increases due Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-71 to a greater chance for human carelessness and historical fire management practices. In other areas, large concentrations of highly flammable brush and grasslands located in flat open spaces are also susceptible to wildfire.

The California Department of Forestry and Fire Protection’s (CAL FIRE) Fire and Resource Assessment Program (FRAP) models map wildfire hazards using a science-based approach and computerized techniques to classify moderate, high, and very high fire severity zones in a Fire Hazard Severity Zone (FHSZ) dataset. The model uses existing CAL FIRE data and hazard information based on fuel, weather, and terrain, explained in more detail in the Extent (Magnitude/Severity) section below.

Figure 4-18 displays the FHSZs falling within State Responsibility Areas, or SRAs, and Local Responsibility Areas, or LRAs, in and surrounding the City. As shown in Figure 4-18, most of the City is within the LRA, and classified as “Unzoned” or within a “moderate” FHSZ. Fire threat zones, which are a combination of fire frequency (or likelihood of a given area burning) and potential fire behavior, are displayed in Figure 4-

19. These two maps provide general indications of potential future fire behavior as well as where fire occurrence might take place. The southern portions of the City, particularly the Fircrest Mobile Home Park area and along Robinson Road show moderate and high severity hazard areas based on fire threat data and the FHSZs mapped for LRA levels. Other potential areas of concern are along the eastern edges of the City’s Planning Area where moderate and high severity zones intermingle.

Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-72 Figure 4-18: FHSZs in State Responsibility Areas (SRAs) and Local Responsibility Areas (LRAs) Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-73 Figure 4-19: Fire Threat Zones in and Near the City of Sebastopol Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-74 Extent (Magnitude/Severity)
Limited – Potential losses from wildfires include human life, structures and other improvements, natural and cultural resources, quality and quantity of water supplies, cropland, timber, recreational opportunities, and impacts to the community’s way of life. Economic losses could also result from reduced tourism and visitation and generally impacted economic sectors. Smoke and air pollution from wildfires can be a severe health hazard. In addition, catastrophic wildfire can create favorable conditions for other secondary hazards such as flooding, landslides, and erosion during the rainy season. Typically, the potential for significant damage to life and property exists in areas designated as “wildland-urban interface” areas, or WUIs, where development is adjacent to densely vegetated area.

Generally, there are three major factors that sustain wildfires and predict a given area’s potential to burn.

These factors are fuel, topography, and weather, as described below. • Fuel - Fuel is the material that feeds a fire and is a key factor in wildfire behavior. Fuel is generally classified by type and by volume. Fuel sources are diverse and include everything from dead tree leaves, twigs, and branches to dead standing trees, live trees, brush, and cured grasses. Manmade structures, such as homes and other associated combustibles are also fuel sources. The type of prevalent fuel directly influences the behavior of wildfire. Fuel is the only factor that is under human control. Fuel types within the City include seasonal grasses, and mature landscaping, such as deciduous and evergreen oaks, and conifers. Fuel types surrounding the City Planning Area include mainly seasonal grasslands and brush.
• Topography - An area’s terrain and land slopes affect its susceptibility to wildfire spread. Both fire intensity and rate of spread increase as slope increases due to the tendency of heat from a fire to rise via convection. The arrangement of vegetation throughout a hillside can also contribute to increased fire activity on slopes.
• Weather - Weather components such as temperature, relative humidity, wind, and lightning also affect the potential for wildfire. High temperatures and low relative humidity dry out fuels that feed wildfires, creating a situation where fuel will more readily ignite and burn more intensely. Thus, during periods of drought, the threat of wildfire increases. Wind is the most treacherous weather factor. The greater a wind, the faster a fire will spread and the more intense it will be. Lightning can also ignite wildfires, often in difficult to reach areas for firefighters.

Wildfires in the broader region (Sonoma County and bordering counties such as Mendocino or Napa) in recent years have resulted in the loss of property as well as human injuries or even deaths. The smoke and air pollution from wildfires are also severe health hazards particularly for sensitive populations including the elderly, children, and people with respiratory and cardiovascular diseases. Wildfires can threaten the health and safety of those fighting the fires, so the overall magnitude or severity of fires can be widereaching and incur many types of impacts. Wildfire severity can usually be quantified in terms of acres burned during an event, number and cost of properties/structures damaged (including critical facilities), money lost from disruption of services, and population affected by the fires (e.g. people displaced, injured or killed). A widely damaging wildland fire within the City is considered to be unlikely, although changing issues and increasing record-high temperatures accompanied by low humidity, strong winds, and drought conditions could worsen the likelihood of fires in the Planning Area, or near the Planning Area in the future.

Previous Occurrences
Wildfires are a significant concern throughout California. According to CAL FIRE, vegetation fires occur across California on a regular basis; most can be controlled and contained early with limited damage. The foothills and mountain areas of California have experienced numerous devastating fires over the last 100 Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-75 years, with the fire risk significantly increasing in recent years due to high fuel loads and expansion of
development into the WUI areas. For those ignitions that are not readily contained and become wildfires, damage
can be extensive. There are many causes of wildfire, from naturally caused lightning fires to human-caused fires
linked to activities such as smoking, campfires, debris burning, equipment use, and arson. Recent studies
conclude that the greater the population density in an
area, the greater the chance of an ignition from human
sources, as well as powerlines or other electrical or utility infrastructure.

Based on Cal FIRE’s wildfire history data, there were no reported fires within a ten-mile buffer around
Sebastopol; however 25 fires were within the vicinity of the City from 1944 to 2020. Table 4-24 summarizes the
fires that occurred around Sebastopol, while Figure 4-20 displays the fires that have occurred close to the City. The fires have been organized by acres burned, with the oldest fire taking place in 1944 and the most recent of record in November 2020. In terms of the largest fires close to the City, the LNU Lightning Complex Fire occurred in August 2020 with 363,219 acres burned, Kincade Fire occurred in 2019 and burned 77,762 acres, and the Glass Fire in northeast Santa Rosa started in September 2020 and burned 67,484 acres.

The NOAA NCEI database was queried for past wildfire events in or near Sebastopol. This NCEI query yielded five records of “Dense Smoke” events between 2018 and 2020. These events were related to the 2018 Camp Fire, 2019 Kincade Fire and 2020 Glass Fire. The 2018 record specifically calls out neighboring community of Santa Rosa as shutting down city operations due to the dense smoke from the Camp Fire.

As such, even events that take place elsewhere can affect the Planning Area as noted herein.

Table 4-24: Summary of Fire History within 10 miles of the City of Sebastopol Fire Name Year Cause of Fire Acres
Burned
Details/Agency in Charge
LNU Lightning
Complex
2020 Lightning 363,219 California Department of Forestry and Fire Protection
Kincade 2019 Power Line 77,762 California Department of Forestry and Fire Protection
Glass 2020 Unknown /
Unidentified
67,484 California Department of Forestry and Fire
Protection
C. Hanly 1964 Unknown /
Unidentified
55,961 California Department of Forestry and Fire
Protection
Nuns 2017 Unknown /
Unidentified
55,798 California Department of Forestry and Fire
Protection
Tubbs 2017 Unknown /
Unidentified
36,702 California Department of Forestry and Fire
Protection
Roadside #44 1961 Unknown /
Unidentified
5,968 California Department of Forestry and Fire
Protection
The Tubbs Fire, as shown above, was a wildfire that
impacted parts of Napa, Sonoma, and Lake counties,
and inflicted the greatest losses on the City of Santa
Rosa. The Tubbs Fire was one of more than a dozen
large fires that started in October 2017. It burned
approximately 36,702 acres and destroyed more than
5,643 homes, half of which were located in Santa Rosa.

Photo Credit: Dante De Prete/2017
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-76 Fire Name Year Cause of Fire Acres
Burned
Details/Agency in Charge
Anderson 1965 Unknown /
Unidentified
4,954 California Department of Forestry and Fire
Protection
P.G.& E.#5 1965 Unknown /
Unidentified
3,250 California Department of Forestry and Fire
Protection
- 1964 Unknown /
Unidentified
2,603 California Department of Forestry and Fire
Protection
Robertson 1961 Unknown /
Unidentified
2,208 California Department of Forestry and Fire
Protection
P.G.& E.#7 1965 Unknown /
Unidentified
1,840 California Department of Forestry and Fire
Protection
P.G.& E. #4 1950 Unknown /
Unidentified
1,133 California Department of Forestry and Fire
Protection
Pressley 2017 Unknown /
Unidentified
792 California Department of Forestry and Fire
Protection
- 1944 Unknown /
Unidentified
541 California Department of Forestry and Fire
Protection
- 1945 Unknown /
Unidentified
526 California Department of Forestry and Fire
Protection
S and T Ranch 1969 Unknown /
Unidentified
210 Contract County
Crane 2013 Unknown /
Unidentified
159 California Department of Forestry and Fire
Protection
Alexander 1982 Unknown /
Unidentified
80 Contract County
Grade 2 2007 Unknown /
Unidentified
60 California Department of Forestry and Fire
Protection
Carmody 1991 Unknown /
Unidentified
54 Contract County
Martinoni 2012 Unknown /
Unidentified
30 Contract County
Oceguera 2004 Unknown /
Unidentified
20 California Department of Forestry and Fire
Protection
Bodega 2017 Unknown /
Unidentified
18 California Department of Forestry and Fire
Protection
Stony 2012 Vehicle 6 California Department of Forestry and Fire Protection
Source, CalFire and National Incident Feature Service (NIFS) 2020 Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-77 Figure 4-20: Fire History and Burn Perimeters near the City of Sebastopol 1944 to 2020 Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-78 Probability of Future Occurrences
Occasional – Considering the local fuels, weather conditions, and the flat topography in the area combined with a lack of extensive WUI development means that fires may only occur occasionally in or immediately surrounding the City. Based on the CAL FIRE Probability and Carbon Accounting mapping, which is based on Mann et al.’s projections for the years 2026-2050 (shown on Figure 4-21 below), the annual probability of fire occurrence is low for the most the City (CalFIRE 2020) and most of the surrounding land uses to the west (grid cells colored white or undefined in Figure 4-21) consist of farmland (e.g., prime farmland, farmland of statewide importance, etc.) and grazing land. However, due to the effects of climate change and because the probability of future occurrences outside the City’s Planning Area ranges from likely to high likely, the City recognizes the probability of future occurrences of wildfires in the City’s Planning Area would increase when taking into consideration climate change and wildfire risk in the region. Recent wildfires in various parts of Sonoma County and in the region have had significant impacts on the City, and in some instances required City residents to evacuate, which was the case during the 2019 wildfires. During these wildfires several of the required evacuations led to gridlock traffic conditions in the City, which limited the ability for the community to evacuate in a timely manner.

These recent wildfires also caused many residents to temporarily evacuate due to poor air quality and smoke. For more information on this CAL FIRE probability mapping methodology and related resources, visit https://frap.fire.ca.gov/frap-projects/fire-probability-and-carbon-accounting/ Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-80 Climate Change Considerations
Increases in GHG emissions coupled with population growth and development are expected to continue impacting California’s forests and natural resources. Likewise, the effects of climate change will impact wildfire behavior, the frequency of ignitions, fire management, and fuel loads. Increasing temperatures will intensify wildfire threat and susceptibility to more frequent wildfires in the grasslands that surround the Planning Area, in addition to wildlands throughout Sonoma County.

Uncertainty exists in how climate change will affect total precipitation, but models suggest that there is a tendency for wetter conditions in the northern part of the state and drier conditions in the south (California Natural Resources Agency 2018). Forests are also sensitive to variable precipitation events, and damaging droughts such as the multi-year event from 2012-2017, which contributed to widespread tree mortality as warmer temperatures stressed trees and made them more susceptible to pests and pathogens (California Natural Resources Agency 2018). While the surrounding hillsides near the City’s Planning Area consist of mostly grasslands, there are emerging studies that indicate that hot and dry winds can influence shrubland and grassland fires. Studies noted in California’s Fourth Assessment indicate climate change impacts on wind patterns may strongly affect forests, potentially serving as a trigger mechanism for conversion of forest to other types of vegetation (California Natural Resources Agency 2018).

Cal-Adapt conducted wildfire risk projections based on statistical modeling from historical data of climate, vegetation, population density, and fire history. The wildfire risk simulations were used in California’s Fourth Climate Change Assessment and based on four models that produced a warm/dry simulation (HadGEM2-ES), cooler/wetter simulation (CNRM-CM5), average simulation (CanESM2), and a simulation that is most unlike the first three for the best coverage of different possibilities (MIROC5). These wildfire risk simulations are shown in Figure 4-22. The upper chart shows the modeled annual averages of area burned in Sonoma County under the RCP 8.5 scenario, while the lower chart shows modeled annual averages of area burned for Sonoma County under the RCP 4.5 scenario. Sonoma County was selected as the data boundary layer to better represent the impact of wildfires in the region and on a County-level, given City may be more affected by nearby wildfires and resulting smoke than wildfires within the City itself.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-81 Figure 4-22: Future Annual Averages of Acres Burned in Sebastopol under Low and High Emission Scenarios
Source: Cal-Adapt 2021
According to the Sonoma County’s 2016 Regional CAP, climate change is expected to result in more frequent and intense wildfires. These risks are expected to continue to rise due to increased dryness of vegetation compounded by the productivity of plants in the spring. Based on the Regional CAP data, by the end of the century, the chances of one or more fires during a 30-year period are projected to increase from 15 to 20 percent to 25 to 33 percent in the mountainous areas of the County.

While the CAL FIRE program actively collaborates with state, local, and national agencies to reduce climate change impacts, current scientific models expect California will be affected by increased numbers of forest fires with added intensity due to longer warmer seasons, reduced distribution of biodiversity, lack of moisture, changes in ecosystems, drought impacts (e.g. pest diseases and continued spread of invasive Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-82 species), and other such impacts in coming years. Due to these increasingly worsening and recurring issues, wildfire hazards should be carefully studied by the City with regards to future negative effects in or near the City Planning Area related to wildfire risk. For these reasons, climate change would have a “high” influence on wildfire hazards.

Vulnerability Assessment
The City’s wildfire risk and vulnerability is a medium concern. Wildfires can affect major transportation roads, such as SRs 116 and 12 by impeding commuters to get to and from their destinations (e.g. to the Bay Area), as well as potentially block emergency responders. As previously mentioned, wildfire can also damage or destroy property and infrastructure, injure people or even cause death. During the May to November fire season, which accounts for seasons during the past decade that now last longer each year (into November and December), the dry vegetation and hot and windy weather, combined with a growing population, results in an increase in the number of potential ignitions. Any fire, once ignited, has the potential to quickly become a large, out-of-control fire. Fires that prevent essential goods or services from entering or leaving the City could negatively affect local residents and businesses by impacting the local economy and the community’s livelihood (e.g. limited access to jobs, daycare, schools, resources, and residences).

The CAL FIRE-produced FHSZs within LRAs displayed in Figure 4-23 were used to assess general wildfire risk in the Planning Area, using methodology detailed below. The results are summarized in the tables and maps that follow. The City’s parcel layer was used as the basis for the inventory of parcels, while the CAL FIRE FHSZs in LRAs, ranked by severity, was used to intersect the parcels and determine general risk based on the severity rank categorization, all in GIS. Centroids were generated for each parcel for simpler overlay analysis, so that a parcel was either “in” a fire threat layer of type “moderate severity,” “high severity,” or “very high severity,” or “out” of any of these fire threat categories (e.g. in Urban Unzoned or NonWildland/Non-Urban areas). For purposes of this analysis, it was assumed that every parcel with an improved value greater than zero was developed in some way, even those stated as “vacant.” This specification ensures parcels such as rights-of-way are discarded for the purposes of determining vulnerability to fire, and to be able to aggregate valuations based on each parcel type (e.g. residential, commercial, agricultural). The assessor’s office data calculates improved values based on the “annual structure value” which relates to the improvements of the parcel; this was the field used to determine improved values for each parcel.

Once parcels in the form of centroids are categorized by property type, next the content values were calculated as follows: a) residential and multi-family properties received contents valued at 50 percent of the parcel improved value; b) commercial and agricultural properties’ contents were valued at 100 percent of the parcel improved value; and, c) vacant parcels (if applicable) received 0 percent content values.

These valuation assignments are based on FEMA’s methodology for estimating contents within their loss estimation software, Hazus-MH.

Properties located within the FHSZs are listed in Table 4-25, along with a summary of all improved structure values, contents values, and total values (which are the aggregated improved structure values plus the content values). Loss estimates are equal to 100 percent of the total building and content values, as wildfires typically result in complete loss. Population at risk is also estimated based on the GIS modeling. As Figure 4-24 illustrates, the areas with parcels exposed to the FHSZs within the LRAs are found along the southern and eastern edges of the City’s Planning Area.

Property
The fire severity zones, and parcel overlay analysis yielded the following results below for fire hazards within LRAs. The highest number of parcels at risk fall under the residential category (with a total of 98 Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-83 parcels), followed by the commercial category (with 17 parcels), multi-family (with 8 parcels), and industrial (with 2 parcels at risk). A total of $78 million is at risk of being affected by potential fires, based on a $45.77 million improved structure value combined between all the parcel types, and $32 million in content values. The moderate FHSZ category contains all the parcels at risk, with 126 parcels. No parcels fall in the high or very high FHSZ areas. Figure 4-24 displays parcels located in the FHSZ areas.

Table 4-25: Parcels in FHSZs within LRAs in Sebastopol
Property
Type
Improved Parcel Improved Value Content Value Total Value Population at Risk
Commercial 17 $16,951,433 $16,454,342 $33,405,775
Industrial 2 $1,153,481 $1,730,222 $2,883,703
Multi-Family 8 $2,291,851 $1,145,926 $3,437,777 25
Religion 1 $900,896 $900,896 $1,801,792
Residential 98 $24,416,846 $12,208,423 $36,625,269 309
Total 126 $45,714,507 $32,439,808 $78,154,315 335
Source: City of Sebastopol, Sonoma County Assessor’s Office; CalFire; Wood Parcel Analysis The wildland fire threat zones and parcel overlay analysis yielded the following results below for fire hazards. There are six residential parcels at risk to high fire threat with a total value of risk at $1.9 million.

The moderate fire threat hazard severity category has the highest risk in Sebastopol. The highest number of parcels at moderate risk fall under the residential category (with a total of 194 parcels), followed by the multi-family category (with 13 parcels), and commercial (with 9 parcels at risk). A total of $136.7 million in parcel value is located within the moderate risk area, based on a $87 million improved structure value combined between all the parcel types, and $49 million in content values. Figure 4-23 displays parcels located in the Wildland Fire Threat areas.

Table 4-26: Parcels in Wildland Fire Threat Areas in Sebastopol Fire Threat Property
Type
Improved
Parcel
Improved
Value
Content Value Total Value Population
at Risk
High Residential 6 $1,295,586 $647,793 $1,943,379 19
Moderate Commercial 9 $9,858,059 $9,360,968 $19,219,027 Industrial 2 $964,660 $1,446,990 $2,411,650
Multi-Family 13 $9,622,722 $4,811,361 $14,434,083 41
Religion 1 $900,896 $900,896 $1,801,792
Residential 194 $65,911,037 $32,955,519 $98,866,556 612 Total 219 $87,257,374 $49,475,734 $136,733,108 653
Low Commercial 1 $50,428 $50,428 $100,856
Multi-Family 1 $133,804 $66,902 $200,706 3
Residential 2 $1,182,335 $591,168 $1,773,503 6
Total 4 $1,366,567 $708,498 $2,075,065 9
Sources: City of Sebastopol, Sonoma County Assessor’s Office; CalFire; Wood Parcel Analysis Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-86 People
Wildfire risk is of greatest concern to populations residing in the moderate, high, and very high wildfire threat zones.

The 2018 U.S. Census estimates were used to show the
average persons per household in the City, so that total persons at risk of each fire threat category could be
calculated, based on property type. For each residential property type (i.e., general residential, multi-family), an average household value of 3.16 people per parcel (based on U.S. Census estimates) was applied to roughly
estimate potential population at risk. Table 4-25 and
Table 4-25 above summarized the estimated population
residing in each fire threat zone along with the parcel analysis summary by LRA FHSZ and Fire Threat type. The
results were estimated by multiplying the average
persons per household in Sebastopol times the number
of residential parcels in each fire threat zone. Based on the analysis, the moderate Fire Threat has 335 potential people at risk and high Fire Threat has a population of 19 at risk with 653 at moderate risk.

Other indirect impacts from wildfires on people are related to dense smoke from fires within the region.

Wildfires in the past three years have decreased the air quality in the Sebastopol area and throughout Sonoma County. Dense smoke poses a risk to both people with compromised health as well as those considered healthy. Studies have also shown an increase in ambulance calls, hospital visits and an increase of people experiencing respiratory or cardiac emergencies (NPR 2020).

Economy
Wildfires can be incredibly destructive depending on the circumstances of the event, particularly the type of resources and populations they affect due to fire size, location, length of the burn, and ongoing or existing weather or hazard conditions. For example, damages to structures and properties are obvious impacts to the economy due to fire, though cascading negative effects on the economic sectors include road closures, lower revenue to the City based on reduced tourism and visitation, or excessive costs of firefighting and relocating people or natural and man-made resources (thus indirectly impacting city revenues). Transportation lifelines being closed and/or damaged could impede a majority of the population’s ability to commute to nearby cities and the Bay Area. Additional direct or indirect impacts to the economy could be further exacerbated by existing hazard issues such as earthquakes, drought, or severe weather, if those make it difficult to control wildfires or reestablish the economic drivers in the Planning Area.

Critical Facilities and Infrastructure
Critical facilities are those community components that are most needed to withstand the impacts of a disaster. An overlay analysis using GIS was performed to determine where critical facilities are located within FHSZs ranked moderate, high, or very high (within the LRAs as defined by CAL FIRE). Only those facilities located in these zones are noted as being at risk. Figure 4-25 shows those critical facilities located in the City that fall in the FHSZs, while Table 4-27 describes the types of facilities. Based on these results, a total of six critical facilities are located in zones of the type “moderate.” No other fire threat zones contain critical facilities in the Planning Area. Of these facilities, three are within the FEMA Community Lifelines category for Food, Water, Shelter and the other three fall under the Transportation category.

The 2020 Glass Fire began in late September in the
North Bay region. Several homes in the Skyhawk
subdivision of Santa Rosa were burned. Dense smoke
from the wildfire decreased air quality, impacting the
City of Sebastopol.

Photo Credit: Black Rock Inn. St. Helena. San Francisco Chronicle 2019 Noah Berger/Associated Press
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-87 Table 4-27: The City of Sebastopol’s Critical Facilities at Risk to Wildfire based on FHSZs in LRAs FEMA Community Lifeline Facility Type Total
Food, Water, Shelter Shelter 2
Wastewater 1
Transportation Bridge 3
Total 6
Source: City of Sebastopol; HIFLD; CalFire FRAP 2010; Wood GIS Analysis An additional analysis was performed on the Wildland Fire Threat layer. Overlay analysis using GIS was performed to determine where critical facilities are located within threat areas ranked moderate, high, or very high. Only those facilities located in these zones are noted as being at risk. Figure 4-25 shows those critical facilities located in the City, while Table 4-28 describes the types of facilities. Based on these results, a total of six critical facilities are found in the “moderate” zone. No other fire threat zones contain critical facilities in the Planning Area. From these facilities, five are within the FEMA Community Lifeline category for Food, Water, Shelter and one falls under the Transportation category.

Table 4-28: City of Sebastopol’s Critical Facilities at Risk to Wildfire based on Wildfire Threat FEMA Lifeline Facility Type Total
Food, Water, Shelter School 1
Shelter 2
Wastewater 1
Water Well 1
Transportation Bridge 1
Total 6
Source: City of Sebastopol; HIFLD; CalFire FRAP 2010a; CalFire FRAP 2010b; Wood GIS Analysis Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-88 Figure 4-25: Critical Facilities and Infrastructure within Local Responsibility Hazard Areas in the City of Sebastopol Figure 4-26: Critical Facilities and Infrastructure within Fire Threat Zones in the City of Sebastopol Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan | January 2021 4-89 Agenda Item Number 19 Agenda Item Number 19
City Council Meeting Packet of June 7, 2022

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Risk Assessment
City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-90 According to California’s Fourth Climate Assessment, wildfire may be the biggest immediate threat to California’s transportation system, as vegetation fuel accumulation continues to increase (California Natural Resources Agency 2018). Wildfires can also lead to mudslides and debris flows, later resulting in the temporary transportation system closures or other key impacts to the community. Studies cited in the most recent climate assessment found that a considerable amount of infrastructure is exposed to wildfire risk, with the highest risk being roads and highways, such as SRs 12 and 116. Railroads may also be at risk of warping during wildfires, and transportation or freight activity disrupted, while smoke and fire-fighting operations can lead to temporary service disruptions that can additionally affect movement of goods and services (California Natural Resources Agency 2018).

Historic, Cultural, and Natural Resources
The City has seven cultural and historic resource places, as summarized under Section 4.2 of this plan.

Since these structures are sensitive in nature and may not have been built according to the latest building codes due to their age, it is expected that they might be at risk of wildfires (e.g. because of their potential inability to withstand significant heat). Several of these historic sites are located outside the City’s Planning Area, and potentially closer to areas prone to wildfires. However, parks or natural spaces, such as the Laguna de Santa Rosa Wetland Preserve, the second largest freshwater marsh in Northern California, could also be at risk of a wildfire, but these places would need to be further studied to determine vulnerability and risk more specifically. Other City park facilities with existing infrastructure, such as park amenities, like playgrounds and ball fields may also be susceptible to wildfires.

Future Development
Any population increases in the Planning Area will continue to make wildfire vulnerability a growing issue, as future development in the WUI will increase risk to this hazard citywide. WUI related risks can however be managed with strong land use regulations and building code requirements. For example, development in the WUI can be limited, or where permitted can require firebreaks between development and grasslands, as well as the enforcement that building construction be compliant with the CBC Chapter 7A:

Materials and Construction Methods for Exterior Wildfire Exposure. The Sebastopol General Plan and Zoning Code can also be amended to address these land use regulations.

Risk Summary
• The overall risk significance of wildfire hazards to the City of Sebastopol is Medium. • The level of wildfire risk will likely increase in the future due to the effects of climate change, and as the City assesses and monitors the level of risk.
• Wildfires are expected to have a probability of occasional occurrence in the future, given the local fuel, topography, and weather conditions and the extent of the WUI. Based on recent CAL FIRE future fire occurrence probability mapping, the City is mostly expected to have a low likelihood of fire from years 2026 to 2050.
• The areas of the City with high and moderate, which in turn pose the highest risk to life and property, are located on the southern portion of the Planning Area. There are no areas of very high fire threat in the Planning Area.
• A majority of parcels fall within the moderate fire threat areas (219 improved parcels) with a total value of roughly $136 million. Six residential parcels were identified as being located in the high fire threat areas in the City.
• Although the probability of future occurrence of wildfire risk is occasional within the City’s Planning Area, the future occurrence outside the City is ranges from likely to highly likely and must account for areas where future occurrence of major wildfires will significantly affect the City through mandated evacuation and poor air quality, among other impacts.

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Risk Assessment
City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-91 • Approximately 355 people may be at risk of the moderate FHSZs within the LRA. • Six critical facilities are in moderate fire threat areas within the City of Sebastopol (no other facilities fall in any additional fire threat zones).
• Seven historic and cultural properties and places are exposed to wildfire risks, based on the NRHP database; however, several of these facilities occur outside the City’s Planning Area. • Population growth in Sebastopol is expected to result in a 5.5 percent increase over the next decade (by 2030) according to the City’s Housing Element, so WUI development may become a larger issue into the future. Building to the current code with regards to materials and structures is recommended based these development trends.

4.3.4 Drought
Hazard Description
Drought is a gradual phenomenon. Although droughts are sometimes characterized as emergencies, they differ from typical emergency events. Most natural disasters, such as floods or forest fires, occur relatively rapidly and afford little time for preparing for disaster response. Droughts occur slowly, many times over a multi-year period, and it is often not obvious or easy to quantify when a drought begins and ends.

Drought is a complex issue involving many factors; it occurs when a normal amount of moisture is not available to satisfy an area’s usual water-consuming activities. Drought can often be defined regionally based on its causes or effects:
• Meteorological drought is usually defined by a period of below average water supply. • Agricultural drought occurs when there is an inadequate water supply to meet the needs of the state’s crops and other agricultural operations such as livestock. • Hydrological drought is defined as deficiencies in surface and subsurface water supplies. It is generally measured as streamflow, snowpack, and as lake, reservoir, and groundwater levels. • Socioeconomic drought occurs when a drought impacts health, well-being, and quality of life, or when a drought starts to have an adverse economic impact on a region.

The California Department of Water Resources (DWR) says the following about drought (DWR 2015):
“One dry year does not normally constitute a drought in California. California’s extensive system of water supply infrastructure—its reservoirs, groundwater basins, and inter-regional conveyance facilities—mitigates the effect of short-term dry periods for most water users. Defining when a drought begins is a function of drought impacts to water users. Hydrologic conditions constituting a drought for water users in one location may not constitute a drought for water users elsewhere, or for water users having a different water supply. Individual water suppliers may use criteria such as rainfall/runoff, amount of water in storage, or expected supply from a water wholesaler to define their water supply conditions.”
The drought issue in California is further compounded by water rights. Water is a commodity possessed under a variety of legal doctrines. The prioritization of water rights between farming and federally protected fish habitats in California is part of this issue.

Drought impacts are wide-reaching and may be economic, environmental, or societal. During a drought, water allocations go down, which results in reduced water availability. Voluntary water conservation measures are typically implemented during extended droughts. A reduction of electric power generation and water quality deterioration are also potential problems. Drought conditions can also cause soil to compact and not absorb water well, potentially making an area more susceptible to flooding.

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Risk Assessment
City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-92 Location
Significant – Drought is a regional hazard, and during severe drought conditions, it can affect the entire state of California with varying levels of dryness. In other words, drought affects all aspects of the economy and environment and the community simultaneously. The most significant impacts associated with drought in the City’s Planning Area are those related to water intensive activities such as municipal usage and general water supply (e.g. irrigation for parks and open spaces), wildfire protection (including relief and response activities), commerce, agriculture, and tourism.

According to City of Sebastopol’s s General Plan Existing Conditions Report the City relies exclusively on groundwater as its water supply source. The City of Sebastopol has five permitted wells, three of which are active for potable, non-potable, irrigation, and industrial water uses. The City has no backup system nor the ability to connect to other water systems. City officials closely monitor their water supply and encourage residents to conserve water through monetary incentives when water-conserving toilets and washing machines are installed and using recycled water for City landscaping. In addition, water saving devices will be incorporated into all new construction, including remodeling of existing kitchens and bathrooms. Water supply sources and projected acre-feet (AF) available for 2025 are displayed in Table 4-

29. Table 4-29: Sebastopol’s Current Water Supply Sources and Projected Availability for 2025 Water Supply 2019 Usage 2025 Projected Availability
Surface Water 0 0 AF
Groundwater 1,000 2,780 AF
TOTAL 1,000 AF 2,780 AF
Sources: City of Sebastopol 2019 Level of Service Report.

Notes:

2. One Acre-Foot = 43,560 cubic feet.

In 2014, the City adopted a Water Shortage Contingency Plan (WSCP) and applied a three-stage conservation plan during declared water shortages (City of Sebastopol Municipal Code (Ord. 1070 § 1, 2014)). Stage 1 involves Voluntary Conservation to achieve a reduction goal of up to 10 percent of water supply conditions, stage 2 builds to a mandatory compliance and involves moderate reductions of 25 percent, and stage 3 requires mandatory compliance and involves severe reductions of 30 percent.

Sustainable Groundwater Management Act of 2014
Groundwater resources play a significant role in the development, growth, and sustainability of the City as 100 percent of its municipal water is from groundwater. The residents of Sebastopol and all of California have been experiencing significant drought and water shortages since 2011 and only recently did the City and the majority of the state come out of drought. In January 2014 the Governor declared an emergency proclamation due to multiple years of drought. The proclamation called on citizens to reduce water use by 20 percent; with a subsequent Executive Order that directed urban water agencies to reduce water use by 25 percent. In September 2014, the Governor signed a three-bill package (California Senate Bills 1168 and 1319, and Assembly Bill 1739), known as the Sustainable Groundwater Management Act of 2014 (SGMA).

The SGMA establishes local Groundwater Sustainability Agencies (GSAs) to manage groundwater sustainability within the groundwater sub-basins defined by the California DWR.

There are three GSAs in Sonoma County: Santa Rosa Plain, Sonoma Valley, and the Petaluma Valley. The City of Sebastopol’s water-producing wells overlay the Wilson Grove Formation, which is part of the Petaluma Valley GSA. However, a small portion of the City covers a part of the Santa Rosa Plain (SRP). The City had a choice to join the Wilson Grove or the Santa Rosa Plain; the City applied to California for inclusion in the Santa Rosa Plain GSA and received admittance in 2019. The California DWR has listed the City as a public water agency located within the Santa Rosa Plan Sub-basin.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-93 The Santa Rosa Plain spans approximately 167,410 acres and includes the Santa Rosa Plain and Rincon Valley groundwater subbasins, as well as parts of the Wilson Grove Formation Highlands (WGFH) groundwater basin, Kenwood Valley groundwater basin, Healdsburg area groundwater subbasin, and Alexander Valley groundwater subbasin. The Santa Rosa Plain drains northwest toward the Russian River and is, therefore, part of the North Coast Hydrologic Region.

Much of the City is located within the Wilson Grove Formation Highlands groundwater basin, which is an area of approximately 140 square miles. The City of Sebastopol occupies its eastern-most boundary. The California DWR monitored wells located outside of the City, and within or near the WGFH. Monitoring data indicates groundwater level recharge and discharge have remained in balance.

Groundwater records indicate that long-term groundwater trends have remained relatively stable in the Wilson Grove basin and adjacent areas within the Santa Rosa Plain groundwater sub-basin. Included in these trends are shorter-term declining trends, ranging up to 6 to 7 years in duration, which subsequently stabilize or recover. These short-term trends may be related to localized variations of groundwater production or climatic conditions such as the 1987-1993 timeframe, which is characterized as a period of below-average rainfall, thereby reducing recharge of the aquifer.

In general, the water quality of the Wilson Grove and Santa Rosa Plain groundwater sub-basin is very good to excellent. However, naturally occurring arsenic and manganese have been detected over state/federal maximum contaminant levels (MCLs) in the upper approximately 100 feet of the well screen.

It has been found that arsenic concentrations drop in deeper portions of the aquifer (PES Environmental, Inc. 2007).

Extent (Magnitude/Severity)
Limited – Extent can be measured according to a scale developed by the United States Drought Monitor, which measures drought in five categories: “abnormally dry,” “moderately dry,” “severely dry,” “extremely dry,” and “exceptionally dry”. The City is vulnerable to all levels of drought, which are further subject to the effects of climate change, precipitation trends, and wet and dry periods. Drought can have a widespread impact on the environment and economy in the Planning Area, but it typically does not result in loss of life or damage to property. Rather drought may have an impact on agriculture, business, and the movement of goods and services related to agricultural, commodities, tourism and recreation, and water supply sectors.

Given that the City’s water users fall within the categories of residential (67 percent of water users) and commercial/office, industrial, and institutional (non-residential represents 33 percent of water users), climatic, demographic, and economic factors have an effect on water demands. These are described below and are expected to influence water demands in the future, as they have in the past. • Climatic. The weather in Sebastopol is mild with a mean annual temperature of 56.7 degrees Fahrenheit. Average annual precipitation is about 25 inches. Climate has the most dramatic annual effect on water demands, and severe deviations from normal temperatures and average rainfall can increase or decrease annual water demands. Although Sebastopol’s municipal supply does not rely on surface water sources, precipitation shortages can have negative effects on what the City receives and can process for potable and other key water uses.
• Demographic. Since water use is related to demographics and population change, an accurate description of population and housing stock in the service area (Planning Area) serves as a basis for water planning activities described in the City’s Planning Documents. According to the City’s Level of Service report, the City’s population was 7,826 in 2019 (City of Sebastopol 2019). Increases to the population are expected to be modest as the current population represents a growth of 40 persons since 2018.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-94 • Economic. Commercial water users have the second highest water demand after residential users (both single family and multi-family). According to the City’s Level of Service report, commercial water use may increase by 2.5 percent of annual production if approved construction occurs and those projects currently under review are approved (City of Sebastopol 2019).

The magnitude or severity of a drought across the City could vary and is difficult to predict. However, understanding the total population affected as well as economy and resources vulnerable provides insight on how to estimate potential losses and damages to the City’s assets; drought related information can be obtained and measured from the National Drought Mitigation Center’s Impact Reporter and Drought Monitor tools (United States Drought Monitor 2020; United States Drought Impact Reporter 2018).

Figure 4-27, Figure 4-28, and Figure 4-29 provide “snapshots in time” of the drought conditions in California as of January 2021, November 2018, and August 2015 (during the period of the last multi-year drought in the state, from 2012- 2017). The snapshots selected are instrumental in depicting both the historic and potential change in drought’s geographic range and severity in the City (circled in blue).

These maps were extracted from the National Drought Mitigation Center and consider several factors including the Palmer Drought Index, Soil Moisture Models, USGS Weekly Stream flows, Standardized Precipitation Index, and Satellite Vegetation Health Index (United States Drought Monitor 2020).

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-95 Figure 4-27: U.S. Drought Monitor Conditions for California, January 5, 2021 Source: National Drought Mitigation Center 2018
City of Sebastopol
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-96 Figure 4-28: U.S. Drought Monitor Conditions for California, November 29, 2018 Source: National Drought Mitigation Center, 2018
City of Sebastopol
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-97 Figure 4-29: U.S. Drought Monitor for California: August 4, 2015 Source: National Drought Mitigation Center, 2018
Previous Occurrences
Historically, California has experienced multiple severe droughts. According to California’s DWR, droughts exceeding three years are relatively rare in Northern California, the source of much of the state’s developed water supply. The 1929-34 drought established the criteria commonly used in designing storage capacity and yield of large Northern California reservoirs. Figure 4-30 depicts California’s multiyear historical dry periods from 2000-2019.

Figure 4-30: Drought Conditions in California – 2000 – 2019 Source: U.S. Drought Monitor https://droughtmonitor.unl.edu/Data/Timeseries.aspx Since the year 2000 there have been several cases of multi-year droughts across California; these are described below:
2007-2009 – Water years 2007-2009 were the seventh driest three-year period in the measured record for state-wide precipitation and the fifteenth driest three-year period for DWR 8-station precipitation City of Sebastopol
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-98 index (a rough indicator of potential water supply available to the State Water Project and Central Valley Project)(California Natural Resources Agency 2010).
2012-2017 – The water years of 2012-14 stand out as California’s driest three consecutive years in terms of statewide precipitation. The drought occurred at a time of record warmth in California, with new climate records set in 2014 for statewide average temperatures. On January 17, 2014, California declared a drought state of emergency and during this time the state assisted farmers and communities that were most impacted by the drought conditions and helped with drinking water shortages. The state also directed all state agencies to use less water and expand their water conservation campaigns. During this time, these factors have led to excessively dry conditions in the City and the surrounding areas than in past years, often requiring disaster declarations to be enacted to combat drought conditions. Sonoma County declared a Proclamation of Local Emergency Due to Drought Conditions from February 2015 to December 2015. In September 2015, Sebastopol imposed Stage 2 (mandatory) restriction, requiring a reduction of water use by 25 percent.

This drought period now marks the second time a statewide proclamation of emergency has been issued for this hazard. On April 17, 2017 Executive Order B-40-17 was issued, which officially ended the drought state of emergency in California, except for Fresno, Kings, Tulare, and Tuolumne counties. Table 4-30 summarizes the drought-related disaster declarations proclaimed for Sonoma County from 1976 through 2020. These declarations include those from FEMA, the USDA’s Secretary of Agriculture, and events noted in the State of California’s 2018 SHMP.

Table 4-30: Disaster Declarations and Proclamations Related to Drought in Sonoma County Declaration or Order Date
1976 Drought (State) 1976
EM-3023 (FEMA) 1/20/1977
S3248 (Secretary of Agriculture) 2012
S3452 (Secretary of Agriculture) 2012
S3565 (Secretary of Agriculture) 2013
S3569 (Secretary of Agriculture) 2013
S3637(Secretary of Agriculture) 2014
S3743 (Secretary of Agriculture) 2014
S3797 (Secretary of Agriculture) 2014
S3784 (Secretary of Agriculture) 2015
S3943 (Secretary of Agriculture) 2015
S3952 (Secretary of Agriculture) 2016
S3964 (Secretary of Agriculture) 2016
S4163 (Secretary of Agriculture) 2016-2017
S4144 (Secretary of Agriculture) 2017
S4691 (Secretary of Agriculture) 2020
S4697 (Secretary of Agriculture) 2020
Source: USDA Disaster Designations 2020) California SHMP 2018; FEMA Figure 4-31 graphically displays the amount of drought-related reported impacts to Sonoma County (United States Drought Impact Reporter 2019). While it is difficult to extract the impacts specifically affecting Sebastopol, a total of 180 reports were made within Sonoma County between January 1, 1950 and December 31, 2020. It is assumed that these drought-related impacts for areas across Sonoma County are likely to have also affected Sebastopol at some point or to some extent. Based on the summary of negative effects to Sonoma County since 1950, the categories of water supply/quality have had the most reports, followed by relief, response, and restrictions operations and society and public health impacts. Agriculture and plants and wildlife have also suffered the effects of drought, but to a lesser extent.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-99 Figure 4-31: Drought Impact Reporter Impacts at the County Level in Sonoma County, 1950-2020 Source: National Drought Mitigation Center Drought Impact Reporter, 2020 Probability of Future Occurrences
Likely – Historical drought data for California and more particularly the Sonoma County municipalities indicate there have been significant droughts and negative effects from water shortages in the past and the present. Based on this data, droughts are likely to affect the City’s Planning Area and surrounding parts approximately every ten years; some of these droughts may persist for multiple years.

Climate Change Considerations
Scientific studies prepared for various California climate assessments and adaptations strategies show that drought conditions in California are likely to become more frequent and persistent over the next century due to climate change. Temperatures are warming, heat waves are more frequent, and precipitation has become increasingly variable (Natural Resources Agency 2018a). Water resources are also already experiencing the following stresses: population growth, poor water quality, groundwater overdraft, and aging water infrastructure.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-100 The recent drought conditions over the past decade underscore the need to examine water supply and distribution management, conservation, and use policies. California and Sonoma County have experienced a succession of dry spells, and with warmer temperatures the impacts of drought conditions have increased (OEHHA 2018). In an average year, approximately 40 percent of the state’s total water supply comes from groundwater, and during a dry year this increases to more than half of the state’s water supply, with groundwater acting as a critical buffer against the impacts of drought and climate change (Natural Resources Agency 2018a). However, The City receives 100 percent of its municipal water supply from groundwater (City of Sebastopol 2020). Historical records show that groundwater declines during dry water years by 5 to15 feet; however, groundwater level declines at this magnitude are not anticipated to affect the current or projected operating capacity of the City’s municipal supply wells. Therefore, water supply will not significantly change during single or multiple dry water years.

According to California’s Climate Adaptation Strategy, also referred to as Safeguarding California Plan:
2018 Update, climate change is likely to significantly diminish California’s future water supply. As a result, the state must change its water management, as climate change will create greater competition for limited water supplies (California Natural Resources Agency 2018b). Similarly, as summarized in the Sonoma County CAP, climate change could result in hotter and drier weather, and more frequent and intense droughts. The CWA (numeric measure of drought stress that quantifies the extent to which plants need for water exceeds moisture available in soil) for the region is projected to increase over this century, producing 10 to 20 percent drier soil conditions in the summer months, leaving less water available for groundwater recharge or runoff into rivers and creeks (SCTA 2016). For these reasons, climate change would have a “high” influence on drought hazards, as well as water shortages.

Vulnerability Assessment
Property
Drought impacts are wide-reaching and may be economic, environmental, and societal. The most significant impacts associated with drought in the City’s Planning Area are those related to water intensive activities, such as agriculture, followed by municipal water use, commerce, tourism, and recreation. The vulnerability of a water intensive activity to the effects of drought usually depends on its water demand, whether the demand is met, and what water supplies are available to meet the demand. During extended droughts, water restrictions and conservation measures are implemented, and these can result in economic impacts on water utilities managed by the City. Drought conditions can also cause soil to compact and not absorb water efficiently, potentially making areas more susceptible to flooding.

According to the Drought Impact Reporter the Sonoma County recorded a total of 180 impacts to drought in the survey period between 1/1/1950 and 12/31/2020 (70-year period). Of these, the majority of the impacts were associated with Water Supply and Quality; and Relief, Response, and Restrictions.

These statistics are shown in Figure 4-31 (above). While the Drought Impact Reporter data reflects impacts at the county-level, the data should be used to develop an ongoing record of drought impacts that can be more specifically tied to events that occur within the City’s Planning Area to better understand cityspecific vulnerable sectors and impacts.

People
According to the California DOF as of 2019 the City population was around 7,826. The City supplies a majority of its water (67 percent) to residential users. The population is expected increase modestly in the future (up to 5.5 percent by 2030); however, the projected growth is not anticipated to strain water supplies. There are also several initiatives (WSCPs and groundwater management policies) that emphasize Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-101 water conservation. Water conservation will ensure that the existing groundwater remains operational during severe drought conditions and readily available during emergencies.

Drought can cause public health problems related to poor water quality, and health problems can become exacerbated due to dust. Generally, drought may require conservation of water resources, which means that water use is restricted to essential uses, which may reduce watering for landscaping. The community may also exhibit a range of abilities to prepare for, respond to, and recover from drought hazards, as these conditions impact populations with health-related issues related to heat-related illness, respiratory problems, and people who work outdoors. These conditions can impact lower-income populations, as food and water prices increase. There are sensitive and socially vulnerable populations residing near the downtown area of the Planning Area that may be the most susceptible to water restrictions, and healthrelated illnesses. Socially vulnerable populations may also be sensitive to increases in water rates and in turn, food prices.

Economy
Drought impacts to the local and regional economy can be difficult to quantify but can be extensive and long-lasting depending on the circumstances during and after a severe drought event. If water resources are limited, effects would be more severe for industries that rely on large amounts of water, and any prolonged drought would intensify these impacts. Sectors critical to the economy such as commerce, distribution, agriculture, tourism, related environmental resources, municipal and industrial water supply, key city assets, energy generation, and even socioeconomic aspects can be affected due to lack of or reduced quality of water resources.

Critical Facilities and Infrastructure
The most direct impact of drought will be on the City’s water supply system. Drought can also directly affect the water storage, treatment, and distribution and conveyance systems. Landscaping around City facilities may no longer be maintained during water restrictions, and the risk within the Planning Area will be largely aesthetic.

Historic, Cultural, and Natural Resources
Severe, prolonged drought can impact the natural
environment. Wildlife and natural habitats including the Laguna de Santa Rosa Wetland Preserve, including the
shrinkage of habitat, habitat fragmentation, reduced
food supply for wildlife, and possibly the migration of species in the nearby hillsides that define the City of Sebastopol. Prolonged drought can also cause poor soil
quality, loss of wetlands, tree mortality (along the
periphery of the City’s Planning Area), and increased soil erosion.

Tree mortality is identified as a cascading impact that can affect (or worsen) other hazards, such as wildfire and wind conditions. For example, drought-impacted trees
can become susceptible to diseases, such as Sudden
Oak Death caused by fungus-like pathogens
Phytophthora ramorum (the disease kills oak and tanoak
species, but primarily coast live oak). Drought-affected trees can also become susceptible to insect infestations that further exacerbate the risk of tree mortality At least 75,000 acres of Sonoma County is affected by
Sudden Oak Death mortality, which equates to 23
percent of the County forests that are at-risk for
infection, including parts of western Sonoma County.

Photo Credit: University of California Cooperative
Extension, Sonoma County Department of Emergency
Services/January 2008
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-102 and those trees resilient to wildfires (Matthews 2020). One of the most prevailing impacts of drought to the natural environment is the increased risk of wildfires, as seen during the 2017-2018 wildfire seasons.

Wildfires now burn larger and more intensely during dry conditions and are happening outside the typical fire season. Lastly, drought conditions can cause soil to compact and not absorb water well, potentially making an area more susceptible to flooding.

Impacts to the City’s seven historic and cultural buildings may be negligible. The City’s open spaces and park and public lands can suffer during droughts, though the ability of the City to integrate more drought-tolerant landscaping and the use of drip irrigation and other conservation efforts can offset this vulnerability.

Future Development
Future development and water conservation are the focus of each update to the Sebastopol General Plan.

The planning process addresses drought conditions and water contingencies. In 2014, the City provided water to approximately 2,650 customers. The Sebastopol General Plan describes how current and future water resources and demands within the City’s service area will be managed to provide adequate and reliable water supply.

As the population grows over time the City will have to revise their reliability and supply projections. The City relies 100 percent on groundwater for its water supply and currently supplies a majority of water supply to single and multi-family residential users. Though future growth is projected to be modest, consistent with Senate Bill 610, any proposed developments in the City are mandated to estimate future water uses and identify water supplies that may be used to meet their uses. This water supply assessment process is intended to ensure that adequate water supplies exist to support new growth.

Risk Summary
• There have been six multi-year droughts since 1950, three of which have occurred since 2000. The most recent drought lasted from 2012 to 2017 and resulted in a declared state of emergency. • 180 drought impact reports were made within Sonoma County between 1950 and 2020. • The City relies exclusively on groundwater as its water supply source, which is provided from five wells. Typically municipalities that rely mostly on groundwater are less vulnerable to drought, but because the City has no backup system or the ability to connect to other water systems, a diversified supply may allow the City to rely on alternative water sources during emergencies. • As of 2019, the City was supplying 323 million gallons of (about 1,000 acre feet), the majority of which is supplied to single family residential properties.
• Population is expected to modestly increase as the City has added about 9 persons per year since 2009. This projected growth will not add additional strain to the water supply, even during future severe drought events.
• Climate change projections indicate the region will experience more frequent and intense droughts due to drier soil conditions in the summer months, leaving less water available for groundwater recharge.
• The enforcement of water conservation policies will help ensure the City is resilient to drought events in the future.
• Overall, the significance of extreme drought is Medium.

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4.3.5 Dam Incidents
Hazard Description
Dams are manmade structures built for a variety of uses, including flood protection, power generation, agriculture, water supply, and recreation. When dams are constructed for flood protection, they are engineered to withstand a flood with a computed risk of occurrence. For example, a dam may be designed to contain a flood at a location on a stream that has a certain probability of occurring in any one year. If prolonged periods of rainfall and flooding occur that exceed the design requirements, that structure may be overtopped and fail. Overtopping is the primary cause of earthen dam incidents and failure in the United States. Dam incidents can also result from any one or a combination of the following causes:
• Earthquake
• Inadequate spillway capacity resulting in excess overtopping flows • Internal erosion caused by embankment or foundation leakage or piping or rodent activity • Improper design
• Improper maintenance
• Negligent operation
• Failure of upstream dams on the same waterway
Water released by a failed dam generates tremendous energy and can cause a flood that is catastrophic to life and property. A catastrophic dam incident or failure could challenge local response capabilities and require evacuations to save lives. Impacts to life safety will depend on the warning time and the resources available to notify and evacuate the public. Major loss of life could result as well as potentially catastrophic effects to roads, bridges, and homes. Associated water quality and health concerns could also be issues. Factors that influence the potential severity of a full or partial dam failure or dam incident are the amount of water impounded; the density, type, and value of development and infrastructure located downstream; and the speed of failure.

Controlled release or spillway flooding: inadequate spillway capacity results in excess overtopping flows, though the potential for flooding as a result of discharge from dam outlet structures or spillways could be expected during excessive rain events. However, controlled releases of water from dams is a measure that can prevent or minimize spillway flooding or structure failure, by regulating capacity in a managed way.

Even controlled releases can lead to unwanted or unpredicted flooding, depending on environmental and weather conditions, or even human error.

In general, there are three types of dams: concrete arch or hydraulic fill, earth-rockfill, and concrete gravity. Each type of dam has different failure characteristics. A concrete arch or hydraulic fill dam can fail almost instantaneously: the flood wave builds up rapidly to a peak then gradually declines. An earthrockfill dam fails gradually due to erosion of the breach: a flood wave will build gradually to a peak and then decline until the reservoir is empty. A concrete gravity dam can fail instantaneously or gradually with a corresponding buildup and decline of the flood wave.

Location
Limited – According to the U.S. Army Corps of Engineers’ National Inventory of Dams (NID) database, last updated in 2018, there is one potential dam of concern upstream of the City of Sebastopol, the Warm Springs Dam (NID 2020). This and other nearby dams have been constructed for flood control, irrigation storage, recreation, and stock watering purposes. This dam of concern is considered to pose a high hazard.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-104 Warm Springs Dam is an earthen dam constructed in 1982 located along Dry Creek in northern Sonoma County approximately 30 miles north of the City. The dam storage capacity is 449,000 acre-feet. This is a high hazard dam owned and operated by the U.S. Army Corps of Engineers and has an Emergency Action Plan (EAP) in place. Dam inundation mapping has been completed for Warm Springs Dam, but the inundation was not available for analysis (Sonoma County 2016).

Table 4-31 below details the dams that could potentially affect the City given their close proximity and potential to inundate if either were to fail. Delta Pond is a significant hazard earthen dam located along a tributary of the Russian River approximately 5 miles north of the City. The dam storage capacity is 1,950 acre-feet and is owned and operated by the City of Santa Rosa. Based on dam inundation mapping data, failure of this dam would not affect the City of Santa Rosa. Figure 4-32 illustrates the location of the closest identified dam of concern near the City, but Meadow Lane Dam is located downstream of the City and associated with the City of Santa Rosa Wastewater Treatment Plant (WWTP).

Table 4-31: Characteristics of the Dams of Concern Upstream of the City of Sebastopol Hazard
Rating
Dam
Name
River
Drainage
Downstream
Community
Dam
Type
Dam
Height
(in Feet)
Storage
Capacity
(AcreFeet)
Emergency
Operations
Plan
Dam
Owner
High Warm
Springs
Dam
Dry Creek Healdsburg Earth 356 449,000 Yes U.S. Army
Corps of
Engineers
Significant Delta
Pond
Russian
River
Tributary
Santa Rosa Earth 27 1,950 Yes City of
Santa
Rosa
Significant Meadow
Lane
Off stream Rohnert Park Earth 30 2,100 No City of
Santa
Rosa
Source: U.S. Army Corps of Engineers’ National Inventory of Dams, 2020; DWR 2021; DWR 2019b Note: 1 acre-foot = 325,851 gallons
Extent (Magnitude/Severity)
Negligible – Standard practice among federal and state dam safety offices is to classify a dam according to the potential impact a dam failure (breach) or mis-operation (unscheduled release) would have on downstream areas (National Performance of Dams Program 2019). The hazard potential classification system categorizes dams based on the probable loss of human life and the impacts on economic, environmental, and lifeline facilities. Dams are classified into the following three categories which identify their potential hazard to life and property:
• High hazard indicates that a failure would most probably result in the loss of life; • Significant hazard indicates that a failure could result in appreciable property damage; • Low hazard indicates that failure would result in only minimal property damage and loss of life is unlikely; and
• Undetermined hazard dams have not been rated or their hazard rating is not known.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-105 Figure 4-32: Dams of Interest in the Vicinity of the City of Sebastopol Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-106 Since there are two potentially hazardous dams upstream of the City (one significant- and one high-rated hazard dam), there is some, though limited, potential for loss of life and/or property damage. Adjacent unincorporated portions of Sonoma County could also be affected by a dam failure upstream of the City, although the specific extent of impacts would depend on the nature of the failure, local emergency response capabilities, people and property found in the path of the dam inundation areas, and other factors. Based on the dam capacity of Delta Pond and the distance of Warm Springs Dam from the City, it is unlikely that much risk would be imposed on those areas near Sebastopol. However, given the moderate size of the downstream population from Warm Springs Dam, in addition to the residential structures, roads, farmland, bridges, and utilities also located downstream, there are consequences that would occur in the event of a dam breach caused by seepage, overtopping, or an earthquake (City of Rohnert Park 2018; Sonoma County 2016). According to the City of Rohnert Park LHMP, portions of the communities of Healdsburg, Windsor, Santa Rosa, Sebastopol, and Guerneville, as well as rural population area in the floodplain immediately downstream of the dam, would be at risk if the dam failed, as these communities and areas are within the 1 to 24-hour flood wave trave time bracket (City of Rohnert Park, 2018). Based on the dam inundation mapping for the Delta Pond dam, failure or breach of this dam will not impact the City of Sebastopol.

Previous Occurrences
There is no history of dam incidents or failures affecting the City.

Probability of Future Occurrences
Unlikely – The City remains at risk to upstream dam failures or incidents, particularly from one of the two dams that are classified as a high hazard structure that has the potential to partially inundate the City.

However, based on the lack of previous dam inundation events, HMPC input, and the fact that the dam posing risk to the City is relatively far from the City, dam failure and dam incidents are unlikely in the area.

Nevertheless, the potential exists for future dam incidents in the City or portions of it, but the likelihood of this is low. Uncontrolled or controlled release flooding as well as spillway flooding below dams due to excessive rain or runoff are more likely to occur than failures.

Climate Change Considerations
The potential for climate change to affect the likelihood of dam failure and incidents is not fully understood at this point in time. With a potential for more extreme precipitation events a result of climate change, this could result in large inflows to reservoirs. However, this could be offset by generally lower reservoir levels if storage water resources become more limited or stretched in the future due to climate change, drought and/or population growth. For these reasons, climate change would have a “low” influence on dam incidents.

Vulnerability Assessment
A dam incident can range from a small, uncontrolled release to a catastrophic failure. Vulnerability to dam failures is confined to the areas and populations subject to inundation downstream of the facility.

Secondary losses would include loss of the multi-use functions of the dam itself and associated revenues that accompany those functions, including potential potable water uses or critical irrigation for crops.

Property
Communities located below a high or significant hazard dam and along a waterway are potentially exposed to the impacts of a dam failure. For reference, high hazard dams threaten lives and property, significant hazard dams threaten property only. Inundation maps that identify anticipated flooded areas Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-107 (which may not coincide with known floodplains) are often produced for all high hazard dams and are contained in the EAP required for each dam. The potential magnitude of a dam incident depends on the time of year and the base flow of the river when the incident or failure occurs. During the winter months, when the river flows are higher, the impact to the area would be much greater and evacuation times even shorter.

Based on the location of the one high hazard dam, Warm Springs Dam, and one significant hazard dam, Delta Pond, both outside and upstream of the City in a sparsely populated area, there are impacts to cities and communities located immediately downstream of each dam. However, the potential impacts to buildings and infrastructure within the City, which is much further downstream are minimal. Spatial GIS dam inundation data for Warm Springs Dam was not publicly available; however, according to mapping and findings in the City of Rohnert Park LHMP and Sonoma County Operational Area HMP, which summarizes dam inundation mapping for the Warm Springs Dam as shown in Figure 4-33, impacts to the City would occur, but be limited to the north-eastern and southern parts of the City along the Laguna de Santa Rosa. (City of Rohnert Park 2018; Sonoma County 2016). Dam inundation data for Delta Pond Dam was available, but the inundation area does not extend into the City of Sebastopol.

Figure 4-33: Dam Failure Inundation in Sonoma County
Source: Sonoma County 2016
People
Persons located underneath or downstream of a dam are at risk of a dam failure, though the level of risk can be tempered by topography (specifically where populations are located within the inundation path of a dam but at higher elevations), amount of water in the reservoir/damming structure, and time of day of the breach. Injuries and fatalities can occur from debris, bodily injury, and drowning. Once a dam has breached, standing water presents all the same hazards to people as floodwater from other sources.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-108 People in the inundation area may need to be evacuated, cared for, and possibly permanently relocated.

Impacts could include hundreds or thousands of evacuations and likely casualties, depending on the dam involved. Inundation may also have a disproportionate impact on socially vulnerable or sensitive populations in the City’s Planning Area.

Economy
Extensive and long-lasting economic impacts could result from a major dam failure including the longterm loss of water in a reservoir after a failure event. A major dam failure or incident and loss of water from the associate reservoir could include direct business and industry damages and indirect disruption of the local economy, including the disruption of irrigation water for crops or even water for livestock which may be key components of the local economy and its sectors.

Critical Facilities and Infrastructure
A total dam failure can cause catastrophic impacts to areas downstream of the water body, including critical infrastructure and essential facilities. Dam incidents may result is less severe downstream impacts, depending on the severity of the incident. Any critical asset located under the dam in an inundation area would be susceptible to the impacts of a dam incident. Of particular risk would be roads and bridges that could be vulnerable to washouts, further complicating response and recovery by cutting off impacted areas. Risk to specific facilities could be considered sensitive information, especially those such as water treatment facilities or water delivery systems which may provide potable water for the local population.

Based on location of the two upstream dams and inundation data, it appears the failure or a major incident at the Warm Springs Dam, a high hazard dam could potentially impact portions of the City’s buildings and infrastructure located along the Laguna de Santa Rosa along the north-eastern side of the City (City of Rohnert Park 2018; Sonoma County 2016).

Historic, Cultural, and Natural Resources
Dam failure effects on the environment would be like those caused by flooding from other causes. Water could erode stream channels and topsoil and cover the environment with debris. For the most part the environment is resilient and would be able to rebound from whatever damages occur, though this process could take years. Historic and cultural resources could be affected just as housing or critical infrastructure would, were a dam to fail and cause downstream inundation that could further erode surfaces or cause scouring of structural foundations. Given only one of the upstream dams would partially inundate the north-eastern portion of the City, most historic, cultural, and natural resources that may be affected are limited to open space lands and a ballfield located within the Laguna de Santa Rosa floodplain.

Future Development
Areas slated for future development should take into consideration potential impacts from dam failure risk upstream and should attempt to overlay the existing digital dam inundation maps (if available) with proposed future development. In the case of a dam failure, inundation would follow existing FEMA mapped floodplains, which contains development restrictions for areas in the one percent annual chance floodplain, but it could exceed those floodplains and affect areas that are not regulated for flood hazards.

Also, added development could compromise dams and reservoir resources if populations depend on them for critical needs such as potable water during or after a dam failure event.

Risk Summary
• The overall significance of dam inundation in the City is Low. • Warm Springs Dam and Delta Pond Dam are two dams of concern located upstream of the City.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-109 • Warm Springs Dam is a high hazard earthen dam constructed in 1982 located along Dry Creek in northern Sonoma County approximately 30 miles north of the City of Sebastopol. The dam storage capacity is 449,000 acre-feet. This dam owned and operated by the U.S. Army Corps of Engineers and has an active EAP in place.
• Due to the lack of historic occurrence data on dam inundation associated with Warm Springs Dam and regular inspection monitoring, and the implementation of the EAP, potential failure of this dam is expected to be unlikely to severely impact the City’s population, property, and critical infrastructure.

4.3.6 Severe Weather
Severe weather is generally any destructive weather event, but usually occurs in the Planning Area as localized thunderstorms that bring heavy rain, hail, lightning, high winds, and dense fog. Severe weather can also include extreme heat events.

The NOAA NCEI has been tracking severe weather since 1950. Their Storm Events Database tracks severe weather events on a county basis and contains data on the following: all weather events from 1993 to current (except from 6/1993-7/1993); and additional data from the Storm Prediction Center, which includes tornadoes (1950-1992), thunderstorm winds (1955-1992), and hail (1955-1992). This database contains 708 severe weather events that occurred in Sonoma County between January 1, 1950, and December 31, 2020. Table 4-32 summarizes these events.

Table 4-32: NCEI Hazard Event Reports for the Sonoma County: 1950-2020 Type # of Events Property Loss ($) Crop Loss ($) Deaths Injuries Debris Flow 60 25,916 20,000 1 0
Dense Fog 14 100,000 0 0 2
Dense Smoke 9 0 0 0 0
Extreme Cold/Wind
Chill
2 0 0 0 0
Flash Flood 44 8,018,000 164,000 1 1
Flood 193 208,097,400 6,150,000 1 0
Frost/Freeze 9 60,000 3,000,000 0 0
Funnel Cloud 1 0 0 0 0
Hail 21 0 $300,000 0 5
Heat 7 0 0 1 0
Heavy Rain 52 383,500 30,250,000 1 2
High Wind 84 713,500 0 2 0
Lightning 3 1,000,000 0 0 0
Strong Winds 174 3,141,200 0 3 5
Tornado 13 1,558,500 500 0 1
Wildfire 22 505,000 5,000 3 14
Total** 708 $224,735,016 $39,889,500 13 30
Source: NOAA’s National Centers for Environmental Information https://www.ncdc.noaa.gov/stormevents/ *Note any reference to a coastal type weather event for Sonoma County has been excluded from this table given Sebastopol’s distance from the coast. **Losses reflect totals for all impacted areas, inclusive of Sonoma County Only a few of the events resulted in state and federal disaster declarations. While the HMPC recognizes these inconsistencies, this data provides value in depicting the City of Sebastopol’s severe weather hazards as they relate to Sonoma County’s “big picture” hazard environment.

As previously mentioned, several state and federal disaster declarations including the City of Sebastopol have been a result of severe weather. For this plan, severe weather is broken down as follows:
• Extreme Heat
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-110 • Heavy Rain/Thunderstorm/Hail/Lightning/Dense Fog
• High Winds

4.3.7 Severe Weather: Extreme Heat
Hazard Description
Extreme heat events can have severe impacts on human health and mortality, natural ecosystems, the agriculture sector and other economic sectors. According to information provided by FEMA, extreme heat is defined as temperatures that hover 10 degrees or more above the average high temperature for the region and last for several weeks. Heat kills by taxing the human body beyond its abilities. In a normal year, about 175 Americans succumb to the demands of summer heat. According to the National Weather Service (NWS), among natural hazards, only the cold of winter takes a greater toll nationally — not lightning, hurricanes, tornadoes, floods, or earthquakes. During the 40-year period from 1936 through 1975, nearly 20,000 people were killed in the United States by the effects of heat and solar radiation. In the heat wave of 1980, more than 1,250 people died. The 2018 California SHMP notes the heat wave during the summer of 2006 lead to 650 deaths in a 13-day period (Cal OES 2018), and in the past 15 years heat waves have claimed more lives in California than all other declared disaster events combined (California Natural Resources Agency 2018a). However, there are a lack of cold weather and extreme cold temperatures events in Sonoma County.

Heat disorders generally have to do with a reduction or collapse of the body’s ability to shed heat by circulatory changes and sweating or a chemical (salt) imbalance caused by too much sweating. When heat gain exceeds the level the body can remove, or when the body cannot compensate for fluids and salt lost through perspiration, the temperature of the body’s inner core begins to rise, and heat-related illness may develop. The elderly, small children, patients with chronic medical conditions, those on prescription medication therapy, and people with weight or alcohol problems are particularly susceptible to heat reactions, especially during heat waves in areas where moderate climate usually prevails. Figure 4-34 illustrates the relationship of temperature and humidity to heat disorders.

Figure 4-34: National Weather Service Heat Index
Source: National Weather Service
Note: Since heat index values were devised for shady, light wind conditions, exposure to full sunshine can increase heat index values by up to 15°F.

Also, strong winds, particularly with very hot, dry air, can be extremely hazardous.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-111 Location
Extensive – Severe weather events have the potential to happen anywhere in the Planning Area.

According to the City and HMPC, extreme heat, occasional heavy rain and thunderstorms, and wind events have occurred in the City.

Extent (Magnitude/Severity)
Limited – The City begins to experience hot weather in June or July of each year, and the heat continues throughout the summer months. According to the Western Regional Climate Center (WRCC), the average high temperature for the City in July is 82.5°F (WRCC 2020). Temperatures that are 10 degrees above normal are considered excessive. The California OES Contingency Plan for Excessive Heat Emergencies (CalOES 2014) indicates that through the use of historical weather and mortality data, the NWS and the California Department of Public Health (CDPH) have identified five major types of climate regions within California to account for climate differences among regions in order to recognize what constitutes an excessive heat event in each of the regions. When temperatures spike for two or more consecutive days without an adequate drop in nighttime temperature to cool the outdoor and indoor environments, there is a significant increase in the risk to vulnerable populations.

The NWS has a system to initiate alert procedures (advisories, watches, and warnings) when high temperatures are expected to have a significant impact on public safety. The expected severity of the heat determines which type of alert is issued. During past heat waves, Sonoma County has designated facilities as Cooling Centers, as have the cities of Santa Rosa and Petaluma. The closest designed cooling centers to the City of Sebastopol are in Petaluma and Santa Rosa. The Petaluma Community Center at Lucchessi Park was designated as a City Cooling Center and the Rincon Valley Library and Salvation Army Senior Center in Santa Rosa. In summary, extreme heat impacts would likely be limited in the Planning Area, with 10 to 25 percent of the Planning Area affected. Extreme heat will have an impact on vulnerable populations and could also impact livestock and crops if the event occurs during certain times of the year.

Previous Occurrences
Information from the closest weather station with the most comprehensive data, the Santa Rosa Weather Station (047965), is summarized below and in Figure 4-35 to illustrate daily temperature averages near the City’s Planning Area.

The City of Santa Rosa (Period of Record 1902 – 2013)
The City of Santa Rosa is approximately seven miles from Sebastopol. Monthly average maximum temperatures in the warmest months (May through October) range from the mid-70s to the low 80s.

Monthly average minimum temperatures from November through April range from the upper 30s to mid40s. The highest recorded daily extreme was 111°F on September 6, 2020. The lowest recorded daily extreme was 16°F on December 14, 1932. In a typical year, maximum temperatures do not exceed 82°F and minimum temperatures do not fall below 36°F.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-112 Figure 4-35: The City of Santa Rosa’s Daily Temperature Averages and Extremes Source: Western Regional Climate Center 2020, www.wrcc.dri.edu/ The California statewide mean temperature departures from the 1900s to mid-2010s are displayed in Figure 4-36. This graphically highlights the general warming trend across the state, and how climate change can have significant implications in future water supply availability, as well as higher mean temperatures.

Figure 4-36: California’s Statewide Mean Temperature Departure, 1900-2014 Source: Drought in California Report (CA DWR; Natural Resources Agency; State of California, 2015) Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-113 Probability of Future Occurrences
Likely – Temperatures of extreme heat are likely to continue to occur annually in the Planning Area.

Climate Change Considerations
Heat waves are likely to become more frequent, which will have direct impacts on human health in terms of heat related illness. With the general trend of increased warming of average temperatures, extreme high temperatures will likely also increase. Cascading impacts include increased stress on water quantity and quality, degraded air quality, and increased potential for more severe or catastrophic natural events such as heavy rain, droughts, and wildfire. Another cascading impact includes increased duration and intensity of wildfires with warmer temperatures.

According to the 2013 document, Preparing California for the Extreme Heat, Cal-Adapt projects that throughout California urban and rural population centers will experience an average of 40 to 53 extreme heat days by 2050 and an average of 40 days by 2099 (Cal-Adapt 2013). This compares to a historical average of four days per year (CalEPA 2013). Cal Adapt also projects that temperatures are expected to rise substantially throughout this century. Future temperature estimates from Cal Adapt for the City under high and low emission scenarios are shown in Figure 4-37. The top graph shows the number of days per year when daily maximum temperature is above the extreme heat threshold of 100°F under the RCP 8.5 scenario (business as usual). The bottom graph shows the number of days per year when daily maximum temperature is above the extreme heat threshold of 103.9°F under the RCP 4.5 scenario.

Figure 4-37: City of Sebastopol – Future Extreme Heat Days in High and Low Emission Scenarios Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-114 Source: Cal-Adapt 2021
Extreme heat has also been shown to accelerate wear and tear on the natural gas and electrical infrastructure (California Natural Resources Agency 2018a). Projected increases in summer demand associated with rising temperatures may increase risks to energy infrastructure and may exceed the capacity of existing substations and distribution line infrastructure and systems.

A recent study on extreme heat released by the Union of Concern Scientists in July 2019 analyzed three global climate scenarios associated with different levels of heat-trapping emissions and future warming (Union of Concerned Scientists 2019). The results of the analysis showed that with no actions taken to reduce heat-trapping emissions by midcentury (2036-2065) the average number of days per year in the United States with a heat index above 100°F will double, while the number of days per year above 105°F will quadruple (Union of Concerned Scientists 2019). The modeling completed for the study showed that the most dramatic transformations will be felt in areas where the climate has been temperate. The City could experience up to 11 more times as many days per year in which the heat feels like 90 degrees (KQED 2019a). According to Cal-Adapt Climate Projections for the Bay Area Region as stated in the 2017 Climate Change Health Profile Report for Sonoma County, by 2100 the number of heat waves in the Bay Area Region is expected to be between 6 to 10 heatwaves per year.

Based on Sonoma County’s 2016 CAP, climate change is also expected to result in higher average temperature and more extreme heat events. If future GHG emissions are mitigated or reduced over time, summer high temperatures are expected to rise by 1 to 2°F. Whereas, if GHG emissions are not mitigated average summer high temperatures will increase by up to 9 to 11°F by 2100 (SCTA 2016). For these reasons, climate change would have a “high” influence on extreme heat hazards.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-115 Vulnerability Assessment
Property
Recent research indicates that the impact of extreme heat, particularly on populations, has been historically under-represented. The risks of extreme heat are profiled as part of larger hazards, such as drought or wildfire. However, as temperature variances may occur independent of other hazards or outside of the expected seasons, but still incur large costs, it is important to examine them as stand-alone hazards. Extreme heat can overload demands for electricity to run air conditioners in homes and businesses during prolonged periods of exposure and presents health concerns to individuals who are outside.

Extreme heat may also be a secondary effect of droughts or may cause temporary drought-like conditions. For example, several weeks of extreme heat increases evapotranspiration and reduces moisture content in vegetation, leading to higher wildfire vulnerability for that time period even if the rest of the season is relatively moist. Extreme heat can cause infrastructure damage to roads. In summary, all property in Sebastopol is vulnerable to extreme heat.

People
Traditionally, the very young and very old are considered at higher risk to the effects of extreme heat, but any population outdoors during periods of extreme temperatures are exposed, including otherwise young and healthy adults and homeless populations. While everyone is vulnerable to extreme heat incidents, some populations are more vulnerable than others. Extreme heat poses the greatest danger to outdoor laborers, such as highway crews, police and fire personnel, and construction workers. The elderly, children, people in poor physical health, and the homeless are also vulnerable to exposure. Arguably, the youngand-otherwise-healthy demographic may also experience a higher vulnerability of exposure due to the increased likelihood that they will be out in temperatures of extreme heat, whether due to commuting for work or school, conducting property maintenance such as lawn care, or for recreational reasons.

It is difficult to isolate the City’s specific vulnerability to this hazard, as the impacts from extreme heat can be spread across an entire state or region. In general, all the population of the City can be considered atrisk to this hazard, but given the senior population in the City, the elderly may be most exposed and vulnerable to extreme heat.

Economy
Extreme heat impacts on the economy may be more indirect compared to other hazards. Infrastructure such as roads could be damaged and lead to increased need for repaving. Critical facilities may be vulnerable to the indirect impact of prolonged excessive heat (i.e., electrical power outages), which may impact response capabilities or care capabilities for hospitals and clinics. Hospitals and clinics may see a surge in patients during the heat event as the exposed population suffers from the effects of the heat, but it is not anticipated that these increases will overwhelm the capacities of hospitals and clinics near the City of Sebastopol. Essential infrastructure, especially the electrical distribution system, is also poised to be stressed during extreme heat events as demand increases to run air conditioning. Peak demand exceeding PG&E’s capacity for supply can lead to blackout or brownout conditions.

Critical Facilities and Infrastructure
Extreme heat can affect road infrastructure, but direct impacts to critical infrastructure is expected to be minimal. Critical infrastructure that relies on public utility systems that could be overloaded may result in impacts during extreme heat events. The loss of utilities or power outages during extreme heat events Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-116 could also result in adverse secondary impacts to sensitive populations, such as the seniors in the community.

Historic, Cultural, and Natural Resources
Extreme heat may cause temporary drought-like conditions. For example, several weeks of extreme heat increases evapotranspiration and reduces moisture content in vegetation, leading to higher wildfire vulnerability for that time period even if the rest of the season is relatively moist. Changing heating and cooling patterns globally can have destructive secondary impacts, intensifying a variety of weather-related disasters that directly impact jurisdictions, which could in turn impact cultural and natural resources.

Future Development
Since structures are not usually directly impacted by severe temperature fluctuations, continued development is less impacted by extreme heat than others in the plan. Continued development implies continued population growth, which raises the number of individuals potentially exposed to temperature variations. Public education efforts should continue to help the sensitive populations understand the risks and vulnerabilities of outdoor activities, property maintenance, and regular exposures during periods of extreme heat.

Risk Summary
• The highest recorded temperature in the Planning Area was 111°F on September 6, 2020. • Extreme heat can have severe impacts on human health, the natural environment, and the economy. • The very young, the elderly, people with poor physical health, and the homeless are more susceptible to the impacts of extreme temperatures.
• The average number of days per year in the United States with a heat index above 100°F will double, while the number of days per year above 105°F will quadruple if no actions to reduce heat-trapping emissions are taken.
• Climate change is expected to result in higher average temperature and more extreme heat events and have a “high” influence on the number of extreme heat days. • Overall, the significance of extreme heat in the City is Medium.

4.3.8 Severe Weather: Heavy Rain/ Thunderstorm/ Hail/ Lightning/ Dense Fog Hazard Description
Severe storms in the Planning Area are generally characterized by heavy rain accompanied by strong winds, and lightning. Approximately 10 percent of the thunderstorms that occur each year in the United States are classified as severe. A thunderstorm is classified as severe when it contains one or more of the following phenomena: hail that is three-quarters of an inch or greater, winds in excess of 50 knots (57.5 mph), or a tornado.

Heavy Rain
Atmospheric rivers, a climate pattern that leads to adverse weather in the City, are responsible for up to 50 percent of California’s precipitation annually and 65 percent seasonally (Arcuni, 2019). An atmospheric river (AR) is a long, narrow region of the atmosphere, like a river in the sky, that transports most of the water vapor outside of the tropics. ARs can be 300 miles wide, a mile deep and more than 1,000 miles long and carry an amount of water vapor roughly the same as the average flow of water at the mouth of Mississippi River (NOAA 2015). Warm water storms over the Pacific Ocean lead to evaporation and create a high concentration of moisture in the air, while prevailing winds create the distinctive river shape, which Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-117 is often compared “to a fire hose pointed at California” (KQED 2019b). When an atmospheric river reaches land, it releases the water vapor in the form of rain or snow. Atmospheric rivers play an important role in the global water cycle and are closely tied to both water supply and flooding risk.

Research suggests that atmospheric rivers contributed to the collapse of both Orville Dam spillways in February 2017 (NASA Global Hydrology Resource Center 2019), as well as the winter flooding in 1861- 1862, which inundated Sacramento and is considered the worst flood event in California’s history (Ingram 2013). When an atmospheric river forms in the tropical regions of the Pacific near Hawaii it is known as a “Pineapple Express”. This type of atmospheric river can produce as much as five inches in one day (NOAA 2018b). In 2018, two Pineapple Express ARs hit California causing significant heavy precipitation events throughout state.

Sonoma Water entered into a cooperative agreement with Scripps Institution of Oceanography and the Center for Western Extremes (CW3E) to advance the research in ocean science and meteorology. Three projects have come from the initial agreement: 1) research to help define the role of atmospheric rivers in filling Lake Mendocino and potentially offering predictability in retaining water without increasing flood risk; 2) a NOAA-funded climate program project to study the role of atmospheric rivers in ending droughts on the Russian River; and 3) cooperation in developing a feasibility assessment for potential use of forecast-informed reservoir operations for Lake Mendocino in cooperation with the U.S. Army Corps of Engineers.

Hail
Hail is formed when water droplets freeze and thaw as they are thrown high into the upper atmosphere by the violent internal forces of thunderstorms. Hail is sometimes associated with severe storms within the Planning Area. Hail falls when it becomes heavy enough to overcome the strength of the updraft and is pulled by gravity towards the earth. Hailstorms occur throughout the spring, summer, and fall in the region, but are more frequent in late spring and early summer. Hailstones are usually less than two inches in diameter and can fall at speeds of 120 mph. Hail causes nearly $1 billion in damage to crops and property each year in the United States. Hail is also one of the requirements which the NWS uses to classify thunderstorms as ‘severe.’ If hail more than ¾ of an inch is produced in a thunderstorm, it qualifies as severe. Severe hailstorms can be quite destructive, causing damage to roofs, buildings, automobiles, vegetation, and crops.

The NWS classifies hail by diameter size, and corresponding everyday objects to help relay scope and severity to the population. Table 4-34 under the Extent subsection below indicates the hailstone measurements utilized by the NWS.

Dense Fog
Fog results from air being cooled to the point where it can no longer hold all of the water vapor it contains. For example, rain can cool and moisten the air near the surface until fog forms. A cloud-free, humid air mass at night can lead to fog formation, where land and water surfaces that have warmed up during the summer are still evaporating water into the atmosphere. This is called radiation fog. A warm moist air mass blowing over a cold surface also can cause fog to form, which is called advection fog.

Sonoma County is made up of three major climactic zones, with the major climatic influence being the Pacific Ocean. The City of Sebastopol falls within the marine zone, which is under direct ocean influence.

The prevailing weather and winds tend to come from the Pacific Ocean from the northwest. Areas such as Sebastopol tend to receive more precipitation in the fall and winter and more wind and fog in early morning of the summer months.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-118 Lightning
Lightning is an electrical discharge between positive and negative regions of a thunderstorm. A lightning flash is composed of a series of strokes with an average of about four. The length and duration of each lightning stroke vary, but typically average about 30 microseconds.

Lightning is one of the more dangerous weather hazards in the United States. Each year, lightning is responsible for deaths, injuries, and millions of dollars in property damage, including damage to buildings, communications systems, power lines, and electrical systems. Lightning also causes forest and brush fires, and deaths and injuries to livestock and other animals. According to the National Lightning Safety Institute, lightning causes more than 26,000 fires in the United States each year. The Institute estimates property damage, increased operating costs, production delays, and lost revenue from lightning and secondary effects to be in excess of $6 billion per year. Impacts can be direct or indirect. People or objects can be directly struck, or damage can occur indirectly when the current passes through or near it.

Intra-cloud lightning is the most common type of discharge. This occurs between oppositely charged centers within the same cloud. Usually it takes place inside the cloud and looks from the outside of the cloud like a diffuse brightening that flickers. However, the flash may exit the boundary of the cloud, and a bright channel, similar to a cloud-to-ground flash, can be visible for many miles.

Cloud-to-ground lightning is the most damaging and dangerous type of lightning, though it is also less common. Most flashes originate near the lower-negative charge center and deliver negative charge to earth. However, a large minority of flashes carry positive charge to earth. These positive flashes often occur during the dissipating stage of a thunderstorm’s life. Positive flashes are also more common as a percentage of total ground strikes during the winter months. This type of lightning is particularly dangerous for several reasons. It frequently strikes away from the rain core, either ahead or behind the thunderstorm. It can strike as far as 5 or 10 miles from the storm in areas that most people do not consider to be a threat (see Figure 4-38). Positive lightning also has a longer duration, so fires are more easily ignited. And, when positive lightning strikes, it usually carries a high peak electrical current, potentially resulting in greater damage.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-119 Figure 4-38: Cloud to Ground Lightning
Source: National Weather Service Pueblo Office
The ratio of cloud-to-ground and intra-cloud lightning can vary significantly from storm-to-storm.

Depending upon cloud height above ground and changes in electric field strength between cloud and earth, the discharge stays within the cloud or makes direct contact with the earth. If the field strength is highest in the lower regions of the cloud, a downward flash may occur from cloud to earth.

Location
Extensive – Heavy rains, hail, lightning, and severe storms have the potential to occur anywhere in the Planning Area. Dense fog is more likely to occur within the western portion of the City, or closer to the coast.

Extent (Magnitude/Severity)
Limited – Extent for severe weather, particularly severe storms that involve heavy rain and hail, can be measured according to hail by diameter size, as it corresponds to everyday objects to define the severity to the population (see Table 4-33).

Common problems associated with severe storms include the loss of utilities or immobility. Loss of life is uncommon but can occur during severe storms. Immobility can occur when roads become impassable due to dense fog, flooding, downed trees, ice, or a landslide. Fog specifically poses a risk to commuters and driving conditions as fog typically forms rapidly in the early morning hours. Nighttime driving in the fog is dangerous and multi-car pileups have resulted from drivers using excessive speed for the conditions and visibility. Within the City’s Planning Area, fog is likely to be problem given the City’s proximity to the Pacific Ocean
Loss of utilities can occur when severe thunderstorms cause trees or tree limbs to fall and damage power lines. Lightning can also cause severe damage and injury, particularly when it causes wildfires.

The NWS classifies hail by diameter size, and corresponding everyday objects to help relay scope and severity to the population. Table 4-33 indicates the hailstone measurements utilized by the NWS.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-120 Table 4-33: Hail Measurements
Average Diameter Corresponding Household Object
.25 inch Pea
.5 inch Marble/Mothball
.75 inch Dime/Penny
.875 inch Nickel

1.0 inch Quarter

1.5 inch Ping-pong ball

1.75 inch Golf-Ball

2.0 inch Hen Egg

2.5 inch Tennis Ball

2.75 inch Baseball

3.00 inch Teacup

4.00 inch Grapefruit

4.5 inch Softball
Source: National Weather Service
There is no clear distinction between storms that do and do not produce hailstones. Nearly all severe thunderstorms produce hail aloft, though it may melt before reaching the ground. Multi-cell thunderstorms produce many hailstones, but not usually the largest hailstones. In the life cycle of the multi-cell thunderstorm, the mature stage is relatively short so there is not much time for growth of the hailstone. Supercell thunderstorms have sustained updrafts that support large hail formation by repeatedly lifting the hailstones into the very cold air at the top of the thunderstorm cloud. In general, hail two inches (5 centimeters) or larger in diameter is associated with supercells (a little larger than golf ball size which the NWS considers to be 1.75 inch.). Non-supercell storms are capable of producing golf ball size hail.

In all cases, the hail falls when the thunderstorm’s updraft can no longer support the weight of the ice.

The stronger the updraft the larger the hailstone can grow. When viewed from the air, it is evident that hail falls in paths known as hail swaths. They can range in size from a few acres to areas 10 miles wide and 100 miles long. In some instances, piles of hail have been so deep that snow plows have been required to remove them, and occasionally hail drifts have been reported.

Lightning is measured by the Lightning Activity Level (LAL) scale, created by the NWS to define lightning activity into a specific categorical scale. The LAL is a common parameter that is part of fire weather forecasts nationwide. The City is at risk to experience lightning in any of these categories. The LAL is reproduced in Table 4-34.

Table 4-34: Lightning Activity Level Scale
LAL 1 No thunderstorms
LAL 2 Isolated thunderstorms. Light rain will occasionally reach the ground. Lightning is very infrequent, 1 to 5 cloud to ground strikes in a five-minute period
LAL 3 Widely scattered thunderstorms. Light to moderate rain will reach the ground. Lightning is infrequent, 6 to 10 cloud to ground strikes in a five-minute period.

LAL 4 Scattered thunderstorms. Moderate rain is commonly produced. Lightning is frequent, 11 to 15 cloud to ground strikes in a five-minute period.

LAL 5 Numerous thunderstorms. Rainfall is moderate to heavy. Lightning is frequent and intense, greater than 15 cloud to ground strikes in a five-minute period.

LAL 6 Dry lightning (same as LAL 3 but without rain). This type of lightning has the potential for extreme fire activity and is normally highlighted in fire weather forecasts with a Red Flag warning.

Source: National Weather Service
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-121 The heavy precipitation that is possible in the City and all of California is often the result of an atmospheric river. Atmospheric rivers are categorized by a unit of measurement known as the Integrated Water Vapor Transport (IVT), which considers the amount of water vapor in the system and the wind that moves it around. For a storm to be classified as an atmospheric river it has to reach an IVT threshold of 250 units; 1,000 IVT or more is considered to be “extreme” (Arcuni, 2019).

A system for categorizing the strength and impacts of atmospheric rivers was developed in 2019 by the Center for Western Weather and Water Extremes (CW3E), out of the Scripps Institution of Oceanography at the University of California San Diego (Scripps Institution of Oceanography 2019). The newly developed scale ranks ARs into five categorizes from weak to exceptional. Unlike the Fujita scale for tornadoes that focuses on potential damages, the AR scale accounts for both storms that may be hazardous and storms that can provide benefits to the local water supply. A category one AR is considered to be primarily beneficial, generally lasting only 24 hours and produces modest rainfall. On the other end of the scale, a category five AR is considered “exceptional” and primarily hazardous, lasting for several days and associated with heavy rainfall and runoff that may cause significant damages. Table 4-35 describes the AR scale. CW3E developed the scale as a tool for officials with an operational need to assess flooding potential in their jurisdictions before the storms makes landfall.

In both February 2018 and 2019 the West Coast experienced six atmospheric rivers. But as Figure 4-39 from the CW3E shows, California experienced vastly different precipitation totals due to the location of where the AR made landfall as well as each atmospheric river’s IVT. Using the AR scale developed by CW3E, the ARs in February 2019 were all considered to be moderate to extreme and concentrated more on California, resulting in heavy precipitation.

Table 4-35: Atmospheric River Categories
Category Potential Impacts
AR Cat 1: Weak Primarily beneficial. For example, a Feb. 2, 2017 AR hit California, lasted 24 hours at the coast, and produced modest rainfall.

AR Cat 2: Moderate Mostly beneficial, but also somewhat hazardous. An atmospheric river on Nov. 19-20, 2016 hit Northern California, lasted 42 hours at the coast, and produced several inches of rain that helped replenish low reservoirs after a drought.

AR Cat 3: Strong Balance of beneficial and hazardous. An atmospheric river on Oct. 14-15, 2016 lasted 36 hours at the coast, produced 5-10 inches of rain that helped refill reservoirs after a drought, but also caused some rivers to rise to just below flood stage.

AR Cat 4: Extreme Mostly hazardous, but also beneficial. For example, an atmospheric river on Jan. 8-9, 2017 that persisted for 36 hours produced up to 14 inches of rain in the Sierra Nevada and caused at least a dozen rivers to reach flood stage.

AR Cat 5: Exceptional Primarily hazardous. For example, a Dec. 29, 1996 to Jan. 2, 1997 atmospheric river lasted over 100 hours at the Central California coast. The associated heavy precipitation and runoff caused more than $1 billion in damages.

Source: Center for Western Weather and Water Extremes, Scripps Institution of Oceanography at UC San Diego. Scale was developed by F. Martin Ralph Director of CW3E in collaboration with Jonathan Rutz of NWS Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-122 Figure 4-39: Atmospheric River Strength and Land Distribution, February 2018 vs. February 2019 Source: Center for Western Weather and Water Extremes, Scripps Institution of Oceanography at UC San Diego 2019 Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-123 Previous Occurrences
Heavy rains and severe storms occur in the Planning Area primarily during the late fall and winter.

According to information obtained from the WRCC the majority of precipitation is produced by storms during January and other winter months. Precipitation during the summer months is in the form of rain showers and is rare. Snowstorms and ice storms occur infrequently in the City. The Storm Events Database records one snow event in the vicinity of the City on January 28, 2002 with one to two inches of snow falling near Petaluma and Sonoma Valley; the database notes this was “quite a rare event”. The only other recorded snow events near Sonoma County occurred in Napa County on January 1, 2011 near the St.

Helena area and near the Napa County/Lake County border. The records 90 hail, heavy rain, lighting and dense fog events that have taken place in Sonoma County in the past 70 years (1950 –2020). Table 4-36 is a summary of the most significant severe weather events for Sonoma County. An asterisk (*) indicates events where the City of Sebastopol was specifically mentioned.

Table 4-36: Severe Weather Events recorded in Sonoma County (1950-2020) Hazard
Type Date Hazard Description
Dense Fog
February 8, 2012
Dense fog is blamed in 11 crashes on Highway 37 near Skaggs Island Rd.

There were 31 vehicles involved in the crashes. Two people suffered minor injuries. $100,000 in property damages were recorded.

December 10-11, 2018
Widespread dense fog impacted the Bay Area blanketing the Bay and interior valleys. Numerous reports of dense fog with visibility less than 1/4 mile. A Dense Fog Advisory was issued for the North and East Bay Valleys as well as the San Francisco Peninsula and surrounding bay coastline. Fog caused numerous diverts at KSFO.

Hail
Jan. 19, 2018
A cold front swept through the region late on the 18th. Small scattered thunderstorms were generated behind the front bringing pea sized hail (0.25 in.) to the region.

Jan. 25, 2018
Isolated thunderstorms developed behind a cold front that passed through the area on the 25th. These thunderstorms caused minor roadway flooding and small hail (0.25 in.)
March 14, 2018
The Press Democrat in Santa Rosa showed multiple reports of accumulating small hail in downtown Petaluma (0.25 in.); An upper level disturbance moved through the area on the afternoon of the 14th. This disturbance created scattered thunderstorms that resulted in lightning and accumulating hail in the North and East Bay areas.

Lightning March 14, 2018
The Press Democrat in Santa Rosa reported that lightning struck a PG&E circuit at 11 am the morning of the 14th causing a power outage for 25 Petaluma residents lasting through the evening. An upper level disturbance moved through the area on the afternoon of the 14th. This disturbance created scattered thunderstorms that resulted in lightning and accumulating hail in the North and East Bay areas.

Heavy Rain
December 15, 2008
Heavy rain caused a fatality of a 32-year-old man when his vehicle collided with another vehicle. Highways 116 and 121 were closed for about three hours after the collision. A cold core low pressure system produced winter storm conditions causing low elevation snow, minor flooding and isolated strong wind through the period December 15 through 17, 2009. $25,000 in property damages is recorded.

December 22, 2012
A series of storm systems, part of a large Atmospheric River type of pattern, impacted the area during late December 2012. From the 21st through 26th of December, heavy rain, gusty winds, flooding, and mudslides occurred across the Bay Area in these consecutive events.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-124 Hazard
Type Date Hazard Description
Downed trees, powerlines, and flooded roadways impacted residents over the Christmas holiday season. $30,000 in property damages were recorded.

September 26, 2014*
On September 26, the approach of an upper level low pressure system brought isolated thunderstorms to the area. Small hail and lightning was observed in several locations. Hail of 1/4 to 1/2 inch was commonly reported in the North Bay as well as near Santa Cruz. Hail fell heavily in Napa and covered the ground to a depth of 3 to 6 inches in some locations. Brief heavy rainfall also accompanied the storms.

December 11, 2014
An Atmospheric River event brought heavy rain and gusty winds with a strong winter storm that impacted the Bay Area for several days in midDecember. Many locations around the entire Bay Area had flooding: urban flooding of streets and highways, flooding of creeks and even one large river in the North Bay. Eventually the NCFR (narrow cold frontal rainband) slowed around the Big Sur Coast. The stalling was likely due to another 'wave' in the atmosphere, farther to the southwest, riding along the boundary. The end result was to have the weakened NCFR lift back northward, almost like a quasi-warm front, producing another round of moderate to locally heavy rainfall around the Bay Area, compounding flooding concerns. The event was followed by several weaker storm systems that week that brought additional rainfall, continued flooding and mudslide concerns to the area.

January 16, 2019
A moderate to strong atmospheric river impacted much of California in the middle of the month. A weak surface low developed off the coast on January 15th bringing moderate to heavy rainfall to portions of the region.

Over the next 24 to 36 hours a second strong low-pressure system moved to the north and east bringing heavy rain, destructive winds, high surf, flooding, and thunderstorms to the Bay Area. Numerous reports were received of downed trees and power lines. Winds were recorded between 60 and 100 mph. Downed trees resulted in two fatalities.

Source: National Centers for Environmental Information, Storm Events Database 2020 *Notes events that were specific to the City of Sebastopol City of Santa Rosa—Period of Record 1893 to 2016)
Information from the closest weather station with the most comprehensive data, the Santa Rosa Station (047965), is summarized below in Figure 4-40 and Figure 4-41. Average annual precipitation in the Planning Area is 30.13 inches per year. The highest recorded monthly precipitation was 19.42 inches in 1998; the highest recorded precipitation for a 24-hour period is 5.23 inches on December 19, 1981. The lowest recorded annual precipitation was 0.03 inches in July 1974.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-125 Figure 4-40: City of Santa Rosa’s Monthly Average Total Precipitation Source: Western Regional Climate Center, www.wrcc.dri.edu/ Figure 4-41: City of Santa Rosa’s Daily Average and Extreme Precipitation Source: Western Regional Climate Center, www.wrcc.dri.edu/ Probability of Future Occurrences
Likely – Heavy rain, thunderstorms, hail, and lightning wind and fog events are well-documented seasonal occurrences that will continue to occur annually in the Planning Area.

Climate Change Considerations
As average temperatures increase over time, this generally will result in higher extreme temperatures and more warming in the atmosphere can trigger climate changes, which could result in more frequent Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-126 extreme weather events. According to California’s Fourth Climate Change Assessment, the number of days each year on which ARs bring “extreme” amounts of rain and snow to the region are expected to increase under the projected climate change for the state, possibly increasing more than a quarter. Pacific Northwest National Laboratory researchers found that ARs will reach the West Coast more frequently if GHG emissions continue to rise under business as usual conditions. Currently, the West receives rain or snow from these ARs between 25 and 40 days each year. By the end of this century, days on which the ARs reach the coast could increase by a third this century, between 35 and 55 days a year. Meanwhile, the number of days each year on which the ARs bring “extreme” amounts of rain and snow to the region could increase by more than a quarter.

Cal-Adapt indicates that on average, projections show little change in total annual precipitation in California; however, the Mediterranean seasonal precipitation pattern is expected to continue, with most precipitation falling during the winter months from North Pacific storms. Cal-Adapt provides extreme future precipitation estimates that summarize the intensity and frequency of events. Future extreme precipitation estimates for the City are shown in Figure 4-42. The upper chart shows estimated intensity of extreme precipitation events under the RCP 8.5 scenario that are exceeded on average every 50 years and how it changes in a warming climate over historical, mid-century, and late-century time periods. This chart shows that emissions rise strongly through 2050 and plateau by 2100 and that extreme precipitation events are days during a water year (October – September) with two-day rainfall totals above an extreme threshold of one inch. The lower chart also shows estimated intensity of extreme precipitation events but under the RCP 4.5 scenario that are exceeded on average every 50 years. This chart shows that emissions peak by 2040 and then decline and that extreme precipitation events are days during a water year (October – September) with two-day rainfall totals above an extreme threshold of 1.53 inches.

Figure 4-42: City of Sebastopol Future Precipitation Estimates in High and Low Emission Scenarios Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-127 Source: Cal-Adapt 2019
It is difficult at this point in time to predict the effects climate change will have on these hazards.

However, as average temperatures increase over time, this generally will result in higher extreme temperatures. More warming in the atmosphere will trigger climate changes, which will result in more frequent extreme weather events. Much of the U.S. has already experienced prolonged periods of heavy downpours and severe flooding as a result of more extreme heavy rain and thunderstorm events. For these reasons, climate change would have a “high” influence on severe weather, specifically more heavy rainfall and precipitation events.

Vulnerability Assessment
Property
Based on historic information, these storms have not directly resulted in significant injury or damages to people and property, or the losses are typically covered by insurance. It is the secondary hazards caused by weather, such as floods, that have had the greatest impact on the City’s Planning Area. But while the primary effects may not result in significant injury or property damage, all property is vulnerable during severe weather events; properties in poor condition or closer to overhead power lines and large trees may be more vulnerable to damage.

People
Exposure is the greatest danger to people from severe thunderstorms. People can be hit by lightning, pelted by hail, and caught in rising waters. However, serious injury and loss of human life is rarely associated with hailstorms.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-128 Reduced visibility is the greatest risk to people when heavy fog is prevalent. Particularly when fog is dense, it can be hazardous to drivers, mariners, and aviators and contributes to numerous accidents each year. To reduce injury and harm, people should avoid driving when dense fog is prevalent, if possible. If driving is pertinent, emergency services advise driving with lights on low beam, avoiding stopping on highways, and avoiding crossing traffic lanes.

While national data shows that lightning causes more injuries and deaths than any other natural hazard except extreme heat, there does not seem to be any trend in the data to indicate that one segment of the population is at a disproportionately high risk of being directly affected. Anyone who is outside during a thunderstorm is at risk of being struck by lightning. Aspects of the population who rely on constant, uninterrupted electrical supplies may have a greater, indirect vulnerability to lightning. As a group, the elderly or disabled, especially those with home health care services rely heavily on an uninterrupted source of electricity. Resident populations in nursing homes, residential facilities, or other special needs housing may also be vulnerable if electrical outages are prolonged. If they do not have a back-up power source, rural residents and agricultural operations reliant on electricity for heating, cooling, and water supplies are also vulnerable to power outages. Thunderstorms have the potential energy and strong winds to topple dead trees and injure people. As a result, power outages that occur from severe weather can be life threatening and these populations could face more exposure and could experience greater secondary effects of the hazard. Refer to the Vulnerability Assessment for Severe Weather: Winds hazards below for analysis related to electricity dependent populations in the City.

Economy
Economic impacts of severe weather are typically short term. Lightning can cause power outages and fires.

Hail can destroy exposed property; an example is car lots, where entire inventories can be damaged.

Generally, long-term economic impacts center around hazards that cascade from a severe thunderstorm, including wildfires ignited by lightning and flooding due to heavy rain.

Critical Facilities and Infrastructure
Due to the unpredictability of severe thunderstorm strength and path, most critical infrastructure that is above ground is equally exposed to the storm’s impacts. According to historical data the Planning Area has experienced power outages in the past due to severe storms, but due to the random nature of these hazards, a more specific risk assessment was not conducted for this plan. Heavy rain and thunderstorms, particularly those that result in hail could significantly impact motorists travelling along SR 116, SR 12, and Bodega Highway. Depending on the severity of the storm, these events could slow traffic, reduce visibility, and increase the likelihood of vehicle accidents along the highway, which may result in greater traffic delays. These effects are also likely to occur along highway segments in adjacent counties.

Fog can have devastating effects on transportation corridors in the City and throughout the County.

Dense fog may increase the potential for transportation accidents along SR 116 which could in turn cause longer traffic delays and timely movement of goods and services. Multi-car pileups have resulted from drivers using excessive speed for the conditions and visibility. These accidents can cause multiple injuries and deaths and could have serious implications for human health and the environment if a hazardous were involved. Other disruptions from fog include delayed emergency response vehicles and school closures.

Historic, Cultural, and Natural Resources
Severe thunderstorms are a natural environmental process. Environmental impacts include the sparking of potentially destructive wildfires by lightning and localized flattening of plants by hail. As a natural process, Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-129 the impacts of most severe thunderstorms by themselves are part of the overall natural cycle and do not cause long-term consequential damage.

Future Development
New critical facilities, such as communication towers, pump stations, and other water or wastewater utilities should be built to withstand heavy rain, lighting, and hail damage. Population and commercial growth in the City will increase the potential for complications with traffic accidents and commerce interruptions associated with dense fog. Future development projects should also consider severe weather hazards at the planning, engineering and architectural design stage with the goal of reducing vulnerability. Storm water master planning and site plan review should account for buildings to withstand severe weather events and be considered for all new development. Future development in the City is not expected to be vulnerable to the hazard, but all development will be affected by severe weather and storm events and population growth will increase potential exposure to hazards such as lightning and hail.

Risk Summary
• Sonoma County has experienced 90 hail, heavy rain, lighting, and dense fog events in past 70 years. • The average annual precipitation is 30.12 inches.
• The highest recorded monthly precipitation was 19.42 inches in 1998. • The highest recorded precipitation for a 24-hour period was 5.23 inches on December 19, 1981. • Overall significance for severe weather hazards such as heavy rain, thunderstorms, hail, lightning, and dense fog is Medium.

4.3.9 Severe Weather: High Winds
Hazard Description
High winds, often accompanying severe thunderstorms, can cause significant property and crop damage, threaten public safety, and have adverse economic impacts from business closures and power loss. The wind patterns in Sebastopol are strongly influenced by the Petaluma Gap, the region from the Estero Lowlands to San Pablo Bay (BAAQMD 2019). The predominant wind pattern is out of the south and southeast and tends to be light in the morning and windier in the afternoon when the sea breeze arrives.

Windstorms in the City are typically straight-line winds. Straight-line winds are generally any thunderstorm wind that is not associated with rotation (i.e., is not a tornado). These winds can exceed 100 miles per hour (mph) and are responsible for most wind damage related to thunderstorms. These winds can overturn mobile homes, tear roofs off houses, topple trees, snap power lines, shatter windows, and sandblast paint from cars. Other associated hazards include utility outages, arcing power lines, debris blocking streets, dust storms, and an occasional structure fire.

High winds and tornadoes can cause damage to property and loss of life. Property damage can include damage to buildings, fallen trees and power lines, broken gas lines, broken sewer and water mains, and the outbreak of fires. Agricultural crops and industries may also be damaged or destroyed. Access roads and streets may be blocked by debris, delaying necessary emergency response.

Location
Extensive – Strong winds have the potential to happen anywhere in the City’s Planning Area. The resulting damage from wind events may be most severe in the downtown area of the City where there are more large trees, infrastructure, and higher density development.

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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-130 Extent (Magnitude/Severity)
Limited –The Beaufort wind scale describes the damaging effects of wind speed, as shown in Table 4-37.

Table 4-37: Beaufort Wind Scale
Wind Speed
(mph) Description—Visible Condition
0 Calm; smoke rises vertically
1-4 Light air; direction of wind shown by smoke but not by wind vanes 4-7 Light breeze; wind felt on face; leaves rustle; ordinary wind vane moved by wind 8-12 Gentle breeze; leaves and small twigs in constant motion; wind extends light flag 13-18 Moderate breeze; raises dust and loose paper; small branches are moved 19-24 Fresh breeze; small trees in leaf begin to sway; crested wavelets form on inland water 25-31 Strong breeze; large branches in motion; telephone wires whistle; umbrellas used with difficulty
32-38 Moderate gale whole trees in motion; inconvenience in walking against wind 39-46 Fresh gale breaks twigs off trees; generally, impedes progress 47-54 Strong gale slight structural damage occurs; chimney pots and slates removed 55-63 Whole gale trees uprooted; considerable structural damage occurs 64-72 Storm very rarely experienced; accompanied by widespread damage 73+ Hurricane devastation occurs
Source: NWS
Based on NCEI records between January 1, 1950 and December 30, 2020 there have been 258 high and strong wind events in Sonoma County, causing a total of $3,859,000 in property damage. The most damaging event took place on December 27, 2006 and was a 30 mph wind event on the Beaufort Wind Scale that resulted in over $1 million of property damage to both commercial and residential structures.

The highest magnitude event recorded occurred on October 27, 2019 and was in association with a series of offshore wind events that were the result of strong surface high pressure and a trough along the California coast that provided one of the strongest offshore winds over the Bay Area since the 2017 North Bay wildfires, as winds remained elevated for 24 hours with gusts in the hills at 60 to 80 miles per hour.

These winds promoted the rapid spread of the Kincade Fire and along with very dry conditions allowed for multiple new wildfires to spark in the greater Bay Area. In response to these winds, PG&E shut off power to over 2 million people across California.

High wind events in Sonoma County have led to five recorded fatalities and five injuries (due to fallen trees). High wind event impacts would likely be limited, with a majority of impacts being related to property damages caused by down trees as well as power outages. Overall, impacts from high wind events would likely be limited, with 10 to 25 percent of property severely damaged.

Previous Occurrences
The City lies north of the Petaluma Gap, and subsequently experiences lower wind speeds. However, high wind events have occurred in the City which led to downed trees and power outages throughout the City.

The following events are recorded in the NCEI Storm Events Database that are specific to the City and nearby areas.

January 25, 2010 - Strong wind brought down a large oak tree along Fitch Mountain Road in Healdsburg. The tree smashed into a home and caused the road to be closed. Some of the other areas experienced downed trees and power lines were Sebastopol, Forestville and Santa Rosa. Approximately 1,200 customers were without power.

Agenda Item Number 19 Agenda Item Number 19
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Chapter 4
Risk Assessment
City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-131 December 2, 2012 - High winds caused a power pole to fall at Hall Road and Third Street in Sebastopol.

It took down a high voltage line and cut power to a significant portion of western Santa Rosa.

January 16, 2019 - Sebastopol Road closed between Fresno Ave and Corporate Center Parkway due to a PG&E power pole that broke and hung over the roadway.

Probability of Future Occurrences
Likely – A total of 258 combined high and strong wind events have occurred in Sonoma County over 70 years of record keeping, which equates to an average of three events in a typical year. Historical wind activity within the Planning Area indicates that the area will likely continue to experience high wind events during adverse weather conditions. The actual risk of a wind event to the City is dependent on the nature and location and the magnitude of a high wind event.

Climate Change Considerations
There presently is not enough data or research to quantify the magnitude of change that climate change may have related to wind frequency and intensity. Studies referenced in California’s Fourth Climate Assessment indicated that extreme fire weather, particularly in the form of hot and dry winds, can strongly influence shrub-land fire regimes. Strong winds have also been associated with severe forest fires in California, meaning climate change impacts on wind patterns may also affect forest health and wildfire susceptibility. Other ongoing research compiled in the recent climate assessment has resulted in different conclusions on the effect of climate change on wind regimes, particularly extreme wind events, such as the Santa Ana and Diablo winds that created some of the most devastating wildfires in recent years (California Natural Resources Agency 2018a). At this time, these changing factors are not well understood and are still being incorporated into state and regional research and risk analysis.

Vulnerability Assessment
Property
General damages from high wind events can be both direct and indirect. Direct impacts refer to what the wind physically destroys, while indirect impacts include additional costs, damages and losses attributed to secondary hazards generated by the event or resulting from the direct damages caused by the wind event. Construction practices and building codes can help maximize the resistance of the structures to damage.

Secondary impacts of damage caused by wind events often result from damage to infrastructure. Downed power and communications transmission lines, coupled with disruptions to transportation, create difficulties in reporting and responding to emergencies. These indirect impacts of a wind event put tremendous strain on a community.

People
Community members are the most vulnerable to high wind events and of the five fatalities associated with high wind and strong wind events, four were associated with wind knocking down trees and one occurred in 2018 when two people fell out of a row boat during a gale warning in 2018 (NCEI 2020).

However, there are also segments of the population that are especially exposed to the indirect impacts of high winds, particularly the loss of electrical power. These populations include the elderly or disabled, especially those with medical needs and treatments dependent on electricity. Nursing homes, communitybased residential facilities, other special needs housing facilities, and other socially susceptible populations are vulnerable if electrical outages are prolonged since backup power generally operates only minimal functions for a short period of time.

Agenda Item Number 19 Agenda Item Number 19
City Council Meeting Packet of June 7, 2022

Chapter 4
Risk Assessment
City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-132 The U.S. Department of Health and Human Services ePOWER Mapping tool provides information on Medicare beneficiaries who rely on electricity-dependent medical equipment such as ventilators to live independently in their homes. According to the tool, there are 8,601 Medicare beneficiaries located in the City (within the zip codes of 95472 and 95473) ((https://empowermap.hhs.gov/) (U.S. Department of Health & Human Services 2021b). Of these individuals, 219 are considered electricity dependent and are highly vulnerable to power outages as a result a high wind event.

Following the unprecedented 2018 wildfire season in California, PG&E announced it will be conducting PSPS when there are high winds and dry conditions and generally a heightened fire risk forecasted. The outages could last several days, and PG&E has suggested customers be prepared for outages that could last longer than 48 hours. A majority of Sonoma County could be affected by the power outages including almost the entirety of the City. PG&E does plan to install a resource area at the Sonoma-Marin Fairgrounds within 24 hours of a PSPS, and will offer power, air conditions and updates for local residents.

Economy
Winds typically don’t have long-term impacts on the economy. The most common problems associated with high winds are loss of utilities. Downed power lines can cause power outages, leaving large parts of the City isolated, and without electricity, water, and communication. Damage may also limit timely emergency response and the number of evacuation routes.

In the event of a PSPS during red flag warnings, as described above, large portions of the City could be without power including several businesses. Economic impacts due to the PSPS depend on the length of the shutoff. Based on the planned PSPS in October 2019, economic impacts were reported across northern California as many businesses and restaurants and other tourism-based operations had to close due to limited to no power supply. In 2018, PG&E abruptly shut down the power in the Napa Valley region and the City of Calistoga reported that numerous small business lost tens of thousands of dollars in missed revenue and inventory (Argus-Courier 2019).

Critical Facilities and Infrastructure
Public gathering places such as schools, community centers, shelters, nursing homes, and churches may have increased impacts at certain times of day due to high wind events and PSPS. Due to the random nature of wind hazard, a more specific risk assessment was not conducted for this plan.

Historic, Cultural, and Natural Resources
High winds can cause massive damage to the natural environment, uprooting trees and other debris. This is part of a natural process, however, and the environment will return to its original state over time. Wind damage to historic or cultural resources on the other hand may result in more severe and permanent damage that could temporarily impact the historic aesthetic of downtown Sebastopol or require extensive restoration and rehabilitation of certain buildings and structures.

Future Development
As the City continues increasing in population, the number of people and housing developments exposed to the wind hazards increases. Proper education on building techniques and the use of sturdy building materials, basements, attached foundations, and other structural techniques may minimize the property vulnerabilities. Public shelters at parks and open spaces may help reduce the impacts of high wind events on the recreational populations exposed to storms.

Agenda Item Number 19 Agenda Item Number 19
City Council Meeting Packet of June 7, 2022

Chapter 4
Risk Assessment
City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-133 Risk Summary
• Increase in post-failure or secondary hazards such as flooding and long-term power outages can occur from high wind events.
• 219 individuals in the City are electricity dependent, and highly vulnerable to power outages due to high wind events.
• Damage to natural resource habitats and other resources may result from severe weather-associated wind.
• Severe wind events could result in the loss of water, communication lines, or power; closures to roads and transportation lifelines, which could impact, strand, and/or impair mobility for emergency responders and/or City residents.
• Economic losses (jobs, sales, tax revenue) associated with loss of commercial structures and/or inability to move through transportation lifelines could occur. • Severe wind hazards could result in loss or damages to historic and cultural resources, which could severely impact the social fabric of Sebastopol.
• Timely removal of debris, specifically downed trees must be addressed, as this can impact the severity of the severe weather events and the secondary impacts (e.g. localized flooding, loss of power). • Overall, the significance of severe weather associated with wind is Medium.

4.4 Human-Health and Human-Caused Hazard Profiles and Risk Assessment The DMA does not require an assessment of human-caused hazards, but the City and HMPC decided to include human-health and human-caused hazards in this LHMP to several reasons. First, the City wants to inform the public about all hazards, including both natural and human-caused hazards. The City is also interested in the impact human-caused hazards could have on their community and on the daily movement of goods and services through the City. The City intends to take a proactive approach to disaster preparedness, and the HMPC indicated that preparation for and response to a human-caused disaster involves the same training and commitment of City resources as a natural hazard. Lastly, the City recognizes that the likelihood of some human-caused hazard events in the Planning Area is greater than several of the natural hazard events identified in the plan.

The City also recognizes that while Sonoma County has several hazardous material management and planning procedures in place through their CUPA administered through their Fire Prevention Division and Hazardous Materials Division (Permit Sonoma), it is equally important to highlight the hazardous material hazards present in the City’s Planning Area in this plan for the purpose of public education and awareness.

The City wants to ensure that these hazards do not exacerbate secondary impacts associated with natural hazard events.

The following human-caused hazards are discussed in this plan:
• Public Health Hazards
• Hazardous Material Releases
Other potential human-caused hazards, such as civil unrest, civil disturbance, and terrorism threats were dismissed from further study. The City and HMPC noted that while these human-caused hazards, specifically civil unrest, or political activism tend to occur in the City, they have not resulted in civil disturbance or other negative impacts over the past two decades.

Agenda Item Number 19 Agenda Item Number 19
City Council Meeting Packet of June 7, 2022

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Risk Assessment
City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-134

4.4.1 Public Health Hazards
Hazard Description
A public health emergency is defined as an emergency need for health care [medical] services to respond to a disaster, significant outbreak of an infectious disease, bioterrorist attack or other significant or catastrophic event. Public health emergencies can occur as primary events by themselves, or they may be secondary to another disaster or emergency, such as tornado, flood, or hazardous material incident.

Public health emergencies have the potential to cause serious illness and death, especially among those who have compromised immune systems due to age or underlying medical conditions. There are several contagious and infectious diseases present in Sonoma County that constitute a public health risk. The Sonoma County Emergency Operations Plan (EOP) provides an organizational framework for public health and medical service preparedness, response, and recovery efforts for various emergency epidemics.

Unlike influenza viruses that have achieved ongoing transmission in humans, the sporadic human infections with avian A (H5N1) viruses are far more severe and result in high mortality. Initial symptoms include high fever and other influenza-like symptoms. It also appears that the incubation period in humans may be longer for avian (H5N1) viruses, ranging from 2 to 8 days, and possibly as long as 17 days. Diarrhea, vomiting, abdominal pain, chest pain, and bleeding from the nose and gums have also been reported. The disease often manifests as a rapid progression of pneumonia with respiratory failure ensuing over several days.

According to the World Health Organization (WHO), a pandemic can be defined as the worldwide spread of a new disease, and specifically as an epidemic occurring worldwide, or over a very wide area, crossing international boundaries and usually affecting a large number of people (WHO 2011). It is a global disease outbreak that attacks a large population across great geographic distances. Pandemics are larger than epidemics in terms of geographic area and number of people affected. Epidemics tend to occur seasonally and affect much smaller areas. Pandemics, on the other hand, are often caused by new subtypes of viruses or bacteria for which humans have little or no natural resistance or immunity, and for which there is no vaccine. The United States Centers for Disease Control and Prevention (CDC) works with the WHO to detect outbreaks that might cause a pandemic and assist in planning and emergency preparation for pandemics. Consequently, pandemics typically result in more deaths, social disruption, and economic loss than epidemics.

There are three conditions that must be met before an influenza pandemic begins:

1. A new virus subtype must emerge that has not previously circulated in humans (and therefore there is no pre-existing immunity),

2. This new subtype must be able to cause disease in humans, and

3. The virus must be easily transmissible from human to human.

As of January 2021, Sonoma County and the nation are continuing to deal with the COVID-19 coronavirus pandemic, confirming that pandemics can have a significant impact on the County and on a global scale.

This hazard risk assessment includes an analysis of pandemic risk in Sonoma County and the City and an analysis of the impacts of the hazards profiled in this plan on public health.

Geographic Location
Extensive – Pandemics occur not only on a county or state level, but on a national and global scale. It is likely that most communities in Sonoma County would be affected, either directly or by secondary impacts. More highly-populated areas may be affected sooner and may experience higher infection rates.

Agenda Item Number 19 Agenda Item Number 19
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Risk Assessment
City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-135 The current COVID-19 pandemic has impacted all 58 California counties. As of January 20, 2020, Sonoma County had reported 23,997 active COVID-19 cases (4.71 percent of the County population); 242 people have died and 17,631 people have recovered (74 percent of the cases) and is currently noted as having a sustained decline in new cases per 100,000 residents (approximately 40 new cases per day per 100,000 people). At the time, this resulted in a 9.6 percent positivity rate, which was similar to the State 8.4 percent positivity rate tracked at the same time (Sonoma County 2021). All communities in the County, including the District’s Planning Area in Sonoma Valley are likely to continue to be impacted, either directly or indirectly. Some indirect consequences may be the diversion of resources that may be otherwise available.

Magnitude/Severity
Critical – The magnitude of a public health emergency will range significantly depending on the aggressiveness of the virus in question and the ease of transmission. Pandemic influenza is more easily transmitted from person-to-person but advances in medical technologies have greatly reduced the number of deaths caused by influenza over time.

Today, a much larger percentage of the world’s population is clustered in cities, making them ideal breeding grounds for epidemics. Additionally, the explosive growth in air travel means a virus could spread around the globe within hours making the speed of onset of a pandemic short and the duration variable, but pandemics typically last for several months to 1-2 years, or until a vaccine becomes available.

Under such conditions, there may be very little warning time. Most experts believe we will have just one to six months between the time that a dangerous new influenza strain is identified and the time that outbreaks begin to occur in the United States. Outbreaks are expected to occur simultaneously throughout much of the nation, preventing shifts in human and material resources that normally occur with other natural disasters. These and many other aspects make influenza pandemic unlike any other public health emergency or community disaster. There is also currently no scientific scale to measure the magnitude of a pandemic, but they are usually measured by the population affected, and by the population that have died from complications associated with the virus. The magnitude and severity of pandemics are also measured based on the Pandemic Intervals Framework (PIF).

The PIF is a six-phased approach to defining the progression of an influenza pandemic. This framework is used to guide influenza pandemic planning and provides recommendations for risk assessment, decisionmaking, and action. These intervals provide a common method to describe pandemic activity which can inform public health actions. The duration of each pandemic interval might vary depending on the characteristics of the virus and the public health response.

The six-phase approach was designed for the easy incorporation of recommendations into existing national and local preparedness and response plans. Phases 1 through 3 correlates with preparedness in the pre-pandemic interval, including capacity development and response planning activities, while Phases 4 through 6 signal the need for response and mitigation efforts during the pandemic interval.

Pre-Pandemic Interval
• Phase 1 is the natural state in which influenza viruses circulate continuously among animals but do not affect humans. Even though such viruses might develop into pandemic viruses, in Phase 1 no viruses circulating among animals have been reported to cause infections in humans. • Phase 2 involves cases of animal influenza that have circulated among domesticated or wild animals and have caused specific cases of infection among humans. In Phase 2 an animal influenza virus circulating among domesticated or wild animals is known to have caused infection in humans and is thus considered a potential pandemic threat.
• Phase 3 represents the mutation of the animal influenza virus in humans so that it can be transmitted to other humans under certain circumstances (usually very close contact between individuals). At this Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-136 point, small clusters of infection have occurred. In Phase 3 an animal or human-animal influenza virus has caused sporadic cases or small clusters of disease in people but has not resulted in human-tohuman transmission sufficient to sustain community-level outbreaks. Limited human-to-human transmission may occur under some circumstances, for example, when there is close contact between an infected person and an unprotected caregiver. Limited transmission under these circumstances does not indicate that the virus has gained the level of transmissibility among humans necessary to cause a pandemic.

Pandemic Interval
• Phase 4 involves community-wide outbreaks as the virus continues to mutate and become more easily transmitted between people (for example, transmission through the air) Phase 4 is characterized by verified human to human transmission of the virus able to cause “community-level outbreaks.” The ability to cause sustained disease outbreaks in a community marks a significant upward shift in the risk for a pandemic.
• Phase 5 represents human-to-human transmission of the virus in at least two countries. Phase 5 is characterized by verified human to human spread of the virus into at least two countries in one WHO region. While most countries will not be affected at this stage, the declaration of Phase 5 is a strong signal that a pandemic is imminent and that the time to finalize the organization, communication, and implementation of the planned mitigation measures is short. • Phase 6 is the pandemic phase, characterized by community-level influenza outbreaks. Phase 6, the pandemic phase, is characterized by community-level outbreaks in at least one other country in a different WHO region in addition to the criteria defined in Phase 5. Designation of this phase will indicate that a global pandemic is underway.

Previous Occurrences
There has been one state and one federal disaster declaration due to a pandemic (DR-4482). These declarations both occurred in 2020. The federal declaration for the COVID-19 pandemic was made on January 20, 2020, and the state declaration followed on March 4, 2020.

Since the early 1900s, five other lethal pandemics have swept the globe:
• 1918-1919 Influenza (H1N1) (Spanish Flu): The Spanish Flu was the most severe pandemic in history. The number of deaths was estimated to be 50-100 million worldwide and 675,000 in the United States. Its primary victims were mostly young, healthy adults. At one point, more than 10 percent of the American workforce was bedridden.
• 1957-1958 Asian Influenza (H2N2): The 1957 Asian influenza killed 1-2 million people worldwide, including about 70,000 people in the United States, mostly the elderly and chronically ill. Fortunately, the virus was quickly identified, and vaccine production began in May 1957. • 1968-1969 Hong Kong Influenza (H3N2): The 1968 Hong Kong Flu killed 34,000 Americans. The elderly were more severely affected. This pandemic peaked during school holidays in December, limiting student-related infections, which may have kept the number of infections down. Also, people infected by the Asian Flu ten years earlier may have gained some resistance to the new virus. The same virus also returned in 1970 and 1972.
• 2009-2010 Swine Influenza (H1N1): The Swine influenza pandemic emerged from Mexico in early 2009 and was declared a public health emergency in the U.S. on April 26. By June, approximately 18,000 cases had been reported in the U.S. and the virus had spread to 74 countries. Most cases were fairly mild, with symptoms similar to the seasonal flu, but there were cases of severe disease requiring hospitalization and a number of deaths. The CDC estimates that 43-89 million people were infected worldwide, with an estimated 8,870 to 18,300 H1N1 related deaths, including 12,469 deaths in the United States. While this is more than those infected by the Spanish Flu, the Swine influenza only Agenda Item Number 19 Agenda Item Number 19
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-137 resulted in about 150,000 to 575,000 deaths. Follow-up studies have since showed that the risk of serious illness resulting from the Swine influenza was no higher than that of the yearly seasonal influenza.
• 2020-Ongoing COVID-19: The COVID-19 or novel coronavirus disease pandemic began in December 2019 and was declared a pandemic in March of 2020. COVID-19 is an infectious disease caused by the newly discovered coronavirus. As of January 20, 2021, it has killed more than 2.14 million people worldwide and more than 400,300 Americans (CDC 2021b). It is now expected to last through 2022 with the development of variants and the potential to impact communities that have large percentages of unvaccinated populations. Figure 4-43 illustrates the number of new cases reported in the County since December 2021. New cases have since declined with the availability of the COVID-19 vaccine, however, there have been recent increases in cases due to new SARS-VoV-2 variants.

Figure 4-43: Sonoma County New Cases per 100,000 Residents by Vaccination Status Source: Sonoma County Emergency Management 2021.

The California Department of Public Health and Environment releases an annual reportable disease summary for each county. The diagnoses with the highest incidences in Sonoma County for 2016 through 2019 are summarized in Table 4-38.

Table 4-38: Annual Cases of Communicable Diseases in Sonoma County: 2015 – 2019 Disease Year of Reported Cases
2015 2016 2017 2018 2019
Ambeiasis 3 9 9 0 11
Anaplasmosis -- 1 2 0 0
Babesiosis 0 0 0 0 0
Botulism (Food-borne) 0 0 0 0 0
Botulism (Wound) -- 0 0 0 0
Brucellosis 1 0 1 1 0
Campylobacteriosis 52 60 184 191 187
Chikungunya Virus 0 0 0 0 0
Cholera 0 0 0 0 0
Ciguatera (Fish Poisoning) 0 0 0 0 0
Coccidioidomycosis 1 2 3 3 12
Creutzfeldt Jakob 0 1 1 0 1
Cryptosporidiosis 5 3 7 6 7
Cyclosporiasis 0 0 0 1 0
Cysticercosis 1 0 0 0 0
Dengue Virus 0 2 1 0 4
Ehrilichiosis 0 0 0 0 0
Flavivirus 0 0 0 0 0
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City of Sebastopol Local Hazard Mitigation Plan Update | 2021 – 2026 Page 4-138 Disease Year of Reported Cases
2015 2016 2017 2018 2019
Giardiasis 50 32 43 49 72
Hantavirus 0 0 0 0 0
Hepatitis E 1 2 3 1 2
Legionellosis 1 7 6 5 2
Leprosy (Hansen’s Disease) 0 0 0 0 0
Leptospirosis 0 0 1 0 0
Listeriosis 1 3 1 2 1
Lyme Disease 13 14 18 11 10
Malaria 0 0 3 0 0
Paralytic – Shellfish Poisoning 0 0 0 1 0
Parathyroid Fever 0 0 0 0 0
Plague – Human 0 0 0 0 0
Psittacosis 0 0 0 0 0
Q Fever 3 1 0 1 1
Rabies (Animal) 5 5 3 5 5
Rabies (Human) 0 0 0 0 0
Relapsing Fever 0 0 1 0 0
Salmonellosis 58 60 80 83 61
Scrombroid Fish Poisoning 0 00 0 0 0
Shiga Toxin-producing E.Coli 27 25 35 50 76
Hemolytic Uremic Syndrome 0 1 0 0 3
Shigellosis 0 12 35 71 51
Spotted Fever Rickettsiosis 0 1 0 0 0
Streptococcal Infection 0 0 0 0 0
Trichinosis 0 0 0 0 0
Tularemia 1 0 0 0 0
Typhoid Fever 0 1 1 0 1
Typhus Fever 0 0 0 0 0
Yersiniosis 0 3 1 2 1
Vibrio Infection 2 0 2 2 10
Zika Virus -- 10 7 1 1
Source: California Department of Public Health 2018.

As shown in Table 4-38 above, common communicable diseases in Sonoma County (including the City of Sebastopol) include Campylobacteriosis, Giardiasis, Lyme Disease, and Salmonellosis. Campylobacteriosis is a common bacterial infection in humans; it is often a food-borne illness. Giardiasis is a diarrheal disease caused by the microscopic parasite Giardia. Once a person has been infected with Giardia, the parasite lives in the intestines and is passed in feces. Lyme disease is a bacterial infection you get from the bite of an infected tick. Salmonellosis is also a common bacterial disease that affects the intestinal tract; humans become infected through contaminated food and water.

The CDPH obtains data on laboratory-confirmed influenza and other respiratory viruses from a number of laboratories throughout the state. These laboratories include the CDPH Viral and Rickettsial Disease Laboratory (VRDL) and 24 local public health laboratories, collectively known as the Respiratory Laboratory Network (RLN), and 16 clinical, academic, and hospital laboratories. According to the CDC, at a national level, the 2019–2020 influenza season (October 1, 2019 – April 4, 2020) was a moderate severity season (CDC 2020a). Influenza activity during the 2019–2020 season began to increase in November and was consistently high through January and February. The season was characterized by two consecutive waves of activity, beginning with influenza B viruses and followed by A(H1N1)pdm09 viruses. Overall, influenza A(H1N1)pdm09 viruses were the most commonly reported influenza viruses this season (CDC Agenda Item Number 19 Agenda Item Number 19
City Council Meeting Packet of June 7, 2022