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Napa River at Zinfandel Lane (RL Area B)
The Zinfandel Lane RLA is located west of the Napa River within a low-gradient agricultural area characterized by broad floodplain conditions and limited drainage infrastructure (Figure 3-15). The flat terrain and proximity to the river result in slow drainage and prolonged inundation following storm events.
Flooding in this area is driven by a combination of Napa River overbank flow and sheet flow runoff from surrounding agricultural lands. During significant rainfall events, water can accumulate across fields and roadways, contributing to localized flooding of residential structures.
Historical impacts have included flooding of access roads, ponding on agricultural lands, and repeated inundation of structures located within the floodway. Limited stormwater infrastructure and minimal topographic relief contribute to extended flood durations.
Past mitigation actions have primarily been limited to site-specific drainage improvements and maintenance of agricultural ditches. No large-scale structural flood control measures have been implemented in this area.
Mitigation opportunities are constrained by the agricultural land use and the need to maintain drainage patterns that support farming operations.
Future recommendations for the Zinfandel Lane RLA include:
▪ Improving local drainage conveyance through ditch maintenance and targeted upgrades. ▪ Evaluating opportunities for floodplain restoration or setback to increase storage capacity. ▪ Encouraging participation in FEMA mitigation programs for elevation and acquisition where feasible.
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Figure 3-15 Zinfandel Ln Repetitive Loss Area
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Napa River at Yount Mill Road (RL Area C)
The Yount Mill Road RLA is located along the Napa River just north of the Town of Yountville, where the river meanders through a relatively broad but low-lying floodplain (Figure 3-16). The area is influenced by both mainstem flooding and backwater effects from nearby tributaries, including Conn Creek and Rector Creek.
During intense storm events, high flows in the Napa River combined with tributary inflows can create complex hydraulic conditions, resulting in elevated flood stages and extended inundation.
Historical flooding has resulted in impacts to residential properties, roadway closures, and ponding in lowlying areas. Limited drainage capacity and flat topography contribute to slow recession of floodwaters.
Mitigation efforts in the broader Napa River corridor, including elements of the Napa River Flood Protection Project, have reduced risk in downstream urban areas; however, this reach remains vulnerable due to its rural setting and limited infrastructure improvements.
Constraints to mitigation include the dispersed development pattern and the need to preserve agricultural land uses.
Future recommendations for the Yount Mill Road RLA include:
▪ Assessing drainage improvements to reduce ponding and improve floodwater recession. ▪ Enhancing coordination with upstream reservoir and watershed management where applicable. ▪ Supporting property-level mitigation measures, including elevation and floodproofing.
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Figure 3-16 Yount Mill Rd. Repetitive Loss Area
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Napa River at Ragatz Lane (RL Area D)
The Ragatz Lane RLA is situated along Hopper Creek near its confluence with the Napa River in a low-lying agricultural area east of State Route 29 (Figure 3-17). The area is subject to both tributary flooding and backwater effects from the Napa River during high-flow conditions.
Flooding is characterized by sheet flow across agricultural lands, overtopping of small drainage channels, and localized inundation of roadways and structures. The flat terrain and presence of irrigation and drainage infrastructure influence flow paths and can exacerbate flooding during storm events.
Historical impacts include roadway flooding, access limitations, and repeated damage to residential structures.
Mitigation actions to date have primarily involved maintenance of agricultural drainage systems and minor site improvements.
Mitigation opportunities are limited by the agricultural setting and the interconnected nature of drainage and irrigation systems.
Future recommendations for the Ragatz Lane RLA include:
Improving coordination of agricultural drainage and flood management practices. Evaluating opportunities for localized detention or retention to reduce peak flows. Encouraging property-level mitigation measures such as elevation and floodproofing.
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Figure 3-17 Ragatz Ln. Repetitive Loss Area
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Napa River at Petra Drive (RL Area E)
The Petra Drive RLA is located near the confluence of Soda Creek and the Napa River, where residential development is situated within a dynamic floodplain environment (Figure 3-18). The confluence setting creates increased flood risk due to the interaction of flows from both watercourses.
During major storm events, backwater effects from the Napa River can impede drainage from Soda Creek, resulting in rapid rises in water levels and increased flood depths.
Historical flooding has included inundation of residential structures, roadway closures, and localized erosion. The proximity of homes to the floodway significantly increases exposure to flood hazards.
Past mitigation efforts have included limited drainage improvements and maintenance of creek channels.
However, no comprehensive flood control project has been implemented in this area.
Mitigation constraints include the established residential development within the floodplain and limited space for structural improvements.
Future recommendations for the Petra Drive RLA include:
▪ Evaluating improvements to Soda Creek conveyance and confluence hydraulics. ▪ Enhancing early warning and emergency access planning. ▪ Promoting FEMA-funded elevation or acquisition of high-risk properties.
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Figure 3-18 Petra Dr. Repetitive Loss Area
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Milliken Creek (RL Area F)
The Milliken Creek RLA is located near the confluence of Milliken Creek and the Napa River within the eastern portion of the City of Napa (Figure 3-19). This area reflects an urban interface where tributary and mainstem flooding interact.
Flooding occurs when high flows in Milliken Creek coincide with elevated stages in the Napa River, reducing drainage capacity and creating backwater conditions. Urban development, roadway crossings, and constrained channels further limit conveyance.
Historical impacts have included roadway flooding, property damage, and localized drainage failures. Debris accumulation in culverts and bridge crossings can exacerbate flooding during storm events.
Past mitigation actions have included maintenance of drainage infrastructure and participation in broader Napa River flood management efforts.
Mitigation constraints include urban development patterns and limited space for channel expansion.
Future recommendations for the Milliken Creek RLA include:
▪ Upgrading culverts and bridge crossings to improve hydraulic capacity. ▪ Continuing vegetation and debris management to maintain conveyance. ▪ Supporting property-level mitigation and stormwater system improvements.
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Figure 3-19 Milliken Creek Repetitive Loss Area
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Napa River at Milton Road (RL Area G)
The Milton Road RLA is located in southern Napa County near the Napa River estuary, where the river transitions into tidally influenced marshlands (Figure 3-20). This area is characterized by very low elevations, reclaimed lands, and extensive levee systems primarily designed for agricultural and habitat management.
Flooding in this area is driven by a combination of riverine flows, tidal influence from San Pablo Bay, and the performance of surrounding levees. During high tides and storm events, water levels can rise significantly, reducing drainage capacity and increasing the likelihood of inundation.
Historical impacts have included roadway flooding, limited access during high water events, and repeated low-level flooding of properties.
Past mitigation measures have largely focused on levee maintenance and wetland management, with limited structural flood protection designed specifically for residential areas.
Mitigation options are constrained by the reliance on non-engineered or aging levee systems and the ecological importance of surrounding wetlands.
Future recommendations for the Milton Road RLA include:
▪ Evaluating levee conditions and opportunities for targeted improvements. ▪ Assessing the impacts of sea level rise and incorporating adaptation strategies. ▪ Enhancing evacuation planning and resilience measures for isolated properties. ▪ Encouraging participation in FEMA mitigation programs where applicable.
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Figure 3-20 Milton Rd. Repetitive Loss Area
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Frequency & Probability of Future Occurrences
Flooding in Napa County is a recurring and well-documented hazard, with historical records and recent events demonstrating that flood occurrences are not isolated or rare. Flood frequency terminology such as the “100-year flood” (1% annual chance event) is often misunderstood. This designation does not imply that such an event occurs only once every 100 years; rather, it indicates that there is a 1% probability of that magnitude flood occurring in any given year. Over a standard 30-year period, such as the life of a mortgage, this corresponds to approximately a 26% chance of occurrence.
Similarly, higher probability events, such as the 10-year (10% annual chance) or 50-year (2% annual chance) floods, have a substantially greater likelihood of occurring over shorter timeframes. As shown in Table 3-15, even relatively infrequent flood events have a significant cumulative probability over time. Importantly, multiple flood events of similar magnitude can occur within short intervals, as evidenced by Napa County’s historical record of repeated flooding (e.g., 1986, 1995, 1997, 2005, and more recent atmospheric river events).
Table 3-15: Flood Probability Annually and Over 30-Year Period Annual
Chance
Common
Name
30-Year Chance
of Occurrence Example
0.2% 500-YR Flood 6%
Like rolling a 100-sided die each year for 30 years, you have about a 1- in-16 chance of getting a “flood” roll.
1% 100-YR Flood 26%
Each year over a 30-year period, you have about a 1-in-4 chance of a flood roll.
2% 50-YR Flood 45% Nearly even odds for each yearly die roll over the span of 30 years. 10% 10-YR Flood 96%
Almost certain to get a flood roll at least once, if not multiple times, in 30 years.
50% 2-YR Flood 100% Expected to get a flood roll many times during a 30-year period.
Frequency of flooding events in Napa County is strongly influenced by regional meteorology and watershed characteristics. The county is subject to intense winter storm systems, including atmospheric rivers, which can deliver high-intensity rainfall over extended durations between approximately November and April.
These events can produce rapid runoff from steep upland areas surrounding Napa Valley, resulting in sharp rises in streamflow within the Napa River and its tributaries. Smaller tributary systems, particularly those draining the eastern and western hillslopes, are characterized by flashy hydrologic response, contributing to localized flooding, debris flows, and downstream backwater effects.
While FEMA Flood Insurance Rate Maps (FIRMs) delineate Special Flood Hazard Areas (SFHAs) based on the 1% annual chance flood, actual flood risk in Napa County extends beyond these mapped boundaries. As documented in recent storm events and reflected in DWR Best Available Mapping (BAM) Flood Awareness Zones, localized flooding can occur outside mapped floodplains due to factors such as:
▪ Intense short-duration rainfall overwhelming drainage infrastructure CAL OES SUBMITTAL - JUNE 2026
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▪ Debris blockage at culverts and bridge crossings
▪ Backwater effects at tributary confluences
▪ Sheet flow across agricultural lands
▪ Tidal influence in southern Napa County near the Napa River estuary In southern Napa County, particularly in low-lying areas influenced by San Pablo Bay, flood frequency is further affected by tidal cycles, storm surge, and sea level rise. These conditions can reduce drainage capacity and increase the duration and extent of inundation during storm events, even in the absence of extreme riverine flows.
The Napa River Flood Protection Project has been designed to reduce flood risk to the City of Napa and surrounding areas for approximately the 1% annual chance event. However, residual risk remains due to the potential for events exceeding design capacity, levee performance variability, and localized drainage limitations. Additionally, upstream and tributary flooding continues to present a risk independent of mainstem river improvements.
Based on historical occurrence, watershed characteristics, and projected climate influences, the probability of future flooding in Napa County is considered likely, corresponding to an annual probability range of approximately 10% to 100% for flood-related impacts of varying magnitudes. Smaller, nuisance-level flooding may occur annually, while more significant and damaging flood events can be expected multiple times over a multi-decade period.
Projected climate change impacts, including increased precipitation intensity, more frequent atmospheric river events, and rising sea levels, are expected to further increase both the frequency and severity of flooding in Napa County. These trends underscore the importance of continued mitigation planning, infrastructure investment, and adaptive land use strategies to reduce long-term flood risk.
Severity
The main factors affecting flood damage are water depth and velocity. Deeper and faster flood flows can cause more damage. Shallow flooding with high velocities can cause as much damage as deep flooding with slow velocity. This is especially true when a channel migrates over a broad floodplain, redirecting high velocity flows and transporting debris and sediment. Flood severity is often evaluated by examining peak discharges; Table 3-16 lists peak flows used by FEMA to map the floodplains of Napa County.
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Table 3-16: Summary of Discharges in Napa County
Drainage
sq. Miles
Peak Discharge (cubic feet/second)
Flooding Source/Location 10-% 2-% 1-% 0.2-%
Blossom Creek
At confluence with Napa River 3.4 1,000 N/A 1,700 2,000 Cyrus Creek
At confluence with Napa River 2.9 900 N/A 1,500 1,900
Garnett Creek
At confluence with Napa River 6.9 1,900 N/A 3,200 3,900 Napa Creek
At mouth 14.9 * * 4,300 *
At the City of Napa corporate limits 14.9 2,200 * 3,500 4,500 Napa River
Upstream of Bull Island 324 25,500 38,900 45,200 69,000 Upstream of Imola Avenue 300 26,770 39,200 44,390 53,450 Upstream of confluence of Tulucay Creek * 25,810 38,010 42,720 51,260 Upstream of confluence of Napa Creek * 20,040 35,600 40,100 48,300 Upstream of Trancas Street * 22,750 33,430 39,000 44,540 Near the City of Napa (Oak Knoll Avenue) 281 18,500 * 33,000 46,000 Upstream of Oak Knoll Avenue 218 18,400 28,100 32,700 50,300 Near the City of St. Helena (Zinfandel Lane) 81.4 13,000 * 21,000 25,000 At Dunweal lane 23 7,800 N/A 12,500 15,000
At State Highway 29 21.6 7,400 N/A 12,000 14,500
At upstream corporate limits 5.4 3,300 N/A 5,100 6,400
Salvador Creek
At confluence with Napa River * * * 3,910 *
At Big Branch Road 1.8 * * 3,213 *
Sulphur Creek
At confluence with Napa River to Sulphur Creek
Tributary
9.2 * * 3,3305 *
Upstream of Sulphur Creek Tributary 7.6 * * 3,110 *
Sulphur Creek Tributary
At confluence with Sulphur Creek 1.1 * * 500 *
Tulucay Creek
At Soscol Avenue * 2,880 * 4,530 5,500
Upstream of Camille Creek * 2,050 * 3,250 4,050
* Data not available
Source: Table 5 Summary of Discharges from FEMA FIS Text, 2016 5 Flow rate reduction due to capacity restriction
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Warning Time
The type and rate of flooding experienced in Napa County varies. In general, warning times for floods can be between 24 and 48 hours to prepare communities to reduce flood damages Seasonal notification for flooding can enhance awareness for citizens at risk, and, when communicated effectively, advance notification can reach target audiences on a large scale Napa County maintains a storm watch program that serves as flood surveillance and an early warning system. The Napa Valley Regional Rainfall and Stream Monitoring System is a real-time rainfall and riverstream data network that relies on data from automated stream gages in rivers and streams throughout Napa County. The Napa County Flood Control and Water Conservation District is responsible for maintaining that data network which provides current and historical rainfall, river, and stream level monitoring data. Napa County coordinates with the National Weather Service in Reno, NV and the California Department of Water Resources for flood forecasting for the Napa River.
DWR Awareness Zones Notification
The Flood Risk Notification Program (FRN Program) is part of the Department of Water Resources’ (DWR) FloodSAFE California Initiative. The program’s key goal is to increase flood risk awareness by effectively communicating that risk to individual property owners, the public, and local, state, and federal agencies.
This includes encouraging people to understand the levee system that protects them; be prepared and aware of their flood risk; and take appropriate actions before, during, and after flooding to protect themselves, minimize damage to their property or personal possessions, and facilitate recovery. To achieve this goal, the FRN Program:
▪ Sends out an annual notice to property owners whose property is at risk of flooding, ▪ Maintains accurate Levee Flood Protection Zone (LFPZ) maps and an associated parcel information database (note: these maps are different from Federal Emergency Management Agency regulatory maps),
▪ Provides people with useful ways to assess risk and reduce flood loss, ▪ Establishes outreach and educational projects with public involvement, ▪ Expands its interactive Flood Risk Notification website, and ▪ Collaborates with federal agencies, local agencies, and communities.
In September 2010, DWR provided the first annual written notice of flood risks to each landowner whose property is protected by State Plan of Flood Control levees and is within an LFPZ. The notice informs recipients of their property’s potential flood risks and potential sources of flooding and offers flood emergency planning and preparedness tips. It encourages recipients to take preventative actions such as purchasing flood insurance, elevating or “floodproofing” their buildings, and preventing blockage of channels, drains, and ditches.
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Flood Vulnerability Assessment
Flood vulnerability in Napa County is influenced by the interaction of flood hazard exposure, land use patterns, population distribution, critical infrastructure, and watershed characteristics. As shown in Figure 3-9, flood-prone areas are concentrated along the Napa River corridor from Calistoga to the Napa-Sonoma Marshes, as well as along major tributaries and low-lying areas in southern Napa County. Development within these floodplains, including residential neighborhoods, agricultural lands, and transportation corridors, contributes to increased exposure to both riverine and localized flooding hazards.
The County’s physical setting further contributes to vulnerability. Steep upland watersheds surrounding Napa Valley generate rapid runoff during intense storm events, while the relatively flat valley floor and southern tidal areas are prone to prolonged inundation. Tributary systems such as Napa Creek, Milliken Creek, and others can produce localized flooding that interacts with mainstem Napa River flooding. In southern Napa County, tidal influence, subsidence of reclaimed lands, and reliance on levee systems—many of which were constructed for agricultural or habitat purposes rather than flood protection—further increase vulnerability to flooding and compound flood hazards.
Population and development patterns also play a significant role. Communities located within or adjacent to Special Flood Hazard Areas (SFHAs), including portions of the City of Napa, Yountville, St. Helena, and unincorporated areas, face heightened risk of structural damage and disruption. Agricultural operations throughout the valley are vulnerable to crop loss, soil erosion, and damage to irrigation infrastructure.
Additionally, repetitive loss properties identified along the Napa River and its tributaries highlight areas of persistent flood risk.
Critical infrastructure and community lifelines are also exposed. Key transportation routes, including State Route 29, Silverado Trail, and local roadways, traverse flood-prone areas and may become impassable during flood events, limiting emergency response and evacuation. Utilities and essential services located within flood hazard areas may experience service interruptions, compounding the impacts on residents and businesses.
This section evaluates the populations, structures, critical facilities, and infrastructure most at risk from flooding, as well as the potential impacts to the local economy, agricultural resources, and natural systems.
Particular attention is given to vulnerable populations, including residents in low-lying or isolated areas, agricultural workers, and individuals with limited mobility. The assessment also considers how flooding may disrupt transportation networks, utilities, and emergency services.
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Figure 3-21: Exposure Summary – Napa County Flood Risk (FEMA) CAL OES SUBMITTAL - JUNE 2026
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3.3.2.9.1 Population
Population at risk from flooding in Napa County was estimated by intersecting FEMA’s 100-year (1% annual chance) and 500-year (0.2% annual chance) floodplain boundaries with U.S. Census Bureau population data.
Using GIS, census blocks, and tracts, populations were weighted according to the percentage of each block lying within the mapped flood hazard areas.
The results show that a substantial share of the county’s population resides within areas that could experience significant flooding during a 100-year event, with additional residents located in areas subject to the less frequent but still damaging 500-year flood. Most at-risk residents are in the broader flood fringe, with smaller portions in high-velocity floodways and low-lying coastal areas vulnerable to storm surge and wave overtopping.
Using this approach, it was estimated that almost two thousand eight hundred county residents are exposed to some sort of FEMA-identified flood risk, in either 100-year or 500-year floodplains, as shown in Figure 3-22 and Table 3-17
Figure 3-22: Napa County Population Exposed to Flood Risk 1,581
1,034
2,615
180
2,795
-
500
1,000
1,500
2,000
2,500
3,000
Flood Fringe Floodway 100-YR Total 500-YR sans 100-YR 500-YR Total Population Exposure
Population Count in the 100-Year and 500-YR Floodplains CAL OES SUBMITTAL - JUNE 2026
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Table 3-17: Napa County Population Exposed to Flood Risk Total Population
Unincorporated County 22,468
Flood Hazard Zone Population Count % of Total
Flood Fringe 1,581 7.04%
Floodway 1,034 4.60%
100-YR Total* 2,615 11.64%
500-YR sans 100-YR 180 0.80%
500-YR Total 2,795 12.44%
*Special Flood Hazard Area (SFHA)
Population exposure to flood hazards in Napa County is concentrated along the Napa River corridor and its primary tributaries, as well as in low lying areas in the southern portion of the county. Communities along the Napa River from Calistoga through St. Helena, Yountville, and the City of Napa contain some of the highest concentrations of residents within FEMA mapped floodplains. Additional exposure occurs along tributary systems such as Napa Creek, Milliken Creek, and Browns Valley Creek, where development extends into areas influenced by both riverine and localized flooding.
In the northern and central portions of the county, smaller but notable population clusters are located along valley floors and creek corridors near Calistoga and St. Helena, where steep surrounding terrain contributes to rapid runoff and localized flood hazards. In the City of Napa and surrounding unincorporated areas, higher density development within the valley floor increases the number of people exposed to flooding, particularly near the Napa River and its tributaries.
In southern Napa County, including areas near American Canyon and the Napa Sonoma Marshes, population exposure is shaped by low elevations, tidal influence, and levee systems. While population density is generally lower in these areas, flood risk can be more persistent due to limited drainage and prolonged inundation during storm events and high tides. These patterns reflect historical settlement along waterways and valley corridors, where developable land has been concentrated.
3.3.2.9.2 Property
Table 3-18 summarizes parcel exposure to flood hazards in Napa County. Within the 100-year floodplain, approximately $890 million in combined structure and content value is exposed to flood risk. Of this total, about $234 million is located within the floodway, where the highest hazard exists due to depth and velocity of flooding, while the remaining exposure is within the flood fringe areas subject to inundation during major storm events. When including the 500-year floodplain, total exposure increases to over $1.05 billion in combined value. These figures highlight the substantial economic risk associated with flooding in Napa County and reinforce the importance of continued mitigation and flood risk management efforts.
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The highest concentration of at-risk assets follows the Napa River corridor, extending from Calistoga through St. Helena, Yountville, and into the City of Napa. Within this corridor, a mix of residential, commercial, and agricultural properties are located within mapped flood hazard areas, reflecting historic development patterns along the valley floor. Additional clusters of exposed assets occur along tributaries such as Napa Creek, Milliken Creek, and Browns Valley Creek, where localized flooding and backwater effects contribute to risk.
In southern Napa County, asset exposure is concentrated in low lying areas near the Napa Sonoma Marshes and the Napa River estuary. Although development is less dense in this area, properties are subject to persistent flood risk due to tidal influence, limited drainage capacity, and reliance on levee systems.
Overall, the distribution of exposed parcels reflects a predominantly riverine flood hazard pattern, with the greatest concentrations of value located along the Napa River and its tributaries. These patterns underscore the need for a combination of watershed based, structural, and property level mitigation strategies to reduce long term flood risk across Napa County.
Table 3-18: Napa County Parcels Exposed to Flood Risk
Total Parcels
Total Market
Value ($)
Total Content
Value ($) Total Value ($)
Unincorporated County 13,668 $11,215,199,644 $9,248,691,039 $20,463,890,683 Flood Hazard Zone Parcel Count % of Total
Market Value
Exposure ($)
Content Value
Exposure ($)
Total Exposure
($)
% of
Total
Flood Fringe 553 4.0% $373,057,827 $282,519,082 $655,576,909 3.2% Floodway 175 1.3% $125,068,121 $109,379,109 $234,447,230 1.1% 100-YR Total 728 5.3% $498,125,948 $391,898,190 $890,024,139 4.3% 500-YR sans 100-YR 146 1.1% $93,347,085 $67,270,532 $160,617,617 0.8% 500-YR Total 874 6.4% $591,473,033 $459,168,723 $1,050,641,756 5.1% Note: The table above does not display loss estimation results; the table exhibits total value at risk based upon the hazard overlay and Napa County Assessor data.
3.3.2.9.3 Critical Facilities & Infrastructure
Essential facilities, high potential loss sites, and lifeline infrastructure in Napa County face notable flood risk due to their location within FEMA mapped 100-year and 500-year floodplains. This assessment evaluates exposure only, meaning facilities are identified based on whether they intersect mapped hazard areas. No first floor elevation or site specific depth data was available, so these results represent potential exposure rather than expected damage.
Flood exposure is concentrated along the Napa River corridor and its major tributaries, where a number of public and private assets intersect flood hazard zones. Essential facilities in flood prone areas include fire CAL OES SUBMITTAL - JUNE 2026
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stations, emergency shelters, and farmworker housing, all of which play critical roles in emergency response and community resilience. While no essential facilities are located within mapped floodways, several are within the 100-year floodplain and may experience operational disruption during significant flood events.
Transportation and lifeline infrastructure represent the largest share of exposed assets. A total of 62 transportation related facilities intersect flood hazard areas, including 58 bridges, which are particularly vulnerable to high flows, debris accumulation, and scour. Flooding at these crossings can restrict access, isolate communities, and delay emergency response. Additional exposed infrastructure includes a wastewater treatment facility, water supply intake, and an airport, all of which are essential to maintaining basic services during and after flood events.
High potential loss facilities are also present within flood hazard areas, including a mix of real property assets, emergency shelters, and farmworker housing. These sites may experience significant economic and social impacts if damaged. Hazardous materials facilities represent another key area of concern, with 27 sites located within flood hazard zones, including several within the floodway. Flooding at these locations could result in the release of hazardous substances, posing risks to public health and the environment.
Overall, the distribution of exposed facilities reflects Napa County’s reliance on infrastructure located within valley floors and along waterways. As shown in Table 3-19, a total of 125 critical facilities and infrastructure assets intersect the 100-year and 500-year floodplains. This concentration of essential services and lifeline systems within flood prone areas highlights the importance of targeted mitigation actions, including infrastructure hardening, redundancy planning, and continued maintenance to reduce the risk of service disruption during future flood events.
Table 3-19: Napa County Critical Facilities Exposed to Flood Risk Infrastructure Type Flood Fringe Floodway 100-YR Total
500-YR sans
100-YR 500-YR Total
Essential Facilities 2 - 2 - 2
EOC - - - - -
Fire Station 2 - 2 - 2
Hospital - - - - -
Law Enforcement - - - - -
High Potential Loss 10 5 15 19 34
Adult Residential Care - - - - -
Child Care Center - - - - -
Church 1 - 1 - 1
Communication Tower - - - - -
Dam - - - - -
Emergency Shelter 2 - 2 - 2
Farm Worker Housing 2 - 2 - 2
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Infrastructure Type Flood Fringe Floodway 100-YR Total
500-YR sans
100-YR 500-YR Total
Historic Building - - - - -
Library - - - - -
Mobile Home Park - - - - -
Nursing Home - - - - -
Private School - - - - -
Public School - - - - -
Real Property Asset* 5 5 10 19 29
Transportation and Lifeline 36 25 61 1 62
Airport 1 - 1 - 1
Bridge 33 25 58 - 58
Bus Stop - - - - -
HMMWC Asset - - - - -
HMMWC Dam - - - - -
HMMWC Hydrant - - - - -
NG Station - - - - -
Power Plant - - - 1 1
Substation - - - - -
Wastewater Treatment Facility 1 - 1 - 1
Water Supply Intake 1 - 1 - 1
Water Tank - - - - -
Water Treatment Facility - - - - -
Hazmat 14 7 21 6 27
Geotracker Cleanup Site - - - - -
HWTS Active Facility 14 7 21 6 27
Landfill - - - - -
Grand Total 62
37 99 26 125
*Real Property Assets are digitized insurance rolls for demonstrating value and ownership and may have overlapping points with other categories such as fire stations and law enforcement.
Lifeline infrastructure in Napa County, including transportation networks, utilities, and levee systems, faces notable flood risk due to its location within FEMA mapped 100-year and 500-year floodplains. Much of this infrastructure is concentrated along the Napa River corridor and across the valley floor, where flood hazards are most prevalent.
Transportation infrastructure represents the largest share of exposed lifeline systems. As shown in Table 3-20, more than 238 miles of roadway are located within the 100-year floodplain, with an additional 26 miles in the 500-year floodplain. These include major roads such as State Route 29 and Silverado Trail, as well as CAL OES SUBMITTAL - JUNE 2026
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an extensive network of local and service roads that traverse flood prone areas. Flooding along these routes can restrict regional mobility, isolate rural and unincorporated communities, and delay emergency response and recovery efforts. Bridges and roadway crossings are particularly vulnerable to high flows, debris accumulation, and scour, increasing the likelihood of closures during storm events. Rail infrastructure is also exposed, with over 10 miles intersecting flood hazard areas, creating potential disruptions to goods movement and regional connectivity.
Utility systems are similarly exposed. Natural gas pipelines, electric transmission lines, and water and wastewater infrastructure cross floodplains throughout the county. Flooding can damage these systems or interrupt service, resulting in power outages, reduced water supply reliability, wastewater overflows, and potential public safety hazards. Critical facilities such as wastewater treatment infrastructure and water supply systems located within flood hazard areas are particularly vulnerable to operational disruption during flood events.
Levee systems, which extend over 100 miles within mapped flood hazard areas, are a key component of Napa County’s flood management system, particularly in southern portions of the county. However, many of these levees were constructed for agricultural or habitat management purposes and do not provide a consistent level of flood protection. Overtopping or failure of these systems can result in rapid inundation of adjacent lands, including transportation corridors and utility infrastructure.
Overall, the widespread exposure of lifeline infrastructure reflects the county’s reliance on systems located within valley floors and along waterways. The concentration of roads, utilities, and levees within flood prone areas underscores the importance of resilient design, regular maintenance, and emergency planning. As summarized in Table 3-20, a total of approximately 264 miles of lifeline infrastructure intersect flood hazard zones, highlighting the potential for significant service disruptions during major flood events and the need for targeted mitigation strategies.
Table 3-20:Napa County Lifeline Infrastructure Exposed to Flood Risk Lifelines (miles) - Flood Risk Exposure
Infrastructure Type (linear) Flood Fringe Floodway 100-YR Total 500-YR sans
100-YR Grand Total
Levee 94.7 6.3 101.0 0.5 101.5
NG Pipeline 3.5 0.7 4.1 0.6 4.8
Railroad 4.8 - 4.8 5.3 10.2
Transmission Line 7.2 1.2 8.4 0.9 9.2
Street OSM 97.3 22.5 119.8 23.6 143.4
Highway & Interstate 1.1 - 1.1 0.1 1.1
Major Road 3.7 0.3 3.9 0.8 4.7
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Lifelines (miles) - Flood Risk Exposure
Residential Road 11.3 1.9 13.2 2.1 15.3
Road - Secondary 6.6 3.6 10.2 2.6 12.8
Road -Tertiary 6.6 1.5 8.1 1.5 9.6
Service Road 60.9 14.7 75.6 16.2 91.9
Unclassified 7.2 0.5 7.7 0.3 8.0
Grand Total 207.5 30.7 238.2 26.1 264.2
*Partially Overlaps “All Roads".
DevelopmentTrends
Napa County faces ongoing challenges in balancing development pressures with flood hazards, particularly as climate change increases the frequency and intensity of storm events and influences downstream tidal conditions. Unlike coastal counties where flood risk is driven by ocean processes, flooding in Napa County is primarily riverine, centered on the Napa River watershed and its tributary systems. However, in southern portions of the county, flood risk is also influenced by tidal backwater effects and sea level rise, creating compound flooding conditions in low lying areas. Future development in these areas must account for multiple pathways of flooding, including extreme rainfall, tributary overflow, and reduced drainage capacity during high tide conditions.
Flood risk management in Napa County relies on a combination of structural and non structural approaches.
The Napa River Flood Protection Project provides significant flood risk reduction along portions of the mainstem river, particularly in and around the City of Napa, but much of the watershed including tributary corridors and unincorporated areas remains vulnerable to flooding. In southern Napa County, levee systems are present but are largely designed for agricultural and habitat management rather than engineered flood protection, limiting their effectiveness during major storm events. As a result, localized flooding continues to occur in areas with constrained channels, aging infrastructure, and limited drainage capacity.
Future growth in Napa County is guided by General Plan policies that emphasize limiting development in flood hazard areas, preserving floodplains for agricultural and open space uses, and requiring mitigation measures such as elevation, floodproofing, and drainage improvements where development does occur.
Development trends continue to reflect a concentration of residential and agricultural uses along the valley floor, where flood exposure is inherently higher. In unincorporated areas, rural development patterns and reliance on local road networks further increase vulnerability to flood related disruptions.
Looking ahead, more intense atmospheric river events, changing watershed conditions, and sea level rise in the southern portion of the county are expected to expand the extent and severity of flooding. These trends highlight the need for continued investment in flood management infrastructure, updated hazard mapping, and proactive land use planning. Ensuring that future development does not increase flood risk will require integrating watershed based approaches, maintaining natural floodplain functions, and prioritizing resilience in both urban and rural areas of Napa County.
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Climate Change Impacts
Flooding is a recurring and costly hazard in unincorporated Napa County, with climate change expected to increase both the frequency and intensity of extreme precipitation events, particularly atmospheric rivers.
These changes are anticipated to produce higher peak flows in the Napa River and its tributaries, expand flood extents, and increase the likelihood of localized flooding in smaller, unmapped watersheds. In southern Napa County, sea level rise and tidal influences will further compound flood risks by reducing drainage capacity and increasing backwater effects. These evolving conditions will disproportionately impact vulnerable populations, critical facilities, and aging infrastructure systems.
Vulnerable Populations & At-Risk Facilities
Several populations in Napa County face elevated flood risk due to their location and limited ability to respond to or recover from flood events. County operated farmworker housing centers located within the Agricultural Preserve are situated in mapped flood zones and house approximately 180 low income residents who may have limited access to evacuation resources or temporary shelter options. Similarly, critical community facilities such as Calistoga Elementary School are located within both the one percent annual chance floodplain and a dam inundation zone, increasing the potential for compounded hazard exposure during extreme events.
Repetitive loss properties also represent a concentration of vulnerability, with 34 properties in unincorporated Napa County accounting for over $2.7 million in cumulative flood insurance claims. These properties highlight persistent exposure to flooding and indicate areas where structural and non structural mitigation measures remain limited or underutilized.
Infrastructure & Access Constraints
Flooding in Napa County frequently affects transportation and drainage infrastructure located within floodplains. Roads, culverts, and local access routes are susceptible to inundation, erosion, and structural damage during flood events, which can isolate rural communities, delay emergency response, and disrupt economic activity. Stormwater management systems, including culverts, channels, and detention capacity, are critical to flood mitigation but may be undersized or impacted by sedimentation, debris, or changing hydrologic conditions. Limitations in stormwater storage and conveyance capacity can exacerbate localized flooding, particularly in developed valley floor areas and agricultural lands.
Systemic & Planning Challenges
Flood risk in Napa County is further influenced by the need to regularly update zoning, floodplain management ordinances, and development standards to reflect changing conditions, including population growth, land use changes, and evolving hazard data. In addition, public awareness of flood risk remains a challenge, as some residents and business owners may not fully understand their exposure or the importance of preparedness measures such as flood insurance, evacuation planning, and property level mitigation.
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Climate Change Trends
Projected increases in the intensity of atmospheric river events will likely result in more frequent high flow conditions, increased sediment transport, and greater strain on both natural and engineered drainage systems. These changes may expand flood hazard areas beyond currently mapped FEMA Special Flood Hazard Areas, particularly in smaller tributary systems and flood awareness zones identified through Best Available Mapping. In southern Napa County, rising sea levels will further reduce the effectiveness of existing drainage infrastructure and levee systems that were not designed for future hydrologic conditions.
Future Considerations & Mitigation Direction
Addressing these risks will require a coordinated approach that prioritizes vulnerable populations and critical facilities while enhancing system wide resilience. Key considerations include improving flood risk awareness and outreach, particularly for underserved communities; investing in upgrades to stormwater and transportation infrastructure; addressing repetitive loss properties through mitigation programs; and maintaining up to date floodplain regulations and mapping. By aligning mitigation actions with identified problem statements, Napa County can reduce long term flood risk, improve emergency response capabilities, and enhance resilience to future climate driven flood events.
Flood Hazard Problem Statements
As part of the flood mitigation planning process, the Napa County Planning Team developed areas of concern (i.e., problem statements) that summarize the county’s most pressing flood-related vulnerabilities and challenges. The problem statements were developed based on the updated risk assessment and vulnerability analysis contained herein, as well as the team’s local knowledge and experience and community input.
These problem statements are not intended to be all-encompassing or to describe each and every issue associated with flood. Instead, they highlight the most urgent concerns that can be addressed within the scope of this MJHMP. Each statement will be linked to at least one mitigation action, ensuring that identified needs are met with specific, measurable, and equitable solutions. Table 3-21 presents the flood problem statements for Napa County, along with their associated area of concern and reference numbers for crosslinking to the mitigation actions in Table 4-4.
Table 3-21: Napa County Flood Hazard Problem Statements Problem No. Hazard Area of
Concern
Primary
Agency
Problem Description Related MA
ps-FL-NCo-102 Flood Victim Napa County Napa County owns 3 farmworker centers located within flood hazard zones, the
residents of which are low-income and
vulnerable to displacement after an event
without assistance.
ma-FL-NCo-59,
ma-FL-NCo-57
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Problem No. Hazard Area of
Concern
Primary
Agency
Problem Description Related MA
ps-FL-NCo-110 Flood Victim Napa County Calistoga Elementary School is located within a high flood risk area and dam inundation
zone, posing risks to children and staff during
a flood event.
ma-FL-NCo-52,
ma-FL-NCo-55,
ma-FL-NCo-56,
ma-FL-NCo-57
ps-FL-NCo-117 Flood Threat Napa County County zoning and floodplain regulations may be inadequate and may not reflect changes in
development patterns, population growth, and
recent hazard mapping updates.
ma-FL-NCo-54,
ma-DR-NCo-26,
ma-CC-NCo-13
ps-FL-NCo-119 Flood Threat Napa County Approximately 34 Repetitive Loss properties with substantial historical losses exist in the
County, underscoring the need for propertylevel mitigation and funding strategies. ma-FL-NCo-51,
ma-FL-NCo-53,
ma-FL-NCo-61,
ma-FL-NCo-54,
ma-FL-NCo-57
ps-FL-NCo-128 Flood Threat Napa County Many residents and business owners lack awareness of flood risk, mitigation options,
and available assistance programs.
ma-AH-NCo-6,
ma-FL-NCo-54
ps-FL-NCo-129 Flood Impact Napa County Critical infrastructure located in the floodplain, especially roads and culverts, can be
inundated and damaged or destroyed during
flood events, hindering emergency response.
ma-FL-NCo-57,
ma-FL-NCo-58,
ma-FL-NCo-60,
ma-AH-NCo-4,
ma-FL-NCo-51
ps-FL-NCo-395 Flood Threat Napa County Accelerating sea-level rise, higher storm surges, and increasing atmospheric and heavy
rain events due to climate change, together,
increase the probability of levee failures in the
Sacramento-San Joaquin Delta.
ma-AH-NCo-6,
ma-DR-NCo-26,
ma-CC-NCo-13,
ma-FL-NCo-51,
ma-FL-NCo-54,
ma-FL-NCo-57
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3.3.3 Dam Failure Hazard Profile
Dam failure can be sudden and catastrophic to all life and property downstream.
Hundreds of dam failures have occurred throughout U.S. history, mostly before safety legislation was passed in 1972, that caused immense property and environmental damages and took thousands of lives. As the nation’s dams age and population increases, the potential for deadly dam failures grows. (Association of State Dam Safety Officials, 2024)
Timber, rock, concrete, earth, steel, or a combination of these materials may be used to build a dam. Dams must have spillway systems to safely convey normal stream and flood stage flows over, around, or through the dam. Spillways are commonly constructed of non-erosive materials such as concrete. Dams also have a drain or other water-withdrawal facility to control the reservoir level and to lower or drain the reservoir for normal maintenance and emergency purposes.
Dam failures can result from earthquakes, structural instability, or intense rain in excess of design capacity, but typically occur in one of four ways:
▪ Overtopping of the primary dam structure, which accounts for 34% of all dam failures, can occur due to inadequate spillway design, settlement of the dam crest, blockage of spillways, and other factors. ▪ Foundation defects due to differential settlement, slides, slope instability, uplift pressures, and foundation seepage can also cause dam failure. These account for 30% of all dam failures. ▪ Failure due to piping, or internal erosion caused by seepage, accounts for 20% of all failures. Piping occurs when seepage through a dam is not properly filtered and soil particles continue to progress and form sink holes within it. Seepage often occurs around hydraulic structures, such as spillways; through animal burrows; around roots of woody vegetation; and through cracks in dams, dam appurtenances, or dam foundations. (Id.)
▪ Failure due to problems with conduits and valves, typically caused by the intrusion of embankment material into conduits through joints or cracks, constitutes 10% of all failures.
Secondary & Compounding Hazards
The main secondary hazard posed by dam failure is rapid and severe downstream flooding, depending on the magnitude of the failure. Other potential secondary hazards of dam failure are landslides around the reservoir perimeter and bank erosion downstream.
Many historical dam failures have been secondary hazards triggered by other disasters, such as earthquakes, landslides, extreme storms, and sabotage. Poor construction, lack of maintenance and repair, and deficient operational procedures are preventable or correctable by a program of regular inspections; however, terrorism and vandalism are serious concerns that all operators of public facilities must plan for.
These threats are under continuous review by public safety agencies. In Napa County, dam failure would most likely be a secondary hazard triggered by a major earthquake, excessive rainfall, or slope failure, which CAL OES SUBMITTAL - JUNE 2026
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are discussed in the Earthquake (Section 3.3.4), Severe Weather (Section 3.3.7), and Slope Failure (Section
3.3.5) Hazard Profiles.
Climate Change
Dams are designed partly based on assumptions about a river’s flow behavior, expressed as hydrographs.
As such, changes in weather patterns can have significant effects on the hydrograph used for the design of a dam. Although climate change will not increase the probability of catastrophic dam failure, it may increase the probability of design failures. If the hygrograph changes, then dam operators may be forced to release increased volumes earlier in a storm cycle to maintain required margins of safety, increasing flood potential downstream. Throughout the western United States, communities downstream of dams are already experiencing increases in-stream flows from early releases.
Policies, Plans & Regulatory Environment
Federal Regulatory Environment
National Dam Safety Act & Dam Safety Program
The potential for catastrophic flooding due to dam failures led to passage of the 1972 National Dam Safety Act. (Public Law 92-367) Then, following a series of deadly dam failures between 1972 and 1977, President Jimmy Carter directed the U.S. Army Corps of Engineers to inspect non-federal, high-hazard dams around the nation. The findings of these inspections helped establish dam safety programs in most states, and, ultimately, resulted in the creation of the National Dam Safety Program. (Id.) The National Dam Safety Program requires a periodic engineering analysis of every major dam in the country. The goal of this program, monitored by FEMA, is to identify and mitigate the risk of dam failure to protect lives and property.
U.S. Army Corps of Engineers (USACE)
Under the National Dam Safety Program, the U.S. Army Corps of Engineers (USACE) is responsible for safety inspections of some federal and non-federal dams in the United States that meet the size and storage limitations specified in the National Dam Safety Act. USACE has inventoried such dams and surveyed each state and federal agency’s capabilities, practices, and regulations regarding design, construction, operation, and maintenance of the dams. USACE also develops guidelines for inspection and evaluation of dam safety.
USACE is also responsible for maintaining the National Inventory of Dams (NID) and works in close collaboration with dam regulating agencies, including FEMA and state agencies, to obtain accurate and complete information about dams in the database.
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Federal Energy Regulatory Commission (FERC) Dam Safety Program The Federal Energy Regulatory Commission (FERC) has the largest dam safety program in the United States.
The FERC cooperates with a large number of federal and state agencies to ensure and promote dam safety and, more recently, homeland security. There are over 3,000 dams that are part of regulated hydroelectric projects in the FERC program. Two-thirds of these are more than 50 years old. As dams age, concern about their safety and integrity grows, so oversight and regular inspection are important. FERC staff inspects hydroelectric projects on an unscheduled basis to investigate:
▪ Potential dam safety problems;
▪ Complaints about constructing and operating a project; ▪ Safety concerns related to natural disasters; and
▪ Issues concerning compliance with the terms and conditions of a license.
Every five years, an independent consulting engineer, approved by the FERC, must inspect and evaluate projects with dams higher than 10 meters (32.8 feet), or with a total storage capacity of more than 2,000 acrefeet.
FERC staff monitors and evaluates seismic research in geographic areas where there are concerns about seismic activity. This information is applied in investigating and performing structural analyses of hydroelectric projects in these areas. FERC staff also evaluates the effects of potential and actual large floods on the safety of dams. During and following floods, FERC staff visits dams and licensed projects, determines the extent of damage, if any, and directs any necessary studies or remedial measures the licensee must undertake. The FERC publication Engineering Guidelines for the Evaluation of Hydropower Projects guides the FERC engineering staff and licensees in evaluating dam safety. The publication is frequently revised to reflect current information and methodologies.
The FERC requires licensees to prepare emergency action plans and conducts training sessions on how to develop and test these plans. The plans outline an early warning system if there is an actual or potential sudden release of water from a dam due to failure. The plans include operational procedures that may be used, such as reducing reservoir levels and reducing downstream flows, as well as procedures for notifying affected residents and agencies responsible for emergency management. These plans are frequently updated and tested to ensure that everyone knows what to do in emergency situations.
Monticello Dam is a federal facility owned and operated by the U.S. Bureau of Reclamation and serves multiple purposes, including water supply, flood control, and recreation. While the dam itself is not regulated by the Federal Energy Regulatory Commission (FERC), it is important to note that the hydroelectric powerplant located at the base of the dam operates under a separate regulatory framework.
The Monticello Dam Powerplant is an approximately 11.5-megawatt facility constructed in 1983 under a FERC license. Although the Bureau of Reclamation owns the dam structure, the powerplant is owned, operated, and maintained by the Solano Irrigation District (SID) under this license. The current FERC license CAL OES SUBMITTAL - JUNE 2026
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for the hydroelectric facility is set to expire on December 31, 2030, at which time relicensing or other regulatory actions will be required.
This distinction is important for hazard mitigation planning purposes, as regulatory oversight differs between the dam structure and the associated power generation facility. While inundation risk is tied to the structural integrity and operation of Monticello Dam under federal oversight, the powerplant component is subject to FERC regulation and licensing requirements, reflecting a shared but distinct operational and regulatory framework.
State Regulatory Environment
California Department of Water Resources (DWR), Division of Safety of Dams (DSOD) The California Department of Water Resources (DWR) Division of Safety of Dams (DSOD) monitors the dam safety program at the state level. When a new dam is proposed, division staff inspects the site. DSOD reviews dam applications and building plans to ensure that the dam is designed to meet minimum requirements and that the design is appropriate for known geologic conditions. It also inspects construction to ensure that the work is done in accordance with the approved plans. Division staff inspects constructed dams on an annual basis to ensure they are performing as intended and are not developing problems. Roughly a third of these inspections include in-depth instrumentation reviews. DSOD periodically reviews the stability of dams and their major appurtenances in light of improved design approaches and requirements, as well as new findings regarding earthquake hazards and hydrologic estimates in California. (DWR, 2025) California Senate Bill 92
Senate Bill 92 was signed into law in 2017 and provides new state requirements focused on dam safety.
Specifically, it requires owners of significant and high hazard potential dams to submit inundation maps to the California DWR. After the map is approved, the dam owner must then submit an emergency action plan (EAP) to the California Office of Emergency Services (Cal OES). An EAP identifies the emergency notification protocols, response process, and responsibilities of the dam operator and other agencies should an incident occur. The plans also outline preparedness activities. EAPs must approved by Cal OES Dam Safety Planning Unit, and updated and resubmitted every 10 years and, in some cases, sooner. (CalOES, 2025) California Senate Bill 854
Passed in 2018, Senate Bill 854 amended California Water Code Section 6161, clarifying requirements for dams with existing EAPs as of March 1, 2017. It stipulates that if the EAP lacked inundation maps for all critical appurtenant structures, dam owners must submit a schedule to the DSOD to complete these maps.
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Local Regulatory Environment
Dam safety and inundation risk management in Napa County are guided by a combination of land use policies, emergency preparedness planning, and interagency coordination frameworks established in the Napa County General Plan and the County’s Emergency Operations Plan (EOP). These regulatory and planning documents collectively support hazard mitigation, emergency response, and long-term resilience and dam safety.
Napa County General Plan
The Napa County General Plan Safety Element (August 2023) provides the primary policy framework for addressing dam failure hazards. The Safety Element identifies dam failure as a potentially highconsequence hazard characterized by rapid-onset flooding, limited warning time, and downstream impacts to people, infrastructure, and transportation systems. The General Plan incorporates several policies specifically relevant to dam failure risk:
▪ Policy SAF-2.16: Requires review of new development proposals related to dam failure inundation maps and allows the County to limit or deny development where necessary to protect life and property .
▪ Policy SAF-2.17: Requires dam operators to maintain and regularly update Emergency Action Plans (EAPs) for high- and significant-hazard dams, supporting warning and response capabilities. ▪ Policy SAF-6.4: Requires development proposals to be reviewed with reference to dam failure inundation maps to identify evacuation routes.
▪ Policy SAF-6.5: Establishes that dam and levee maintenance is the responsibility of the owner/operator, with the County supporting coordination to ensure proper maintenance and repairs. ▪ Policy SAF-6.11: Calls for improving risk assessments within the County to better understand and reduce hazard exposure.
Additional supporting policies reinforce evacuation planning and emergency access, including Policy SAF- 1.6 (development of intra-county evacuation routes) and Policy SAF-2.18 (prioritization of improvements to evacuation routes). These policies are consistent with California Government Code §65302(g) and AB 747 requirements for evacuation planning.
Collectively, these policies demonstrate an integrated approach that combines hazard mapping, land use regulation, infrastructure planning, and interagency coordination to reduce dam failure risk.
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Napa County Emergency Operations Plan
The 2024 Napa County Emergency Operations Plan (EOP) provides the operational framework for preparedness, response, and recovery related to dam failure incidents and other hazards. The EOP adopts an all-hazards approach and incorporates the National Incident Management System (NIMS), Standardized Emergency Management System (SEMS), and Incident Command System (ICS) to ensure coordinated multiagency response.
Dam failure is specifically addressed through hazard-specific annexes within the EOP structure, which define roles, responsibilities, and procedures for emergency response, including evacuation, warning, and resource coordination. The EOP establishes Napa County as the Operational Area responsible for coordinating information sharing, resource requests, and mutual aid among local jurisdictions, special districts, and state and federal partners. Key elements relevant to dam safety include:
▪ Emergency coordination and communication: The County’s Emergency Operations Center (EOC) serves as the central coordination point for situational awareness, damage assessment, and resource deployment during dam-related incidents.
▪ Evacuation and warning systems: The EOP outlines alert and notification procedures, including mass notification systems and coordination with state and federal warning systems to support timely evacuation of at-risk populations.
▪ Multi-agency collaboration: The plan emphasizes coordination with dam owners/operators (e.g., Bureau of Reclamation, Solano Irrigation District), as well as regional, state, and federal agencies responsible for dam safety oversight.
▪ Whole community approach: The EOP incorporates coordination with schools, healthcare facilities, and community organizations to ensure inclusive and effective emergency response.
Together, the General Plan and EOP provide a complementary regulatory and operational framework that supports dam safety, risk reduction, and emergency preparedness in Napa County. These documents ensure that both long-term planning and real-time response capabilities are aligned to minimize risk to life, property, and critical infrastructure from potential dam failure events.
Location
Dam hazard classifications, or hazard potential classifications, are used to categorize dams based on the potential consequences of failure rather than the likelihood of failure. As defined in Table 3-22, dams are categorized as Low, Significant, or High Hazard depending on the potential for loss of life, economic damage, and environmental impacts. The California DSOD also includes an additional classification of “Extremely High” for facilities with particularly severe downstream consequences.
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Table 3-22: Dam Hazard Potential Classifications
Hazard Potential Classification Loss of Human Life Economic, Environmental, Lifeline Losses Low None expected Low and generally limited to owner
Significant None expected Yes
High Probable; one or more expected Yes (but not necessary for this classification) Extremely High
One or more and inundating an
area of 1,000+ population
Inundation of facilities or infrastructure, posing
significant threat to public safety determined
case-by-case by DSOD
Source: Federal Guidelines for Dam Safety, Hazard Potential Classification Systems for Dams, 2004; DSOD Jurisdictional Dams
According to the California DSOD and the National Inventory of Dams, Napa County contains over 50 jurisdictional dams, the majority of which are classified as High Hazard Potential (Table 3-23), indicating that failure could result in probable loss of life and significant property damage . These dams are owned and operated by a mix of entities, including local jurisdictions (e.g., City of Napa, City of St. Helena, City of Vallejo), state agencies (e.g., California Department of Veterans Affairs), federal agencies (e.g., U.S. Bureau of Reclamation), water companies, and numerous private landowners and vineyard operators. This diverse ownership structure requires coordinated oversight between DSOD, federal regulators, and local emergency management agencies.
Reservoir capacities range from small agricultural impoundments of less than 50 acre-feet to major regional facilities such as Monticello Dam, which exceeds 1.9 million acre-feet and represents the largest storage facility in the county. Many dams were constructed between the late 1800s and mid-20th century, reflecting a system that includes both aging infrastructure and more recently constructed facilities.
Areas of Napa County most susceptible to dam failure inundation are concentrated along the Napa River corridor and its tributaries, including developed areas within the Cities of Calistoga, St. Helena, Yountville, and Napa, where downstream flow paths converge and population exposure is greatest.
Table 3-23: Dams in or Adjacent to Napa County
Dam Name Owner River or Stream Year
Completed
Storage
(Acre-Ft)
Hazard
Classification
Condition
Assessment
Lake Curry City of Vallejo Gordon Valley
Cr 1926 10,700 High Poor
Peju Reservoir
No. 2
Persephone Ranch
Ltd. And Peju
Province Winery
Burton Creek 96 High Poor
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Dam Name Owner River or Stream Year
Completed
Storage
(Acre-Ft)
Hazard
Classification
Condition
Assessment
Araujo
Reservoir No. 1 Kerwin Estate, LLC Unnamed 1950 21.9 High Fair Araujo
Reservoir No. 2 Kerwin Estate, LLC 1950 19.8 High Fair
Bell Canyon City of Saint Helena Bell Creek 1959 2530 High Fair Conn Creek City of Napa Conn Creek 1946 31,000 High Fair Hestan
Vineyards Dam Stanley Cheng 146 High Fair
Veterans Home
California
Department of
Veteran Affairs
Tr Napa River 1908 39 High Fair
Angwin Pacific Union College Moore Creek 1967 156 High Satisfactory B. J. Robinson Massimo De Simoni Tulocay Creek 1957 49 High Satisfactory BlanchardOffner Jim Collins Tr Pope Creek 1963 274 High Satisfactory Circle S Circle R Ranch LLC Tr Milliken Cr 1979 131 High Satisfactory Crystal Glendale Ranch
Vineyards Tr Crystal Crk 1952 105 High Satisfactory
Deer Lake
Howell Mountain
Mutual Water
Company
Tr Angwin
Creek 1958 248 High Satisfactory
Duvall Jim Collins Tr Pope Creek 1940 242 High Satisfactory Eaton H.
Magoon Lake Langtry Farms, LLC Routan Creek 1965 3,760 High Satisfactory Foss Valley Atlas Peak Vineyards Tr Milliken Cr 1988 800 High Satisfactory Heitz Heitz Wine Cellars Tr Pope Creek 1991 272 High Satisfactory Henne
Howell Mountain
Mutual Water
Company
Angwin
Stream 1959 109 High Satisfactory
Kimball Creek City of Calistoga Kimball Creek 1939 405 High Satisfactory Lake Camille Department of State
Hospitals-Napa Tr Tulucay Cr 1880 42.9 High Satisfactory Lake Cynthia Krupp Brothers
Vineyard Tr Soda Creek 1955 92 High Satisfactory
Lake Leticia William E. Jarvis Tr Milliken Cr 1960 115 High Satisfactory Lake Marie County of Napa Tr Tulucay Cr 1908 170 High Satisfactory Lake William William E. Jarvis Tr Milliken Cr 1960 340 High Satisfactory Maxville Maxville Lake Winery Maxwell
Creek 1953 183 High Satisfactory
Metcalf Skalli Corporation Tr Maxwell Cr 1974 1,045 High Satisfactory CAL OES SUBMITTAL - JUNE 2026
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Dam Name Owner River or Stream Year
Completed
Storage
(Acre-Ft)
Hazard
Classification
Condition
Assessment
Milliken City of Napa Milliken Creek 1924 1,980 High Satisfactory Olson Precision Vineyards,
LLC
Ledgewood
Creek 1955 200 High Satisfactory
Orville
Howell Mountain
Mutual Water
Company
Angwin Creek 1959 89 High Satisfactory
Rector Creek
California
Department of
Veteran Affairs
Rector Creek 1946 4,587 High Satisfactory
Robert L.
Matheson Sage Canyon, LLC Elder Creek 1954 90 High Satisfactory Turkey Ranch Juliana Mutual Water
Company Tr Pope Creek 1999 240 High Satisfactory
Usibelli No. 2 Usibelli Coal Mine,
Inc. Tr Maxwell Cr 1973 900 High Satisfactory
Will Lake #1 C. Mondavi and Son,
Inc. Tr Napa River 2004 89 High Satisfactory
Winery Lake Rene Di Rosa Tr Carneros
Creek 1953 310 High Satisfactory
Monticello
Dam
Bureau Of
Reclamation
PUTAH
CREEK 1957 1,902,086
High Not Available
Bassett Brown Brown Estate
Vineyards Sage Creek 1990 16.41 Significant Fair
Hudson
Vineyards Hudson Vineyards Tr Huichica Crk 1983 74.12 Significant Fair Burns Buckman Vineyards
LLC Tri Swartz Cr 1956 62 Significant Satisfactory
Davis Temple Family
Vineyards, LLC Tr Pope Creek 1955 140 Significant Satisfactory Dick Week Juliana Mutual Water
Company Tr Pope Creek 1955 3,140 Significant Satisfactory Hardester
North
Juliana Mutual Water
Company
Tr Burton
Crrek 2003 150 Significant Satisfactory
Jameson
Vineyards
John Anthony
Family of Wines
Napa Valley LLC
Tr Fagan
Creek 75 Significant Satisfactory
Lake La Verne Clinton and Nancy
Pridmore
Tr Capell
Creek 1956 54 Significant Satisfactory
Linda Vista Elizabeth C.
Williamson
Tr Chiles
Creek 1959 52 Significant Satisfactory
Naz Lake Huneeus-Chantre
Properties Tr Napa River 1955 150 Significant Satisfactory CAL OES SUBMITTAL - JUNE 2026
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Dam Name Owner River or Stream Year
Completed
Storage
(Acre-Ft)
Hazard
Classification
Condition
Assessment
Scotts Canyon Stephen Juelsgaard Tr Carneros
Creek 1948 58 Significant Satisfactory
Veeder Hills
Dam
The Hess Persson
Estates
Pickle Canyon
Creek 23.9 Significant Satisfactory
Deer Creek Juliana Mutual Water
Company Tr Pope Creek 1980 103 Low Satisfactory
La Herradura L.P.R.P. Legacy Trust Tr Conn Creek 1948 110 Low Satisfactory Ler Twin Lake Juliana Mutual Water
Company Tr Pope Creek 1985 127 Low Satisfactory
Long Valley
West No. 2
Juliana Mutual Water
Company Tr Pope Creek 1982 177 Low Satisfactory
Montana Lake Terrence Wilson Tr Lake Curry 2009 150 Low Satisfactory Morgan William and Barbara
Morgan
Tr Gordon Val
Cr 94 Low Satisfactory
Moskowite George Moskowite Tr Capell
Creek 1955 472 Low Satisfactory
Napa Valley
Lake Langtry Farms, LLC Tr Putah Creek 1990 200 Low Satisfactory Upper Twin
Lake
Juliana Mutual Water
Company Tr Pope Creek 1987 63 Low Satisfactory
Source: National Inventory of Dams
Hazard Classification and Tributary-Based Risk Patterns Dam hazard classifications reflect the consequences of failure rather than likelihood, with High Hazard dams posing the greatest potential risk to life safety. A review of Napa County’s dam inventory indicates several notable concentrations of High Hazard dams within shared tributary systems, which may increase cumulative downstream risk during extreme events or cascading failures. ▪ Pope Creek Watershed Cluster: One of the most significant concentrations of dams occurs along tributaries to Pope Creek, where multiple High Hazard dams—including Blanchard-Offner, Duvall, Heitz, and Turkey Ranch—are located within the same drainage system. This clustering, combined with several additional Significant hazard dams (e.g., Dick Week, Davis), suggests the potential for cascading impacts that could affect downstream areas connected to Lake Berryessa and ultimately the Napa River system.
▪ Milliken Creek Watershed Cluster: Another concentration of High Hazard dams is present along Milliken Creek and its tributaries, including Circle S, Foss Valley, Lake Leticia, and Lake William, as well as the larger Milliken Dam operated by the City of Napa. This watershed drains directly into the Napa River near the City of Napa, indicating a direct pathway for downstream flooding impacts affecting urbanized areas and critical infrastructure.
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▪ Napa River Tributary Cluster: Several High Hazard dams are located directly on or adjacent to tributaries of the Napa River, including St. Helena Ler, Veterans Home, and Will Lake #1, as well as Significant hazard facilities such as Naz Lake. These upstream facilities have the potential to contribute to localized flood surges that could propagate downstream through the Napa Valley floor. ▪ Putah Creek / Lake Berryessa System: The Monticello Dam, located on Putah Creek, represents the most regionally significant facility affecting Napa County. Its High Hazard classification reflects the extreme consequences of a failure scenario, although the facility is subject to federal oversight and rigorous safety standards.
▪ Additional Localized Clusters: Other smaller groupings of High Hazard dams occur along Tulucay Creek (Lake Camille, Lake Marie) and Maxwell Creek (Maxville, Metcalf, Usibelli No. 2), where multiple facilities within the same drainage could contribute to compounded downstream effects.
The presence of multiple High Hazard dams within shared tributary systems highlights the importance of considering cumulative and cascading failure scenarios in hazard mitigation planning. While individual dam failures are rare, the spatial clustering of dams—particularly within the Pope Creek and Milliken Creek watersheds—introduces the potential for compound flood waves, reduced warning times, and increased downstream exposure.
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Figure 3-23: Napa County Dam Failure Risk Map
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Past Events
No dams have failed in Napa County to date. As the risk assessment for Napa County illustrates, the chances of a dam failure area low, but the consequences of such are quite severe.
The most recent concerning dam failure in California was the 2017 collapse of a spillway on the Oroville Dam in Butte County, California. Oroville Dam is the largest facility within the State Water Project in California; the dam stores 3.5-million-acre feet of water, and serves as important flood control for the Feather River. In 2017, after substantial runoff from the Sierra Nevadas, Lake Oroville was full and the spillway was opened to release extra water downstream. “Spillways” are dam safety features that allow water to overtop the dam if the reservoir fills too quickly. Spillway overflow events, often referred to as “design failures,” result in increased discharges downstream and increased flooding potential.
The force of the release gouged a large
crater in the concrete spillway and required
the California Department of Water
Resources (DWR) to halt water releases via
that conduit. The high lake level then
created fear that erosion would
compromise the integrity of the auxiliary
spillway and flood the city of Oroville and
surrounding communities. Thousands
were evacuated, and eventually runoff
receded without further issue. Ultimately,
an independent analysis concluded that
poor design and construction and
inadequate state oversight contributed to
the collapse of the concrete spillway.
(Water Education Foundation, 2020)
The Oroville Dam spillway failure triggered inspection of 93 dam spillways across California through the new Spillway Re-evaluation Program, including three in Napa County: Conn Creek, Bell Canyon, Lake Curry, and Rector Creek dams. (Cal. Dep't of Water Resources, 2019) Frequency & Probability of Future Occurrences
Dams are constructed with engineered safety features, including spillways, that are designed to safely convey excess water when reservoir levels exceed storage capacity. Spillway overflow events, sometimes referred to as “design exceedance” events, can result in elevated downstream flows and increased flooding potential. The residual risk associated with dams reflects the possibility that an event exceeds design standards or that structural or operational failures occur despite safeguards. In modern regulatory environments, including oversight by the California DSOD and federal agencies, the overall probability of Figure 3-24. Oroville Dam Spillway after 2017 failure.
Source: https://www.watereducation.org/aquapedia/oroville-dam CAL OES SUBMITTAL - JUNE 2026
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dam failure remains low. Failures are most often associated with extreme or compound events such as earthquakes, landslides, or unusually intense rainfall and runoff.
Currently available information indicates that the overall probability of dam failure in Napa County is low (generally less than 1% annual probability); however, this probability is not uniform across all facilities. The likelihood of failure is influenced by several key factors, including dam condition, age, design, maintenance practices, and hazard potential classification. While hazard classification reflects the consequences of failure rather than failure likelihood, dams classified as High Hazard Potential often receive increased regulatory scrutiny due to the severe consequences associated with failure. At the same time, condition assessments (e.g., Fair or Poor) provide important insight into structural integrity and operational reliability.
Dams in Poor condition, particularly those classified as High Hazard, may present a relatively elevated probability of failure compared to well-maintained facilities, especially under stress from extreme hydrologic or seismic events.
As shown in Table 3-23, Napa County contains several dams where high hazard classification coincides with Fair or Poor condition ratings. This combination represents a higher-priority concern for risk management and emergency planning, as these facilities may be more susceptible to performance issues during extreme events. For example, dams such as Lake Curry, Peju Reservoir No. 2, and St. Helena Ler are classified as High Hazard and Poor condition, indicating both significant downstream consequences and potential structural or maintenance deficiencies. Additional dams, including Araujo Reservoir No. 1 and No. 2, Bell Canyon, Conn Creek, Hestan Vineyards Dam, and Veterans Home, are classified as High Hazard with Fair condition, suggesting that while they are currently functional, continued monitoring, maintenance, and potential upgrades are necessary to ensure long-term reliability.
Although the dam failure risk assessment for Napa County illustrates that the chances of a failure are low, the consequences are quite severe. Given the monitoring protocols at all the dams within and adjacent to the county with significant or high-hazard potential, the probability of dam failure in Napa County is unlikely (less than 1% annual probability).
Severity
Even though the probability of dam failure in Napa County is low, the potential consequences are severe to catastrophic, particularly for areas located downstream of large-capacity reservoirs and dams situated near populated areas. The severity of a dam failure event is strongly influenced by the volume of stored water, proximity to developed areas, and downstream channel characteristics, all of which affect the magnitude and speed of inundation.
Within Napa County, the most significant risk is associated with larger impoundments located upstream of populated areas, including facilities such as Monticello Dam (Lake Berryessa), Conn Creek Dam (Lake Hennessey), and Lake Curry Dam. These dams store substantial water volumes and are hydraulically connected to downstream waterways that flow through or toward developed areas, including the Napa Valley floor and incorporated jurisdictions such as the Cities of Napa, St. Helena, Yountville, and Calistoga.
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A failure at one of these facilities could result in rapid-onset flooding, limited warning time, and widespread impacts to life safety, property, and critical infrastructure.
Warning Time
Warning time for dam failure depends on the cause of failure. In an event of extreme precipitation, such as during an atmospheric river event, evacuations can be planned with sufficient time, but in the event of a structural failure due to earthquake, there may be no warning time. A dam’s structural type also affects warning time.
Earthen dams do not tend to fail instantaneously. Once a breach is initiated, discharging water erodes the breach until the reservoir water is depleted, or the breach resists further erosion. Concrete gravity dams also tend to have an initial partial breach. The time of breach formation ranges from a few minutes to a few hours. Several participating jurisdictions have established protocols in their emergency operations plans for warning and response to imminent dam failure. These protocols are tied to emergency action plans (EPA) created by the dam owner.
EAPs identify areas downstream from dams requiring warning and evacuation in the event of a failure.
Documented cases have demonstrated that warning and evacuation time for EAPs can dramatically influence the loss of life, varying from 0.02% of the threatened population when the warning time is 90 minutes or more, to 50% when there is less than 15 minutes of warning. (Graham, 1988) Dam failure inundation mapping studies usually assume one of two scenarios:
▪ Flows from a dam failure during “fair weather” or “sunny day” conditions with the reservoir at the normal pool level and receiving normal inflow (usually insignificant). A fair weather failure is generally considered to have the most potential for loss of human life, primarily due to the element of surprise.
▪ Flows from a dam failure during flood conditions or the inflow design flood. Failure during flood conditions is considered to show the upper limit of inundation and to have less potential for loss of human life because the operators and downstream population are on alert. However, the flood conditions scenario is used less frequently because of the expense in additional watershed and spillway analysis.
Inundation mapping shows a continuous line of inundation identifying the area potentially at risk. It starts at the dam and continues downstream to a point where the breach flood no longer poses a risk to life or property, such as a large river or reservoir with the capacity of storing the flood waters. The domino effect of a dam failure causing failures of any downstream dams is studied on a case-by-case basis.
EAPs are not publicly available but are on file with local emergency management agencies. Information provided on flooding conditions at downstream locations includes: distance downstream; arrival time of the leading edge of the breach flood wave; peak flow depth, incremental rise, or water surface elevation (as appropriate); and peak velocity.
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Dam Failure Vulnerability Assessment
The primary danger associated with dam failure is high-velocity flooding downstream of the dam and limited warning times for evacuation. Vulnerability varies by community and depends on the particular dam and stream profiles, and the nature and extent of the failure. A summary of dam failure risk exposure in Napa County to people, property, and critical facilities and infrastructure is provided in Figure 3-25.
3.3.3.8.1 Population
The potential for loss of life resulting from a dam failure in Napa County is influenced by the volume of water impounded, proximity of downstream populations, warning time, and the availability and capacity of evacuation routes. While the probability of dam failure remains low, all populations located within mapped dam inundation areas are considered exposed to risk, particularly where large-capacity dams are hydraulically connected to developed areas.
As illustrated in Figure 3-25 and summarized in Table 3-24, a notable portion of the County’s population is located within dam inundation zones, with exposure concentrated along the Napa Valley floor where development is densest.
Population exposure is not evenly distributed across all dams, but is instead concentrated downstream of a limited number of higher-capacity facilities. In particular, Conn Creek Dam represents the most significant population exposure, accounting for the largest share of residents within a single inundation area.
Additional dams with comparatively high population exposure include Rector Creek, Summit Reservoir, Lake Marie, Milliken Dam, and Bell Canyon Dam, all of which are located upstream of or adjacent to developed portions of the Napa Valley corridor. These dams are hydraulically connected to areas with higher population density, increasing the potential life safety risk in the event of a failure.
In contrast, many smaller dams—particularly those located in rural or upland areas—have relatively limited population exposure, often affecting only small numbers of residents. However, even these facilities may pose localized risks, particularly where evacuation routes are constrained or warning times are limited.
Disproportionately Impacted Populations
Certain populations within inundation zones may be disproportionately vulnerable during a dam failure event. These include children, elderly individuals, and persons with access and functional needs, particularly those who may require assistance to evacuate. In addition, individuals with limited English proficiency, limited access to transportation, or reduced access to emergency alert systems may face increased challenges in receiving warnings and responding effectively. Schools, care facilities, and other community-serving institutions located within inundation areas require coordinated emergency planning, communication systems, and evacuation procedures to ensure life safety.
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Figure 3-25: Exposure Summary – Napa County Dam Failure Risk CAL OES SUBMITTAL - JUNE 2026
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Table 3-24: Napa County Population Exposure to Dam Failure Dam Inundation Zone Population Count % of Total
Angwin 57 0.04%
Araujo Reservoir 1 2 0.00%
Araujo Reservoir 2 2 0.00%
B J Robinson 304 0.22%
Bell Canyon 1,848 1.35%
Blanchard 4 0.00%
Burns 1 0.00%
Circle S 10 0.01%
Conn Creek 15,136 11.02%
Deer Lake 36 0.03%
Dick Week 1 0.00%
Duvall 3 0.00%
Eaton H Magoon Lake 1 0.00%
Foss Valley 15 0.01%
Guenoc Lake 1 0.00%
Heitz 6 0.00%
Henne 47 0.03%
Homestake Tailings 4 0.00%
Hudson Vineyards 1 0.00%
Jamieson Vineyards 19 0.01%
Kimball Creek 818 0.60%
Lake Camille 910 0.66%
Lake Curry 3 0.00%
Lake Cynthia 80 0.06%
Lake La Verne 1 0.00%
Lake Marie 2,262 1.65%
Linda Vista 1 0.00%
Milliken 2,012 1.46%
Naz Lake 7 0.01%
Orville 12 0.01%
Rector Creek 2,546 1.85%
Scotts Canyon 2 0.00%
St Helena Lower 404 0.29%
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Dam Inundation Zone Population Count % of Total
Summit Reservoir 2,277 1.66%
Turkey Ranch 3 0.00%
Usibelli No 2 1 0.00%
Veeder Hills Dam 60 0.04%
Veterans Home 68 0.05%
Willow Lake 1 39 0.03%
Wine Lake 139 0.10%
Total* 2,471 11.00%
*Total population is not equal to sum of all dam inundation zones due to dissolved overlapping inundation areas.
3.3.3.8.2 Property
Vulnerable properties include those located within or immediately adjacent to dam inundation areas, where flood depths and velocities would be greatest. These areas would experience the most destructive impacts from a dam failure, including structural damage, foundation failure, and loss of access. Properties located in low-lying areas along stream corridors and the Napa River floodplain are particularly susceptible, as these areas tend to concentrate floodwaters during an inundation event.
As summarized in Table 3-25, approximately 1,006 parcels (7.4% of all unincorporated parcels) are located within dam inundation areas. These parcels represent a combined exposure value of approximately $1.16 billion, including both structural (market value) and content value. This indicates that while a relatively small percentage of parcels are exposed, the total economic value at risk is substantial, reflecting the presence of higher-value residential, agricultural, and estate properties within the Napa Valley.
Similar to population exposure, property risk is highly concentrated downstream of a limited number of dams. The Conn Creek Dam inundation area accounts for the largest share of property exposure, with over $600 million in total value at risk, representing more than half of the total exposure within the unincorporated area. Additional dams with significant property exposure include Milliken Dam (approximately $269 million), Bell Canyon Dam (over $100 million), and Rector Creek (approximately $69 million). These facilities are located upstream of or adjacent to developed portions of the Napa Valley corridor, where property values and development intensity are highest.
In contrast, many smaller dams, particularly those located in rural or upland areas, are associated with minimal property exposure, often affecting only a small number of parcels with comparatively lower total value. However, localized impacts may still be significant for affected landowners, particularly where access routes, agricultural operations, or critical utilities are disrupted.
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It is also important to note that overlapping inundation areas between dams may result in cumulative exposure that is not equal to the sum of individual dam totals. This highlights the importance of considering compound and cascading impacts, particularly in watersheds with multiple upstream facilities.
Table 3-25: Napa County Unincorporated Parcels Exposed to Dam Inundation Zones Total Parcels Total Market Value ($)
Total Content Value
($) Total Value ($)
Unincorporated County 13,668 $11,215,199,644 $9,248,691,039 $20,463,890,683 Dam Inundation Zone Parcel Count % of
Total
Market Value
Exposure ($)
Content Value
Exposure ($)
Total Exposure
($)
% of
Total
Angwin 3 0.0% $9,852,564 $9,852,564 $19,705,127 0.1%
B J Robinson 21 0.2% $9,113,700 $4,699,650 $13,813,350 0.1% Bell Canyon 94 0.7% $64,043,445 $39,879,150 $103,922,595 0.5% Blanchard 2 0.0% $515,911 $261,124 $777,035 0.0%
Conn Creek 438 3.2% $335,128,769 $301,113,263 $636,242,031 3.1% Deer Lake 18 0.1% $5,982,193 $2,991,415 $8,973,608 0.0% Duvall 2 0.0% $515,911 $261,124 $777,035 0.0%
Eaton H Magoon Lake 2 0.0% $1,030,200 $515,100 $1,545,300 0.0% Foss Valley 1 0.0% $1,003,850 $501,925 $1,505,775 0.0%
Heitz 2 0.0% $515,911 $261,124 $777,035 0.0%
Henne 24 0.2% $7,104,339 $3,552,490 $10,656,829 0.1%
Kimball Creek 33 0.2% $19,309,936 $11,036,432 $30,346,368 0.1% Lake Camille 7 0.1% $3,005,176 $1,502,589 $4,507,765 0.0% Lake Curry 23 0.2% $16,723,654 $10,802,391 $27,526,045 0.1% Lake La Verne 2 0.0% $767,125 $383,563 $1,150,688 0.0%
Lake Marie 21 0.2% $7,384,381 $3,692,192 $11,076,573 0.1% Metcalf 2 0.0% $2,367,998 $1,831,648 $4,199,646 0.0%
Milliken 324 2.4% $177,005,495 $91,702,468 $268,707,963 1.3% Orville 4 0.0% $349,663 $175,148 $524,811 0.0%
Rector Creek 66 0.5% $36,729,897 $32,415,192 $69,145,089 0.3% Robert L Matheson 3 0.0% $589,900 $589,900 $1,179,800 0.0% Scotts Canyon 1 0.0% $394,740 $394,740 $789,480 0.0%
Turkey Ranch 3 0.0% $990,211 $735,423 $1,725,634 0.0%
Usibelli No 2 2 0.0% $2,367,998 $1,831,648 $4,199,646 0.0% Veeder Hills Dam 1 0.0% $564,400 $282,200 $846,600 0.0% Wine Lake 33 0.2% $22,436,815 $15,209,690 $37,646,506 0.2% Dam Inundation Area* 1,006 7.4% $662,690,682 $494,743,052 $1,157,433,734 5.7% *Totals are not equal to sum of all dam inundation zones due to dissolved overlapping inundation areas.
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3.3.3.8.3 Critical Facilities & Infrastructure
Critical facilities and infrastructure located within dam inundation areas in Napa County are vulnerable to severe damage or complete loss of function during a dam failure event. Facilities located in low-lying areas along the Napa River corridor and its tributaries are particularly susceptible due to the potential for highvelocity floodwaters and rapid inundation. While the overall number of critical facilities within mapped inundation zones is relatively limited, the presence of essential services and high-consequence assets within these areas presents a significant risk to emergency response and community recovery.
As summarized in Table 3-26, a total of 120 critical facility assets are located within dam inundation areas.
These include essential facilities such as fire stations (2), as well as high potential loss facilities (37), hazardous materials sites (27), and other community-serving assets such as childcare centers, emergency shelters, and a mobile home park. In addition, 54 transportation and lifeline point assets—including bridges, airports, and wastewater infrastructure—are located within inundation zones. Although relatively few essential facilities are directly exposed, the presence of hazardous materials facilities and high potential loss sites introduces the potential for secondary impacts, including hazardous material releases, fire, or environmental contamination during a dam failure event.
Transportation and lifeline infrastructure are particularly vulnerable to dam failure impacts. As shown in Table 3-27, approximately 196 miles of linear infrastructure are located within inundation areas. This includes extensive roadway networks (over 130 miles of local streets and additional mileage across major, secondary, and service roads), as well as levees, railroads, natural gas pipelines, and electrical transmission lines. Damage to this infrastructure could result in significant disruptions to evacuation, emergency response, and regional mobility, particularly in areas where routes are limited by valley topography or where multiple systems converge.
The loss or impairment of critical infrastructure may also lead to cascading impacts, including power outages, communication failures, and reduced access to emergency services. For example, damage to bridges or key roadway segments could isolate communities or delay evacuation efforts, while impacts to utilities such as pipelines and transmission lines could complicate response and recovery operations.
Overall, while the number of exposed critical facilities in Napa County is relatively modest compared to other hazards, the strategic importance and interconnected nature of these assets—particularly within the Napa Valley corridor—elevates their significance in dam failure planning.
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Table 3-26: Napa County Critical Facilities Exposure to Dam Failure Critical Infrastructure - Dam Inundation
Infrastructure Type Total Feature Count
Essential Facilities 2
EOC -
Fire Station 2
Hospital -
Law Enforcement -
High Potential Loss 37
Adult Residential Care -
Child Care Center 1
Church -
Communication Tower -
Dam 6
Emergency Shelter 1
Farm Worker Housing 1
Historic Building -
Library -
Mobile Home Park 1
Res Elder Care Facility 1
Private School -
Public School -
Real Property Asset 26
Transportation and Lifeline 54
Airport 3
Bridge 50
Bus Stop -
HMMWC Asset -
HMMWC Dam -
HMMWC Hydrant -
NG Station -
Power Plant -
Substation -
Wastewater Treatment Facility 1
Water Supply Intake -
Water Tank -
Water Treatment Facility -
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Critical Infrastructure - Dam Inundation
Infrastructure Type Total Feature Count
Hazmat 27
Geotracker Cleanup Site 1
HWTS Active Facility 26
Landfill -
Grand Total 120
*Real Property Assets are digitized insurance rolls for demonstrating value and ownership and may have overlapping points with other categories such as fire stations and law enforcement.
Table 3-27: Napa County Critical Infrastructure Exposure to Dam Failure Critical Infrastructure - Dam Inundation
Infrastructure Type (linear) Total Dam Exposure
Levee 46.4
NG Pipeline 3.4
Railroad 4.0
Transmission Line 8.6
Street OSM 133.6
Highway & Interstate -
Major Road 3.7
Residential Road 20.2
Road - Secondary 12.5
Road -Tertiary 10.9
Service Road 82.9
Unclassified 3.3
Grand Total 196.0
Miles of exposure
Development Trends
Flooding resulting from a dam failure event in Napa County has the potential to exceed mapped Special Flood Hazard Areas (SFHAs) regulated through local floodplain ordinances. As such, the County and participating jurisdictions should consider dam failure inundation zones in addition to FEMA floodplain maps when reviewing development proposals, particularly in areas located downstream of High and Significant Hazard dams. Land use decisions in these areas should account for the potential for rapid-onset, high-velocity flooding that may exceed typical regulatory flood protection standards.
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communities. As a result, future development, particularly infill and redevelopment within urbanized areas, may be exposed to dam failure risk, especially where it occurs downstream of major facilities such as Conn Creek, Milliken, and Rector Creek Dams. The County’s growth management framework generally directs higher-density development toward existing urbanized areas with access to infrastructure; however, several of these areas are also located within or near mapped inundation zones.
Most areas susceptible to dam failure impacts overlap with mapped flood hazard zones along the Napa River and its tributaries. Consequently, existing floodplain management policies—such as requirements for elevation, setbacks, and floodproofing—provide a baseline level of protection for new development.
However, dam failure events can produce flooding conditions that exceed the 1-percent annual chance (100- year) flood standard, which forms the basis for most regulatory requirements. Therefore, reliance solely on floodplain regulations may not fully address the risks associated with dam failure.
Incorporating dam inundation mapping into land use planning and development review processes would enhance risk reduction by ensuring that site design, access, and emergency planning consider worst-case flooding scenarios. This is particularly important for critical facilities, high-occupancy uses, and vulnerable populations located within inundation areas. Additionally, as development occurs downstream of existing dams, hazard classifications may change over time, with Low Hazard dams potentially being reclassified as Significant or High Hazard due to increased downstream consequences.
Overall, integrating dam failure considerations into planning, permitting, and capital improvement decisions represents an important opportunity to reduce long-term risk in Napa County, particularly in areas where flood and dam inundation hazards overlap.
Climate Change Impacts
Dam failure is a low-probability but high-consequence hazard in Napa County, with climate change expected to increase stress on dam systems and downstream environments, potentially influencing both the likelihood and consequences of failure. Projected increases in extreme precipitation, atmospheric river events, and runoff variability are expected to result in higher reservoir levels, more frequent spillway use, and increased hydraulic loading on dam infrastructure. In addition, more intense storm events occurring after prolonged dry periods may increase erosion, sedimentation, and debris loading, placing additional strain on both dams and downstream conveyance systems. These conditions may exacerbate risks associated with aging infrastructure and dams in fair or poor condition, while also increasing the potential for cascading impacts in watersheds with multiple upstream facilities.
Vulnerable Populations & At-Risk Facilities
Several populations in Napa County face elevated risk from dam failure due to their location within inundation zones and limited capacity to respond to rapid-onset flooding. As identified in the dam failure exposure analysis, a notable portion of the County’s population resides within mapped inundation areas, CAL OES SUBMITTAL - JUNE 2026
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particularly along the Napa Valley floor downstream of major facilities such as Conn Creek and Milliken Dams.
Vulnerable populations include children, elderly individuals, and persons with access and functional needs, particularly those who may require assistance evacuating within limited warning timeframes. Schools, childcare facilities, and community-serving institutions located within or near inundation zones are of particular concern due to the need for coordinated evacuation and communication procedures. In addition, individuals with limited English proficiency, limited transportation access, or reduced access to emergency alerts may face additional barriers in receiving warnings and taking protective action.
Hazardous materials facilities and high potential loss sites located within inundation areas present an additional layer of risk. Damage to these facilities during a dam failure event could result in secondary impacts, including hazardous material releases, environmental contamination, and fire hazards, further affecting nearby populations.
Infrastructure & Access Constraints
Dam failure would likely result in significant disruption to transportation and lifeline infrastructure, particularly within the Napa Valley corridor where infrastructure networks are concentrated. Roads, bridges, and key evacuation routes located within inundation zones are vulnerable to rapid flooding, erosion, and structural damage, which may limit evacuation capacity and delay emergency response.
As identified in the risk assessment, a substantial extent of transportation and utility infrastructure, including roads, pipelines, and transmission lines, is located within dam inundation areas. Damage to these systems could result in cascading impacts, including power outages, communication failures, and restricted access to critical services. These impacts are particularly concerning in areas where limited ingress and egress routes already constrain evacuation options, increasing overall life safety risk.
Systemic & Planning Challenges
Dam failure risk in Napa County is influenced by a combination of aging infrastructure, evolving land use patterns, and regulatory limitations. While dam safety is regulated by state and federal agencies, local jurisdictions play a key role in land use planning, emergency preparedness, and downstream risk reduction.
Current floodplain regulations provide a baseline level of protection; however, dam failure events may exceed regulatory flood standards, including the 1-percent annual chance flood. As a result, reliance solely on floodplain management tools may not fully address dam-related risks. In addition, continued development downstream of existing dams may increase hazard classifications over time, elevating both exposure and regulatory requirements.
Public awareness of dam failure risk is also limited compared to more frequent hazards, which may reduce preparedness and response effectiveness. Increased outreach and education are needed to ensure that residents understand inundation risk, evacuation procedures, and available warning systems.
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Climate Change Trends
Climate change is expected to influence dam safety risk through increased hydrologic variability and more extreme weather conditions. More frequent and intense atmospheric river events may lead to higher peak reservoir levels and increased spillway usage, while prolonged drought conditions followed by intense storms may contribute to soil instability and increased sediment transport.
These changing conditions may also increase the likelihood of compound hazard events, such as flooding coinciding with dam stress or infrastructure failure. In watersheds with multiple dams, these dynamics may elevate the potential for cascading or sequential failures, particularly where facilities are located along shared tributaries.
Future Considerations & Mitigation Direction
Addressing dam failure risk under changing conditions will require a coordinated, multi-agency approach that prioritizes vulnerable populations, critical facilities, and high-consequence dams. Key considerations include:
▪ Enhancing early warning systems and Emergency Action Plan (EAP) coordination with dam operators
▪ Improving evacuation planning and route capacity, particularly in high-exposure areas ▪ Incorporating dam inundation mapping into land use and development review processes ▪ Prioritizing inspection, maintenance, and rehabilitation of dams in fair or poor condition ▪ Expanding public outreach and education, particularly for underserved and at-risk populations ▪ Evaluating cascading and compound risk scenarios in watershed-scale planning efforts By aligning mitigation strategies with identified dam failure problem statements, Napa County can reduce life safety risk, minimize economic impacts, and enhance resilience to climate-driven stressors affecting dam systems and downstream communities.
Dam Failure Hazard Problem Statements
As part of the mitigation action identification process, the Napa County Planning Team developed areas of concern (i.e., problem statements) that summarize the county’s most pressing dam failure-related vulnerabilities and challenges. The problem statements were developed based on the updated risk assessment and vulnerability analysis contained herein, as well as the team’s local knowledge and experience and community input. The problem statements address a broad spectrum of concerns, from direct impacts on residents and vulnerable populations to threats to critical infrastructure and long-term recovery.
These problem statements are not intended to be all-encompassing or to describe each and every issue associated with dam failure hazards. Instead, they highlight the most urgent concerns that can be addressed within the scope of this MJHMP. Each statement will be linked to at least one mitigation action, ensuring CAL OES SUBMITTAL - JUNE 2026
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that identified needs are met with specific, measurable, and equitable solutions. Table 3-28 presents the dam failure problem statements for Napa County, along with their associated areas of concern and reference numbers for cross-linking to the mitigation actions in Table 4-4.
Table 3-28: Napa County Dam Failure Hazard Problem Statements Problem No. Hazard Area of
Concern
Primary
Agency
Problem Description Related MA
ps-DF-NCo-25 Dam Failure Threat Napa County Approximately 120 critical facilities and 200 miles of transportation and lifeline
infrastructure are located within dam
inundation zones, including essential facilities,
roads, bridges, water and sewer pipelines, and
hazardous materials sites, creating potential for
secondary impacts such as hazardous material
release and service disruption.
ma-FL-NCo-52,
ma-FL-NCo-55,
ma-FL-NCo-56,
ma-DF-NCo-120
ps-DF-NCo-384 Dam Failure Threat Napa County Several high-capacity dams (e.g., Conn Creek, Milliken, Lake Curry) are located upstream of
developed areas along the Napa Valley floor,
with some located in shared tributary systems,
creating potential for cascading failures and
high-consequence flooding.
ma-FL-NCo-55
ps-DF-NCo-385 Dam Failure Threat Napa County Approximately 15% of the County’s population and over $1 billion in property value are located
within mapped dam inundation zones, with
exposure concentrated downstream of a small
number of dams including Conn Creek and
Milliken, increasing potential for significant life
safety and economic impacts.
ma-AH-NCo-6,
ma-DR-NCo-26,
ma-FL-NCo-55,
ma-FL-NCo-54,
ma-FL-NCo-57
ps-DF-NCo-386 Dam Failure Threat Napa County Many dams in Napa County were constructed prior to modern engineering standards and
some are rated in fair or poor condition, which
may increase the probability of failure under
extreme conditions such as earthquakes or
intense storm events.
ma-FL-NCo-55
ps-DF-NCo-387 Dam Failure Victim Napa County Vulnerable populations, including children, elderly individuals, and persons with access and
functional needs, are located within dam
inundation zones and may face challenges
evacuating due to limited warning time,
mobility constraints, and constrained
evacuation capacity in some areas.
ma-AH-NCo-6,
ma-CC-NCo-15,
ma-FL-NCo-55
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3.3.4 Earthquake Hazard Profile
Earthquake is the sudden shaking of the ground caused by the passage of seismic waves through Earth’s rocks. Seismic waves are produced when some form of energy stored in Earth’s crust is suddenly released, usually when masses of rock straining against one another suddenly fracture and “slip.” Earthquakes associated with this type of energy release are called tectonic earthquakes. The energy also can be released by elastic strain, gravity, chemical reactions, or even the motion of massive bodies. Earthquakes occur most often along geologic faults, narrow zones where rock masses move in relation to one another. The seismic conditions in Napa County include the West Napa and Hunting Creek- Berryessa Fault Zones.
Earthquakes have different properties depending on the type of fault that causes them (Figure 3-26). The usual fault model has a “strike” (the direction from north taken by a horizontal line in the fault plane) and a “dip” (the angle from the horizontal shown by the steepest slope in the fault). The lower wall of an inclined fault is called the footwall. Lying over the footwall is the hanging-wall. When rock masses slip past each other parallel to the strike, the
movement is known as strike-slip
faulting. Movement parallel to the dip is
called dip-slip faulting. In dip-slip faults,
if the hanging-wall block moves
downward relative to the footwall block,
it is called “normal” faulting; the opposite
motion, with the hanging-wall moving
upward relative to the footwall, produces
reverse or thrust faulting.
As a fault rupture progresses along or up
the fault, rock masses are flung in
opposite directions and thus spring back
to a position where there is less strain.
Earthquake Classifications
Earthquakes are typically classified in one of two ways: by the amount of energy released, measured as magnitude; or by the impact on people and structures, the most common method of measuring released energy is magnitude. While the majority of scientists now use either the Moment Magnitude (Mw) Scale or Modified Mercalli Intensity (MMI) Scale to measure an earthquake, the Richter scale is the most well-known; however, the Richter scale has been largely replaced with the more accurate Mw and MMI scales.
Figure 3-26: Earthquake Faulting
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Magnitude
The magnitude of an earthquake is related to the total area of the fault that ruptured, as well as the amount of offset (displacement) across the fault. Table 3-29 summarizes the seven earthquake magnitude classes on the Mw scale, ranging from micro to great. A magnitude class of great can cause tremendous damage to infrastructure, compared to a micro class, which results in minor damage to infrastructure.
Table 3-29: Moment Magnitude (Mw) Scale
Intensity
The effects of an earthquake in a particular location are measured by intensity. Earthquake intensity decreases with increasing distance from the epicenter of the earthquake. The MMI value assigned to a specific site after an earthquake has a more meaningful measure of severity to the nonscientist than the magnitude because intensity refers to the effects experienced at that place.
The lower numbers of the intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers of the scale are based on observed structural damage. Structural engineers usually contribute information for assigning intensity values of VIII or above. Table 3-30 is an abbreviated description of the levels of MMI.
Magnitude Class Magnitude Range
(M = Magnitude) Description
Great M > 8 Tremendous damage
Major 7 <= M < 7.9 Widespread heavy damage
Strong 6 <= M < 6.9 Severe damage
Moderate 5 <= M < 5.9 Considerable damage
Light 4 <= M < 4.9 Moderate damage
Minor 3 <= M < 3.9 Rarely causes damage.
Micro M < 3 Minor damage
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Table 3-30: Modified Mercalli Intensity (MMI) Scale
Intensity Shaking Description/Damage
I Not Felt Not felt except by a very few under especially favorable conditions.
II Weak Felt only by a few persons at rest, especially on upper floors of buildings.
III Weak
Felt quite noticeably by persons indoors, especially on upper floors of buildings.
Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.
IV Light
Felt indoors by many, outdoors by few during the day. At night, some awakened.
Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
V Moderate Felt by nearly everyone; many awakened. Some dishes, windows broken.
Unstable objects overturned. Pendulum clocks may stop.
VI Strong Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
VII Very
Strong
Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.
VIII Severe
Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
IX Violent
Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
X Extreme Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.
Source: USGS, Abridged from The Severity of an Earthquake, USGS General Interest Publication 1989-288-913 Ground Motion
Earthquake hazard assessment is also based on expected ground motion. This involves determining the annual probability that certain ground motion accelerations will be exceeded, then summing the annual probabilities over the time period of interest. The most commonly-mapped ground motion parameters are the horizontal and vertical peak ground accelerations (PGA) for a given soil or rock type. Instruments called accelerographs record levels of ground motion due to earthquakes at stations throughout a region. These readings are recorded by state and federal agencies that monitor and predict seismic activity.
Maps of PGA values form the basis of seismic zone maps that are included in building codes such as the International Building Code (IBC). Building codes that include seismic provisions specify the horizontal force due to lateral acceleration that a building should be able to withstand during an earthquake. PGA values are directly related to these lateral forces that could damage “short period structures” such as singleCAL OES SUBMITTAL - JUNE 2026 Item #6.5.
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family dwellings. Longer-period response components determine the lateral forces that damage larger structures with longer natural periods such as apartment buildings, factories, high-rises, and bridges. Table 3-31 lists damage potential and perceived shaking by PGA factors, compared to the MMI scale.
Table 3-31: Modified Mercalli Intensity (MMI) Scale and Peak Ground Acceleration (PGA) MMI Scale Perceived Shaking
Potential Structure Damage
Resistant Buildings Vulnerable Buildings Estimated PGA (%g) I Not Felt None None <0.17%
II-III Weak None None 0.17% - 1.4%
IV Light None None 1.4% - 3.9%
V Moderate Very Light Light 3.9% - 9.2%
VI Strong Light Moderate 9.2% - 18%
VII Very Strong Moderate Moderate/Heavy 18% - 34%
VIII Severe Moderate/Heavy Heavy 34% - 65%
IX Violent Heavy Very Heavy 65% - 124%
X - XII Extreme Very Heavy Very Heavy >124%
Note: PGA is measured in percent of g, where g is the acceleration of gravity.
Source: USGS, 2008; USGS, 2010
Artificial Induction
Earthquakes are sometimes caused by human activities, including the injection of fluids into deep wells, pumping of ground water, the excavation of mines, and the filling of large reservoirs. In fluid injection, the slip is thought to be induced by premature release of elastic strain, as in the case of tectonic earthquakes, after fault surfaces are lubricated by the liquid.
Secondary & Compounding Hazards
Earthquakes can trigger secondary hazards that compound impacts, including soil liquefaction, tsunamis, dam failures, fires, and slope failure. Many of these secondary hazards relevant to Napa County are discussed in their respective profiles. These include dam failure (Section 3.3.3), wildfire (Section 3.3.1), and slope failure (Section 3.3.5). Although the secondary and compounding hazards discussed in this section are considerations in mitigation planning for Napa County, liquefaction and tsunami were found not to have a high enough probability or impact potential to warrant profiling in this plan.
Urban Fires
Gas leaks, explosions, and fires are common after an earthquake, especially in urban areas. As seen during the 1906 San Francisco Earthquake, the damage from an earthquake is not usually the end of the devastation. The fires that ensued following this event, which resulted from and were fueled by broken gas CAL OES SUBMITTAL - JUNE 2026
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lines and a lack of water pressure, caused additional impacts. According to the Earthquake Country Alliance, around one in four fires after an earthquake is related to a gas leak.
Tsunami
A tsunami is a series of traveling ocean waves of extremely long length, generated by disturbances associated primarily with earthquakes occurring below or near the ocean floor. Subduction zone earthquakes at plate boundaries often cause tsunamis. However, tsunamis can also be generated by submarine landslides, submarine volcanic eruptions, the collapse of volcanic edifices, and, in very rare instances, large meteorite impacts in the ocean.
Within Napa County, tsunami risk is considered minimal and highly localized due to its inland position relative to the Pacific Ocean. The County is situated along the northern portion of San Pablo Bay, which is partially sheltered by surrounding landforms and the broader San Francisco Bay system. This geographic configuration significantly attenuates tsunami wave energy before it can reach inland areas. As a result, tsunami impacts within the planning area are expected to be limited in both extent and severity compared to open-coast jurisdictions.
Available mapping from the California Geological Survey (CGS) identifies only small, low-probability inundation zones confined to the immediate vicinity of the Napa River mouth and adjacent low-lying tidal areas. No substantial wave amplification or deep inland penetration is anticipated under modeled scenarios.
Consequently, tsunami hazards in Napa County are most appropriately characterized as a secondary or compounding hazard, with potential to contribute to localized flooding impacts when coinciding with other events such as high tides, storm surge, or fluvial flooding.
Subhazards
For Napa County, there are specific subhazards of earthquake that are more relevant to the planning area than others. This includes liquefaction and which are profiled as part of this section, but it also includes mention of thunderstorms and hail.
Soil Liquefaction
Liquefaction is a process by which sediments below the water table temporarily lose strength during an earthquake and behave as a viscous liquid rather than a solid. Liquefaction is restricted to certain geologic and hydrologic environments, primarily recently deposited sand and silt in areas with high groundwater levels, where these soils tend to be saturated, low-density, and loose. The process of liquefaction involves seismic waves passing through saturated granular layers, distorting the granular structure, and causing the particles to collapse. This causes the granular layer to behave temporarily as a liquid, resulting in liquefaction.
Saturated or partially-saturated soil substantially loses strength and stiffness in response to an applied stress, such as ground shaking or other sudden changes in stress condition. Loose sand has a tendency to CAL OES SUBMITTAL - JUNE 2026
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compress when a load is applied. Dense sands, by contrast, tend to expand in volume or 'dilate'. If the soil is saturated by water, which often occurs when soil is below the water table or sea level, then water fills the pore spaces between soil grains.
Liquefaction can cause the soil beneath a structure to lose strength, spread laterally, or lurch in the direction of an opening. Lateral spreading is lateral ground movement, with some vertical component, caused by liquefaction. In effect, the soil rides on top of the liquefied layer. Lateral spreading can occur on relatively flat sites with slopes less than 2%, under certain circumstances, and can cause ground cracking and settlement. Lurching is the movement of the ground surface toward an open face when the soil liquefies.
An open face could be a graded slope, stream bank, canal face, gully, or other similar feature.
Severe property damage can be caused by soil liquefaction, including damaging pipes, compromising building foundations, and bucking roads and airport runways. For buildings, liquefaction may result in the loss of foundation-bearing capacity. This loss of strength commonly causes the structure to settle or tip.
Loss of bearing strength can also cause light buildings with basements, buried tanks, and foundation piles to rise buoyantly through the liquefied soil. Soil liquefaction and related phenomena have been responsible for tremendous amounts of damage in historical earthquakes around the world.
Policies, Plans & Regulatory Environment
Federal Regulatory Environment
National Earthquake Hazards Reduction Program (NEHRP)
The National Earthquake Hazards Reduction Program (NEHRP) was established by the U.S. Congress when it passed the Earthquake Hazards Reduction Act of 1977, Public Law 95–124. In establishing the NEHRP, Congress recognized that earthquake-related losses could be reduced through improved design and construction methods and practices; land use controls and redevelopment; prediction techniques and early warning systems; coordinated emergency preparedness plans; and public education and involvement programs. The four basic goals of NEHRP remain unchanged:
1. Develop effective practices and policies for earthquake loss reduction and accelerate their implementation.
2. Improve techniques for reducing earthquake vulnerabilities of facilities and systems.
3. Improve earthquake hazards identification and risk assessment methods and their use.
4. Improve the understanding of earthquakes and their effects.
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State Regulatory Environment
California Alquist‐Priolo Earthquake Fault Zoning Act & Seismic Hazards Mapping Act The Alquist-Priolo Earthquake Fault Zoning Act (Public Resources Code [PRC] Sections 2621 to 2630) was passed in 1972 to provide a statewide mechanism for reducing the hazard of surface fault rupture to structures used for human occupancy. The main purpose of the Act is to prevent the siting of buildings used for human occupancy across the traces of active faults. It should be noted that the Act addresses the potential hazard of surface fault rupture and is not directed toward other earthquake hazards, such as seismically induced ground shaking or landslides.
The law requires the State Geologist to identify regulatory zones (known as Earthquake Fault Zones or Alquist-Priolo Zones) around the surface traces of active faults and to depict these zones on topographic base maps, typically at a scale of one inch to 2,000 feet. Earthquake fault zones vary in width, although they are often 0.75-mile wide. Once published, the maps are distributed to the affected cities, counties, and state agencies for their use in planning and controlling new or renewed construction. With the exception of single-family wood-frame and steel-frame dwellings that are not part of a larger development (i.e., four units or more), local agencies are required to regulate development within the mapped zones. In general, construction of new buildings within 50 feet of an active fault zone is prohibited.
California Building Standards Code
The California Building Code (CBC), adopted by napa County, is another name for the body of regulations known as the California Code of Regulations, Title 24, Part 2, which is a portion of the California Building Standards Code. The CBC incorporates by reference the International Building Code (IBC) requirements with necessary California amendments. Title 24 is assigned to the California Building Standards Commission, which, by law, is responsible for coordinating all building standards.
Compliance with the CBC requires (with very limited exceptions) that structures for human occupancy be designed and constructed to resist the effects of earthquake motions. The Seismic Design Category for a structure is determined in accordance with either CBC Section 1613, Earthquake Loads, or the American Society of Civil Engineers Standard No. 7-05, Minimum Design Loads for Buildings and Other Structures. In brief, based on the engineering properties and soil types of a proposed site, plus location-specific ground acceleration information, a Seismic Design Category is assigned ranging from A to F, of which D, E, and F represents the most severe conditions.
The Code also requires that a geotechnical investigation be prepared for projects on sites categorized C or greater, all new buildings that are 4,000 square feet or larger, and new smaller buildings if they meet certain criteria. A California registered Geotechnical Engineer must determine design categories and prepare geotechnical investigations, as well as determine requirements for geotechnical designs necessary to meet standards for reducing exposure to geological hazards.
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Benchmark Years
Revisions to the Uniform Building Code (UBC) and California Building Code (CBC) mark important milestones in the evolution of seismic design standards. These benchmarks are critical for understanding the vulnerability of Napa County’s building stock, much of which was constructed before modern seismic requirements. (See Section 1.5.3.2 for building stock ages across the county.) Table 3-32 summarizes the major revisions to the Uniform Building Code (UBC), California Building Code (CBC), and International Building Code (IBC) that shaped modern seismic design standards. These benchmarks highlight the evolution from the first voluntary seismic regulations in 1927 to the adoption of comprehensive, strength-based seismic design provisions in the late 20th century, and ongoing refinements through updated seismic hazard maps.
This context is critical for Napa County because much of the existing housing stock was constructed prior to these milestones, leaving many buildings without essential seismic safety features. Structures built before 1978 (prior to Title 24) and especially those constructed before 1997 (prior to the modern overhaul of seismic design provisions) are considered more vulnerable to earthquake damage. Buildings constructed after adoption of the CBC/IBC framework benefit from more rigorous seismic standards; however, ongoing updates underscore the continued need for seismic retrofits and strict code compliance to reduce risk in older structures.
Table 3-32: Seismic Benchmark Years
Year(s) Code / Reference Key Seismic Change
1927 UBC First voluntary seismic regulations introduced following the 1925 Santa Barbara earthquake.
1933 UBC / State Law Field Act (schools) and Riley Act (all buildings) mandated seismic design statewide after the Long Beach earthquake.
1967 & 1976 UBC Requirements added to limit building drift and reduce damage to non-structural components.
1978 CBC Enactment of Title 24, unifying California’s building standards and incorporating seismic provisions.
1988 UBC Establishment of benchmark base shear values, used as reference in later seismic codes.
1997 UBC Final UBC edition; major overhaul with near-source effects, soil amplification factors, redundancy/reliability factors, and strength-based seismic design. 2000 IBC / NEHRP International Building Code (IBC) introduced, incorporating NEHRP provisions; CBC amended from the IBC.
2007 & 2017 IBC / CBC Major updates to seismic hazard maps based on new research, refining design requirements.
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Local Regulatory Environment
Napa County General Plan
The Napa County General Plan includes the following goals and policies to mitigate the effects of earthquake:
▪ Goal SAF-2 (Safety): To the extent reasonable, protect residents and businesses in the unincorporated area from hazards created by earthquakes, landslides, and other geologic hazards. ▪ Policy SAF-8: Consistent with County ordinances, require a geotechnical study for new projects and modifications of existing projects or structures located in or near known geologic hazard areas, and restrict new development atop or astride identified active seismic faults in order to prevent catastrophic damage caused by movement along the fault. Geologic studies shall identify site design (such as setbacks from active faults and avoidance of on-site soil-geologic conditions that could become unstable or fail during a seismic event) and structural measures to prevent injury, death and catastrophic damage to structures and infrastructure improvements (such as pipelines, roadways and water surface impoundments not subject to regulation by the Division of Safety of Dams of the California Department of Water Resources) from seismic events or failure from other natural circumstances.
• Action Item SAF-8.1: The County’s seismic fault maps shall be reviewed regularly to ensure that they reflect the latest information available.
• Action Item SAF-8.2: Updated maps should be made available to the public at County offices, on the County’s Web site, and through other appropriate channels. ▪ Policy SAF-43: Consistent with state and federal requirements, critical facilities should be provided with additional earthquake resistance and damage control to allow such facilities to remain operative after a disaster.
▪ Policy CC-29 (Community Character): Significant historic resources that are damaged by flood, fire, neglect, earthquake, or other natural disaster should be carefully evaluated by a structural engineer with preservation experience before they are determined to be beyond repair and destroyed.
Napa County Code & Seismic Retrofit Ordinances
Communities in the Napa County Operational Area have all adopted Seismic Retrofit ordinances to reinforce historic buildings. During a previous update of the building and fire codes across the county, all jurisdictions in the county adopted the same county-wide regulations to streamline permitting and enforcement. Along with the seismic retrofit ordinances, the Napa County Code includes requirements for telecommunications facilities to be constructed to withstand the forces of the “maximum credible earthquake.” (§ 18.119.080) Past Events
Numerous earthquakes have occurred in and near Napa County over the last 25 years. See Table 3-33 for earthquake events 4.0 magnitude or greater since 2000. Two large-scale earthquakes in Napa County caused CAL OES SUBMITTAL - JUNE 2026
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damage, death, and injuries over the last twenty years. In September 2000, Napa County residents experienced a 4.9 earthquake on the West Napa Fault. Damages were estimated at $30-50 million. The August 24, 2014 South Napa earthquake was the largest in the San Francisco Bay Area since the 1989 Loma Prieta earthquake, registering at 6.0 on the moment magnitude scale and with a maximum Mercalli intensity of VIII (Severe). The epicenter of the earthquake was located northwest of the City of American Canyon on the West Napa Fault. Total damage in the southern Napa Valley and Vallejo areas was in the range of $362 million to $1 billion, with one person killed and 200 injured.
Table 3-33: Earthquakes near Napa County M4.0 or Greater Since 2000 Date Location Magnitude
1/2/2025 4 km NW of Cobb 4.7
6/08/2024 7 km NW of The Geysers 4.5
9/14/2022 3 km N of Santa Rosa 4.4
10/15/2019 1 km SSE of Pleasant Hill 4.5
1/4/2018 2 km from Berkeley 4.4
5/22/2015 10 km ENE of Yountville 4.1
8/24/2014 South Napa 6.0
8/3/2006 Northern California 4.5
5/8/2005 Northern California 4.1
5/25/2003 Northern California 4.1
9/3/2000 4.8 km SSW of Yountville 4.9
Source: USGS and EarthquakeTrack.com
Location
The Alquist-Priolo Act established earthquake fault zones in California. These Alquist-Priolo Earthquake Fault Zones encompass surface traces of active faults that have a potential for future surface fault rupture and are mapped across California. These zones have been established by the State Geologist and indicate an active fault within the zone. The fault may pose a risk to existing or future structures from a surface fault rupture. Figure 3-27 shows the location of fault zones as well as the underlying quaternary faults6 near the County.
According to USGS, the most likely faults that are likely to produce strong ground shaking in Napa County include the Northern Hayward/Rodgers Creek in the west, the Mayacama in the northwest, the Hunting Creek-Berryessa in the north, the Green Valley in the southeast and the West Napa in the south central. The Concord-Green Valley and the West Napa Fault are the only two major faults that pass through Napa County. (Association of Bay Area Governments, 2014) See Figure 3-27 for locations of regional fault lines. 6 Quaternary faults are those active faults that have been recognized at the surface and which have evidence of movement in the past 1.6 million years - the duration of the Quaternary Period.
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Areas susceptible to liquefaction consist of softer, alluvial soils generally located in proximity to waterways like the Napa River (Figure 3-28).
Figure 3-27: Regional Fault Lines
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Figure 3-28: Napa County Liquefaction Risk
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Frequency & Probability of Future Occurrences
According the California State Hazard Mitigation Plan, earthquakes large enough to cause moderate damage to structures—those of M5.5 or larger—occur three to four times a year statewide. Strong earthquakes of M6 to 6.9 strike on an average of once every two to three years, and major earthquakes of M7 to 7.9 occur in California about once every decade. The probability of an earthquake occurring in Napa County is likely (between 10 and 100% annual probability).
30-Year Earthquake Probability (UCERF3)
To clarify the extent of future earthquake risk, a partnership of the USGS, the California Geologic Survey (CGS), and the Southern California Earthquake Center, known as the Working Group on California Earthquake Probabilities, evaluated, and systemized currently available historic and paleo-seismic information to produce a probabilistic seismic hazards analysis to indicate the type of future earthquakes.
The resulting model, the Uniform California Earthquake Rupture Forecast Version 3 (UCERF3), 7is a comprehensive model of earthquake occurrence for California and represents the best available science for estimates of the magnitude, location, and likelihood of potentially damaging earthquakes (M6.7 or greater).
This rupture forecast model tells us where and when the Earth might slip along the state’s many faults.
The results of the UCERF3 for the San Francisco region are shown in Table 3-34. The table indicates the average time between earthquakes and the likelihood of having one or more such earthquakes in the next 30 years (starting from 2014). It is important to note that the repeat times are averages and actual repeat times will certainly vary significantly. The fault systems producing these earthquakes are located throughout the San Francisco Bay Area region including Napa County.
Table 3-34: 30-Year Probability of Earthquakes in the San Francisco Region Source: UCERF3
Probability of earthquake events is based on the approximate location of earthquake faults within and outside the Napa region. The Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3) is a comprehensive model of earthquake occurrence for California. It represents the best available science for 7 Quaternary faults are those active faults that have been recognized at the surface and which have evidence of movement in the past 1.6 million years (the duration of the Quaternary Period).
Magnitude 30-Year Probability Average Repeat Time
5 100% 1.3 Years
6 98% 8.9 Years
6.7 72% 29 Years
7 51% 48 Years
7.5 20% 124 Years
8 4% 825 Years
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authoritative estimates of the magnitude, location, and likelihood of potentially damaging earthquakes in California. According to the USGS’s earthquake probability maps, shown in Figure 3-29, the Hunting CreekBerryessa fault has a 5% chance of producing an earthquake of 6.7 magnitude or greater in the next 30 years, while the Rodgers Creek / Hayward North fault, located just outside county boundaries, has a more than 10% chance.
The combined probability of a major earthquake occurring on one of the major faults passing through Napa County is 63% over the next thirty years. (Association of Bay Area Governments, 2014). Figure 3-30 shows likely shaking intensity in Napa County in any 50-year period from all possible faults. It is an equivalent risk to a 500-year flood event. 10% in 50 years was chosen as it most closely aligns to the levels of shaking the current building code is designed to withstand. The predicted shake intensity map, Figure 3-30, is not intended to be site-specific. Rather, it depicts the general risk within neighborhoods and the relative risk from community to community.
Figure 3-29: Fault Probability Map for Napa County Area CAL OES SUBMITTAL - JUNE 2026
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Severity
Damage from earthquakes varies with the local geologic conditions, the quality of construction, the energy released by the earthquake, the distance from the earthquake’s focus, and the type of faulting that generates the earthquake. Ground motion is the primary cause of damage and injury during earthquakes and can result in surface rupture, liquefaction, landslides, lateral spreading, differential settlement, tsunamis, building failure and broken utility lines, leading to fire and other collateral damage. Typically, areas underlain by thick, water-saturated, unconsolidated material will experience greater shaking motion than areas underlain by firm bedrock, but in some cases, relief may intensify shaking along ridge tops.
Movement along local faults can cause fault-related surface deformation (e.g., surface fault rupture) where the fault reaches the surface of the ground. Within the mapped fault zones in the county, it is likely that movement along these faults will damage structures, roads, utilities, and other fixed facilities. The mapping of these zones has not changed for this plan update.
In addition to these zones, other ground cracking was observed during the 1989 Loma Prieta earthquake and the 1906 San Francisco earthquake. Many of these ground cracks can be attributed to movement or consolidation of large and moderate-sized landslides while other ground cracks were most likely related to ridge spreading.
As demonstrated during past events, even a “moderate” earthquake occurring in or near the Napa County region could result in deaths, casualties, property and environmental damage, and disruption of normal services and activities. The severity of the event could be aggravated by collateral emergencies such as fires, hazardous material spills, utility disruptions, landslides, and transportation emergencies.
As we know from past events, even a “moderate” earthquake occurring in or near the Napa County region could result in deaths, casualties, property and environmental damage, and disruption of normal services and activities. The severity of the event could be aggravated by collateral emergencies such as fires, hazardous material spills, utility disruptions, landslides, transportation emergencies, and the possible failure of the Napa County dams.
For this plan, a representative magnitude 6.7 West Napa Fault earthquake scenario8 was used to show possible shake severity in the region in the earthquake vulnerability analysis, modeled after the South Napa Earthquake in 2014. This scenario was chosen from a range of regional, scenario-based shakemaps available from USGS. The shakemap data consist of peak ground velocity, peak ground acceleration, peak spectral accelerations in an earthquake scenario. See Figure 3-31 for potential shake intensities for the chosen scenario. If a 6.7 magnitude earthquake were to occur again along the West Napa fault, the County would experience moderate to severe shaking, as shown in and as felt in 2014. 8 As described at the beginning of this profile, the severity of an earthquake can be expressed in terms of intensity or magnitude.
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Figure 3-30: Napa County Earthquake Risk (M 7.1 Hunting Creek Berryessa Scenario) CAL OES SUBMITTAL - JUNE 2026
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Figure 3-31: Napa County Earthquake Risk (M6.7 West Napa Scenario) CAL OES SUBMITTAL - JUNE 2026
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Warning Time
There is currently no reliable way to predict the day or month that an earthquake will occur at any given location, but research is being done with warning systems that use the low energy waves that precede major earthquakes. Currently, these systems are able to provide seconds and sometimes minutes of advance warning, which can allow people and systems to take actions to protect life and property from destructive shaking. Even a few seconds of warning can enable protective actions such as:
▪ Public: Individuals, including schoolchildren, drop, cover, and hold on; turn off stoves; and safely stop vehicles.
▪ Businesses: Personnel move to safe locations; automated systems ensure elevator doors open; production lines are shut down; and sensitive equipment is placed in a safe mode. ▪ Medical Services: Surgeons, dentists, and others stop delicate procedures. ▪ Emergency Responders: Open firehouse doors; personnel prepare and prioritize response decisions. ▪ Power Infrastructure: Protect power stations and grid facilities from strong shaking.
ALERT Napa County Notification System
ALERT Napa County is a system that enables the agencies within Napa County to provide the public with critical and non-critical information in a variety of situations. This includes situations such as severe weather, planned and unexpected road closures, missing persons, evacuations, natural disasters, and public health concerns. Many agencies across Napa County, including Emergency Services, the local jurisdictions, and Office of Education use ALERT Napa County to communicate with the public.
Earthquake Vulnerability Assessment
The vulnerability of a community to earthquake is based on a variety of factors, including proximity to active and inactive faults; the age and construction materials of structures; the density of the population and development; and the location of critical facilities. Recent history indicates that Napa County has a very high vulnerability to earthquakes due to proximity to faults and density of population.
One or more moderate to large sized earthquakes (M6.5 to 7.5) will likely shake the entire County of Napa during the life span of most residents. Many older homes that were damaged but not destroyed in 1989 were improved seismically but are still at a higher risk than newer construction using the latest building code standards. Modern homes will normally fare better in earthquakes but 1989 showed that this is not necessarily the case. Some older homes fared better than newer ones due to location and design.
A major earthquake on the West Napa Fault or other major fault in the regional will:
▪ Damage roads, bridges, and critical structures, and could severely damage most homes in the county. ▪ Potentially cause ruptures and ground cracking around the major faults and through nearby hillslopes.
▪ Potentially activate landslides, cause urban fires, or trigger other secondary hazards.
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The following subsections summarize the population, property, and critical infrastructure and facilities exposed to earthquake hazards. Figure 3-32 and Figure 3-33 summarize the potential exposure of the population and these assets to two hypothetical earthquake scenarios: M7.1 Hunting Creek Berryessa and M6.7 West Napa.
Community needs would quickly exceed the response capability of the county's emergency management organization, requiring mutual assistance from volunteer and private agencies, the Governor's Office of Emergency Services, and the Federal Emergency Support Functions.
In any earthquake, the primary consideration is saving lives. Time and effort must also be given to providing for people's mental health by reuniting families, providing shelter to the displaced persons, and restoring basic needs and services. A major effort will be needed to remove debris and clear roadways, demolish unsafe structures, assist in reestablishing public services and utilities, and provide continuing care and temporary housing for affected citizens.
After any earthquake there will be a loss of income both in private and public sectors. Individuals can lose wages due to businesses inability to function because of damaged goods or facilities. Due to business losses, Napa County and the municipalities in the planning area will lose revenue. Economic recovery from even a minor earthquake is critical.
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Figure 3-32: Exposure Summary – Napa County Earthquake Risk (M7.1 Hunting Creek Berryessa Scenario) CAL OES SUBMITTAL - JUNE 2026
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Figure 3-33: Exposure Summary – Napa County Earthquake Risk (M6.7 West Napa Scenario) CAL OES SUBMITTAL - JUNE 2026
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3.3.4.9.1 Population
The population exposed to earthquake in the planning area was estimated for two scenarios: a M6.7 earthquake on the West Napa Fault and a M7.1 earthquake on the Hunting Creek-Berryessa Fault. Figure 3-34 and Table 3-35 summarize population exposure results for the West Napa Fault scenario. Figure 3-35 and Table 3-36 summarize population exposure results for the Hunting Creek Berryessa Fault scenario The entire population of Napa County is potentially exposed to direct and indirect impacts from earthquakes.
Approx. 100% of the population of Napa County is in either Very Strong, Severe, or Violent probabilistic shake intensity zones.
M6.7 earthquake on the West Napa Fault Scenario
Figure 3-34: Napa County Population Exposure to Earthquake (M6.7 West Napa Scenario) Table 3-35: Napa County Population Exposure to Earthquake (M6.7 West Napa Scenario) Total Population
Unincorporated County 22,468
Shake Severity Zone Population Count % of Total
VIII - Severe 7,858 34.97%
VII - Very Strong 6,574 29.26%
VI - Strong 7,052 31.39%
V - Moderate 984 4.38%
Total 22,468 100.00%
7,858
6,574
7,052
984
-
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
VIII - Severe VII - Very Strong VI - Strong V - Moderate CAL OES SUBMITTAL - JUNE 2026
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M7.1 earthquake on the Hunting Creek-Berryessa Fault
Figure 3-35: Napa County Population Exposure to Earthquake (M7.1 Hunting Creek-Berryessa Scenario) Table 3-36: Napa County Population Exposure to Earthquake (M7.1 Hunting Creek-Berryessa Scenario) Total Population
Unincorporated County 22,468
Shake Severity Zone Population Count % of Total
VIII - Severe 615 2.74%
VII - Very Strong 9,470 42.15%
VI - Strong 12,383 55.12%
Total 22,468 100.00%
Disproportionately Impacted Populations
The degree of earthquake population exposure depends on many factors, including the age and construction type dwelling residents are inhabiting, the soil types on which their homes are constructed, and proximity to fault location. Whether directly or indirectly impacted, the entire population of the county will have to deal with the consequences of a large-scale localized earthquake. Business interruption could keep people from working, road closures could isolate populations, and loss of functions of utilities could impact populations that suffered no direct damage from an event itself. 615
70,579
66,190
-
10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000
VI - Strong VII - Very Strong VIII - Severe
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Although earthquakes expose everyone in a specific geographic area to ground shaking, populations that live or work in older buildings that predated modern building code standards are more likely to exposed to loss that populations that live or work in more recently constructed buildings. Older buildings, particularly those of unreinforced masonry construction, are more likely to experience significant structural failings. In the aftermath of the 2014 West Napa earthquake, more than 150 buildings in Napa County were “red tagged” or deemed unsuitable for human habitation. Nearly all of them were older buildings.
In addition to structural and geographic factors, earthquake impacts in Napa County may disproportionately affect certain populations based on social and economic characteristics. As discussed in Section 1.5.3, individuals with access and functional needs, including older adults, persons with disabilities, and those with limited mobility, may face greater challenges during and after an earthquake event, particularly in evacuating damaged structures or accessing emergency services. Similarly, low-income households and renters are more likely to reside in older or more vulnerable housing stock and may have limited financial capacity to retrofit structures or recover following a damaging event. Language barriers, lack of transportation, and limited access to resources may further compound these challenges, particularly in the immediate aftermath when communication systems and lifelines are disrupted.
3.3.4.9.2 Property
Earthquake-related property vulnerability in Napa County is driven by both the intensity of anticipated ground shaking and the structural characteristics of the built environment. Scenario-based analysis indicates that all parcels in the County would be exposed to ground shaking during a major earthquake event, with large portions of the unincorporated area expected to experience strong to very strong shaking, and localized areas experiencing severe shaking under the M6.7 West Napa Fault and M7.1 Hunting Creek– Berryessa Fault scenarios. These levels of shaking are capable of causing widespread structural damage, particularly in buildings that do not meet modern seismic design standards.
To quantify potential property exposure, an inventory of current market values and content value was completed using County Assessor’s parcel data. An inventory of current market values and content value was completed using County Assessor’s parcel data. GIS was used to create centroids, or points, to represent the center of each parcel polygon, assumed to be the location of the structure for analysis purposes. The centroids were then superimposed with the USGS probabilistic shaking severity zones to determine the atrisk structures. Table 3-37 and Table 3-38 reveal the count of at-risk parcels and their associated building and content exposure values to the two earthquake scenarios.
All parcels within Napa County would be exposed to shaking during a damaging earthquake, which equates to approximately $54.3 billion in assessed assets. In areas projected to receive severe shaking during such an event, there is almost $20 billion in total exposure to a M6.7 earthquake on the West Napa Fault and a M7.1 earthquake on the Hunting Creek-Berryessa Fault. This concentration of high-value assets in areas of strong to severe shaking indicates a significant potential for widespread structural damage and economic loss during a major earthquake event.
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Table 3-37: Parcel Exposure to M6.7 West Napa Earthquake Shake Scenarios Total
Parcels Total Market Value ($) Total Content Value ($) Total Value ($) Unincorporated County 13,668 $11,215,199,644 $9,248,691,039 $20,463,890,683 Shake Severity Zone
Improved
Res. Parcel
Count
% of
Total
Market Value Exposure
($)
Content Value Exposure
($) Total Exposure ($) % of Total
VIII - Severe 2,310 16.9% $3,234,845,964 $3,489,055,810 $6,723,901,774 32.9% VII - Very Strong 5,770 42.2% $4,126,072,672 $3,158,327,813 $7,284,400,485 35.6% VI - Strong 5,082 37.2% $3,575,651,143 $2,431,735,902 $6,007,387,045 29.4% V - Moderate 506 3.7% $278,629,864 $169,571,514 $448,201,378 2.2% Total 13,668 100.0% $11,215,199,644 $9,248,691,039 $20,463,890,683 100.0% Table 3-38: Parcel Exposure to M7.1 Hunting Creek-Berryessa Earthquake Shake Scenarios Total
Parcels Total Market Value ($) Total Content Value ($) Total Value ($) Unincorporated County 13,668 $11,215,199,644 $9,248,691,039 $20,463,890,683 Shake Severity Zone
Improved
Res. Parcel
Count
% of
Total
Market Value Exposure
($)
Content Value Exposure
($) Total Exposure ($) % of Total
VIII - Severe 556 4.1% $281,940,469 $153,263,510 $435,203,979 2.1% VII - Very Strong 6,199 45.4% $4,387,335,537 $3,108,030,980 $7,495,366,517 36.6% VI - Strong 6,913 50.6% $6,545,923,638 $5,987,396,549 $12,533,320,187 61.2% V - Moderate 0 0% $0 $0 $0 0%
Total 13,668 100.0% $11,215,199,644 $9,248,691,039 $20,463,890,683 100.0% Building Stock vs. Benchmark Years
While the tables above illustrate the extent of exposure to ground shaking, the severity of damage is strongly influenced by the characteristics of the building stock, particularly construction era and seismic design standards.
According to the Napa County Assessor’s Office, a substantial portion of the unincorporated county’s housing stock was constructed prior to the adoption of modern seismic design standards (Table 3-39). As discussed in Section 1.5.3.2, approximately two-thirds of structures in the unincorporated county were built before 1980, including roughly 24% constructed in 1939 or earlier. These structures predate both the 1978 establishment of California’s Title 24 building standards and the significant seismic design improvements incorporated in the 1997 Uniform Building Code (UBC).
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As a result, a large share of the housing stock may not incorporate modern seismic safety provisions unless retrofitted. Older structures, particularly those constructed prior to 1978, are more likely to experience significant structural damage during strong ground shaking due to limited lateral force resistance, outdated construction practices, and lack of consideration for site-specific hazards such as soil conditions and nearfault effects.
In addition to structural vulnerability, building age is often correlated with broader social vulnerability factors described in Section 1.5.3. Lower-income households and renters are more likely to reside in older or more vulnerable structures and may have limited capacity to retrofit or recover following a damaging earthquake. Similarly, individuals with access and functional needs may face increased risks in structurally compromised buildings. These factors can contribute to disproportionate impacts, even where exposure to ground shaking is widespread.
Table 3-39: Building Stock Ages and Benchmark Years
Construction
Era Seismic Code Context
Approx. Share of
Building Stock Vulnerability Notes
Pre-1950
Built prior to modern seismic
awareness; many structures
predate enforced seismic
provisions
29%
Very high vulnerability — includes
unreinforced masonry and nonductile construction; often associated with older, lower-value housing stock
1950–1979
Limited seismic provisions;
predates Title 24 (1978) or early
implementation period
38%
High vulnerability — improved
construction practices but still lacks
modern lateral design; represents a
large share of exposed housing
1980–1996
Early Title 24 era; incremental
seismic improvements prior to 1997
UBC
20%
Moderate vulnerability — better
performance but lacks modern nearfault and soil amplification considerations
Post-1997
Built under modern UBC (1997) and
later CBC/IBC standards 13%
Lower vulnerability — incorporates
modern seismic design; however,
social vulnerability factors may still
influence impacts and recovery
Source: Napa County Assessor’s Roll; Seismic Code Benchmarks from UBC/CBC/IBC history Newer construction under CBC/IBC standards provides stronger protection against seismic forces. For the large share of older structures that predate modern code requirements, the County’s role will focus on public education, outreach, and incentives to encourage homeowners to pursue voluntary retrofits. By providing information on available programs, technical guidance, and potential funding opportunities, the County can help reduce seismic vulnerability in existing housing stock and reduce potential economic losses associated with future earthquake events, while ensuring new development continues to meet updated code standards.
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3.3.4.9.3 Critical Facilities & Infrastructure
Earthquakes pose numerous risks to critical facilities and infrastructure. Seismic risks, or losses, that are likely to result from exposure to seismic hazards include:
▪ Utility outages.
▪ Economic losses for repair and replacement of critical facilities, roads, buildings, etc. ▪ Indirect economic losses such as income lost during downtime resulting from damage public infrastructure.
▪ Roads or railroads that are blocked or damaged can prevent access throughout the area and can isolate residents and emergency service providers needing to reach vulnerable populations or to make repairs.
Linear utilities and transportation routes are vulnerable to rupture and damage during and after a significant earthquake event. The cascading impact of a single failure can have affects across multiple systems and utility sectors. Degrading infrastructure systems and future large earthquakes with epicenters near critical regional infrastructure could result in system outages that last weeks for the most reliable systems, and multiple months for others.
All critical facilities in Napa County are exposed to the earthquake hazard. Table 3-40 and Table 3-41 list the number of each type of facility in the Violent, Severe, and Very Severe M6.7 West Napa and M7.1 Hunting Creek Berryessa Earthquake Shake Scenarios within the County.
All of the critical facilities and infrastructure in the county would be exposed to strong, very strong, or severe shaking during a major earthquake, such as the M6.7 West Napa Earthquake scenario. Notably, a significant portion of essential, high-potential-loss, and hazardous materials facilities would be subject to severe shaking during such an event, including five fire stations and two dams, potentially driving up casualties and damage due to disruptions to emergency services and the triggering of secondary hazards like dam failures or hazardous material releases. In addition, over 370 miles of critical infrastructure is exposed to shaking, including roadways, transmission lines, sewer lines, and natural gas pipelines. Damage to these systems would not only disrupt mobility and emergency response during and after the event but could also contribute to additional hazards such as downed power lines, urban fires, and environmental contamination.
Utilities
Linear utilities and transportation infrastructure would likely suffer considerable damage in the event of an earthquake. Due to the amount of infrastructure and sensitivity of utility data, linear utilities are difficult to analyze without further investigating individual system components.
Table 3-42 and Table 3-43 provide best available linear utility data for the two earthquake scenarios; it should be assumed that these systems are exposed to breakage and failure.
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Water
Water availability and distribution for supporting life and treating the sick and the injured are of major concern to Napa County. The three reservoirs in Napa are on solid ground and are expected to be usable after a major earthquake, but Lake Hennessey may be inaccessible due to damage to the pipelines that distribute potable water. Napa is connected to the State Water Project at Jameson Canyon and has a tertiary source in Milliken Dam water treatment facility. Any one of these three facilities remaining in operation is able to supply the emergency potable water needs to the City of Napa and its immediately contiguous County areas, if the distribution system can be repaired.
There are three water reservoirs within the City of Napa that have all been recently retrograded and covered, and one reservoir in the City of St Helena. If the reservoirs and water tanks remain intact, they will likely provide ample potable water to meet demands during the time the water treatment stations are being repaired. However, water tanks in some participating jurisdictions have low survivability; potable water will most likely have to be supplied.
Natural Gas
Several common characteristics of earthquakes and their impacts on natural gas safety are:
▪ Earthquake ground shaking will generally lead to substantially more instances of building damage than fire ignitions.
▪ Ground motions that are sufficient enough to damage buildings are the most likely to impact utility and customer gas systems and create a potential for gas-related fire ignitions. ▪ The number of post-earthquake fire ignitions related to natural gas can be expected to be 20% to 50% of the total post-earthquake fire ignitions.
▪ The consequences of post-earthquake fire ignitions for residential gas customers are largely financial. A fire ignition only becomes a life safety concern when inhabitants are unable to exit the building following earthquakes. Experience in past earthquakes indicates that egress from earthquake damaged single-family homes is generally possible because of the limited structure height, low numbers of occupants, and multiple direct escape paths through doors and windows. ▪ The potential life safety dangers from post-earthquake fires are considerably more serious in seismically vulnerable apartment or condominium buildings since they provide a greater chance for damaging the structure and trapping the occupants.
Pacific Gas and Electric Company (PG&E), Napa County’s natural gas utility, is responsible for designing, constructing, maintaining, and operating the natural gas system safely and efficiently. This includes all the facilities used in the delivery of gas to any customer up to and including the point of delivery to the customers’ gas piping system.
Gas customers and Napa County residents are responsible for using gas safely on their property and within their buildings and other facilities. Customers meet this responsibility by maintaining their gas appliances in good working condition, assuring that only qualified individuals are engaged to modify or CAL OES SUBMITTAL - JUNE 2026
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maintain their gas service and facility piping, and knowing what to do before and after earthquakes to maintain the safe operation of their natural gas service.
The following conditions, when combined, pose the greatest risk for post-earthquake fire damage:
▪ Buildings are unoccupied and individuals are not present to mitigate damage to gas systems or control small fires.
▪ High building density or dense, fire-prone vegetation. ▪ High wind and low humidity weather conditions.
▪ Damage to water systems that severely limits firefighting capabilities. ▪ Reduced responsiveness of firefighting resulting from impaired communications, numerous requests for assistance, direct damage to fire stations, restricted access because of traffic congestion and damaged roadways, and delays in mutual aid from neighboring fire districts.
Telecommunication
Telecommunication systems will be affected by system failure, overloads, loss of electrical power and possible failure of some alternate power systems. Immediately following an event, numerous failures will occur, compounded by system use overloads. This will likely disable up to 80% of the telephone system for one day. County UHF/VHF and microwave radio systems are expected to operate at 40% effectiveness the first 12 hours following an earthquake, increase to 50% for the second 12 hours then begin to slowly decline to approximately 40% within 36 hours.
Microwaves systems will likely be 30% or less effective following a major earthquake. Damage to natural gas facilities serving the Napa communities will consist primarily of isolated breaks in major transmission lines. Breaks in mains and individual service connections within the distribution system will be significant, particularly near the fault zones, especially in the cities of American Canyon and Napa. These many leaks pose a fire threat in these susceptible areas of intense ground shaking and/or poor ground near the shoreline. Breaks in the system will affect large portions of the County and restoration of natural gas service could be significantly delayed.
Transportation
Significant damage is expected on the road systems with Napa County following an earthquake. State Highway 12 is expected to be impassable from Cordelia to the Highway 29 Intersection. Interstate 80 could suffer severe surface distortion in the Fairfield and Vacaville areas, as well as damage to its numerous bridges and viaducts in the greater Bay Area. Highway 128 is subject to landslides both up valley toward Geyserville and in the hills around Lake Berryessa. Highway 29 leaving the County to the north is subject to landslides and debris flows to the south where it crosses over old bay mud and fills areas and is subject to soil liquefaction and surface distortion. Any combination of failures of these main highways could isolate the County for up to 72 hours with complete road restoration taking perhaps several weeks. Vehicular traffic will be limited on the foothill roads due to potential and actual landslides. (Napa County Operational Area Hazard Mitigation Plan 2013 Update, 2013)
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Soil liquefaction problems could cause the closure of several roads in American Canyon and areas of other cities built on unconsolidated river soils. (Napa County Operational Area Hazard Mitigation Plan 2013 Update, 2013).
Table 3-40: Critical Facilities Exposure to Earthquake (M6.7 West Napa Scenario) Critical Infrastructure - M6.7 West Napa
Infrastructure Type VIII - Severe VII - Very Strong VI - Strong V - Moderate Essential Facilities 5 9 13 -
EOC 1 - - -
Fire Station 2 9 9 -
Hospital - - 1 -
Law Enforcement 2 - 3 -
High Potential Loss 87 78 118 14
Adult Residential Care - - - -
Child Care Center 2 5 4 -
Church 1 8 8 -
Communication Tower 5 7 4 1
Dam - 10 33 7
Emergency Shelter 1 1 7 -
Farm Worker Housing 2 - 2 -
Historic Building 1 6 4 -
Library - - 1 -
Mobile Home Park - 2 5 -
Res Elder Care Facility - 3 4 -
Private School - - 3 -
Public School - - - -
Real Property Asset 71 36 43 6
Transportation and Lifeline 22 44 54 28
Airport 1 2 4 1
Bridge 16 35 34 11
Bus Stop - - - -
HMMWC Asset - - - -
HMMWC Dam - - - -
HMMWC Hydrant - - - -
NG Station 1 1 - -
Power Plant 2 2 - 3
Substation - 3 1 2
Wastewater Treatment Facility - - 6 6
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Water Supply Intake - - 1 1
Water Tank 2 1 5 3
Water Treatment Facility - - 3 1
Hazmat 66 88 80 7
Geotracker Cleanup Site - 2 1 1
HWTS Active Facility 65 86 77 6
Landfill 1 - 2 -
Grand Total 180 219 265 49
*Real Property Assets are digitized insurance rolls for demonstrating value and ownership and may have overlapping points with other categories such as fire stations and law enforcement.
Table 3-41: Napa County Critical Facilities Exposure to Earthquake (M7.1 Hunting Creek Berryessa Scenario) Critical Infrastructure - M7.1 Hunting Creek Berryessa
Infrastructure Type VIII - Severe VII - Very Strong VI - Strong Essential Facility 3 11 13
EOC - - 1
Fire Station 2 9 9
Hospital - - 1
Law Enforcement 1 2 2
High Potential Loss 19 123 155
Adult Residential Care - - -
Child Care Center - 4 7
Church 2 7 8
Communication Tower - 3 14
Dam 5 28 17
Emergency Shelter 2 5 2
Farm Worker Housing - 2 2
Historic Building - 6 5
Library - 1 -
Mobile Home Park 3 - 4
Res Elder Care Facility - 2 5
Private School - 2 1
Public School - - -
Real Property Asset 7 63 86
Transportation and Lifeline 31 57 60
Airport 3 3 2
Bridge 11 42 43
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Bus Stop - - -
HMMWC Asset - - -
HMMWC Dam - - -
HMMWC Hydrant - - -
NG Station - - 2
Power Plant - 3 4
Substation 1 2 3
Wastewater Treatment Facility 11 1 -
Water Supply Intake 1 1 -
Water Tank 3 3 5
Water Treatment Facility 1 2 1
Hazmat 13 104 124
Geotracker Cleanup Site 1 1 2
HWTS Active Facility 11 103 120
Landfill 1 - 2
Grand Total 66 295 352
Table 3-42: Napa County Infrastructure Exposure to Earthquake (M6.7 West Napa Scenario) Lifelines (miles) - M6.7 West Napa
Infrastructure Type (Linear) V-Moderate VI-Strong VII-Very Strong VIII-Severe Levee - - 6.6 96.3
NG Pipeline - 5.7 18.7 19.8
Railroad - 0.0 8.1 15.6
Transmission Line 14.0 44.5 41.8 6.0
Street OSM 113.6 737.8 566.4 240.6
Highway & Interstate - - 2.7 2.2
Major Road 12.3 7.6 24.7 17.8
Residential Road 17.6 136.3 90.9 30.4
Road - Secondary 3.9 59.9 25.9 6.5
Road -Tertiary 8.8 76.2 42.2 19.3
Service Road 43.5 410.9 367.2 154.6
Unclassified 27.5 46.9 12.8 9.8
Grand Total 127.6 788.1 641.6 378.3
Table 3-43: Napa County Infrastructure Exposure to Earthquake (M7.1 Hunting Creek Berryessa Scenario) Lifelines (miles) - M7.1 Hunting Creek Berryessa
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Infrastructure Type (Linear) VI-Strong VII-Very Strong VIII-Severe Levee 11.0 91.8 -
NG Pipeline 31.1 13.2 -
Railroad 10.6 13.1 -
Transmission Line 72.1 28.3 6.0
Street OSM 826.4 713.4 118.7
Highway & Interstate 4.5 0.4 -
Major Road 46.1 16.3 -
Residential Road 156.3 109.9 8.9
Road - Secondary 40.5 44.3 11.5
Road -Tertiary 59.2 62.0 25.2
Service Road 489.4 424.3 62.6
Unclassified 30.4 56.1 10.5
Grand Total 951.2 859.7 124.7
Development Trends
Land use in the planning area will be directed by general plans adopted under California’s General Planning Law. The safety elements of the general plans establish standards and plans for the protection of the community from hazards. The information in this plan provides the participating partners a tool to ensure that there is no increase in exposure in areas of high seismic risk. Development in the planning area will be regulated through building standards and performance measures so that the degree of risk will be reduced.
The geologic hazard portions of the planning area are heavily regulated under California’s General Planning Law. The International Building Code establishes provisions to address seismic risk.
Napa County has a number of compact urban communities as well as extensive areas of agricultural land and forested hillsides and many rural villages and towns. As dictated by Measure D, most new development has occurred within or adjacent to the urban services line (i.e., the boundary point for such infrastructure as water and sewage service). As with most communities, increased housing costs has resulted in the need to provide higher density housing. In Napa County, all development of this type occurs where urban services are available; other development is mostly in-fill or reuse development, and development of existing rural residential properties.
Growth management policies including the Geologic Hazards Ordinance limit development from occurring where hazards are present and, in most cases, require substantial setbacks from these hazards. Seismic safety standards are a requirement for all building permits, and as infrastructure is repaired or replaced, updated seismic safety standards are incorporated. Development on existing lots of record is required to avoid hazards and incorporate appropriate setbacks and other requirements to mitigate potential impacts CAL OES SUBMITTAL - JUNE 2026
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from earthquake and liquefaction hazards. The Planning, Building, and Environmental Services Department, staffed by resource planners, specializes in reviewing each application for new residential and commercial structures to ensure that new development does not occur in earthquake and liquefaction hazards and that development on existing lots of record avoid, minimize, and mitigate potential impacts from identified earthquake and liquefaction hazards.
No changes in these development regulations or patterns occurred that would affect the county’s overall vulnerability since the previous plan was adopted. As noted above, most new development of residential structures and virtually all new development of commercial structures occurs within the urban services line, and always outside of the highest risk earthquake hazard areas.
Development Impacts
New development within Napa County will increase exposure of residents and workers to earthquake hazards such as strong ground shaking. However, adherence to modern building codes serves to mitigate a substantial aspect of that risk.
Climate Change Impacts
Unlike hazards such as flooding, wildfire, and drought, earthquakes are not directly influenced by climate change. However, climate change can compound earthquake impacts by stressing the same populations and systems that are most vulnerable to seismic risk. Residents living in older, unretrofitted housing, mobile homes, or low-income communities already face disproportionate earthquake risks, and these same groups are often more exposed to climate-driven hazards like extreme heat, wildfire smoke, or flooding. Repeated climate stressors can erode financial stability, limit access to insurance or retrofitting resources, and reduce community capacity to prepare for and recover from seismic events. In this way, climate change intensifies underlying social vulnerabilities, making recovery from earthquakes more difficult for disadvantaged and dependent populations.
Earthquake Hazard Problem Statements
As part of the mitigation action identification process, the Napa County Planning Team developed areas of concern (i.e., problem statements) that summarize the county’s most pressing earthquake-related vulnerabilities and challenges. The problem statements were developed based on the updated risk assessment and vulnerability analysis contained herein, as well as the team’s local knowledge and experience and community input. The problem statements address a broad spectrum of concerns, from direct impacts on residents and vulnerable populations to threats to critical infrastructure and long-term recovery.
These problem statements are not intended to be all-encompassing or to describe each and every issue associated with earthquake. Instead, they highlight the most urgent concerns that can be addressed within the scope of this MJHMP. Each statement will be linked to at least one mitigation action, ensuring that CAL OES SUBMITTAL - JUNE 2026
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identified needs are met with specific, measurable, and equitable solutions. Table 3-44 presents the earthquake hazard problem statements for Napa County, along with their associated areas of concern and reference numbers for cross-linking to the mitigation actions in Table 4-4.
Table 3-44: Napa County Earthquake Hazard Problem Statements Problem No. Hazard Area of
Concern
Primary
Agency
Problem Description Related MA
ps-EQ-NCo-56 Earthquake Threat Napa County Many older public safety buildings, including fire and law enforcement facilities, require
retrofits to improve seismic resilience and
ensure continuity of emergency services
during a major earthquake.
ma-AH-NCo-4,
ma-EQ-NCo-38,
ma-CC-NCo-15,
ma-DR-NCo-26,
ma-EQ-NCo-37
ps-EQ-NCo-57 Earthquake Impact Napa County The 2014 South Napa earthquake highlighted gaps in mapping and contingency planning
and exposed the vulnerability of critical utility
systems to ground failure, the fire hazards
from system damage, and the financial
challenges public agencies face in improving
seismic resilience.
ma-EQ-NCo-38,
ma-EQ-NCo-37
ps-EQ-NCo-58 Earthquake Victim Napa County Home and business owners often lack clear information and understanding about
earthquake insurance and seismic retrofits,
limiting participation in programs that help
improve structure safety and protect private
assets. The South Napa event highlighted
significant gaps insurance coverage and
funding for private retrofits.
ma-EQ-NCo-36,
ma-EQ-NCo-37,
ma-AH-NCo-6,
ma-DR-NCo-26
ps-EQ-NCo-62 Earthquake Impact Napa County Deaths and injuries sustained in the South Napa earthquake point to continuing gaps in
public awareness and education on
earthquake safety and preparedness.
ma-AH-NCo-4,
ma-EQ-NCo-36,
ma-EQ-NCo-37,
ma-AH-NCo-6
ps-EQ-NCo-77 Earthquake Threat Napa County Emergency communications systems, such as on Oat Hill, and fire protection infrastructure
(e.g., water tanks, pumps, and hydrants) may
be damaged or rendered inoperable after
earthquakes, hindering emergency response.
ma-EQ-NCo-38,
ma-AH-NCo-4
ps-EQ-NCo-392 Earthquake Threat Napa County Areas of high and very high liquefaction potential are located primary in the Napa
Valley, along the Napa River, other waterways,
and the coastline, where a significant portion
of the county population resides.
ma-DR-NCo-26,
ma-EQ-NCo-37,
ma-EQ-NCo-38
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3.3.5 Slope Failure Hazard Profile
Slope failure is a general term that describes a wide variety of mass downslope movements of soil and rock in response to gravity, including many types of landslide. Slope failures occur in a variety of ways, as falls, topples, slides, spreads, flows, and a combination of these categories, and may change from one form of failure to another during their movement. Factors that contribute to slope failures include the soil and rock strengths, orientation and steepness of elements on the slope, weathering and erosion, moisture content, recent fire activity, and human activities such as the removal of vegetation and inappropriate grading. Slope failures are a worldwide hazard, but Napa County’s unique geologic conditions make large portions of the planning area particularly susceptible to many forms of slope failure. Factors that contribute to the county’s slope failure susceptibility include:
▪ Local climatology and weather patterns that include wet winters; ▪ Terrain, geology, and weather patterns that cause significant erosion; ▪ High seismicity in the region;
▪ Wildfire susceptibility in the mountainous areas and burn scars on steep terrain; and ▪ Human activities such as vegetation removal and grading in areas of slope instability.
Slope failures can also be caused by land or aging stormwater infrastructure, particularly in the county’s mountain regions. In areas burned by forest and brush fires, a lower threshold of precipitation will initiate failures. The deterioration of old timber harvest roads may also result in concentration of drainage that induces slope failure.
Although they can sometimes present a threat to human life, slope failures often result in a disruption of everyday services, including emergency response capabilities, blocking transportation routes, and impacting water supplies. When slope failure hazards affect transportation routes, which is one of the most common impacts, they are frequently expensive to clean up and can have significant economic consequences to the county.
Secondary & Compounding Hazards
There are some hazards that can trigger or exacerbate slope failure; namely, wildfire, heavy rain, and flooding. These compounding hazards are briefly discussed in this section in relation to slope failure and detailed in their respective profiles, in Sections 3.3.1, 3.3.2, and 3.3.7. Further, these compounding hazards are exacerbated by climate change (Section 3.3.9); consequently, slope failure is also exacerbated by climate change to this extent.
Heavy Rain & Flooding
Heavy rain and associated flood events can lead to landslides, mudslides, debris flows, and other types of slope failure, particularly in steep mountain watersheds where saturated soils lose cohesion and fail, as well CAL OES SUBMITTAL - JUNE 2026
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as stormwater infrastructure exceeding capacity or culverts failing. Flood events can undercut the toe of a slope which can remove the support for the slope and cause a landslide or rockfall.
In Napa County, landslides and other slope failure hazards pose a considerable threat to everyday services, especially emergency response capabilities and transportation facilities. Generally, the highestsusceptibility areas are in the hilly regions bordering the Napa Valley. Landslides are most frequently triggered during periods of high rainfall, typically between November and April. These heavy rainstorms, sometimes associated with an “atmospheric river” event, can deliver over one inch of rain per hour for extended time frames and trigger slope failures as well as flash floods in the county’s steep, mountainous terrain.
Wildfires & Water Quality Degradation
Wildfires can create short-term and long-term impacts on landscapes by altering the soil structure, impeding its ability to absorb moisture, and destroying vegetation that binds the soil with roots and absorbs rainfall and runoff. Post-wildfire, even small rain events can create devastating mudflows, debris flows, and landslides. Areas that are currently mapped as low to moderate risk of slope failure may have elevated risk following a wildfire.
Slope failures, especially post-wildfire, can significantly degrade local water quality by introducing high sediment loads, contaminants such as metals, and excess nutrients into waterways and water bodies, causing turbidity and damage to aquatic ecosystems. (WWSC, 2023) Sedimentation in watersheds also disrupts water supply and treatment infrastructure, increasing repair and maintenance costs.
Earthquakes
Seismic waves from earthquakes can trigger landslides and other slope failures. Landslides caused by earthquakes are usually located on steeper and longer slopes than those triggered by heavy rainfall. (AGI, 2023) Earthquake-induced slope failures generally consist of rock falls and slides, lateral spreads, and slumps. Areas having the potential for earthquake-induced landslides generally occur in areas of previous landslide movement, or where local topographic, geological, geotechnical, and subsurface water conditions indicate a potential for permanent ground displacements.
Types of Slope Failure
In Napa County, there are four primary types of slope failure that pose significant threats to people, property, and critical facilities and infrastructure: landslide, debris flow, rockfall, and bank failure.
Landslide
Landslides are the downward and outward movement of slopes composed of natural rock, soils, artificial fills, or combinations thereof and are classified into many different types based on the nature of the slope materials, type of movement, and speed of the failure. They range from slow-moving rotational slumps and CAL OES SUBMITTAL - JUNE 2026
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earth flows, which can slowly distress structures but are less threatening to personal safety, to fast-moving rockfalls and debris flows that are a serious threat to structures and have been responsible for the most slope failure-related fatalities. Many large landslides are complex and a combination of more than one landslide type. (CalOES, 2018)
Debris Flow
According to NOAA, debris flows—also known as mudslides, mudflows, or debris avalanches—are the most common type of slope failure in Napa County. These fast-moving landslides occur when slope material becomes saturated with water. As such, they typically occur during or after intense rainfall.
Debris flows originate on steep hillsides as liquefied, shallow flows that become concentrated in steep canyons. They can accelerate from approximately 10 miles per hour to over 35 miles per hour and carry a variety of materials such as boulders, trees, and even vehicles. Debris flows vary in consistency from watery mud to thick, rocky mud with entrained debris, and can severely damage road surfaces and structures. As debris flows converge from adjacent channels, they accumulate more materials and muddy water, thus increasing in destructive force. From a geologic perspective, there are generally two types of debris flows. ▪ Shallow Landslide Debris Flows: These debris flows occur due to soil failure from oversaturation, causing the soil to liquify and run downhill. This type of debris flow generally results from a shallow landslide (less than 15 feet deep) and has a discrete initiation zone depositional area. Shallow landslides tend to occur in winter but are most likely after prolonged periods of heavy rainfall when soil materials are saturated.
▪ Post-Wildfire Debris Flows: This type of debris flow is a result of post-wildfire conditions, where burned and baked soil surfaces increase rainfall runoff that concentrates in a channel and picks up debris as it moves. Under extreme wildfire conditions, soils can be damaged to the point of becoming hydrophobic, preventing water absorption or infiltration over large areas where the fire was most intense, such as steep hillsides and drainages with dense vegetation. Post-wildfire debris flows have a less discrete initiation zone but are otherwise similar in form to a debris flow derived from hillslopes.
An example of a catastrophic post-fire debris flow is the event that occurred in Santa Barbara County on January 9, 2018, when, after the Thomas Fire, numerous canyons deposited debris flows onto urbanized alluvial fans in Montecito and Carpinteria with little warning time resulting in significant fatalities. (CalOES, 2018)
Rockfall
Rockfalls are the falling of a newly detached mass of rock from a cliff or rock outcrop or a loose rock that erodes out of unconsolidated debris on a hillside and rolls or falls down a very steep slope. Over-steepened slopes such as at roadcuts or in glaciated terrain are susceptible to rockfall due to the steep slopes that are CAL OES SUBMITTAL - JUNE 2026
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not highly vegetated or benched, which can help attenuate rockfall. Rock outcrops that are highly fractured or undercut by weaker rock layers are also susceptible to rockfall.
Bank Failure
Bank failure refers to the collapse or erosion of a stream or riverbank caused by hydraulic forces, geologic instability, or loss of vegetative cover. Failures can occur gradually through chronic undercutting and slumping, or suddenly during high-flow events when saturated soils give way. This hazard specifically includes stream bank failure, which is of particular concern in Napa County where many homes, businesses, and roads were developed prior to modern setback requirements. Development constructed close to streams, as well as roadways built directly along stream corridors, are especially vulnerable to erosion and failure, with numerous examples distributed throughout the county.
Policies, Plans & Regulatory Environment
State Regulatory Environment
California Building Standards Code
The California Building Code (CBC), adopted by Napa County, is another name for the body of regulations known as the California Code of Regulations, Title 24, Part 2, which is a portion of the California Building Standards Code. The CBC incorporates by reference the International Building Code (IBC) requirements with necessary California amendments. The CBC includes requirements for materials, construction methods, and maintenance standards for slope failure protection and resiliency, especially in relation to adequate drainage.
Local Regulatory Environment
Napa County General Plan
The 2023 Napa County General Plan Safety Element includes the following goals and policies to mitigate slope failure hazards. Goal SAF-7 (Safety) states: To the extent reasonable, protect residents and businesses in the unincorporated area from hazards created by earthquakes, landslides, and other geologic hazards. ▪ Policy SAF-7.1: Require a geotechnical study for new projects and modifications along known hazard areas.
▪ Policy SAF-7.2: Plant native vegetation on unstable slopes to minimize erosion and landslide potential.
▪ Policy SAF-7.3: Prohibit extensive grading where geological hazards are present. ▪ Policy SAF-7.4: Require new hillside parcels be large enough for site flexibility. ▪ Policy SAF-7.5: Prohibit road dedication where geological hazards would require excessive county maintenance.
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▪ Policy SAF-7.7: Conduct regular review of slope failure maps. ▪ Policy SAF-7.8: Implement slope stabilization projects. ▪ Policy SAF-7.9: Require private critical facilities to evaluate and address geological hazard resilience. ▪ Policy SAF-7.10: Conduct seismic retrofitting for County-owned critical facilities.
Napa County Code & Hazard Evaluation Requirements
Napa County Code includes regulations prohibits construction, improvement, grading, earthmoving activity, or vegetation removal associated with the development or use of land in areas characterized by steep slopes, high erosion potential, unstable soils, combustible vegetation, or other sensitive environmental resource areas. County Code Chapter 18.108, Conservation Regulations, states that no extensive grading shall be permitted on slopes over 15% where geologic hazards are present unless the hazards are eliminated or effectively mitigated.
For development projects in slope failure-susceptible areas, the County requires the preparation of a landslide hazard evaluation for review and approval by the County. The evaluation must be prepared by a California Registered Geologist or Certified Engineering Geologist and include a filed survey as well as a report. The hazard evaluation is conducted to determine the effect of slope failures on proposed development and, more importantly, the effect of the proposed project on slope stability and the threat to both existing and proposed improvements. In addition, such evaluations provide information that helps determine how the project will affect sediment loads in from the property into nearby drainage ways.
Past Events
Napa County’s slope failure hazard is amplified by its winter storm and rainfall patterns, especially between November and April, when heavy precipitation increases the likelihood of slope failure. Historical data reveals a correlation between heavy rain events and debris flows in the Napa Valley foothills and surrounding mountainous areas mostly during the winter months (Table 3-45).
There have been nine federal disaster declarations that included some kind of slope failure hazard event; however, all of these major county-wide incidents occurred in conjunction with severe winter storms and flooding. Recent climate change projections suggest these events may intensify in both frequency and magnitude over the coming decades. See the Severe Weather and Climate Change Hazard Profiles (Sections
3.3.7 and 3.3.9) for more information on heavy rain events and the compounding effects of climate change.
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Table 3-45: Slope Failure and Storm Records for Napa County Since 2015 Date Event Type Cause Location Event ID
1/6/2016 Debris Flow Heavy Rain Woodleaf 608298
10/28/2016 Debris Flow Heavy Rain Rutherford 663605
1/3/2017 Debris Flow Heavy Rain Imola 671865
1/18/2017 Debris Flow Heavy Rain Lokoya 675443
1/22/2017 Debris Flow Heavy Rain St Helena 675772
2/7/2017 Debris Flow Heavy Rain Calistoga 681160
2/13/2017 Mudslides Severe Winter Storms Napa County DR-4301-CA 2/20/2017 Debris Flow Heavy Rain Salvador 682383
3/15/2017 Mudslides Severe Winter Storms Napa County DR-4305-CA 3/31/2017 Mudslides Severe Winter Storms Napa County DR-4308-CA 1/8/2018 Debris Flow Heavy Rain Lokoya 730501
11/29/2018 Debris Flow Heavy Rain Woodleaf 788893
1/6/2019 Debris Flow Heavy Rain Salvador 796259
1/6/2019 Debris Flow Heavy Rain Salvador 796262
2/13/2019 Debris Flow Heavy Rain Angwin 800877
2/13/2019 Debris Flow Heavy Rain Salvador 800990
2/14/2019 Debris Flow Heavy Rain Salvador 801988
2/14/2019 Debris Flow Heavy Rain Yountville 801991
2/26/2019 Debris Flow Heavy Rain Krug 802703
2/26/2019 Debris Flow Heavy Rain Krug 802712
2/27/2019 Debris Flow Heavy Rain Salvador 802832
4/30/2019 Landslides, Mudslides Severe Winter Storms Napa County DR-4431-CA 5/16/2019 Landslides, Mudslides Severe Winter Storms Napa County DR-4434-CA 1/26/2021 Debris Flow Heavy Rain Yountville 932426
1/8/2023 Mudslides Severe Winter Storms Napa County EM-3591-CA 1/13/2023 Landslides, Mudslides Severe Winter Storms Napa County DR-4683-CA 3/9/2023 Landslides, Mudslides Severe Winter Storms Napa County EM-3592-CA 4/2/2023 Landslides, Mudslides Severe Winter Storms Napa County DR-4699-CA Source: NWS Storm Events Database, FEMA Disaster Declarations Database CAL OES SUBMITTAL - JUNE 2026
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Location & Extent
The best available predictor of where movement of slides and earth flows might occur is the location of past movements. Past slope failures can be recognized by their distinctive topographic shapes, which can remain in place for thousands of years. Most landslides recognizable in this fashion range from a few acres to several square miles, show no evidence of recent movement, and are not currently active. A small proportion of them may become active in any given year, with movements concentrated within all or part of the landslide masses or around their edges.
Recognizing ancient dormant mass movement sites is important to identify current areas susceptible to flows and slides, because they can be reactivated by earthquakes or exceptionally wet weather. Those ancient scars also consist of broken materials, frequently involve disruption of groundwater flow, and are vulnerable to construction-triggered sliding.
Figure 3-36 shows low, moderate, and high landslide susceptibility in Napa County from the California Geological Survey (CGS), and Table 3-46 describes these classifications. This map should be used with caution, however. Some areas labeled as low or moderate risk may still be highly unstable due to subsurface conditions or poor drainage, while some areas mapped as high risk may be stabilized through engineering or natural conditions. As a result, site-specific geotechnical investigations are often required prior to development, grading, or mitigation projects to accurately determine slope stability.
Slope failures in Napa County are located primarily in the foothills and steeper terrain of the two mountain ranges that border the Napa Valley: the Vaca Range to the east and Mayacamas to the west. There is a long history of geologic activity in the region, from the formation of these mountain ranges to the megaslides off the Vaca Mountains that formed the hills and knolls seen in the valley floor today. The valley's topography changes with its length, from the windswept estuarine flats and gentle hills in the south to the valley's narrow tip at the town of Calistoga, cradled between the sheer walls of the Palisades at the foot of Mount St.
Helena to the east and the forested Mayacamas Mountains to the west.
The locations and types of slope failures that occur in Napa can be summarized as: rivers and streams throughout the county experience shallow bank failures, rotational landslides, and lateral spreading; or steep slopes and drainages experience large, deep composite landslides and debris flows.
Table 3-46: Slope Failure Susceptibility Classifications Susceptibility Native Class Description
Low 1-5
These classes express the generalization that on very low slopes, landslide susceptibility is low even in weak materials, and that landslide susceptibility increases with slope and in weaker rocks.
Medium 6-7
Very high landslide susceptibility, classes VIII, IX, and X, includes moderate and steep slopes in hard rocks and weak rocks.
High 8-10 Very high landslide susceptibility, classes VIII, IX, and X, includes very steep slopes in hard rocks and moderate to very steep slopes in weak rocks.
Source: CGS Susceptibility to Deep-Seated Landslides in California CAL OES SUBMITTAL - JUNE 2026
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Figure 3-36: Napa County Landslide Susceptibility
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Frequency & Probability of Future Occurrences
Measuring the frequency and probability of slope failure hazards requires a combination of historical data analysis, geologic mapping, climate trends, and probabilistic modeling, as these hazards are influenced by factors such as precipitation, seismic activity, topography, and human development. To assess hazard frequency, researchers examine past events recorded in geological surveys, historical documents, and remote sensing data. For example, slope failure occurrences in Napa County are often linked to intense rainfall, and analyzing past precipitation patterns and climate change models helps estimate future probabilities. Probabilistic models, such as landslide susceptibility mapping, incorporate variables like slope gradient, soil composition, and vegetation cover to predict the likelihood of future events.
Slope failures are most frequently triggered in periods of high rainfall during the winter months, November to April, in Napa County. The hazard is greatest in steeply sloped areas, although slides may occur on slopes of 15% or less if the conditions are right. Slope steepness and underlying soils are the most important factors affecting the landslide hazard. However, surface and subsurface drainage patterns also affect the hazard, and vegetation removal can increase the likelihood of slope failure. Human activities contribute to instability through grading of steep slopes, overloading with artificial fill, extensive irrigation, construction of impermeable surfaces, and excessive groundwater withdrawal. (USGS, 2020) As discussed in Section 3.3.5.1, other natural hazards such as earthquakes, heavy rain, flood, or wildfire, can also trigger slope failures or compound their impacts. As such, slope failure frequency is often related to the frequency of these other hazards. Seismic hazards, including fault movements and ground shaking, are assessed using data from the USGS and other monitoring networks. The probability of debris flows is often measured through field assessments and computer simulations that consider factors such as freeze-thaw cycles, extreme weather events, and wildfire burn scars, like the LNU Lightening Complex Fire, which has increased erosion and debris flow risks in affected areas.
Slope failures will continue to affect the county, especially during El Niño weather patterns and atmospheric river events. El Niño weather patterns will continue approximately every seven years, and the San Andreas Fault, as well as other faults, will generate earthquakes, which will contribute to the formation of landslides.
The probability of slope failure occurring in Napa County is highly likely (100% annual probability).
Failure is more likely in areas that have recently experienced flooding or wildfires, but it can occur on any susceptible terrain given the right conditions—such as “ideal” underlying geology, steeper slopes, and saturation. Areas with unstable conditions, like over-steepened slopes, may regularly experience debris flows during heavy rainfall. Additionally, decadal variance and climate change driving increase in frequency and intensity of heavy rain events as well as changes in vegetation composition and wildfire, which could impact slope stability in unpredictable ways.
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Severity
Slope failures, including landslides, debris flows, and bluff failures, pose significant hazards in Napa County, often disrupting essential services and emergency response capabilities. The severity of slope failure hazards depends on local geologic and soil conditions. Hazards are greatest in steeply sloped areas, although slides may occur on slopes of 15% or less if the conditions are right. Slope steepness and underlying soils are the most important factors affecting landslide hazards. However, surface and subsurface drainage patterns also affect landslide hazards, and vegetation removal can increase the likelihood of a landslide.
The primary impacts from slope movement are on roads and any underlying or aboveground utilities including water and sewer infrastructure. While landslides occasionally threaten human life, their more common impact lies in blocking transportation routes, damaging buildings and infrastructure, damming creeks, and contaminating water supplies. The cleanup of these hazards on transportation routes can be costly and have substantial economic repercussions for the county.
A significant amount of linear infrastructure (or lifelines) are exposed to slope failure, as further discussed in the vulnerability assessment section. Impacts to these lifelines can include:
▪ Roads: Access to major roads is crucial to life-safety, response, and recovery operations after a disaster event. Landslides can block egress and ingress on roads, causing isolation for neighborhoods, traffic problems, and delays for public and private transportation. This can also result in economic losses for businesses.
▪ Bridges: Landslides can significantly impact bridges, by knocking out bridge abutments or weakening the soil supporting them. Debris flows can also clog drainages at choke points such as bridges with downed trees, boulders, and other materials picked up along their path, triggering additional flooding impacts.
▪ Power Lines: Power lines are generally elevated above steep slopes, but the towers and poles supporting them can be subject to landslides. A landslide could trigger failure of the soil underneath support structures, causing them to collapse and rip down the lines. Power and communication failures due to landslides can create problems for first responders, vulnerable populations, and businesses.
▪ Water and Sewer Pipes: The underground pipes delivering water or collecting raw sewage can be severely damaged if disturbed by a slope failure, leading to loss of service and contamination of the surrounding environment and water supply. These failures can impact agricultural users, businesses, residents, and tourism.
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Warning Time
Some slope failure hazards occur slowly, including creeping slides and bank erosion, but can have significant property or health consequences. Identifying these hazards generally requires site-specific analysis to determine if the site's soil and geology are susceptible to failure and to identify the most relevant and prudent mitigation measures. For these types of slope failure hazards, warning time is long.
However, for other types of slope failure, such as sudden and fast-moving debris flows, warning time is often very short. Identifying areas with known hazards (i.e., where past events have occurred) or with characteristics conducive to these hazards can help with preparedness when triggering events like prolonged or intense rainfall occur. Post-fire debris flow mapping is one of the primary ways in which Napa County has worked to identify potential hazard areas for proactive mitigation and evacuation ahead compounding events like atmospheric rivers. While this identification does not increase the warning time for an actual failure event, it does enhance proactive responses to potential triggers, save lives, and help prevent damage. (Manconi, 2016)
Slope Failure Vulnerability Assessment
Past experience has shown that many areas of the county are susceptible to the effects of slope failures.
Most of the damage caused by landslides has been to privately-owned structures; although, a fair number of county roads are also at risk. Napa County’s underlying seismic activity and diverse topography, which includes hilly and mountainous terrain, makes certain communities and infrastructure particularly vulnerable to slope failure hazards. The exposure of both population and assets to these hazards depends on factors such as development patterns, land use, and the presence of critical facilities and infrastructure in high-risk areas.
Figure 3-37 summarizes exposure to landslide susceptibility for population, property, and critical facilities and infrastructure in Napa County.
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Figure 3-37: Exposure Summary – Napa County Slope Failure Risk CAL OES SUBMITTAL - JUNE 2026
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3.3.5.9.1 Population
Even though Napa County’s population is primarily located in the valley, where slope failure risks are lower, just under 12,000 people, or about 53% of the unincorporated county population, are exposed to some level of slope failure susceptibility area as identified by CGS (Figure 3-38 and Table 3-47). Over one-quarter of the population is exposed to high risk.
Exposed populations are located in the foothills and steeper terrain of the Vaca and Mayacamas Mountain Ranges to the east and west of the valley.
Figure 3-38: Napa County Population Exposure to Slope Failure Table 3-47: Napa County Population Exposure to Slope Failure Total Population
Unincorporated County 22,468
Landslide Susceptibility Population Count % of Total
High 6,178 27.49%
Moderate 2,382 10.60%
Low 3,358 14.95%
Total 11,918 53.04%
6,178
2,382
3,358
0 1,000
2,000
3,000
4,000
5,000
6,000
7,000
High Moderate Low
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Disproportionately Impacted Populations
Certain populations in Napa County may be disproportionately impacted by slope failure hazards, particularly in areas where landslide susceptibility intersects with socially vulnerable communities. Slope failure risk is generally concentrated in the County’s hillside and mountainous areas, including the eastern and northern portions of the planning area such as the wildland-urban interface near Lake Berryessa, Angwin, Pope Valley, and other steep terrain locations. These areas often have limited access routes, increasing the potential for isolation when landslides block roadways or disrupt critical infrastructure.
As described in Section 1.5.3, individuals with access and functional needs, including older adults, persons with disabilities, and those with limited mobility, may face increased challenges during slope failure events, particularly where evacuation routes are constrained or emergency response access is limited. Residents in hillside areas may also experience disruptions to essential services such as power, water, and transportation, which can have more severe consequences for medically dependent individuals or those requiring regular access to care.
In addition, lower-income households and renters may be more vulnerable to slope failure impacts due to limited financial capacity to implement mitigation measures such as drainage improvements, slope stabilization, or vegetation management. Rural and hillside properties may also require ongoing maintenance to reduce landslide risk, which can present financial and logistical challenges for some property owners.
Transportation limitations further compound vulnerability in these areas. Many hillside communities rely on a limited number of access roads, and landslides have the potential to isolate residents from emergency services, employment centers, and essential resources. This risk is particularly significant for individuals without reliable transportation or those who may require additional assistance to evacuate.
These combined physical and social factors indicate that while slope failure hazards are geographically concentrated, their impacts may be disproportionately experienced by populations with reduced capacity to prepare for, respond to, and recover from hazard events. As such, targeted mitigation strategies, outreach, and emergency planning efforts are necessary to support vulnerable populations in high-risk areas.
3.3.5.9.2 Property
Property vulnerability to slope failure in Napa County is primarily influenced by topography, soil conditions, and proximity to steep slopes, waterways, and areas of recent wildfire activity. Landslides and debris flows are the primary slope failure hazards and can result in significant structural damage to buildings and infrastructure, particularly in areas with unstable soils, high erosion potential, or wildfire burn scars.
Slope failure hazards are most prevalent in the County’s hillside and mountainous regions, where steep terrain and variable geologic conditions increase susceptibility to ground movement. Buildings located on or below steep slopes are at elevated risk of damage due to slope instability. Landslides can undermine foundations, damage or collapse retaining walls, and shift structures downslope, resulting in costly repairs CAL OES SUBMITTAL - JUNE 2026
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or complete structural loss. Debris flows—especially in post-wildfire conditions—pose an additional threat by rapidly mobilizing soil, rock, and vegetation, which can bury structures, damage roadways, and disrupt critical infrastructure.
To quantify property exposure, parcel-level data from the Napa County Assessor’s Office was analyzed using GIS to identify structures located within areas of mapped landslide susceptibility. Table 3-48 summarizes the number of exposed parcels and associated market and content values within high, moderate, and low landslide susceptibility zones.
Table 3-48: Napa County Property Exposure to Slope Failure Total Parcels Total Market Value
($)
Total Content Value
($) Total Value ($)
Unincorporated County 13,668 $11,215,199,644 $9,248,691,039 $20,463,890,683 Landslide Susceptibility Parcel Count % of Total Market Value Exposure ($)
Content Value
Exposure ($) Total Exposure ($) % of Total
High 1,969 14.4% $1,640,696,735 $1,164,220,757 $2,804,917,492 13.7% Moderate 1,497 11.0% $910,986,195 $598,065,540 $1,509,051,735 7.4% Low 2,896 21.2% $2,158,569,579 $1,522,890,346 $3,681,459,925 18.0% Total 6,362 46.5% $4,710,252,509 $3,285,176,643 $7,995,429,152 39.1% More than 46% of parcels in the unincorporated county are located within mapped landslide susceptibility zones, representing nearly $8.0 billion in combined market and content value exposure. Properties within high susceptibility areas account for a significant share of this exposure and are at the greatest risk of damage from slope instability and associated hazards.
The predominant land uses within these areas include single-family residential, vacant land, and manufactured housing, indicating that slope failure hazards primarily threaten residential properties and associated infrastructure. Damage to these properties may result not only from direct ground movement, but also from secondary impacts such as loss of access, damage to utilities, and erosion-related degradation over time.
Overall, the concentration of development within landslide-prone areas highlights the potential for localized but significant property damage during slope failure events, particularly where steep slopes, wildfire impacts, and intense precipitation combine to increase instability.
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3.3.5.9.3 Critical Facilities & Infrastructure
Numerous critical facilities and extensive segments of infrastructure in Napa County are located within areas susceptible to slope failure hazards. These hazards are most prevalent in hillside and mountainous regions of the County, where steep terrain, variable geologic conditions, and wildfire impacts increase the likelihood of landslides and debris flows.
Critical infrastructure, including transportation networks, utilities, and essential service facilities, is particularly vulnerable to slope failure due to its linear nature and frequent placement along slopes and drainage corridors. Roadways, bridges, and utility alignments are often constructed in or adjacent to steep terrain, making them highly susceptible to ground movement, erosion, and debris deposition.
Slope failures can significantly disrupt transportation systems by blocking roadways, damaging bridges, and isolating communities. Even small landslides can render roads impassable, delaying emergency response and limiting access for residents and service providers. Many hillside areas in Napa County rely on a limited number of access routes, increasing the potential for isolation during and after a slope failure event.
Utility infrastructure is also highly vulnerable. Ground movement can damage or sever underground pipelines, transmission lines, and communication systems, resulting in service disruptions. Water and wastewater systems located on unstable slopes may rupture, causing service outages and potential environmental impacts. Similarly, power and communication infrastructure, including transmission lines, substations, and towers—may be displaced or damaged, increasing the likelihood of prolonged outages.
Table 3-49 summarizes the number and types of critical facilities located within areas of varying landslide susceptibility. Over 200 critical facilities are located within mapped landslide susceptibility zones, including essential facilities, high potential loss facilities, and lifeline infrastructure components. Notably, facilities such as fire stations, hospitals, dams, communication towers, and hazardous materials sites are among those exposed to slope failure risk.
Table 3-49: Napa County Critical Facilities Exposure to Slope Failure Critical Infrastructure - Landslide Susceptibility
Infrastructure Type High Moderate Low
Essential Facilities 3 4 2
EOC - - -
Fire Station 2 4 1
Hospital 1 - -
Law Enforcement - - 1
High Potential Loss 27 27 40
Adult Residential Care - - -
Child Care Center - 2 -
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Church 3 1 5
Communication Tower 5 1 6
Dam 9 8 8
Emergency Shelter - 2 -
Farm Worker Housing - 1 -
Historic Building 2 1 2
Library - - 1
Mobile Home Park - 1 -
Res Elder Care Facility - 1 2
Private School - - -
Public School - - -
Real Property Asset 8 9 16
Transportation and Lifeline 14 37 12
Airport - 2 1
Bridge 2 30 1
Bus Stop - - -
HMMWC Asset - - -
HMMWC Dam - - -
HMMWC Hydrant - - -
NG Station - - 1
Power Plant 3 - -
Substation 2 1 2
Wastewater Treatment Facility 2 2 2
Water Supply Intake - - -
Water Tank 4 2 3
Water Treatment Facility 1 - 2
Hazmat 17 18 26
Geotracker CleanupSite - 1 -
HWTS Active Facility 16 16 26
Landfill 1 1 -
Grand Total 61 86 80
*Real Property Assets are digitized insurance rolls for demonstrating value and ownership and may have overlapping points with other categories such as fire stations and law enforcement.
In addition to point-based facilities, a substantial amount of linear infrastructure is exposed to slope failure hazards. Table 3-50 summarizes miles of infrastructure located within landslide susceptibility zones, including roads, transmission lines, pipelines, and rail corridors. Of particular note, hundreds of miles of CAL OES SUBMITTAL - JUNE 2026
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roadway—including residential roads, service roads, and secondary roads—are located within moderate to high susceptibility areas.
Table 3-50: Napa County Critical Infrastructure Exposure to Slope Failure Lifelines (miles) - Landslide Susceptibility
Infrastructure Type (Linear) High Moderate Low
Levee 0.1 1.1 0.0
NG Pipeline 0.6 4.9 5.5
Railroad 0.1 1.0 0.2
Transmission Line 45.2 15.5 17.1
Street OSM 292.6 244.2 339.1
Highway & Interstate 0.8 0.8 0.5
Major Road 7.4 7.0 8.2
Residential Road 50.0 38.9 62.3
Road - Secondary 16.9 24.6 15.0
Road -Tertiary 26.9 25.7 33.6
Service Road 170.5 129.3 194.1
Unclassified 20.0 18.0 25.4
Grand Total 338.6 266.7 361.9
In total, more than 600 miles of infrastructure are located within mapped landslide susceptibility zones, with a significant portion in areas of moderate to high risk. Because transportation and utility systems are highly interconnected, damage to a single segment can have cascading impacts across the broader network, affecting emergency response, service delivery, and economic activity.
Overall, the extent of critical facility and infrastructure exposure highlights the potential for localized but severe disruptions resulting from slope failure events. Impacts may include loss of access to emergency services, prolonged utility outages, and damage to key community assets, particularly in areas where steep terrain and limited redundancy increase system vulnerability.
DevelopmentTrends
Napa County’s development patterns are shaped by a combination of geographic constraints, regulatory policies, and long-standing growth management strategies. Much of the County’s recent and future development is expected to occur within incorporated cities and established urbanized areas, while rural and hillside regions, where slope failure susceptibility is highest, remain subject to more limited and carefully regulated development.
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The Napa County General Plan and associated policies include provisions to guide development in areas susceptible to geologic hazards, including slope failure. These policies, along with County Code Title 16 and the California Building Standards Code, require site-specific geotechnical investigations and the incorporation of appropriate engineering measures for development proposed in areas with potential slope instability. Such measures may include slope setbacks, drainage controls, grading restrictions, and structural stabilization techniques.
Despite these regulatory frameworks, development continues to occur in areas with inherent landslide susceptibility, particularly in rural residential and hillside settings. Existing parcels of record may be developed in accordance with current standards, but these areas often present ongoing challenges due to steep terrain, variable soil conditions, and limited infrastructure redundancy. As a result, even welldesigned development may retain an elevated level of risk relative to areas outside of slope failure zones.
Future growth in Napa County is expected to continue emphasizing infill development and redevelopment within established communities, consistent with regional growth management objectives. However, some level of rural residential development and infrastructure expansion in hillside areas is anticipated. These projects will require continued integration of geologic assessments and engineering solutions to minimize slope failure risk.
Advances in technology, including high-resolution topographic mapping (e.g., LiDAR), improved geotechnical analysis, and remote monitoring systems, are enhancing the County’s ability to identify and manage slope instability. These tools support more informed planning decisions and can improve the effectiveness of mitigation measures over time.
Development Impacts
Development within or adjacent to areas of slope failure susceptibility has the potential to influence both the likelihood and consequences of slope-related hazards. Grading activities, vegetation removal, and changes to natural drainage patterns associated with development can increase slope instability if not properly designed and managed. Additionally, the expansion of roads, utilities, and other infrastructure into hillside areas may introduce new vulnerabilities, particularly where systems lack redundancy or are exposed to ground movement.
As development continues in areas with existing landslide susceptibility, the overall exposure of property and infrastructure to slope failure hazards may increase. While regulatory requirements and engineering practices are intended to reduce risk, they cannot eliminate it entirely. This is particularly true in areas affected by compounding factors such as wildfire and intense precipitation, which can significantly alter slope stability conditions over time.
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networks, and loss of utility services. In hillside areas with limited access routes, slope failures may isolate communities and delay emergency response, further increasing risk.
Overall, while Napa County’s regulatory framework helps manage development in hazard-prone areas, continued growth in or near slopes will require ongoing attention to geologic conditions, infrastructure resilience, and long-term maintenance of mitigation measures to limit increases in vulnerability.
Climate Change Impacts
Slope failures, including landslides and debris flows, are a persistent hazard in Napa County due to its steep terrain, variable geologic conditions, and history of intense winter storm events. These hazards are most pronounced in hillside and mountainous areas, particularly in the eastern and northern portions of the County. Climate change is expected to exacerbate slope failure risks by increasing the frequency and intensity of extreme precipitation events, extending periods of soil saturation, and compounding erosion in areas affected by wildfire burn scars. These conditions increase the likelihood of both rapid slope failures, such as debris flows, and slower forms of ground movement, posing ongoing risks to structures, infrastructure, and essential services.
Direct Impacts
Landslides and debris flows can damage or obstruct transportation routes, particularly in hillside areas where roads are often constructed along steep slopes or within narrow corridors. Even relatively small slope failures can block roadways, limiting access for residents and delaying emergency response. Larger events may result in structural damage to roadbeds, bridges, and adjacent properties.
Slope instability can also undermine building foundations, retaining structures, and other improvements located on or near steep slopes. In some cases, this may result in partial or complete structural failure.
Properties located near drainage channels or at the base of slopes may be especially vulnerable to debris flows, which can inundate structures with mud, rock, and vegetation.
Water, wastewater, and stormwater infrastructure are also susceptible to damage from slope movement.
Landslides can rupture pipelines, damage drainage systems, and increase sediment loads in surface water supplies, affecting water quality and system reliability. These impacts may lead to service disruptions, increased maintenance costs, and potential environmental consequences.
Secondary Hazards
Slope failure hazards in Napa County are often compounded by secondary processes, particularly in areas affected by wildfire. Burn scar areas are especially vulnerable to debris flows, as the loss of vegetation reduces slope stability and increases runoff during storm events. Intense rainfall on these areas can trigger rapid slope failures with little warning.
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Inadequate or poorly maintained drainage systems can further increase slope failure risk by allowing water to accumulate and saturate soils. Blocked culverts and channels may cause water to overtop or redirect, triggering localized slope instability. Additionally, landslide debris can obstruct waterways and drainage infrastructure, contributing to localized flooding, erosion, and additional slope failures downstream.
Disproportionate Considerations (Optional if you want to keep it here, otherwise remove) While slope failure hazards are geographically concentrated, their impacts may be amplified in areas where access is limited and infrastructure redundancy is low. Disruptions to transportation and utilities can have cascading effects, particularly in rural and hillside areas where alternative routes and services are limited.
Slope Failure Hazard Problem Statements
As part of the mitigation action identification process, the Napa County Planning Team developed areas of concern (i.e., problem statements) that summarize the county’s most pressing slope failure-related vulnerabilities and challenges. The problem statements were developed based on the updated risk assessment and vulnerability analysis contained herein, as well as the team’s local knowledge and experience and community input. The problem statements address a broad spectrum of concerns, from direct impacts on residents and vulnerable populations to threats to critical infrastructure and long-term recovery.
These problem statements are not intended to be all-encompassing or to describe each and every issue associated with slope failure hazards. Instead, they highlight the most urgent concerns that can be addressed within the scope of this MJHMP. Each statement will be linked to at least one mitigation action, ensuring that identified needs are met with specific, measurable, and equitable solutions. Table 3-51 presents the slope failure problem statements for Napa County, along with their associated areas of concern and reference numbers for cross-linking to the mitigation actions in Table 4-4.
Table 3-51: Napa County Slope Failure Hazard Problem Statements Problem No. Hazard Area of
Concern
Primary
Agency
Problem Description Related MA
ps-SF-NCo-151 Slope Failure Threat Napa County Napa County has many areas susceptible to landslides but lacks comprehensive mapping
of slope failure risks.
ma-SF-NCo-84,
ma-AH-NCo-6,
ma-DR-NCo-26
ps-SF-NCo-152 Slope Failure Threat Napa County Over 600 miles of roads and other critical infrastructure and almost 150 critical facilities
are located in moderate or high landslide
susceptibility areas, including the Angwin VFD
storage facility, threatening emergency
response capabilities.
ma-SF-NCo-85,
ma-SF-NCo-84
ps-SF-NCo-396 Slope Failure Impact Napa County Slope failures can damage or destroy critical infrastructure, including access routes and
transmission lines, isolating communities and
disrupting emergency and other services.
ma-SF-NCo-84,
ma-SF-NCo-85
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3.3.6 Drought Hazard Profile
California's water resources have been stressed by periodic drought cycles, but climate change is expected to increase the frequency, severity, and duration of droughts across the state. While the length of drought periods is always in question, it is certain that California and Napa County will continue to be impacted by drought. (California Drought Contingency Plan, 2021)
Napa County surface and groundwater resources provide drinking water for agriculture, residents and visitors, critical habitat to numerous plant and animal species, and opportunities for recreational and commercial activities. Like many other areas of California, the county faces water resource challenges, including inadequate water supply particularly during droughts, impaired water quality, over-drafted groundwater basins, depleted streams, and degraded riparian habitat.
Napa County has four water sources: surface, imported, groundwater, and recycled. The majority of Napa County’s potable water supply is locally derived from surface water. For example, the City of Calistoga relies on Kimball Reservoir while the City of Napa relies on Lake Hennesy. Certain jurisdictions such as the City of American Canyon and the City of Napa receive State Water Project water or City of Vallejo water.
Agricultural and rural users often rely on groundwater wells. Certain jurisdictions, including the Cities of American Canyon, Calistoga, and Napa also have recycled water systems for non-potable uses such as irrigation.
Drought has impacted almost every county in California at one time or another, causing more than $2.6 million in damage. Droughts exceeding three years are relatively rare in northern California, the source of much of the state’s water supply. The 1929 to 1934 drought established the criteria commonly used in designing storage capacity and yield for large northern California reservoirs. The driest single year in California’s measured hydrologic history was 1977.
Past experience shows that drought impacts in California are felt first by those most dependent on annual rainfall: agencies fighting wildfires, farmers and ranchers, rural residents relying on wells, or small water systems lacking a reliable water source.
Most of California’s precipitation comes from storms moving across the Pacific Ocean. The path followed by the storms is determined by the position of an atmospheric high-pressure belt that normally shifts southward during the winter, allowing low pressure systems to move into the state. On average, 75% of California’s annual precipitation occurs between November and March, with 50% occurring between December and February. If a persistent Pacific high-pressure zone takes hold over California mid-winter, the water year tends to be dry.
A typical water year produces about 100 inches of rainfall over the North Coast, 50 inches of precipitation (combination of rain and snow) over the Northern Sierra, 18 inches in the Sacramento area, and 15 inches in the Los Angeles area. In extremely dry years, these annual totals can fall to as little as one half, or even one third of these amounts.
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Napa County and other public agencies have been monitoring local groundwater resources since the mid 1900s. Based on recent studies and bi-annual monitoring of groundwater levels in over 100 volunteered wells, level trends in the Napa Valley Sub-basins of the Napa-Sonoma Valley Groundwater Basin are stable in the majority of wells with long-term records. Although some wells show a response to drought conditions, levels in recent drought years are generally higher than those during the 1976 to 1977 drought. Elsewhere in the county, long-term groundwater level records are more limited, with the exception of the Milliken-SarcoTulocay (MST) Subarea. Groundwater level declines observed in the MST Subarea as early as the 1960s and 1970s have stabilized since about 2008. The observation that groundwater level responses differ within the MST Subarea and even within the north, central, and southern sections of this subarea indicate that localized conditions, whether geologic or anthropogenic in nature, might be the primary influence on conditions in the subarea. (What is Known About Groundwater, n.d.) Secondary & Compounding Hazards
Drought is compounded by climate change and contributes to secondary hazards like wildfire. These hazards are briefly discussed in this section in relation to drought; detailed assessments of wildfire and climate change are found in their respective profiles, in Sections 3.3.1 and 3.3.9.
Climate Change
The long-term effects of climate change on regional water resources are unknown, but global water resources are already experiencing stresses without climate change, including growing populations, increased competition for available water, poor water quality, groundwater overdraft, and aging urban water infrastructure. With a warmer climate, droughts could increase in severity, frequency, and duration, contributing to declining streamflow in surface waterbodies, which in turn, reduces groundwater recharge as well as water availability for humans, wildlife, and vegetation. It can also contribute to sea level risedriven seawater intrusion into aquifers.
According to the UC Davis Center for Watershed Sciences, water shortages in 2016 were projected to cost the agricultural industry a total of $550 million in direct costs and 1,815 in lost jobs. More frequent extreme events such as droughts could end up being more cause for concern than the long-term change in temperature and precipitation averages.
Wildfire
The secondary hazard most associated with drought is wildfire. A prolonged lack of precipitation stresses and dries out vegetation, which becomes increasingly susceptible to ignition as the duration of the drought extends. Increased temperatures from climate change during drought conditions further intensify wildfire danger by warming and accelerating the desiccation of vegetative fuels. As climate conditions shift toward longer fire seasons and drier fuels, secondary wildfire effects from drought are expected to increase in both frequency and severity across Napa County. A changing climate is expected to subject forests to increased CAL OES SUBMITTAL - JUNE 2026
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stress due to drought, disease, invasive species, and insect pests (e.g., glassy-winged sharpshooter and sudden oak death). These stressors are likely to make forests more vulnerable to catastrophic fire.
Policies, Plans & Regulatory Environment
State Regulatory Environment
State Water Resources Control Board
Through a ballot initiative in the early 20th century, state law-makers passed a Constitutional amendment declaring that California’s water resources “shall put water to the highest beneficial use possible and shall not waste water or use it unreasonably.” In 1967, the State Legislature recognized that California would not have enough clean water for agricultural, municipal, industrial, environmental, and other uses unless water quality and quantity decision-making were coordinated. It was at that time that the State Water Resources Control Board (State Water Board) was created and given the broad authority and responsibility to protect water quality, balance competing demands on water resources, and attempt to resolve decades-long water disputes.
This new regulatory board merged the functions of two previous state boards: the State Water Quality Control Board and the State Water Rights Board. The former had its roots in the late 1940s, when legislators created a streamlined regulatory agency to address rising water quality problems with the state’s explosive industrial and population growth. A water rights commission, which preceded the water rights board, was created in the early 1900s to arbitrate and resolve the state's water battles, which began during the 1849 Gold Rush. Back then, prospectors from throughout the world raced to the Sierra Nevada Mountains to stake their claims, using the cold mountain streams as a tool to unearth gold.
Today the five-member State Water Board allocates water rights, adjudicates water right disputes, develops statewide water protection plans, establishes water quality standards, and guides the nine Regional Water Quality Control Boards located in California’s major watersheds. The Regional Boards, each comprised of seven members, serve as the front-line for state and federal water pollution control efforts. In Napa County, the San Francisco Bay Area Regional Water Quality Board has jurisdiction.
Today’s challenges are California’s rapid population growth, and the continuing struggle over water flows.
The State Water Board faces tough demands: to fix ailing sewer systems; build new wastewater treatment plants; and tackle the cleanup of underground water sources harmed by the technology and industry that has catapulted the state into global prominence. (State Water Board, 2018) California Sustainable Groundwater Management Act (SGMA) In 2014, Governor Jerry Brown signed into law a package of senate bills (SB) and assembly bills (AV), including SB 1168, AB 1739, and SB 1319, collectively called the Sustainable Groundwater Management Act (SGMA). SGMA requires governments and water agencies of high and medium priority basins to halt CAL OES SUBMITTAL - JUNE 2026
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overdraft and bring groundwater basins into balanced levels of pumping and recharge. Under SGMA, these basins should reach sustainability within 20 years of implementing their sustainability plans. For critically overdrafted basins, that will be 2040. For the remaining high and medium priority basins, 2042 is the deadline.
Executive Order B-37-16
Governor Jerry Brown issued Executive Order B-37-16, Making Water Conservation a California Way of Life, in 2016 that included, among other directives, drought hazard and planning related actions. The order established long-term, permanent water conservation measures in response to the ongoing drought and changing climate, and mandated the California State Water Resources Control Board permanently prohibit practices like using hoses to clean driveways and sidewalks, overwatering lawns, and serving water by default in restaurants, with a focus on urban water use. In addition, directive number 10 in the order specified that, “for areas not covered by a Water Shortage Contingency Plan, the [board] shall work with counties to facilitate improved drought planning for small water suppliers and rural communities.” In 2016, the State Water Board adjusted emergency water conservation regulations indefinitely in recognition of the differing water supply conditions and ongoing drought across the state. Executive Order B-37-16 updates temporary emergency water restrictions and transitions to permanent, long-term improvements in water use by using water more wisely; eliminating water waste; strengthening local drought resilience; and improving agricultural water use efficiency and drought planning.
California Water Plan
The California Water Plan, which dates back to the late 1800s, is the state's strategic plan for sustainably managing and developing water resources for current and future generations. As required by State Water Code Section 10005(a), the California Department of Water Resources (DWR) presents in the plan the status and trends of California’s water-dependent natural resources; water supplies; and agricultural, urban, and environmental water demands for a range of plausible future scenarios. The plan is updated every five years and provides a framework for collaboration among local government officials, tribes, water suppliers and resource managers, businesses, academia, and the general public. Although the plan does not mandate actions, authorize spending, or make project-specific recommendations, it does require policy-makers to take definitive steps to authorize specific actions proposed in the plan and appropriate the funding needed for their implementation.
The plan was last updated in 2023. This update promotes a vision where “all Californians benefit from water resources that are sustainable, resilient to climate change, and managed to achieve shared values and connections to our communities and the environment.” To tackle this ambitious vision, the 2023 update focuses on three intersecting themes: addressing climate urgency, strengthening watershed resilience, and achieving equity in water management. (DWR, 2025)
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Local Regulatory Environment
Napa County Authority & Role
Napa County has no direct authority over the entities that provide water supply to county residents. The largest suppliers are governed by the city councils elected by those city residents. Independently elected agency boards govern the other public entities, and the California Public Utilities Commission oversees privately-owned water systems to some extent. All water systems are governed by state and federal safe drinking water regulations. The larger systems, with more than 200 connections, are regulated directly by the California Department of Public Health.
The county serves as an agent of the state, ensuring compliance with the state regulations for small water systems with between five and 199 connections. The county also permits individual water systems to serve new homes in rural areas, typically on wells. Although well yield standards for new development are conservative, it is possible that individual wells and older wells serving small water systems may experience diminished yield or go dry during an extended drought. The county has no ongoing oversight of water use for individual water systems after the initial development permit is approved.
Groundwater Sustainability Plans
The Sustainable Groundwater Management Act (SGMA) establishes a statewide framework for managing groundwater resources to achieve long-term sustainability. Under SGMA, local agencies are responsible for developing and implementing Groundwater Sustainability Plans (GSPs) or approved alternatives for groundwater basins designated by the California Department of Water Resources (DWR) as high or medium priority. These plans serve as long-term roadmaps to ensure that groundwater is managed in a manner that avoids undesirable results such as chronic lowering of groundwater levels, reduced water supply reliability, land subsidence, and water quality degradation.
As part of SGMA implementation, DWR has identified five groundwater basins and subbasins within Napa County, each assigned a priority level based on factors such as population, land use, and reliance on groundwater resources:
▪ Napa-Sonoma Valley – Napa Valley Subbasin – High Priority - This basin coincides with the floor of the Napa Valley and encompasses nearly 46,000 acres between the City of Napa and the City of Calistoga. The GSP for this basin was approved by DWR in 2023. ▪ Napa-Sonoma Valley – Napa-Sonoma Lowlands Subbasin – Very Low Priority ▪ Pope Valley Basin – Very Low Priority
▪ Berryessa Valley Basin – Very Low Priority
▪ Suisun-Fairfield Valley Basin – Low Priority
Of these, only the Napa Valley Subbasin is classified as high priority and is therefore subject to SGMA requirements for the preparation and implementation of a GSP. High-priority basins must comply with CAL OES SUBMITTAL - JUNE 2026
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ongoing monitoring, reporting, and planning requirements, including annual reporting to DWR and periodic plan updates every five years. In contrast, low and very low priority basins are not currently required to develop GSPs, although they may still be subject to local groundwater management efforts.
The Napa Valley Subbasin GSP, approved by DWR in 2023, provides a comprehensive framework for managing groundwater resources across the valley floor, which extends from the City of Napa to the City of Calistoga. The plan incorporates extensive groundwater monitoring data, including groundwater levels, water quality, and recharge conditions, to establish sustainable management criteria and guide long-term water resource decisions. Local monitoring efforts, including those coordinated through Napa County watershed and groundwater programs, support ongoing assessment of groundwater conditions and inform adaptive management strategies.
Groundwater plays a critical role in Napa County’s overall water supply, particularly for agricultural uses and rural residential areas that rely on private wells. As a result, effective implementation of the GSP is essential for reducing vulnerability to drought. By improving understanding of groundwater conditions, establishing sustainability thresholds, and guiding water use and recharge practices, the County and its partners can better manage water resources during prolonged dry periods.
Urban Water Management Plans (UWMP)
Jurisdictions either supplying over 3,000 acre-feet of water annually or serving more than 3,000 urban connections are required to submit an Urban Water Management Plan (UWMP) for review and approval by the California Department of Water Resources (DWR), pursuant to California Water Code, § 10610-10656 and § 10608. The plans must be updated every five years and support the suppliers’ long-term resource planning to ensure that adequate water supplies are available to meet existing and future water needs. The information collected for UWMPs is useful for local, regional, and statewide water and drought planning.
UWMPs also explore stormwater capacity and assist in planning and funding future stormwater needs. (DWR, 2025)
All jurisdictions participating in this MJHMP which are also municipal water suppliers have an UWMP, including the Cities of American Canyon, Calistoga, Napa, and St. Helena. As a more rural water agency, the Howell Mountain Mutual Water Company has a 2022 Drought Contingency Plan. These jurisdiction-specific plans are explored in more detail in the annexes in Volume 2.
Napa County General Plan
The Napa County General Plan Conservation Element sets forth several goals and policies relevant to drought:
▪ Goal CON-10: Conserve, enhance and manage water resources on a sustainable basis to attempt to ensure that sufficient amounts of water will be available for the uses allowed by this General Plan, for the natural environment, and for future generations.
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▪ Goal CON-11: Prioritize the use of available groundwater for agricultural and rural residential uses rather than for urbanized areas and ensure that land use decisions recognize the long-term availability and value of water resources in Napa County. ▪ Goal CON-12: Proactively collect information about the status of the county’s surface and groundwater resources to provide for improved forecasting of future supplies and effective management of the resources in each of the County’s watersheds. ▪ Policy CON-42: The County shall work to improve and maintain the vitality and health of its watersheds [through education, restoration, and protection]. ▪ Policy CON-51: Recognizing that groundwater best supports agricultural and rural uses, the County discourages urbanization requiring net increases in groundwater use and discourages incorporated jurisdictions from using groundwater except in emergencies or as part of conjunctive-use programs that do not cause or exacerbate conditions of overdraft or otherwise adversely affect the County’s groundwater resources.
▪ Policy CON-52: Groundwater is a valuable resource in Napa County. The County encourages responsible use and conservation of groundwater and regulates groundwater resources by way of its groundwater ordinances.
▪ Policy CON-60.5: All aspects of landscaping from the selection of plants to soil preparation and the installation of irrigation systems should be designed to reduce water demand, retain runoff, decrease flooding, and recharge groundwater.
Regional Climate Action and Adaptation Plan
Napa County’s approach to climate change and drought mitigation has evolved from the 2018 Climate Action Plan (CAP) into the Draft Regional Climate Action and Adaptation Plan (RCAAP), which establishes a comprehensive, region-wide framework to reduce greenhouse gas emissions while improving resilience to climate-related hazards. The RCAAP identifies drought and water supply impacts as a key climate risk and outlines a coordinated set of adaptation strategies, measures, and actions to enhance long-term water supply reliability and reduce vulnerability to increasingly frequent and severe drought conditions.
The RCAAP uses a structured adaptation framework consisting of overarching strategies supported by specific measures and implementation actions. One of the primary strategies directly addresses drought by focusing on water conservation, supply diversification, and system resilience, while additional crosscutting measures support water resource sustainability through land use planning, infrastructure improvements, and ecosystem protection. Key drought-related adaptation measures identified in the RCAAP include:
▪ Water Conservation and Reuse (DROUGHT-1): Expands water conservation programs and promotes water reuse through recycled water, greywater systems, stormwater capture, and managed aquifer recharge. The plan also emphasizes public education, incentive programs (e.g., rebates, turf CAL OES SUBMITTAL - JUNE 2026
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replacement), and exploration of policies such as zero water footprint requirements for new development.
▪ Agricultural Water Resilience (DROUGHT-2): Supports the agricultural sector, particularly vineyards, through improved irrigation efficiency, soil health practices, and groundwater recharge strategies.
Measures include promoting drip irrigation, conducting irrigation system testing, implementing cover crops, and coordinating with the Napa County Groundwater Sustainability Agency (NCGSA) to implement water conservation and groundwater management workplans. ▪ Water Supply Reliability and Infrastructure Resilience (DROUGHT-3): Calls for evaluation of water supply system vulnerabilities and development of resilience strategies, including infrastructure upgrades, adoption of resiliency standards, and implementation of drought contingency planning efforts such as the Napa Valley Drought Contingency Plan and SB 552–related planning. ▪ Public Outreach and Water Conservation Culture (DROUGHT-4): Expands education and outreach efforts to promote “water conservation as a way of life,” targeting both municipal water users and selfsupplied groundwater users through coordinated regional messaging and accessible information platforms.
▪ Regional Water Supply Coordination (DROUGHT-5): Encourages collaboration with local, state, and federal agencies to secure future water supplies, expand groundwater recharge, and pursue alternative water sources. This includes leveraging grant funding and coordinating with agencies such as the California Department of Water Resources and the State Water Resources Control Board.
In addition to drought-specific measures, the RCAAP includes broader resilience strategies that support water resource sustainability. These include promoting green infrastructure and low-impact development to enhance groundwater recharge, protecting ecosystems and watershed function, and integrating climate considerations into land use planning, infrastructure design, and emergency management.
Napa County Code
Chapters 13 and 18 of the County Code set forth requirements relevant to drought, including permitting for new groundwater wells, implementation groundwater conservation practices for new wells, and landscaping and irrigation standards.
Past Events
California experienced massive changes over the course of the twentieth century as evidenced by dramatic population increases and land use conversion. (Cal. Dep't of Water Resources, 2015) The California Department of Water Resources has state hydrologic data back to the early 1900s. The hydrologic data show multi-year droughts from 1912 to 1913, 1918 to 1920, 1922 to 1924, 2007 to 2009, and 2014 to 2017.
The most recent major drought in California spanned 2014 to 2017. With California facing water shortfalls in the driest year in recorded state history, California State Governor Jerry Brown, declared a drought state of CAL OES SUBMITTAL - JUNE 2026
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emergency on January 17, 2014. In the State of Emergency declaration, Governor Brown directed state officials to assist farmers and communities that are economically impacted by dry conditions and to ensure the state can respond if Californians face drinking water shortages. The Governor also directed state agencies to use less water and hire more firefighters and initiated a greatly expanded water conservation public awareness campaign. On April 17, 2017, Brown issued Executive Order B-40-17, officially ending the drought state of emergency in all California counties except Fresno, Kings, Tulare, and Tuolumne.
The National Drought Monitor provides drought data and maps nationally and on a localized scale. The National Drought Monitor is the product of eleven agencies, from the NDMC, NOAA and USDA, and is available at http://droughtmonitor.unl.edu/. Figure 3-39 depicts the U.S. Drought Monitor conditions in California for both December 2017 and December 2018. In December 2017, as the declared drought ended in California, Napa County did not experience any drought conditions. One year later, however, the County already experienced abnormally dry to moderate drought conditions again.
A drought is a period of dry weather that persists long enough to cause serious problems such as crop damage and water supply shortages. In recent history, Napa County has experienced five main drought periods: 1976 to 1977, 1987 to 1992, 2007 to 2009, 2012 to 2016, and 2020 to 2023. The only drought period declared a federal disaster was 1977, triggering FEMA’s Public Assistance (PA) program. It is expected that the effects of climate change will result in more severe droughts of longer duration.
Figure 3-39 shows a time series of the level of drought in Napa County between 2000 and 2026 according to the National Drought Monitor. The National Drought Monitor provides drought data and maps nationally and on a localized, watershed-scale. The National Drought Monitor is the product of 11 agencies, including the National Drought Mitigation Center (NDMC) at the University of Nebraska-Lincoln, National Oceanic and Atmospheric Administration (NOAA), and U.S. Department of Agriculture (USDA). The National Drought Monitor categorizes the level of drought from D0 through D4, with D4 being the highest “exceptional drought.” Table 3-52 depicts drought classifications and impacts from the level of drought occurrence in California.
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Table 3-52: Drought Classifications and Impacts for California Category Description Possible Impacts
D0
Abnormally
Dry
▪ Soil is dry; irrigation delivery begins early
▪ Dryland crop germination is stunted
▪ Active fire season begins
▪ Winter resort visitation is low; snowpack is minimal
D1
Moderate
Drought
▪ Dryland pasture growth is stunted; producers give supplemental feed to cattle ▪ Landscaping and gardens need irrigation earlier; wildlife patterns begin to change ▪ Stock ponds and creeks are lower than usual
D2
Severe
Drought
▪ Producers increase water efficiency methods and drought-resistant crops ▪ Grazing land inadequate
▪ Fire season is longer, with high burn intensity, dry fuels, and large fire spatial extent; more fire crews on staff
▪ Lake- and river-based tourism declines; boat ramps close ▪ Trees are stressed; plants increase reproductive mechanisms; wildlife diseases increase ▪ Water temperatures increase; programs to divert water to protect fish begin ▪ River flows decrease; reservoir levels are low and banks are exposed D3
Extreme
Drought
▪ Federal water not adequate to meet irrigation contracts; extracting supplemental groundwater is expensive
▪ Fire season lasts year-round; fires occur in typically wet parts of the state; burn bans are implemented
▪ Ski and rafting business is low; mountain communities suffer ▪ Low water levels impede fish migration and cause lower survival rates ▪ Wildlife encroach on developed areas; little native food and water is available for bears, which hibernate less
Figure 3-39: Napa County Drought History and Severity Chart CAL OES SUBMITTAL - JUNE 2026
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Category Description Possible Impacts
▪ Water sanitation is a concern; reservoir levels drop significantly; surface water is nearly dry, flows are very low; water theft occurs
▪ Livestock need supplemental feed, cattle and horses are sold; little pasture remains ▪ Well and aquifer levels decrease; homeowners drill new wells D4
Exceptional
Drought
▪ Fire season is very costly; number of fires and areas burned are extensive ▪ Many recreational activities are affected
▪ Fields are left fallow; orchards are removed; vegetable yields are low; honey harvest is small; agricultural unemployment is high, food aid is needed ▪ Fish rescue and relocation begins; pine beetle infestation occurs; forest mortality is high; wetlands dry up; wildlife death is widespread; algae blooms appear ▪ Poor air quality affects health; greenhouse gas emissions increase as hydropower production decreases; West Nile outbreaks rise
▪ Water shortages are widespread; surface water is depleted; federal irrigation water deliveries are curtailed; water prices are extremely high; wells are dry, more and deeper wells are drilled; water quality is poor
Source: Adapted from U.S. Drought Monitor Drought Classifications and Impacts Location
Drought is one of the few hazards with the potential to directly or indirectly impact the entire population of Napa County, be it from water restrictions, higher water and food prices, reduced air or water quality, restricted access to recreational areas, or economic stress from crop loss. No portion of the County is immune from drought conditions. Lack of winter snowfall in the mountains can eventually lead to agricultural impacts due to decreased stream flows. Droughts of just a few weeks during critical periods of plant development can have disastrous effects on agriculture production. Reduced reservoir storage from decreased runoff in the mountains can lead to water shortages. Droughts that occur in populated areas may not have direct effects to the residents but may increase the threat of wildfire in the wildland urban interface areas.
According to the Napa Valley Subbasin GSP, there are several wells located near the Napa Valley Subbasin margin in the northeastern Napa, southwestern Yountville, and southeastern St. Helena areas that show periods of groundwater decline during times of drought. (Napa Valley Subbasin, 2022) Frequency & Probability of Future Occurrences
Droughts may not be predictable, but they should be expected. They occur with some regularity and varying levels of severity. The magnitude and duration of a drought, as shown in Figure 3-39, is something that can be predicted based on historical records and should be taken into account in water resources planning.
According to the results of the hazard risk ranking exercises conducted by the participating jurisdictions, the probability of drought occurring in Napa County is likely (between 10 and 100% annual probability).
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Currently there is no data on the probability of drought akin to data for predicting earthquakes or flood probability. Empirical studies conducted over the past century have shown that meteorological drought is never the result of a single cause. It is the result of many causes, often synergistic in nature; these include global weather patterns that produce persistent, upper-level high-pressure systems along the West Coast with warm, dry air resulting in less precipitation.
The expected effects of climate change will also increase the risk of drought. Numerous climate models have been run with various predictions for the Napa County area. Although it is unclear whether the average amount of rainfall will increase, it is apparent that the timing and intensity of rainfall will change, which will lead to more severe and extended droughts. More intense rainfall (as opposed to longer, less intense events) will not contribute as much to groundwater recharge or dry season stream baseflows and will ultimately have adverse impacts on water supply quality through flooding and erosion.
Severity
The severity of a drought depends on the degree of moisture deficiency, the duration, and the size and location of the affected area. The longer the duration of the drought and the larger the area impacted, the more severe the potential impacts. Droughts are not usually associated with direct impacts on people or property, but they can have significant impacts on agriculture, local economies, and natural resources, impacting people indirectly.
Drought eventually affects groundwater sources but generally not as quickly as surface water supplies; although, groundwater supplies take longer to recover. Reduced precipitation during a drought means that groundwater is not replenished at a normal rate, leading to problems such as reduced pumping capacity, wells going dry, seawater intrusion, and land subsidence. Shallow wells are more susceptible than deep wells. Reduced replenishment of groundwater also affects streams and other surface water supplies. Much of the flow in streams comes from groundwater, especially during the summer when there is less precipitation. Reduced groundwater levels mean that even less water will enter streams when steam flows are lowest.
On average, the nationwide annual impacts of drought are greater than the impacts of any other natural hazard. They are estimated to be between $6 and $8 billion annually in the United States and occur primarily in the agriculture, transportation, recreation and tourism, forestry, and energy sectors. Water-dependent industries are commonly forced to shut down all or a portion of their facilities, resulting in layoffs. Social and environmental impacts are also significant, although it is difficult to put a precise cost on these impacts.
Droughts that occur in populated areas may not have direct effects to the residents but may increase the threat of wildfire.
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Warning Time
Unlike most disasters, droughts normally occur slowly but last a long time. Droughts are climatic patterns that occur over extended periods of time. Only generalized warning can take place due to the numerous variables that scientists have not pieced together well enough to make accurate and precise predictions.
Scientists at this time do not know how to predict drought more than a month in advance for most locations.
Predicting drought depends on the ability to forecast precipitation and temperature. Anomalies of precipitation and temperature may last from several months to several decades. How long they last depends on interactions between the atmosphere and the oceans, soil moisture and land surface processes, topography, internal dynamics, and the accumulated influence of weather systems on the global scale.
Drought Vulnerability Assessment
Drought vulnerability in Napa County varies based on water supply source, system capacity, and reliance on groundwater or surface water. Water agencies that depend more heavily on surface water are generally more susceptible to supply shortages during extended dry periods, while groundwater-dependent systems may experience delayed but longer-lasting impacts due to reduced recharge and declining water levels.
Countywide reductions in groundwater levels and streamflow highlight the ongoing need for water conservation, demand management, and supply diversification. While all populations, properties, and natural resources in Napa County are exposed to drought conditions, the severity of impacts depends on water demand and the availability of reliable water supplies.
Drought is a slow-onset hazard with widespread and long-term impacts across multiple sectors. In Napa County, the agricultural sector is particularly vulnerable due to its reliance on consistent water supplies.
Reduced water availability can affect crop production, increase operational costs, and contribute to broader economic impacts, including reduced employment in agriculture and related industries.
3.3.6.8.1 Population
The entire population of Napa County is directly or indirectly affected by drought conditions. Direct impacts include reduced water availability and potential limitations on water use, while indirect impacts include increased water costs, reduced agricultural production, and economic effects on water-dependent industries.
Rural residents who rely on private wells or small water systems are particularly vulnerable to drought.
These systems often lack redundancy and are more susceptible to declining groundwater levels during prolonged dry periods. Shallow wells, in particular, may experience reduced yield or go dry, especially as groundwater demand increases.
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drought increases wildfire risk due to dry vegetation, further compounding risks to communities, infrastructure, and natural resources.
Disproportionately Impacted Populations
Certain populations in Napa County may be disproportionately impacted by drought conditions due to economic, physical, and social factors that limit their ability to prepare for, respond to, and recover from water shortages. As described in Section 1.5.3, low-income households may be more sensitive to increases in water costs associated with drought, including higher utility rates, well maintenance expenses, and costs associated with water conservation measures or alternative water supplies.
Residents in rural areas, particularly those dependent on private wells, face increased vulnerability due to limited water supply reliability and the potential for wells to go dry during extended drought conditions.
These households may also have limited access to financial resources needed to deepen wells, install storage systems, or secure alternative water sources.
Individuals with access and functional needs, including older adults, persons with disabilities, and medically dependent individuals, may experience additional challenges during drought. Reliable access to water is essential for health, sanitation, and medical needs, and disruptions or limitations in water supply may have more severe consequences for these populations.
Additionally, renters may have limited control over water infrastructure or conservation upgrades, such as irrigation systems or plumbing improvements, and may be indirectly impacted by increased housing costs or reduced property maintenance during drought conditions.
Overall, while drought affects the entire population, these factors indicate that impacts may be more severe for populations with reduced financial capacity, limited mobility, or reliance on vulnerable water supply systems.
3.3.6.8.2 Property
Drought does not typically cause direct structural damage to buildings; however, it can result in significant indirect impacts. These include increased wildfire risk, loss of landscaping, and economic impacts to waterdependent properties and businesses.
Napa County’s economy is highly dependent on agriculture and tourism, both of which are sensitive to water availability. Water shortages or restrictions during peak seasons may affect agricultural production, hospitality operations, and other tourism-related businesses, potentially resulting in revenue losses and reduced economic activity.
Property owners relying on groundwater—particularly those with shallow wells—may experience increased costs or reduced water availability during drought conditions. Additionally, shifts from surface water to CAL OES SUBMITTAL - JUNE 2026
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groundwater use during drought periods can place further stress on aquifers, contributing to long-term supply challenges.
3.3.6.8.3 Critical Facilities & Infrastructure
Critical facilities and infrastructure in Napa County are expected to remain operational during drought conditions; however, the systems that support essential services are vulnerable to reduced water availability and associated secondary impacts. Key systems at risk include water supply, wastewater, energy, communications, and emergency services.
Drought can reduce the availability and quality of water supplies needed to support these systems. Lower reservoir levels and reduced streamflow may concentrate pollutants, increase water temperatures, and promote harmful algal blooms, all of which can degrade water quality and complicate treatment processes.
In some cases, reduced freshwater flows may also contribute to saltwater intrusion, further affecting water supply reliability.
Water and wastewater infrastructure may also be indirectly impacted by drought-related conditions.
Reduced groundwater levels can increase stress on pipelines and wells, while extreme drying may contribute to localized ground instability, potentially affecting buried infrastructure. Additionally, wildfire— closely associated with drought—poses a significant secondary threat to critical facilities and lifelines.
Drought also highlights the interdependency of infrastructure systems. Water is essential for energy production, including cooling and operational processes at power facilities, while energy is required to extract, treat, and distribute water. Communications infrastructure, including data centers, also relies on water for cooling. Disruptions to water supply can therefore have cascading effects across multiple sectors.
Emergency services may face additional constraints during severe drought. Reduced water availability and pressure can limit firefighting capabilities, particularly in areas dependent on hydrant systems or stored water supplies. These limitations, combined with increased wildfire risk, can significantly strain emergency response capacity.
Impacts on Agriculture
The agricultural sector demonstrates the potential site-specific impacts of drought. Droughts of just a few weeks during critical periods of plant development can have disastrous effects on agriculture production. A drought can result in farmers not being able to plant crops or the failure of planted crops, leading to loss of work for agricultural workers and those in food industries.
Agricultural drought impacts are normally felt earliest by those relying on unmanaged water supplies:
entities carrying out dryland grazing and non-irrigated crop production, usually grain crops. Impacts to irrigated agriculture depend on the source and nature of the irrigation water supply, whether it be local groundwater, local surface water, or imported surface water, and any water rights or contractual provisions that may be associated with the source. The extent to which producers may mitigate water shortage impacts CAL OES SUBMITTAL - JUNE 2026
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