Atwater City Council
June 22, 2026

Merced County Multi-Jurisdictional Hazard Mitigation Plan Hazard Identification and Risk Assessment
2021-2026 Update Page 4-10
Jurisdiction
Property
Type
Improved
Parcel
Count
Building
Count Improved Value Estimated Content Value Total Value Livingston
Agricultural 8 9 $713,550 $713,550 $1,427,100
Commercial 82 235 $75,954,862 $75,954,862 $151,909,724
Exempt 48 114 $8,331,817 $8,331,817 $16,663,634
Industrial 19 33 $103,753,191 $155,629,787 $259,382,978 Mixed Use 3 4 $53,004 $53,004 $106,008
Residential 3,177 3,605 $519,238,129 $259,619,065 $778,857,194 Vacant
Land 7 7 $142,582 $142,582 $285,164
Total 3,344 4,007 $708,187,135 $500,444,666 $1,208,631,801 Los Banos
Agricultural 6 8 $519,029 $519,029 $1,038,058
Commercial 441 889 $287,223,390 $287,223,390 $574,446,780 Exempt 189 270 $15,571,378 $15,571,378 $31,142,756
Industrial 23 43 $29,327,893 $43,991,840 $73,319,733
Mixed Use 9 9 $175,156 $175,156 $350,312
Residential 11,554 11,861 $2,158,859,942 $1,079,429,971 $3,238,289,913 Vacant
Land 31 31 $530,525 $530,525 $1,061,050
Total 12,253 13,111 $2,492,207,313 $1,427,441,289 $3,919,648,602 Merced
Agricultural 23 26 $1,112,724 $1,112,724 $2,225,448
Commercial 911 2,819 $1,045,871,699 $1,045,871,699 $2,091,743,398 Exempt 632 1,313 $48,779,381 $48,779,381 $97,558,762
Industrial 120 220 $167,440,322 $251,160,483 $418,600,805 Mixed Use 16 17 $259,279 $259,279 $518,558
Residential 20,611 27,288 $3,488,757,480 $1,744,378,740 $5,233,136,220 Vacant
Land 49 61 $1,410,697 $1,410,697 $2,821,394
Total 22,362 31,744 $4,753,631,582 $3,092,973,003 $7,846,604,585 Unincorporated
Agricultural 9,875 13,314 $2,230,504,412 $2,230,504,412 $4,461,008,824 Commercial 689 1,703 $277,981,955 $277,981,955 $555,963,910 Exempt 427 769 $56,580,563 $56,580,563 $113,161,126
Industrial 158 250 $322,872,531 $484,308,797 $807,181,328 Mixed Use 70 81 $2,168,754 $2,168,754 $4,337,508
Residential 17,020 19,083 $2,737,477,431 $1,368,738,716 $4,106,216,147 Vacant
Land 104 127 $1,311,415 $1,311,415 $2,622,830
Total 28,343 35,327 $5,628,897,061 $4,421,594,611 $10,050,491,672 Grand Total 77,584 97,953 $15,466,815,549 $10,554,980,566 $26,021,796,115 Source: Merced County Assessor’s Office, Wood Analysis
Critical Facility/Lifeline Inventory
A significant aspect of the 2021 HIRA update was the update of critical facilities and an alignment/classification with the FEMA Lifelines framework. The critical facilities/lifelines GIS database was based on a combination of county-provided data, Homeland Infrastructure Foundation Level Data (HIFLD), and local input. Jurisdictions were able to browse map based critical facility data, edit descriptive attributes, and add new point data through a password protected interface built with the ArcGIS Online Experience Builder application setup by the County GIS coordinator. The results are summarized here and provided the basis for GIS based vulnerability analyses, where data permitted.

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For the purposes of this plan, a critical facility is defined as one that is essential in providing utility or direction either during the response to an emergency or during the recovery operation. FEMA sorts critical facilities into seven lifeline categories as shown in Figure 4-1.

Figure 4-1 Lifeline Categories
These lifeline categories standardize the classification of critical facilities and infrastructure that provide indispensable service, operation, or function to a community. A lifeline is defined as providing indispensable service that enables the continuous operation of critical business and government functions, and is critical to human health and safety, or economic security. These categorizations are particularly useful as they:
● Enable effort consolidations between government and other organizations (e.g., infrastructure owners and operators).
● Enable integration of preparedness efforts among plans, easier identification of unmet critical facility needs.
● Refine sources and products to enhance awareness, capability gaps, and progress towards stabilization.
● Enhance communication amongst critical entities, while enabling complex interdependencies between government assets.
● Highlight lifeline related priority areas regarding general operations as well as response efforts.

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Table 4-7 shows a summary of the critical facilities analysis. Figure 4-2 illustrates the location of critical facilities in Merced County.

Table 4-7 Summary of Critical Facilities by Jurisdiction and Lifeline Jurisdiction
CommunicationsEnergyFood, Water, ShelterHazardous MaterialHealth and MedicalSafety and SecurityTransportationTotal Atwater 7 5 3 1 1 34 11 62
Dos Palos 1 - - - - 7 - 8
Gustine 6 - 2 1 - 8 3 20
Livingston 6 1 3 3 - 14 5 32
Los Banos 15 2 20 6 3 53 6 105
Merced 36 3 52 17 22 87 53 270
Unincorporated 113 216 31 24 1 164 467 1,016
Total 184 227 111 52 27 367 545 1,513
Source: HIFLD, Merced County, City of Atwater, City of Dos Palos, City of Gustine, City of Livingston, City of Los Banos, City of Merced

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Figure 4-2 Merced County Critical Facilities

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Other critical facilities unique to the county are the California Aqueduct, the Castle Air Museum, and the Federal Bureau of Prisons located in Atwater. These facilities are better addressed in the emergency operations plans for the county, the two municipalities, and federal emergency plans.

The California Aqueduct, part of the California State Water Project, runs through the western part of Merced County. The State Water Project is a water storage and delivery system of reservoirs, aqueducts, power plants, and pumping plants. Its main purpose is to store water and distribute it to 29 urban and agricultural water suppliers in Northern California, the San Francisco Bay Area, the San Joaquin Valley, the Central Coast, and Southern California. Seventy percent of the contracted water supply goes to urban users and 30 percent goes to agricultural users. The State Water Project makes deliveries to two-thirds of California's population. Earthquakes, landslides, flooding, or other hazard events that disrupt the aqueduct’s ability to deliver water could have serious impacts to agriculture in the county and water users in many areas of California.

The Castle Air Museum is one of Merced County’s treasures drawing visitors from across the country.

Located on the site of the former Castle Air Force Base (known as Merced Army Airfield during WWII), the museum has a collection of 53 World War II, Korean War, and Vietnam Era aircraft. The indoor museum contains many additional Air Force and Army Air Corps artifacts and a B-52 simulator.

The Federal Bureau of Prisons in Atwater is one of the larger employers in the city with over 100 employees and approximately 1,600 prisoners. Although this plan recognizes the critical assets of this facility and its role in the county’s economy, as federally owned property, the prison develops its separate emergency management plans.

Cultural, Historical and Natural Resources
Assessing Merced County’s vulnerability to disaster also involves inventorying the natural, historical, and cultural assets of the area. This step is important for the following reasons:
● The community may decide that these types of resources warrant a greater degree of protection due to their unique and irreplaceable nature and contribution to the overall economy. ● In the event of a disaster, an accurate inventory of natural, historical and cultural resources allows for more prudent care in the disaster’s immediate aftermath when the potential for additional impacts is higher.
● The rules for reconstruction, restoration, rehabilitation, and/or replacement are often different for these types of designated resources.
● Natural resources can have beneficial functions that reduce the impacts of natural hazards, for example, wetlands and riparian habitat which help absorb and attenuate floodwaters and thus support overall mitigation objectives.

Historic and Cultural Resources
Historical resources are buildings, structures, objects, places, and areas that are eligible for listing in the National Register of Historic Places (NRHP), the California Register of Historic Resources, or the County’s List of Historic Resources, have an association with important persons, events in history, or cultural heritage, or have distinctive design or construction method.

For purpose of federal actions, a qualified historic resource is defined as a property listed in or formally determined eligible for listing in the NRHP before a disaster occurs. The NRHP is part of a national program to coordinate and support public and private efforts to identify, evaluate, and protect historic and archeological resources. Properties listed include districts, sites, buildings, structures, and objects that are significant in American history, architecture, archeology, engineering, and culture. The National Register is administered by the U.S. Department of the Interior National Park Service. Local and state

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agencies may consider a broader definition of qualified historic properties in the review, evaluation, and treatment of properties damaged during a disaster.

The State of California Office of Historic Preservation can provide technical rehabilitation and preservation services for historic properties affected by a natural disaster. Depending on the hazard, protection could range from emergency preparedness, developing a fire safe zone around sites susceptible to wildfires, or seismically strengthening or structurally reinforcing structures.

State and local registers of historic resources provide designated Historical Landmarks, Points of Historical Interest, and Historic Buildings. These resources include, but are not limited to:
● The California Register of Historical Resources
● The California Historical Landmarks
● The California Inventory of Historical Resources
● The California Points of Historical Interest
Historical Resources designated on a federal, state or local level are listed in Table 4-8.

Table 4-8 Merced County Historical Resources
Property Name Register Jurisdiction Date Listed
Bank of Italy National Merced 10/12/2004
Bank of Los Banos
Building National Los Banos 8/24/1979
Bloss Mansion National Atwater 9/3/1981
Buhach Grammar School National Merced 4/7/1983
Canal Farm Inn State Landmark Los Banos 1/27/1956
Church of St. Joseph National Los Banos 7/8/2004
Cook, Maj. George
Beecher, House National Merced 9/15/1983
G.B. Neighbors Home Point of Interest Snelling 6/2/1967 George S. Bloss Mansion Point of Interest Atwater 3/15/1974 Gwin Post Office, Paddy
Bennett Place Point of Interest Le Grand 1/19/1972
Kaehler-Rector House National Merced 1/4/192
Lake Yosemite Water
Tower Point of Interest Merced 1/19/1972
Leggett House National Merced 10/25/1979
Leggett, Thomas H. House National Merced 7/8/1982
Los Banos State Landmark Los Banos 3/1/1956
Los Banos Creek Point of Interest Los Banos 6/2/1967
Merced County
Courthouse Point of Interest/National Merced 6/2/1967/10/29/1975 Merced County High
School National Merced 5/31/1984
Merced County Justice
Courthouse Building Point of Interest Gustine 8/25/1995 Merced Masonic
Temple/Welch & Griffith State Merced 11/3/2000
Merced Theatre National Merced 5/1/2009
Pacheco Pass State Landmark Los Banos 5/29/1969
San Luis Gonzaga
Archeological District National Los Banos 5/7/1973

Merced County Multi-Jurisdictional Hazard Mitigation Plan Hazard Identification and Risk Assessment
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Property Name Register Jurisdiction Date Listed
Snelling Community
Recreational Hall Point of Interest Snelling 6/2/1967
Snelling Courthouse State Landmark Snelling 11/15/1948
Temporary Detention
Camps for Japanese
Americans-Merced
Assembly Center
State Landmark Merced 5/13/1980
Tioga Hotel National Merced 10/3/1980
U.S. Post Office National Merced 2/10/1983
Source: California Office of Historic Preservation
Other historic sites of local importance also exist. These include several cemeteries and churches located in the larger cities and other rural areas in the county. The five additional sites designated as California Historical Landmarks include the Canal Farm Inn, Los Banos Park, Merced Assembly Center at Merced County Fairgrounds, Pacheco Pass at Romero Outlook, and Snelling Courthouse. Other notable sites include Applegate Park; one of Merced’s central parks located along the Bear Creek Bikeway. The park is an excellent destination on bike or foot. It offers 32 acres of recreation with a zoo, outdoor theater, picnic tables, barbeque grills, volleyball nets, tennis/basketball courts, a skate park, rental facilities, a large playground, a rose garden and fountain, and the Kiwanas-sponsored Kiddieland amusement park.

Applegate Park is located along Bear Creek and the Bear Creek bikeway, between M and R Streets. The southern edge of the park follows 25th Street, P Street, and 26th Street.

Lists of designated historical resources change periodically, and they may not include those currently in the nomination process and not yet listed. Additionally, as defined by the National Environmental Policy Act (NEPA), any property over 50 years of age is considered a historic resource and is potentially eligible for the National Register. Thus, in the event that the property is to be altered, or has been altered, as the result of a major federal action, the property must be evaluated under the guidelines set forth by NEPA.

Structural mitigation projects are considered alterations for the purpose of this regulation.

Cultural resources defined in California Environmental Quality Act Section 15064.5 as include prehistoric and historic archeological resources; historic-period resources (buildings, structures, area, place, or objects). Archeological resources reflect past human activity extending from Native American prehistoric cultures throughout the early 20th century. The artifacts left by previous occupants may be encountered in small to large residential sites, or special use areas.

Many cultural and historical resources in the County are vulnerable to several hazards due to location and the nature of their construction. Some of these risks include earthquakes, wildfires, coastal storms, or adverse weather.

Natural Resources
Natural resources are important to include in benefit/cost analyses for future projects and may be used to leverage additional funding for mitigation projects that also contribute to community goals for protecting sensitive natural resources. Inventory and awareness of natural resource assets is vital to meeting conservation objectives. For example, protecting wetland areas provides sensitive habitat protection as well as floodwater conveyance and storage, which further enhances public safety.

The Merced National Wildlife Refuge which encompasses 10,258 acres of wetlands, native grasslands, vernal pools, and riparian areas. It was established in 1951 under the Lea Act to attract wintering waterfowl away from adjacent farmland where foraging ducks and geese were causing extensive damage

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to crops. Much of the Refuge is located in flood hazard areas and benefits the surrounding area by absorbing floodwaters and recharging aquifers.

To further understand natural resources that may be particularly vulnerable to a hazard event, as well as those that need consideration when implementing mitigation activities, it is important to identify at risk species (endangered and threatened species) in the Planning Area. The US Fish and Wildlife Service (USFW) maintains a list of federally listed threatened and endangered species for the country, which can be queried at the state or even county levels. The California Department of Fish and Wildlife also maintains species lists and accounts for threatened and endangered species. State and federal laws protect the habitat of these species through the environmental review process. Species of special concern may additionally include species that meets the State definition of threatened or endangered but has not been formally listed, experiences seriously population declines, or habitat decline, or has naturally small populations exhibiting high susceptibility to population decline (USFW 2019).

Table 4-9 summarizes Merced County’s special status animal species as indicated in the Fish and Wildlife Service database, within the Environmental Conservation Online System.

Table 4-9 Threatened and Endangered Species in Merced County Group Common Name Scientific Name Status
Amphibians California tiger Salamander Ambystoma californiense Threatened California red-legged frog Rana draytonii Threatened
Birds California condor Gymnogyps californianus Endangered Crustaceans
Conservancy fairy shrimp Branchinecta conservatio Endangered Vernal pool fairy shrimp Branchinecta lynchi Threatened Vernal pool tadpole shrimp Lepidurus packardi Endangered Fishes Delta smelt Hypomesus transpacificus Threatened
Flowering Plants
Greene's tuctoria Tuctoria greenei Endangered
Keck's Checker-mallow Sidalcea keckii Endangered
Colusa grass Neostapfia colusana Threatened
Hartweg's golden sunburst Pseudobahia bahiifolia Endangered Fleshy owl's-clover Castilleja campestris ssp.
succulenta Threatened
Hoover's spurge Chamaesyce hooveri Threatened
Hairy Orcutt grass Orcuttia pilosa Endangered
San Joaquin Orcutt grass Orcuttia inaequalis Threatened Insects
Monarch Butterfly Danaus plexippus Candidate
Valley elderberry longhorn
beetle Desmocerus californicus dimorphus Threatened
Mammals
Giant kangaroo rat Dipodomys ingens Endangered
San Joaquin kit fox Vulpes macrotis mutica Endangered
Fresno kangaroo rat Dipodomys nitratoides exilis Endangered Reptiles Blunt-nosed leopard lizard Gambelia silus Endangered Giant garter snake Thamnophis gigas Threatened
Source: USFW – Environmental Conservation Online System Growth and Development Trends
Population and Projected Growth
Merced County is the fastest growing county in the state, and the region as a whole is projected to grow significantly through 2060 (CA 4th CC Assessment). According to the Department of Finance (DOF) the 2020 population of Merced County as a whole was 283,521. DOF projects the total population will

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increase by 86% to 329,635 by 2030. While total households in the County is also projected to increase from 89,454 in 2020 to 97,063, people per households is project to slightly decrease from 3.43 to 3.31 person per household.

All of the incorporated communities expect Dos Palos (slight decrease -0.1) experienced an increase or no change in population between 2019 and 2020. The City of Merced is listed by the California DOF as one of the top ten cities under 300,000 with the largest numeric change (+2,209) in population between 2019 and 2020 (88,120).

Increases in population growth also increase exposure to severe weather-related hazards, pandemic/epidemics as well as earthquakes. This increase in growth also puts more demand on water resources and can increase vulnerability to drought.

Social Vulnerability
Social vulnerability considerations were included in the update of this plan in 2021 to identify areas across the County that might be more vulnerable to hazard impacts based on a number of factors. A social vulnerability index (SoVI) was developed by the Center for Disease Control’s (CDC) Agency for Toxic Substances and Disease Registry and their Geospatial Research, Analysis & Services Program teams, as a way to portray communities’ capacities to prepare for and respond to natural and manmade disasters. The SoVI does so by providing insight into particularly vulnerable populations to in turn assist emergency response planners and even public health officials identify communities more likely to require additional support before, during, and after a hazardous event. The CDC’s SoVI create county- and state level maps to show relative vulnerability and hence provide key socially and spatially relevant information on communities’ populations, and these maps compare the SoVI based on Census Tracts. The overall social vulnerability based on the SoVI data is shown for Merced County by Census Tracts in Figure 4-5 below, based on statewide ranking. This overall index combines four main themes of vulnerability, namely:
socioeconomic status; household composition and disability; minority status and language; and housing and transportation, which in turn are comprised of subcategories for a total of 15 variables accounting for various vulnerability factors. For additional information on the CDC’s SoVI, refer to their documentation and materials online at https://svi.cdc.gov/.

Based on this data, the areas with the highest level of social vulnerability in the County are in the western and southeastern portion of the County, as well as some of the incorporated jurisdictions including Merced and Los Banos.

Merced County Multi-Jurisdictional Hazard Mitigation Plan Hazard Identification and Risk Assessment
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Figure 4-3 Overall Social Vulnerability in Merced County based on the SoVI, by Census Tracts

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Development Trends
As noted, the County has experienced population growth in the last decade and since the last plan update. There has been an increase in housing unit development, primarily in the cities of Merced, Gustine, Livingston and the unincorporated county. This new development built to modern codes should be less vulnerable to severe weather and earthquakes. Development in flood hazard areas should be mitigated to withstand the 1% annual chance flood; however, development in the 0.2% flood hazard zone is not regulated at this time. Although most development in the City of Merced and around the UC Merced campus has occurred in downtown Merced and in areas outside the floodplain.

Refer to Table 4-10 for comparison of housing units from the 2010 census to the ACS 5-Year estimates for 2015-2019 in the incorporated and unincorporated areas of Merced County. This comparison provides additional information on changes in development trends since the 2015 MJHMP was approved, for both the municipalities and the Census Designated Places (CDP) in the unincorporated areas.

Table 4-10 Housing Unit Development, 2010-2019
Jurisdiction 2010 2019 % Change
Merced County 83,698 85,251 2%
City of Atwater 9,771 9,577 -2%
Ballico CDP 132 182 38%
Bear Creek CDP 90 61 -32%
Cressey CDP 123 113 -8%
Delhi CDP 2,854 3,030 6%
City of Dos Palos 1,700 1,545 -9%
Dos Palos Y CDP 123 94 -24%
El Nido CDP 105 97 -8%
Franklin CDP 1,907 1,976 4%
City of Gustine 2,087 2,249 8%
Hilmar-Irwin CDP 1,841 2,001 9%
Le Grand CDP 503 554 10%
City of Livingston 3,320 3,699 11%
City of Los Banos 11,375 11,293 -1%
McSwain CDP 1,409 1,590 13%
City of Merced 27,446 26,873 -2%
Planada CDP 1,207 1,338 11%
Santa Nella CDP 493 614 25%
Snelling CDP 112 69 -38%
South Dos Palos CDP 488 629 29%
Stevinson CDP 101 154 52%
Tuttle CDP 39 48 23%
University of California Merced CDP 0 0 0%
Volta CDP 106 69 -35%
Winton CDP 3,056 3,305 8%
Source: California Dept. of Finance
The areas located in the Sphere of Influence (SOI) for each incorporated jurisdiction are areas each City plans to grow into and potentially slated for future development. Understanding the potential hazard exposure in each area can help to mitigate the impacts of events before development occurs in those areas. During this plan update process parcel analysis was conducted using the SOI areas for each incorporated jurisdiction and overlaid with available hazard risk layers to determine where future development may be at risk of natural hazard events. The results of the analysis have been integrated into

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the applicable hazard sections: dam, flood, wildfire and landslide. Table 4-10 is the summary of the SOI total exposure by jurisdiction.

Table 4-11 Total Exposure Summary by Jurisdiction Sphere of Influence Areas Jurisdiction Improved
Parcel Count
Building
Count Improved Value Estimated Content Value Total Value Atwater 675 837 $150,375,296 $124,680,732 $275,056,028
Dos Palos 545 688 $53,185,264 $32,248,327 $85,433,591
Gustine 97 126 $17,005,380 $15,800,283 $32,805,663
Livingston 60 85 $13,899,670 $12,073,468 $25,973,138
Los Banos 194 253 $28,473,256 $23,323,506 $51,796,762
Merced 2,528 2,896 $554,502,737 $328,069,420 $882,572,157 Total 4,099 4,885 $817,441,603 $536,195,735 $1,353,637,338 ● City of Dos Palos - Received approval for a grant that will fund the construction of a new Water Treatment facility. Once the new facility is completed and operational, development can begin on new residential and commercial projects.
● City of Merced - City Council voted in February of 2021 to move forward the annexation of UC Merced. This would allow for additional annexations to potentially occur and would include up to 7,800 acres of land. Currently, much of this land is rolling grass land or grazing land. Much of this property will likely be developed into commercial, retail and high-density housing to support the demands of UC Merced.
● Merced County – The HMPT noted the growth and development trends associated with UC Merced, high-speed rail and related transit-oriented growth and growth at Castle Air Force Base associated with autonomous vehicle testing.

4.2 Hazard Analysis and Risk Summary
Requirement §201.6(c)(2)(i):
[The risk assessment shall include a] description of the type, location and extent of all natural hazards that can affect the jurisdiction. The plan shall include information on previous occurrences of hazard events and on the probability of future hazard events.

4.2.1 Dam Incidents
Hazard/Problem Description
Dams are manmade structures built for a variety of uses, including flood protection, power generation, agriculture, water supply, and recreation. When dams are constructed for flood protection, they usually are engineered to withstand a flood with a computed risk of occurrence. For example, a dam may be designed to contain a flood at a location on a stream that has a certain probability of occurring in any one year. If prolonged periods of rainfall and flooding occur that exceed the design requirements, that structure may be overtopped and fail. Overtopping is the primary cause of earthen dam failure in the United States. Dam failures can also result from any one or a combination of the following causes:
● Earthquake
● Inadequate spillway capacity resulting in excess overtopping flows ● Internal erosion caused by embankment or foundation leakage or piping or rodent activity ● Improper design
● Improper maintenance

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● Negligent operation
● Failure of upstream dams on the same waterway
Water released by a failed dam generates tremendous energy and can cause a flood that is catastrophic to life and property. A catastrophic dam failure could challenge local response capabilities and require evacuations to save lives. Impacts to life safety will depend on the warning time and the resources available to notify and evacuate the public. Major loss of life could result as well as potentially catastrophic effects to roads, bridges, and homes. Associated water quality and health concerns could also be issues. Factors that influence the potential severity of a full or partial dam failure are the amount of water impounded; the density, type, and value of development and infrastructure located downstream; and the speed of failure.

Controlled release or spillway flooding: Spillways are designed to relieve pressure on dams and prevent dam failures. Flooding downstream often results when spillways flow, though the potential for flooding as a result of discharge from dam outlet structures can also result from excessive rain events. However, controlled releases of water from dams is a measure that can prevent or minimize spillway flooding or structure failure, by regulating capacity in a managed way. Even controlled releases can lead to unwanted or unpredicted flooding, depending on environmental and weather conditions, or even human error.

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

Geographic Area
According to the California Department of Water Resources’ Jurisdictional Dams and the National Inventory of Dams (NID) databases there are 44 dams of concern to Merced County; ten of which are in the County and 34 upstream of the County. Table 4-11 lists the high and significant hazard dams within and upstream of Merced County.

Figure 4-4 shows where the high and significant hazard dams are located, and the potential inundation areas based on the best available data at the time of this plan update. It is important to note that the inundation areas shown do not represent all dams that pose a risk; some of this information is not available in GIS or allowed for release in a public document. Virtually no urban area in the County is free from flooding in the event of dam failure. Potential impacts are greatest for Merced, Livingston and the central areas of the unincorporated Merced County.

Table 4-12 Dams of Concern Within and Upstream of Merced County County Dam Name River Downstrea
m City
Normal
Storage
Capacity
(Acre Feet)
Emergency
Action
Plan
(EAP)?
NID
Hazard
Rating

CA DSOD
Hazard
Rating*
Merced
Lake Yosemite Merced 10,323 No High Extremely High
Castle Canal Creek 7,510 No High Extremely High
Merced Falls Merced Snelling 603 Yes High Low
Burns Burns Creek Planada 6,800 Yes High -
Los Banos Creek
Detention
Los Banos
Creek
Los Banos 34,500 Yes High -

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County Dam Name River Downstrea
m City
Normal
Storage
Capacity
(Acre Feet)
Emergency
Action
Plan
(EAP)?
NID
Hazard
Rating

CA DSOD
Hazard
Rating*
O’Neill Forebay San Luis
Creek
Volta 56,430 Yes High -
B.F. Sisk Dike San Luis
Creek
- Yes High -
B.F. Sisk San Luis
Creek
2,094,900 Yes High -
Mustang Creek Mustang
Creek
700 No Significant Significant
Kelsey S. Fork Dry 1,000 No Significant Significant
Fresno
Alluvial Drain
Detention Big Dry Creek 833 No High
Big Dry Creek Big Dry Creek
& Dog Creek 30,200 No High
Fancher Creek
Fancher
Creek and
Hog Creek
9,600 No High
Fancher Creek
Detention
Fancher
Creek 1,891 No High
Friant Dike 3 520,500 Yes High
Friant Millerton
Road
Embankment A
San Joaquin
River Fresno 520,500 Yes High
Griffen Reservoir Holland
Creek
900 Yes High
Little Panoche
Detention
Little
Panoche
Creek
Oro Loma
5,580
Yes High
Pine Flat Dam Kings Sanger 1,000,000 Yes High
Redbank Redbank
Creek
1,100 No High
Redbank Creek
Detention Basin
Redbank
Creek
940
No High
Madera
Buchanan Dam Chowchilla
River Chowchilla 105,000 Yes High
Friant San Joaquin
River Fresno 520,500 Yes High
Hidden Dam Fresno River Madera 90,000 Yes High
Madera Lake Fresno River 2,300 No High
Berenda Slough Berenda
Slough 2,450 No Significant
Black Hawk Coarse Gold
Cr 740 No Significant
Mendota
Diversion
San Joaquin
River 3,000 No Significant
Oakhurst
Wastewater
Treatment
Tr Fresno
River 110 No Significant

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County Dam Name River Downstrea
m City
Normal
Storage
Capacity
(Acre Feet)
Emergency
Action
Plan
(EAP)?
NID
Hazard
Rating

CA DSOD
Hazard
Rating*
Spring Longhollow
Creek 152 Yes Significant
Upper Wilcox
Unnamed
Tributary to
Picayunne
Creek
200 No Significant
Mariposa
Bear Dam Bear Creek Planada 7,700 Yes High
Exchequer Dike Merced River Snelling 1,024,600 Yes High Exchequer Main Merced River Snelling 1,024,600 Yes High Mariposa Dam Mariposa
Creek Le Grand 15,000 Yes High
Mcswain Merced River Snelling 8,255 Yes High
Owens Dam Owens Creek Planada 3,600 Yes High
Stockton Creek Stockton
Creek 368 No High
Stanislaus Turlock Lake Tuolumne
River 63,406 Yes High
Tuolumne
Don Pedro Dike
B
Tuolumne
River La Grange 2,030,000 Yes High
Don Pedro Main Tuolumne
River La Grange 2,030,000 Yes High
Gasburg Creek
Dike
Tuolumne
River La Grange 1 Yes High
Don Pedro Dike
A
Tuolumne
River La Grange 2,030,000 Yes Significant
Don Pedro Dike
C
Tuolumne
River La Grange 2,030,000 Yes Significant
Source: California Department of Water Resources Division of Safety of Dams (DSOD) and the NID * DSOD added a fourth category (“Extremely High”) to the NID categories for downstream hazard potential. Extremely High is defined as expected to cause considerable loss of human life or would result in an inundation area with a population of 1,000 or more.

Analysis using inundation data and mapping associated with the O’Neil Forebay, Los Banos, and San Luis dams was not possible for the plan due to information being considered sensitive and restricted, per communication with OES and the dam owners. The following is a summary of the downstream communities that would be affected by each dam in a dam failure or incident based on information in dam emergency action plans, including depth of inundation and time to inundation. ● O’Neil Forebay Dam
- Santa Nella: 4-10 feet within 5 hours of failure
- Ingoma: 0-2 feet within 10 hours
- Volta: 2-4 feet within 5 hours
● Los Banos Dam
- Los Banos: 5-6 feet within 2.5 hours
- Volta: 5-6 feet within 2.5 hours
- Trent: 5-6 feet within 2.5 hours

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● San Luis Dam
- Santa Nella: 10-20 ft within 2 hours
- Volta: 10-20 ft in less than 10 hours
- Ingomar: 10-20 ft in less than 10 hours
- Trent: 2-4 ft in 5 hours
- Gustine: 2-10 ft in less than 10 hours
- Stevenson: 2-10 in 15 hours

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Figure 4-4 Dams of Concern to Merced County and Best Available Inundation Mapping

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Extent (Magnitude/Severity)
Standard practice among federal and state dam safety offices is to classify a dam according to the potential impact a dam failure (breach) or mis-operation (unscheduled release) would have on downstream areas. The hazard potential classification system categorizes dams based on the probable loss of human life and the impacts on economic, environmental and lifeline facilities. The US Army Corp of Engineers uses three categories to classify dams’ potential hazard to life and property:
● High hazard indicates that a failure would most probably result in the loss of life; ● Significant hazard indicates that a failure could result in appreciable property damage; ● Low hazard indicates that failure would result in only minimal property damage and loss of life is unlikely.
● Undetermined hazard dams have not been rated or their hazard rating is not known In addition to these, high, significant, and low hazard classifications the California Division of Dam Safety (DSOD) adds the fourth category of “Extremely High”. The DSOD defines this fourth category as “expected to cause considerable loss of human life or would result in an inundation area with a population of 1,000 or more” (DSOD 2020). As shown in Table 4-11 two dams in Merced County, Lake Yosemite and Castle dams are classified as “Extremely High”.

In 2017 the State of California established the Spillway Re-evaluation Program after the Orville Dam incident, to assess spillways to confirm they meet minimum safety standards. The Don Pedro dam located in Tuolumne County was listed as being part of this program and poses a risk to central portions of Merced County.

Since the County has several high and significant hazard dams, there is potential for loss of life and property damage. The inundation areas for each of the dams are generally downstream and include large rural and urban areas on the valley floor below the dams. Adjacent jurisdictions could also be affected by a dam failure in Merced County. The extent of impacts depends on the nature of failure and location of the dam. The largest populations potentially at risk would be in Merced, Livingston, and Atwater. The central portions of the unincorporated areas county would also be impacted were a dam to fail and flood downstream.

Past Occurrences
There is no history of dam failure affecting the county, but according to the HMPT there have been recurring issues with flooding due to high flows released below dams in the area.

Probability of Future Occurrences
Occasional—The County remains at risk to dam failures from numerous dams under a variety of ownership and control and of varying ages and conditions. However, based on historical experience dam failure is unlikely in the area. Nevertheless, given the number of dams of concern in the County the potential exists for future dam failures in Merced County, but the likelihood of this is low. Uncontrolled or controlled release flooding as well as spillway flooding below dams due to excessive rain or runoff are more likely to occur than failures.

Climate Change Considerations
The potential for climate change to affect the likelihood of dam failure is not fully understood at this point in time. With a potential for more extreme precipitation events a result of climate change, this could result in large inflows to reservoirs. However, this could be offset by generally lower reservoir levels if storage

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water resources become more limited or stretched in the future due to climate change, drought and/or population growth.

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

Secondary losses would include loss of the multi-use functions of the dam itself and associated revenues that accompany those functions.

General Property
In general, communities located below a high or significant hazard dam and along a waterway are potentially exposed to the impacts of a dam failure. Inundation maps that identify anticipated flooded areas (which may not coincide with known floodplains) are typically produced for all high hazard dams and included in the Emergency Action Plan (EAP) required for each dam. A GIS layer that contained inundation maps for four of the 34 high hazard dams that affect the County was analyzed to quantify risk across the planning area. Table 4-13 through Table 4-16 summarize the estimated building exposure (more details on the parcel analysis methodology under the Asset Summary section). Table 4-12 summarizes the results of the GIS analysis, and includes a population estimate based on the number of residential parcels exposed, multiplied by an average household size estimate from the census. Exposure estimates from failure of other high hazard dams were not possible due to data restrictions and/or limitations.

Table 4-13 Summary of Dam Inundation by Dam and Jurisdiction Dam Jurisdiction Improved
Parcel Count
Building
Count Population
Don Pedro Dam
Livingston 1 1 -
Unincorporated 3,888 4,909 8,353
Total 3,889 4,910 8,353
McSwain Dam
Livingston 1 1 -
Unincorporated 160 180 309
Total 161 181 309
New Exchequer Dam
Atwater 37 37 -
Livingston 3,340 4,001 14,216
Unincorporated 8,869 11,138 19,505
Total 12,246 15,176 33,721
Yosemite Dam
Atwater 21 23 67
Merced 4,768 6,080 18,784
Unincorporated 1,024 1,359 3,214
Total 5,813 7,462 22,065
Grand Total 22,109 27,729 64,447
Source: California DSOS, NID, Merced County Assessor’s Office Table 4-14 Don Pedro Dam Inundation
Jurisdiction Property Type Improved
Parcel Count Building Count
Livingston Agricultural 1 1
Total 1 1
Unincorporated Agricultural 1,352 1,968
Commercial 97 333

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Jurisdiction Property Type Improved
Parcel Count Building Count
Exempt 40 56
Industrial 15 19
Mixed Use 2 2
Residential 2,365 2,514
Vacant Land 17 17
Total 3,888 4,909
Grand Total 3,889 4,910
Source: California DSOS, NID, Merced County Assessor’s Office Table 4-15 McSwain Dam Inundation
Jurisdiction Property Type Improved
Parcel Count Building Count
Livingston Agricultural 1 1
Total 1 1
Unincorporated
Agricultural 48 59
Commercial 13 18
Exempt 7 8
Industrial 1 1
Residential 90 93
Vacant Land 1 1
Total 160 180
Grand Total 161 181
Source: California DSOS, NID, Merced County Assessor’s Office Table 4-16 New Exchequer Dam Inundation
Jurisdiction Property Type Improved
Parcel Count
Building Count
Atwater Residential 37 37
Total 37 37
Livingston
Agricultural 8 9
Commercial 79 231
Exempt 48 113
Industrial 19 33
Mixed Use 3 4
Residential 3,176 3,604
Vacant Land 7 7
Total 3,340 4,001
Unincorporated
Agricultural 3,183 4,415
Commercial 223 603
Exempt 101 144
Industrial 41 64
Mixed Use 12 15
Residential 5,273 5,860
Vacant Land 36 37
Total 8,869 11,138
Grand Total 12,246 15,176
Source: California DSOS, NID, Merced County Assessor’s Office

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Table 4-17 Lake Yosemite Dam Inundation
Jurisdiction Property Type Improved
Parcel Count
Building Count
Atwater
Commercial 1 2
Residential 20 21
Total 21 23
Merced
Agricultural 5 5
Commercial 24 30
Exempt 110 134
Industrial 22 38
Mixed Use 1 1
Residential 4,603 5,869
Vacant Land 3 3
Total 4,768 6,080
Unincorporated
Agricultural 36 46
Commercial 34 305
Exempt 10 12
Industrial 18 22
Mixed Use 3 3
Residential 917 965
Vacant Land 6 6
Total 1,024 1,359
Grand Total 5,813 7,462
Source: California DSOS, NID, Merced County Assessor’s Office People
Persons located underneath or downstream of a dam are at risk of a dam failure, though the level of risk can be tempered by topography (specifically where populations are located within the inundation path of a dam), amount of water in the reservoir and time of day of the breach. Injuries and fatalities can occur from debris, bodily injury and drowning. Once a dam has breached, standing water presents all the same hazards to people as floodwater from other sources. People in the inundation area may need to be evacuated, cared for, and possibly permanently relocated. Impacts could include thousands of evacuations and likely hundreds of casualties, depending on the dam involved. Specific population impacts are noted in Table 4-12 , and total people at risk were calculated by multiplying the average number of persons per household in Merced County (3.32) and each applicable incorporated jurisdiction (Atwater 3.18, Livingston

3.94, and Merced 3.20) times the number of residential parcels where dam inundation occurs. Based on the best available data accessible to the planning team at this time of this analysis, an incident at the New Exchequer Dam would potentially put the most people at risk (33,721), followed by Lake Yosemite dam (22,064) and Don Pedro dam (8,353). The unincorporated areas of Merced County have the most people (31,381) potentially exposed to a dam inundation event followed by City of Merced with potentially 18,784 exposed to the Lake Yosemite dam inundation. Population impacts from other high hazard dams were not possible due to data restrictions and/or limitations.

Critical Facilities
A total dam failure can cause catastrophic impacts to areas downstream of the water body, including critical infrastructure. Any critical asset located under the dam in an inundation area would be susceptible to the impacts of a dam failure. Of particular risk would be roads and bridges that could be vulnerable to washouts, further complicating response and recovery by cutting off impacted areas. Based on the critical facility inventory considered in the updating of this plan and intersected with the dam inundation extents

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available, 272 critical facilities were found to be at risk. Unincorporated County has the greatest number of facilities at risk followed by City of Merced and City of Livingston. These at risk facilities are listed in the tables below by jurisdiction and critical facility classification as based on the FEMA Lifeline categories.

Table 4-18 Dam Inundation Effects on Critical Facilities – Estimates by Jurisdiction and FEMA Lifeline
Jurisdiction
CommunicationsEnergyFood, Water, ShelterHazardous MaterialHealth and MedicalSafety and SecurityTransportationTotal Atwater - - - - - - 1 1
Dos Palos - - - - - - - 0
Gustine - - - - - - - 0
Livingston 6 1 3 3 - 13 5 31
Los Banos - - - - - - - 0
Merced 4 2 6 3 3 4 13 35
Unincorporated 21 8 16 8 - 64 88 205
Total 31 11 25 14 3 81 107 272
Source: HFILD, Merced County, Cities of Atwater, Dos Palos, Gustine, Livingston, Los Banos, Merced As noted under the Geographic Area section, inundation data is not available for the O’Neil Forebay, Los Banos or San Luis Dams due to being restricted and sensitive information. The following is a summary of the potential impacts on critical facilities from a dam failure or inundation event. ● O’Neil Forebay Dam
- Potential Impacts to Transportation infrastructure:
o Interstate 5
o Highway 33
o Highway 140
- Potential Impacts to Water Systems infrastructure:
o Delta Mendota
o The California Aqueduct
● Los Banos Dam
- Potential impacts to Transportation infrastructure:
o Interstate 5
o Highway 152
o Highway 165
- Potential impacts to Water Systems infrastructure:
o Delta Mendota
o The California Aqueduct
● San Luis Dam
- Potential impacts to Transportation infrastructure:
o Interstate 5
o Highway 152

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o Interstate 165
o Highway 33
o Highway 140
- Potential impacts to Water Systems infrastructure:
o Delta Mendota
o The California Aqueduct
- Potential impacts to Power/Energy infrastructure:
o PG&E substations
o Solar fields
Economy
Extensive and long-lasting economic impacts could result from a major dam failure including the longterm loss of water in a reservoir after a failure event. Based on the analysis there is 6,504 buildings, at a minimum, that are designated as agricultural properties potentially exposed to dam inundation event. A major dam incident and loss of water from the associated reservoir could include direct business and specifically impacts to the agricultural industry, resulting in damages and indirect disruption of the local economy.

Historic, Cultural and Natural Resources
Dam failure effects on the environment would be similar to those caused by flooding from other causes.

Water could erode stream channels and topsoil and cover the environment with debris. For the most part the environment is resilient and would be able to rebound from whatever damages occurred, though this process could take years. However, historic and cultural resources could be affected just as housing or critical infrastructures would, were a dam to fail and cause downstream inundation that could further erode surfaces or cause scouring of structural foundations.

Future Development
GIS was used to analyze parcels exposed to dam inundation areas that are located within each city’s SOI.

The results shown in Table 4-18 show that there is a total of 6,917 buildings and 18,247 currently exposed to dam inundation within the SOI areas.

Table 4-19 Total Dam Inundation Summary by Dam within Sphere of Influence Areas Dam Inundation Sphere of
Influence
Improved
Parcel Count
Building
Count Population
Don Pedro Dam
Delhi 55 55 183
Hilmar 1,450 1,778 4,731
Livingston 1 3 0
Total 1,506 1,836 4,914
McSwain Dam
Cressey 1 1 3
Livingston 1 1 0
Snelling 104 111 286
Total 106 113 289
New Exchequer Dam
Atwater 10 12 7
Ballico 59 65 166
Cressey 78 92 262
Delhi 1,021 1,319 3,788
Hilmar 1,450 1,778 4,731
Livingston 59 83 149
Snelling 145 170 412

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Dam Inundation Sphere of
Influence
Improved
Parcel Count
Building
Count Population
Stevinson 53 65 149
Winton 98 113 375
Total 2,973 3,697 10,040
Yosemite Dam
Atwater 42 44 120
Franklin 774 1,080 2,520
Merced 128 147 365
Total 944 1,271 3,005
Grand Total 5,529 6,917 18,247
Source: California DSOS, NID, Merced County Assessor’s Office The areas located in the SOI for each incorporated jurisdiction are areas each City plans to grow into and potentially slated for future development. In addition to the SOI areas there are also 71 vacant parcels countywide that are within dam inundation areas. These areas should take into consideration potential impacts from dam failure risk upstream and should attempt to overlay the existing dam inundation maps with proposed future development. In the case of a dam failure, inundation would likely follow some existing FEMA mapped floodplains, which contains development restrictions for areas in the 1% annual chance floodplain, but it could exceed those floodplains and affect areas that are not regulated for flood hazards. Also, of note is that development below a low or undetermined hazard dam could increase its hazard rating, though there is only one low hazard and one undetermined hazard dam in the County (while the majority are high or significant hazard dams). Finally, added development could compromise dams and reservoir resources if populations depend on them for critical needs such as potable water during or after a dam failure event.

Risk Summary
● There are 44 dams of concern for Merced County. 10 are within the county (8 High Hazard Potential and two Significant) and 34 dams are upstream of the county (26 High Hazard and eight Significant). ● Of the eight high hazard potential rated dams in the County, 2 (Castle and Yosemite dams) are rated as “Extremely High” by the State of California DSOD.
● Countywide there are a minimum of 27,729 parcels at risk based on limited available inundation mapping, with greatest number of buildings exposed in unincorporated areas (17,586) followed by the City of Merced (6,080), City of Livingston (4,003) and City of Atwater (60). ● Residential property types make up the most buildings (18,963) at risk of flooding from dam failures, agricultural (6,504), and commercial (1,522).
● Based on limited available mapping, the New Exchequer Dam puts the most parcels at risk of dam inundation (33,721) countywide (most in Unincorporated and Livingston), followed by the Yosemite dam (22,065).
● An estimated 64,447 people countywide might be displaced from their homes based the location of their residences along the inundation areas; actual amounts would vary. ● A total of 272 critical facilities are within dam inundation areas. The greatest number in unincorporated Merced County (205) and Transportation lifeline is most at risk countywide. ● Related Hazards: drought, flooding, earthquake/liquefaction, landslide, and severe weather.

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Table 4-20 Hazard Risk Summary by Jurisdiction - Dam Incidents Jurisdiction Geographic Area
Probability of
Future
Occurrence
Magnitude/
Severity
Overall
Significance
Merced County Significant Occasional Catastrophic Medium City of Atwater Extensive Occasional Limited Medium
City of Dos Palos Limited Occasional Limited Medium
City of Gustine Extensive Occasional Limited Low
City of Livingston Limited Occasional Limited Medium
City of Los Banos Extensive Occasional Catastrophic Medium City of Merced Extensive Occasional Catastrophic High

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4.2.2 Drought
Hazard/Problem Description
Drought is a gradual phenomenon. Normally, one dry year does not constitute a drought in California, but rather serves as a reminder of the need to plan for droughts. California's extensive system of water supply infrastructure (reservoirs, groundwater basins, and interregional conveyance facilities) generally mitigates the effects of short‐term dry periods for most water users.

Drought can have secondary impacts. For example, drought is a major determinant of wildfire hazard, in that it creates greater propensity for fire starts and larger, more prolonged conflagrations fueled by excessively dry vegetation, along with reduced water supply for firefighting purposes. Drought is also an economic hazard. Significant economic impacts on California’s agriculture industry can occur as a result of short‐ and long‐term drought conditions; these include hardships to farmers, farm workers, packers, and shippers of agricultural products. In some cases, droughts can also cause significant increases in food prices to the consumer due to shortages.

Drought is a complex issue involving many factors—it occurs when a normal amount of moisture is not available to satisfy an area’s usual water-consuming activities. Drought can often be defined regionally based on its effects:
● Meteorological drought is defined by a period of substantially diminished precipitation duration and/or intensity. The commonly used definition of meteorological drought is an interval of time, generally on the order of months or years, during which the actual moisture supply at a given place consistently falls below the climatically appropriate moisture supply. ● Agricultural drought occurs when there is inadequate soil moisture to meet the needs of a particular crop at a particular time. Agricultural drought usually occurs after or during meteorological drought, but before hydrological drought and can affect livestock and other dry-land agricultural operations. ● Hydrological drought refers to deficiencies in surface and subsurface water supplies. It is measured as stream flow, snowpack, and as lake, reservoir, and groundwater levels. There is usually a delay between lack of rain or snow and less measurable water in streams, lakes, and reservoirs. Therefore, hydrological measurements tend to lag other drought indicators. ● Socioeconomic drought occurs when physical water shortages start to affect the health, well-being, and quality of life of the people, or when the drought starts to affect the supply and demand of an economic product.

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

Drought impacts are wide-reaching and may be economic, environmental, and/or societal. The most significant impacts associated with drought in the planning area are those related to water intensive activities such as agriculture, wildfire protection, municipal usage, commerce, tourism, recreation, and

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wildlife preservation. Also, during a drought, allocations go down, which results in reduced water availability. Voluntary water conservation measures are typically implemented during extended droughts.

A reduction of electric power generation and water quality deterioration are also potential problems that can occur as a result of drought conditions. The reduced demand for electrical power generation is commonly linked to higher electricity costs due to the loss of hydropower supplies. Water quality deterioration is due to lower levels of precipitation and limited water storage supply. Drought conditions can also cause soil to compact and not absorb water well, potentially making an area more susceptible to flooding.

Geographic Area
Drought is a regional hazard, and at its worst can affect the entire state of California with varying levels of dryness and drought activity (as will be covered in more detail under the sections to follow). It is safe to assume that unless the drought event is at its very beginning or very end, if any area of the Merced County is affected by any level of drought, the other areas of the county are experiencing varying effects as well.

Sustainable Groundwater Management Act (SGMA) of 2014
In January 2014, Governor Brown declared an emergency proclamation due to multiple years of drought.

The proclamation called on citizens to reduce water use by 20 percent; with a subsequent executive order in April 2015 that directed urban water agencies to reduce water use by 25 percent (Ken Topping, 2016).

In September 2014, the Governor signed a three-bill package (California Senate Bills 1168 and 1319, and Assembly Bill 1739), known as the SGMA. The SGMA provides for the establishment of local GSAs to manage groundwater sustainability within the groundwater subbasins defined by the California DWR.

Within Merced County there are portions of four groundwater subbasins, Merced, Turlock, Delta Mendota, and Chowchilla, that are required under SGMA to conduct sustainable groundwater planning and management. Each of the four subbasins are required to develop and implement a GSP by 2020 or 2022.

As of April 2021, all four subbasins have formed 14 GSAs as shown in Table 4-20.

Table 4-21 Groundwater Sustainability Agencies in Merced County Subbasin Groundwater Sustainability Agency (GSA)
Merced
Merced-Irrigation-Urban GSA
Merced Subbasin GSA
Turner Island Water District GSA #1
Turlock West Turlock GSA
East Turlock GSA
Delta Mendota
City of Dos Palos GSA
City of Gustine GSA
City of Los Banos GSA
Central Delta Mendota GSA
Turner Island Delta Mendota GSA
Northwestern Delta Mendota GSA
Grassland Water District GSA
San Joaquin River Exchange Contractors GSA
Chowchilla Chowchilla Water District GSA
County of Merced Chowchilla GSA
Source: Merced County Water Resources https://www.co.merced.ca.us/3140/Sustainable-Groundwater---SGMA

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Extent (Magnitude/Severity)
The magnitude of a drought’s impact is directly related to the severity and length. The severity of a drought depends on water availability and moisture deficiency, the time period, and the size and location of the affected area. The longer the drought persists and the larger the area impacted, the more severe the potential impacts. Droughts can be a short-term event over several months or a long-term event that lasts for years or even decades. In Merced County, the onset of drought is often signaled by a lack of significant winter precipitation and snowfall (moisture deficiency) in the Sierra Nevada Mountains.

Droughts typically do not result in direct impacts on people or property, but they can have significant impacts on agriculture, which can indirectly impact people and property. Hot and dry conditions that persist into spring, summer, and fall can aggravate drought conditions, making the effects of drought more pronounced as water demands increase during the growing season and summer months. Impacts increase with the length of a drought, as carry over supplies in reservoirs are depleted and water levels in groundwater basins decline (California DWR 2012).

The U.S. Drought Monitor is an accepted and widely used site for obtaining and summarizing drought information, as it integrates data from several other sources including the Palmer Drought Index, Soil Moisture Models, U.S. Geological Survey Weekly Stream Flows, Standardized Precipitation Index, and the Satellite Vegetation Health Index. It includes drought intensity categories for measuring dry conditions across counties, states, and regions of the U.S., so that drought can be quantified. These categories range from “abnormally dry” to “exceptional drought.” The following figures provide “snapshots in time” of the drought conditions in California as of August 2015 (during the period of the last multi-year drought in Merced County and the state, from 2012- 2017) and March 30, 2021. The snapshots selected are instrumental in depicting both the historic and potential change in drought’s geographic range and severity in the County (circled in blue).

Note: The Drought Monitor maps integrate data from several sources including the Palmer Drought Index, Soil Moisture Models, U.S. Geological Survey Weekly Stream flows, Standardized Precipitation Index, and Satellite Vegetation Health Index. The drought status in California is noted on the following graphic from the National Drought Mitigation Center.

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Figure 4-5 U.S. Drought Monitor for California: August 4, 2015

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Figure 4-6 U.S. Drought Monitor for California: March 30, 2021 Past Occurrences
Historically, California has experienced severe drought conditions. The state’s available record for determining hydrologic risks is short, only going back about 100 years. Recent droughts affecting Merced County are summarized below using data from Cal OES and from the County Agricultural Commissioner’s Office.
● 1928-1937—This drought affected the entire state and is the longest, most severe drought on record with a recurrence interval of greater than 100 years.
● 1947-1950—Drought affected the entire state but was most extreme in Southern California. The drought in winter of 1950 affected the area from the Kern River basin north to the American River basin. The drought caused two deaths and $33 million in damages. ● 1976-1977—The drought of 1976-1977 was most severe in the northern three-quarters of California, but the impact was experienced statewide because of the dependence of Southern California on water transfers from the north. The water year 1977 was the driest year of record at almost all gauging stations in the affected area in California, and the water year 1976 was among the five driest in the central and northern Sierra Nevada. The two-year deficiency in runoff accumulated during the drought is unequalled at gauging stations in the affected area; and this deficiency has a recurrence interval that exceeds 80 years. Crop damages statewide were $2.67 billion. ● 1987-1992—California experienced a serious drought due to low precipitation and runoff levels.

During this multi-year, multi-county drought, the runoff from the San Joaquin Valley was 47 percent of average. In 1991, the U.S. Department of Agriculture Economic Research Report Agricultural Outlook reported that the Merced River flow would be inadequate to provide sufficient water for

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agricultural uses for the fifth consecutive year. A U.S. Department of Agriculture (USDA) drought disaster declaration was declared.
● 2004-2005—On January 15, 2005, the USDA designated 53 of California’s 58 counties as natural disaster areas due to damages and losses caused by extreme drought. ● 2008-2009—On June 12, 2008, the Governor proclaimed Merced County as a state disaster area due to the Central Valley Drought.
● 2012 – 2017—In September 2012, the USDA designated Merced County a contiguous disaster area due to drought that occurred since January 1, 2012. In January 2014, the Governor proclaimed a State of Emergency and directed state officials to take all necessary actions to prepare for these drought conditions: 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 an expansive water conservation public awareness campaign. Drought produced severe impacts to water wells throughout the planning area, with a high number of wells running dry. Land subsidence due to increased groundwater pumping also occurred in areas of the San Joaquin Valley. Crop damage was widespread as well. Water allotments were drastically reduced in many towns and water agencies, with extremely high costs for procuring water. In addition, job loss occurred with many families requiring food supply assistance, and water supply assistance provided to homeowners with dry wells.

According to a report released by UC Davis Center for Watershed Sciences, the 2014 California drought cost the state's agriculture industry about $1 billion in lost revenue, with a total statewide economic cost of the drought calculated to be $2.2 billion. The 2014 drought, the report says, is responsible for the greatest water loss ever seen in California agriculture - about one-third less than normal. The report calls the groundwater situation in California "a slow-moving train wreck." (NCICS 2019). According to the 2018 State of California Hazard Mitigation Plan, by the time the drought was declared officially in 2017, the state been declared a federal disaster area and had expended $6.6 billion in drought response and mitigation programs.

As shown in Table 4-4, of the 18 total Secretary of Agricultural Disaster Designations for Merced County in the past 8 years, 15 were specific to drought. In addition to the USDA designations there have also been two federal disaster declarations due to drought in the county (Refer to Table 4-3).

The National Drought Mitigation Center developed the Drought Impact Reporter in response to the need for a national drought impact database for the United States. Information comes from a variety of sources:
online, drought-related news stories and scientific publications, members of the public who visit the website and submit a drought-related impact for their region, members of the media, and members of relevant government agencies. The database is being populated beginning with the most recent impacts and working backward in time. The Drought Impact Reporter contains information on 195 impacts from droughts that affected Merced County between 2000 and 2020. Most of the impacts (113) were classified as “water supply and quality” and “relief, response and restrictions” (101). Other impacts included “agriculture” (79), “society and public health” (72), “plants and wildlife” (23), “fire” (16), “business and industry” (13), “tourism and recreation” (7) and “energy” (6). These categories are described as follows:
● Water Supply and Quality (113) —Drought effects associated with water supply and water quality include dry wells, voluntary and mandatory water restrictions, changes in water rates, increasing of water restrictions, increases in requests for new well permits, changes in water use due to water restrictions, greater water demand, decreases in water allocation or allotments, installation or alteration of water pumps or water intakes, changes to allowable water contaminants, water line damage or repairs due to drought stress, drinking water turbidity, change in water color or odor, declaration of drought watches or warnings, and mitigation activities.

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● Relief, Response, and Restrictions (101) —This category refers to drought effects associated with disaster declarations, aid programs, requests for disaster declaration or aid, water restrictions, or fire restrictions. Examples include disaster declarations, aid programs, USDA Secretarial disaster declarations, Small Business Association disaster declarations, government relief and response programs, state level water shortage or water emergency declarations, county-level declarations, a declared “State of Emergency,” requests for declarations or aid, nonprofit organization based relief, water restrictions, fire restrictions, National Weather Service (NWS) Red Flag Warnings, and declaration of drought watches or warnings.
● Agriculture (79) —Drought effects associated with agriculture, farming, aquaculture, horticulture, forestry, or ranching. Examples of drought-induced agricultural impacts include damage to crop quality; income loss for farmers due to reduced crop yields; reduced productivity of cropland; insect infestation; plant disease; increased irrigation costs; cost of new or supplemental water resource development (wells, dams, pipelines) for agriculture; reduced productivity of rangeland; forced reduction of foundation stock; closure/limitation of public lands to grazing; high cost or unavailability of water for livestock, Christmas tree farms, forestry, raising domesticated horses, bees, fish, shellfish, or horticulture.
● Society and Public Health (72) —Drought effects associated with human, public and social health include: health-related problems related to reduced water quantity or quality, such as increased concentration of contaminants; loss of human life (e.g., from heat stress, suicide); increased respiratory ailments; increased disease caused by wildlife concentrations; increased human disease caused by changes in insect carrier populations; population migration (rural to urban areas, migrants into the United States); loss of aesthetic values; change in daily activities (non-recreational, like putting a bucket in the shower to catch water); elevated stress levels; meetings to discuss drought; communities creating drought plans; lawmakers altering penalties for violation of water restrictions; demand for higher water rates; cultural/historical discoveries from low water levels; cancellation of fundraising events; cancellation/alteration of festivals or holiday traditions; stockpiling water; public service announcements and drought information websites; protests; and conflicts within the community due to competition for water.
● Plants and Wildlife (23)—Drought effects associated with unmanaged plants and wildlife, both aquatic and terrestrial, include: loss of biodiversity of plants or wildlife; loss of trees from rural or urban landscapes, shelterbelts, or wooded conservation areas; reduction and degradation of fish and wildlife habitat; lack of feed and drinking water; greater mortality due to increased contact with agricultural producers (as predators seek food from farms and producers are less tolerant of the intrusion); disease; increased vulnerability to predation (from species concentrated near water); migration and concentration (loss of wildlife in some areas and too much wildlife in others); increased stress on endangered species; salinity levels affecting wildlife; wildlife encroaching into urban areas; and loss of wetlands.
● Fire (16)—Drought often contributes to forest, range, rural, or urban fires, fire danger, and burning restrictions. Specific impacts include enacting or increasing burning restrictions, fireworks bans, increased fire risk, occurrence of fire (number of acres burned, number of wildfires compared to average, people displaced, etc.), state of emergency during periods of high fire danger, closure of roads or land due to fire occurrence or risk, and expenses to state and county governments of paying firefighters overtime and paying equipment (helicopter) costs. ● Business and Industry (13) —This category tracks drought’s effects on non-agriculture and nontourism businesses, such as lawn care, recreational vehicles, or gear dealers, and plant nurseries.

Typical impacts include reduction or loss of demand for goods or services, reduction in employment, variation in number of calls for service, late opening or early closure for the season, bankruptcy, permanent store closure, and other economic impacts.

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● Tourism and Recreation (7) —Drought effects associated with recreational activities and tourism include closure of state hiking trails and hunting areas due to fire danger; water access or navigation problems for recreation; bans on recreational activities; reduced license, permit, or ticket sales (e.g., hunting, fishing, ski lifts, etc.); losses related to curtailed activities (e.g., bird watching, hunting and fishing, boating, etc.); reduced park visitation; and cancellation or postponement of sporting events. ● Energy (6)—This category concerns drought's effects on power production, rates and revenue.

Examples include production changes for both hydropower and non-hydropower providers, changes in electricity rates, revenue shortfalls and/or windfall profits, and purchase of electricity when hydropower generation is down.

Probability of Future Occurrences
Likely – Historical drought data for the County planning area and the San Joaquin Valley region indicate there have been five significant multi-year droughts in the last 91 years. This equates to a multi-year drought every 18 years on average, or a 5 percent chance of a drought in any given year. Based on this data, droughts will likely affect the planning area. Given the historical occurrence of severe drought impacts throughout Merced County and across the state, the HMPC understands that drought will continue to pose a high degree of risk to the entire planning area, potentially impacting crops, livestock, water resources, the natural environment at large, buildings and infrastructure (from cascading or compound hazards), and local economies.

In addition, although drought affects the entire planning equally, the potential impacts may be variable and specific to each jurisdiction, depending on contextual factors such as the degree of assets and activities historically impacted by drought within each jurisdiction, such as the agricultural and parks and tourism industries.

Climate Change Considerations
Water resources in California area already experiencing stresses related to population growth, increased water demand, poor water quality, groundwater overdraft, and aging infrastructure. Scientific studies prepared for various California climate assessments and adaptations strategies also already show that drought conditions in California are likely to become more frequent and persistent over the next century due to climate change. Temperatures are warming, heat waves are more frequent, and precipitation has become increasingly variable (California Natural Resources Agency 2018a). According to California’s Climate Adaptation Strategy, also referred to as “Safeguarding California Plan: 2018 Update”, climate change is likely to significantly diminish California’s future water supply. As a result, the state must change its water management, as climate change will create greater competition for limited water supplies (California Natural Resources Agency 2018b). The recent drought conditions over the past decade underscore the need to examine water supply and distribution management, conservation, and use policies. California and the San Joaquin Valley region have experienced a succession of dry spells and with warmer temperatures and periodic droughts that frequently contribute to water shortages in the region.

Climate change projections of extreme prolonged droughts will exacerbate the San Joaquin Valley’s existing water supply challenges. In an average year, approximately 40 percent of the state’s total water supply comes from groundwater, and during a dry year this increases to more than half of the state’s water supply, with groundwater acting as a critical buffer against the impacts of drought and climate change (California Natural Resources Agency 2018).

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Table 4-22 Summary of Climate Change Impacts on Water Resources Resource Type of
Impact Description
Soil Moisture Direct Prolonged dry seasons can lead to decreases in soil moisture; drier vegetation
Vegetation Indirect Longer and more intense fire season with increased extent of area burned
Stream Conditions Direct Increases in water temperature; potential effects on fish
Snowpack Indirect Increases in temperature will lead to decreases in snowpack
Runoff Direct
Warmer temperatures are likely to lead to a shift in
peak runoff from spring to winter and a likely
decrease in summer baseflow
Hydropower Indirect
Decreased summer flows resulting from earlier
snowmelt and a shift in peak runoff could affect
hydropower generation during summer months
Precipitation Direct
Warmer winter temperatures will result in a greater
percentage of precipitation falling as rain rather than as snow
Groundwater Indirect
Reduction in snowpack and extended periods of
drought are likely to increase dependency on
groundwater
Source: http://frap.fire.ca.gov/data/assessment2010/pdfs/3.1water.pdf p. 140 Vulnerability Assessment
All of Merced County is vulnerable to drought. Drought is one of the few hazards with the potential to impact all the citizens of the county through water restrictions, economic losses, and increased energy costs. The urbanized areas of the county and the agriculture industry are most likely to experience hardships associated with reduced water supply.

Impacts include water restrictions associated with domestic supplies, agricultural and livestock losses and economic impacts, hydroelectric power reductions, and increased costs for water. Secondary effects include susceptibility to wildfires and increased groundwater pumping that can contribute to land subsidence problems and degraded water quality.

General Property
Based on the USDA’s Risk Management Agency (RMA) Crop Indemnity Reports, which were collected for the years 2007-2020, crop losses due to drought were only reported between 2007 and 2014 across the county. Table 4-22 summarizes the agricultural losses experienced across the county communities. A total of $885,477 was indemnified for 14,660 acres of insured crops.

Table 4-23 Crops Loss Due to Drought, Risk Management Agency Crop Indemnity Reports, 2007-2020
Year Crop
Net
Determined
Acres
Indemnity
Amount
2007
Wheat 301.9 $16,130
Oats 228.3 $1,241
Barley 1,345.9 $32,857

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Year Crop
Net
Determined
Acres
Indemnity
Amount
Total 1,876 $50,228
2008
Wheat 683.2 $75,727
Oats 1,376.3 $34,192
Barley 960.9 $45,245
Total 3,020 $155,164
2009
Wheat 2,179.4 $280,639
Oats 882.7 $32,334
Barley 1,534.2 $74,455
Total 4,596 $387,428
2010 Wheat 860.2 $5,599
Total 860.2 $5,559
2011 Wheat 170 $22,806
Total 170 $22,806
2012
Wheat 1,910.3 $167,880
Oats 1,482 $56,827
Total 3,392 $224,707
2013
Wheat 243.9 $8,907
Oats 35 $633
Total 279 $9,540
2014
Wheat 313.9 $26,623
Oats 151.4 $3,382
Total 465 $30,005
Grand Total 14,660 $885,477
Source: USDA RMA Crop Indemnity Reports, 2007-2020
People
According to this California DOF the 2020 County population was 283,521 people. The County’s population is projected to continue to increase by 1 percent to 1.48 percent annually until 2060 (CBPR 2016). This projected population growth would add additional strain to the surface water supplies and already depleted groundwater supplies but through the County Groundwater ordinance which prohibits the unstainable extraction or conveyance may help with reducing the impacts on drought on the regional groundwater supply.

The historical and potential impacts of drought on populations include agricultural sector job loss, secondary economic losses to local businesses and public recreational resources, increased cost to local and state government for large-scale water acquisition and delivery, and water rationing and water wells running dry for individuals and families. As drought is often accompanied by prolonged periods of extreme heat, negative health impacts such as dehydration can also occur, where children and elderly are most susceptible. Air quality often declines in times of drought which can affect those with respiratory ailments.

Critical Facilities
Drought impacts to critical facilities include water shortfalls for facility operations and critical functions, and potential structural destabilization and damage resulting from land subsidence. Refer to the section on Subsidence for more information.

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

Agriculture in the San Joaquin Valley relies on artificial irrigation using mostly imported water and/or groundwater. Local droughts are expected and accommodated for; however, a prolonged statewide drought could exceed local capabilities to handle reductions of imported surface water supplies and potentially lead to reductions in distribution from local water storage districts.

The costs of drought are difficult to quantify because the impacts affect so many different sectors including wildlife and natural resources, business and industry, tourism and recreation, agriculture, and individual households. Agriculture often suffers the most financial losses from drought and is the major component of the Merced County economy. For example, according to the California Integrated Climate Adaptation and Resiliency Program’s 2020 Report and 2020 Recommendations, the 2014-2017 multi-year drought was estimated to cost over $6.6 billion once it was over and the loss of 5,000 jobs in 2016 (OPR 2020).

Historic, Cultural and Natural Resources
The Merced National Wildlife Refuge encompasses over 10,000 acres of wetlands, native grasslands, vernal pools and riparian areas and is home to various bird species and other wildlife including the largest wintering population of lesser Sandhill cranes and Ross’ geese, and both species are vulnerable to drought. The impacts of drought on vegetation and wildlife can include death from dehydration and spread of invasive species or disease because of stressed conditions, loss of biodiversity, loss of trees in rural and urban landscapes, loss of wetlands, and degradation of habitat.

In general, environmental impacts from drought are more likely at the interface of the human and natural world. The loss of crops or livestock due to drought can have far reaching economic effects on communities, wind and water erosion can alter the visual landscape, and dust can damage property.

Water-based recreational resources are also heavily affected by drought conditions. Indirect impacts from drought arise from increased wildfire risk and greater occurrence of fire.

Future Development
With the County’s population projected to continue to grow while climate change projections are showing an increased duration an intensity of drought events for the San Joaquin Valley, it will be important for each new development application to be reviewed with existing and future water supplies in mind.

Because future development encompasses all forms of property, buildings, infrastructure, critical facilities and all related populations and their functions, drought impacts to future development align with the historical and potential impacts to populations, property, natural environment, and critical facilities discussed (above).

Risk Summary
● Due to the widespread impacts in San Joaquin Valley, it can have, drought is considered a High significance hazard.
● There have been five multi-year droughts in the past 91 years. Most recent drought lasted from 2012 to 2017 and resulted in a declared State of Emergency.

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● 15 USDA Disaster Designations due to drought have been made in the last 8 years. ● 195 reports of impacts related to drought were made within Merced County between, 2000 and 2020. ● Between 2007-2014 a total of $885,477 crop indemnity claims was paid due to crop losses for a total of 14,660 acres due to drought impacts.
● Climate change projections show that extreme prolonged drought is likely to continue and will exacerbate existing water supply challenges.
● Related Hazards: Extreme Heat, Wildfire, Subsidence, Agricultural Pest Infestation and Disease Table 4-24 Hazard Risk Summary – Drought
Jurisdiction Geographic Area
Probability of
Future
Occurrence
Magnitude/
Severity
Overall
Significance
Merced County Extensive Likely Critical High
City of Atwater Extensive Likely Limited Medium
City of Dos Palos Extensive Likely Limited Medium
City of Gustine Extensive Likely Limited Medium
City of Livingston Extensive Likely Limited Medium
City of Los Banos Extensive Likely Limited Medium
City of Merced Extensive Likely Negligible Low

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4.2.3 Earthquake
Hazard/Problem Definition
An earthquake is caused by a sudden slip on a fault. Stresses in the earth’s outer layer push the sides of the fault together. Stress builds up and the rocks slip suddenly, releasing energy in waves that travel through the earth’s crust and cause the shaking that is felt during an earthquake. The amount of energy released during an earthquake is usually expressed as a magnitude and is measured directly from the earthquake as recorded on seismographs. The magnitude of earthquakes is usually measured using the Richter scale; a logarithmic scale calculated from the amplitude of the largest seismic wave recorded for the earthquake.

Another measure of earthquake severity is intensity. Intensity is an expression of the amount of shaking at any given location on the ground surface. Seismic shaking is typically the greatest cause of damage to structures during earthquakes. Seismologists have developed the Mercalli scale to quantify the shaking intensity of an earthquake’s effects, which is measured by how an earthquake is felt by humans and the damage to buildings.

Earthquakes can cause structural damage, injury, and loss of life, as well as damage to infrastructure networks such as water, power, gas, communication, and transportation lines. Other damage-causing effects of earthquakes are surface rupture, fissuring, settlement, and permanent horizontal and vertical shifting of the ground. Secondary impacts can include landslides, seiches, liquefaction, and dam failure.

Liquefaction occurs when ground shaking causes the mechanical properties of some fine grained, saturated soils to liquefy and act as a fluid (liquefaction). It is the result of a sudden loss of soil strength due to a rapid increase in soil pore water pressures caused by ground shaking, in areas of shallow groundwater (within 10’ of surface or less). Liquefaction that produces surface effects generally occurs in the upper 40 to 50 feet of the soil column, although the phenomenon can occur deeper than 100 feet.

The duration of ground shaking is also an important factor in causing liquefaction to occur. The larger the earthquake magnitude, and the longer the duration of strong ground shaking, the greater the potential there is for liquefaction to occur.

In populated areas, the greatest potential for loss of life and property damage can come as a result of ground shaking from a nearby earthquake. The degree of damage depends on many interrelated factors.

Among these are the moment magnitude, focal depth, distance from the causative fault, duration of shaking, type of surface deposits or bedrock, presence of high ground water, topography, and finally, the design, type, and quality of building construction.

Geographic Area
The nearest faults of major significance are the San Andreas to the west of Merced County, a distance of approximately 15 miles from the county line; the Hayward and Calaveras faults to the northwest; the White Wolf, Garlock, and Sierra Nevada faults to the south; and the Bear Mountain Fault Zone about 5 miles east of and parallel to the eastern border of Merced County. These faults have been and will continue to be the principal source of seismic activity affecting the County of Merced.

The only fault known inside the County is the "Ortigalita", also known as the "Telsa-Ortigalita Fault", located in the western quarter of the County, dissecting the Coast Range in a northwesterly direction. This fault has not been active in historic times, however, there is no guarantee that it will never become active again.

There have been no records of major seismic activity originating in the county, with most epicenters in the county being below a magnitude 4.0. However, the county has been shaken by earthquakes originating elsewhere. There is documented evidence of six earthquakes that shook the area, those of 1872, 1906,

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1952, 1966, 1984, and 1989. Major damage occurred in Los Banos in 1906, with minor damage recorded throughout the county from other occurrences.

The San Andreas Fault occurs where the North American and Pacific plates come together and grind in a side-by-side motion relative to each other. Another large known fault, the White Wolf fault, is located to the south near Arvin and Bakersfield in Kern County and produced a severe M 7.7 earthquake in 1952.

Figure 4-7 below shows the known faults and potential for ground shaking resulting from earthquakes in and near Merced County, based on USGS probabilistic ground shaking with a 2% in 50-year occurrence (AKA 2500-year probabilistic ground shaking).

Figure 4-7 Merced County Earthquake Ground Shaking Potential and Nearby Faults Earthquakes can occur at any time of the day or night and any time of the year. Earthquakes are particularly dangerous due to their rapid onset, generally without warning. Aftershocks can occur for days, weeks, and even months following a major earthquake. This additional damage to structures already weakened by the main earthquake increases the danger to rescue and recovery personnel.

Extent (Magnitude/Severity)
For extent, the severity of an earthquake, or the amount of energy released during an earthquake is usually expressed in terms of intensity or magnitude as described further below.

Intensity: Intensity represents the observed effects of ground shaking at any specified location and earthquake shaking decreases with distance from the earthquake epicenter. Intensity is an expression of the amount of shaking at any given location on the ground surface based on felt or observed effects.

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Seismic shaking is typically the greatest cause of losses to structures during earthquakes. Intensity is measured with the Modified Mercalli Intensity (MMI) scale. The intensity of ground shaking at a particular site or structure is a function of many factors including: 1) earthquake magnitude, 2) distance from the epicenter, 3) duration of strong ground motion, 4) local geologic conditions (soil type and topography), and 5) the fundamental period of the structure. A brief description of those factors is presented below.

The Modified Mercalli Intensity scale is summarized in Table 4-24, along with the effects associated with the MMI scale. Damage typically occurs in MMI of scale VII or above.

Earthquake Magnitude: Magnitude represents the amount of seismic energy released at the hypocenter of an earthquake. It is based on the amplitude of the earthquake waves recorded. Seismologists have developed several magnitude scales; one of the first was the Richter Scale, developed in 1932 by the late Dr. Charles F. Richter of the California Institute of Technology. The Richter scale is numeric and has a logarithmic relationship between scale factors, so that a difference of one scale number represents a tenfold increase in measured amplitude, which in turn corresponds to an approximate 31x energy release difference when compared to the next whole number value. The Moment Magnitude scale (Mw, or M), which is a measurement of energy released by the movement of a fault and is the modern method used by seismologists to measure earthquakes. Overall, as the amount of energy released by an earthquake increases, the potential for ground shaking impacts also increases.

Distance from Epicenter: Earthquake energy generally dissipates (or attenuates) with distance from a fault. Over long distances, this loss of energy can be significant, resulting in a significant decrease in ground shaking with increased distance from the epicenter.

Duration of Strong Shaking: The duration of the strong ground shaking constitutes a major role in determining the amount of structural damage and the potential for ground failure that can result from an earthquake. Larger magnitude earthquakes have longer durations than smaller earthquakes.

Local Geologic Conditions: The geologic and soil conditions at a particular site have the potential to substantially increase the effects of ground shaking. The thickness, density, and consistency of the soil, as well as shallow ground water levels, have the potential to amplify the effects of ground shaking depending on the characteristics of the earthquake. In general, the presence of unconsolidated soils above the bedrock surface can amplify the ground shaking caused by an earthquake.

Fundamental Periods: Every structure has its own fundamental period or natural vibration. If the vibration of ground shaking coincides with the natural vibration period of a structure, damage to the structure can be greatly increased. The extent of damage suffered during an earthquake can also depend on non-geologic factors. The type of building and its structural integrity will influence the severity of the damage suffered. Generally, small, well-constructed, one- and two-story wood and steel frame buildings have performed well in earthquakes because of their light weight and flexibility. Reinforced concrete structures will also usually perform well. Buildings constructed from non-flexible materials, such as unreinforced brick and concrete, hollow concrete block, clay tile, or adobe, are more vulnerable to earthquake damage.

Effects of Ground Shaking: The primary effect of ground shaking is the damage or destruction of buildings, infrastructure, and possible injury or loss of life. Building damage can range from minor cracking of plaster to total collapse. Disruption of infrastructure facilities can include damage to utilities, pipelines, roads, and bridges. Ruptured gas and water lines can result in fire and produce scour/inundation damage, respectively, to structures, as can fire from other causes, such as electrical damages. Secondary effects can include geologic impacts such as co-seismic fault movement along nearby faults, seismically induced slope instability, liquefaction, lateral spreading, and other forms of ground failure and seismic response. These secondary effects were demonstrated in Oceano by the San Simeon 2003 earthquake.

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Table 4-25 Earthquake Magnitude and Intensity Measurements and Intensity Characteristics Magnitude Mercalli
Intensity Effects Frequency
Less than 2.0 I Microearthquakes, not felt or rarely felt; recorded by seismographs. Continual

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

3.0-3.9 II to IV Often felt by people; rarely causes damage; shaking of indoor objects noticeable.

Over 100,000
per year

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

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

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

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

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

One per year

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

One per 10-
50 years
Source: USGS
Past Occurrences
There have not been any damaging earthquakes greater than M 6.0 recorded in Merced County in over 200 years and the County has not been included in any federal disaster declarations involving earthquakes, though several have been very close. The most recent large earthquake near Merced County was 16 miles east north-east of King City at a magnitude of 5.3 on October 20, 2012. Several aftershocks followed the main earthquake, including a 3.1 aftershock. This quake was preceded by two earthquakes located approximately 100 miles from Merced County, the 1982 New Idria earthquake (M 5.4) and the 1983 Coalinga (M 6.5). There was a low level of ground shaking reported.

Major earthquakes have occurred near Merced County and resulted in ground shaking felt in the county.

The Fort Tejon earthquake in 1857 of M 7.9 was one of the greatest earthquakes ever recorded in the United States and the largest in California. It left an extensive surface rupture scar over 215 miles in length along the San Andreas Fault. The epicenter is now thought to have been located near Cholame, approximately 134 miles southwest of the City of Merced. During the Fort Tejon earthquake, strong shaking lasted from one to three minutes. As a result of the shaking, the flow of the Kern River was turned upstream, and water ran four feet deep over its banks. The waters of Tulare Lake were thrown upon its shores, stranding fish miles from the original lakebed. Property loss was heavy at Fort Tejon, one of the only settlements at the time, an Army post in south-central Kern County about four miles from the San

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Andreas Fault. In 1857, two buildings were declared unsafe, three others were damaged extensively but were habitable, and still others sustained moderate damage. One person was killed in the collapse of an adobe house at Gorman.

Figure 4-8 below displays the common areas damaged by earthquakes based on historic evidence dating back to the year 1800. The occurrences are color-coded by damaging shaking intensity across the state of California, and Merced County is enclosed within a black circle. The figure shows that, per the MMI scale noting occurrences equal to or greater than an Intensity of VII, the county has experienced 1 to 3 earthquake events of this kind, mostly affecting the far western and southwestern portions of the County.

Figure 4-8 Areas Damaged by Earthquakes from 1800 to 2017 Source: California State Hazard Mitigation Plan, 2018; California Geologic Survey

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Probability of Future Occurrences
It is estimated that major earthquakes (ranging from a magnitude of 7 to 7.9) occur in California one out of every 10 years. However, strong earthquakes (from magnitudes 6 to 6.9) strike the state about once every two to three years. A strong earthquake can cause major damage depending on the epicenter’s location with regards to populated areas, and can lead to billions of dollars in disasters, deaths, injuries, and disruptions in services and communities’ way of life. Moderate earthquakes (around magnitudes of

5.5) can occur three to four times a year in the state.

Along the San Andreas Fault, segments exist where no large earthquakes have occurred for long intervals of time. These areas accumulate potential energy and provide clues as to where the next earthquake may occur and when. Scientists term these segments “seismic gaps” and, in general, have been successful in forecasting the time when some of the seismic gaps will produce large earthquakes. Geologic studies show that over the past 1,400 to 1,500 years, large earthquakes have occurred at about 150-year intervals on the southern San Andreas Fault. As the last large earthquake on the southern San Andreas was the Fort Tejon earthquake in 1857, that section of the fault is considered a likely location for an earthquake within the next few decades (USGS 1997).

Based on the earthquake shaking potential mapped for Merced County, the proximity to the San Andreas Fault with a history of shaking, but no surface rupture, the probability of damaging seismic ground shaking in Merced County is occasional. The shaking levels expected with a 2% in 50 years probability are depicted in Figure 4-7.

Climate Change Considerations
Climate change is not expected to directly affect earthquake frequency or intensity.

Vulnerability Assessment
Earthquake loss estimation for the 2021 HMP update utilized FEMA’s Hazus-MH 4.2 natural hazard loss estimation software. Hazus is a GIS based, standardized, nationally applicable multi-hazard loss estimation methodology and software. Local, state and federal government officials use Hazus for preparedness, emergency response, and mitigation planning. A level 1 Hazus analysis was performed which estimates damage based on an inventory database compiled at a national level aggregated to Census Tracts. As with any model there are uncertainties, and the results should be considered approximate for planning purposes.

To evaluate potential losses associated with earthquake activity in the planning area, a Hazus 2,500-year probabilistic scenario was run for Merced County. The 2,500-year scenario represents the level of shaking considers multiple faults in the region instead of a single scenario. The methodology utilizes probabilistic seismic hazard contour maps developed by the U.S. Geological Survey (USGS) for the 2018 update of the National Seismic Hazard Maps that are included with HAZUS-MH. The USGS maps provide estimates of potential ground acceleration and spectral acceleration at periods of 0.3 second and 1.0 second, respectively. The 2,500-year return period analyzes ground shaking estimates from the various seismic sources in the area with a 2 percent probability of being exceeded in 50 years. The International Building Code uses this level of ground shaking for building design in seismic areas.

Hazus estimates the number of people displaced, the number of buildings and facilities/infrastructure damaged, the number of casualties, and the damage to transportation systems and utilities. Results produced by Hazus are reported at the census tract level. Figure 4-9 below displays the Census Tracts within the county which were analyzed in this scenario, color-coded by the amount of total losses each tract experienced. The map is followed by the Hazus results broken up in respective sections.

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Figure 4-9 Hazus Earthquake Analysis for Merced County – Total Losses by Census Tract (in Thousands of Dollars)
General Property
Unreinforced Masonry Building (URM)s: Unreinforced masonry building type structures consist of buildings made of unreinforced concrete and brick, hollow concrete blocks, clay tiles, and adobe. Buildings constructed of these materials are heavy and brittle, and typically provide little earthquake resistance. In small earthquakes, unreinforced buildings can crack, and in strong earthquakes, they have a tendency to collapse. These types of structures pose the greatest structural risk to life and safety of all general building types.

Per Hazus, there are an estimated 77,000 buildings in Merced County with a total building replacement value (excluding contents) of $24 Billion. Approximately 94% of these buildings (and 82% of the building value) are associated with residential housing. In terms of building construction types found in the region, wood frame construction makes up 87% of the building inventory. The remaining percentage is distributed between the other general building types.

The categories of damages defined by Hazus are:

Slight damage includes diagonal hairline fractures on most shear wall surfaces and hairline cracks on most infill walls.

Moderate damage includes cracks on most walls and failure of some shear walls.

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Extensive damage means that most shear wall surfaces in the structure have reached or exceeded their capacity exhibited by large, through-the-wall diagonal cracks.

Complete damage means that the structure has collapsed or is in danger of collapse.

The building losses are broken into two categories: direct building losses and business interruption losses.

The direct building losses are the estimated costs to repair or replace the damage caused to the building and its contents. The business interruption losses are the losses associated with inability to operate a business because of the damage sustained during the earthquake. Business interruption losses also include the temporary living expenses for those people displaced from their homes because of the earthquake.

The total building-related losses were $2.4 billion. By far, the largest loss was sustained by the residential occupancies which made up over 61% of the total loss. Figure 4-10 and Figure 4-11 below provide a summary of the losses associated with the building damage.

Figure 4-10 Earthquake Losses by Loss Type and Occupancy Type (in Millions of Dollars) Source: Hazus-MH 4.2

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Figure 4-11 Hazus 2,500 Year Probabilistic Earthquake Scenario Structure Damage by Occupancy Type
Source: Hazus-MH 4.2
Hazus estimates that about 17,106 buildings will be at least moderately damaged. This is over 22% of the total building stock in the county. There are an estimated 1,386 buildings that will be damaged beyond repair. Table 4-25 summarizes the expected damage by property occupancy type in more detail, whereas Table 4-26 contains the results of the expected building damage by building material type.

Table 4-26 Expected Building Damage by Occupancy Class
Source: Hazus-MH 4.2

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Table 4-27 Expected Building Damage by Building Material (All Design Levels) Source: Hazus-MH 4.2
Hazus also estimates the amount of debris that will be generated by the earthquake event analyzed. The model subdivides the debris into two general categories: a) Brick/Wood, and b) Reinforced Concrete/Steel. This distinction is made because of the different types of material handling equipment required to handle the debris.

The model estimates that a total of 442,000 tons of debris will be generated. Of that total amount, Brick/Wood comprises 38% of the debris, while the remainder would be Reinforced Concrete/Steel. If the debris tonnage was converted to estimates of truckloads required to remove it, debris generated would convert to about 17,680 truckloads, with each truckload carrying 25 tons. Figure 4-12 summarizes the debris generation estimate and material type for this earthquake event.

Figure 4-12 Debris Generation in Millions of Tons and by Material Type Source: Hazus-MH 4.2

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People
Shelter Requirement: Hazus estimates the number of households that are expected to be displaced from their homes due to the earthquake and the number of displaced people that will require accommodations in temporary public shelters. The model estimates 918 households to be displaced due to the earthquake.

Of these, approximately 882 people (out of a total population of 255,793) will seek temporary shelter in public shelters.

Casualties: Ground movement during an earthquake is seldom the direct cause of death or injury. Most earthquake-related injuries result from collapsing walls, flying glass, and falling objects as a result of the ground shaking, or people trying to move more than a few feet during the shaking. Hazus estimates the number of people that will be injured and killed by the earthquake. The casualties are broken down into four severity levels that describe the extent of the injuries. The levels are described as follows:

Severity Level 1: Injuries will require medical attention, but hospitalization is not needed.

Severity Level 2: Injuries will require hospitalization but are not considered life-threatening.

Severity Level 3: Injuries will require hospitalization and can become life-threatening if not promptly treated.

Severity Level 4: Victims are killed by the earthquake.

The casualty estimates are provided for three times of day: 2:00 AM, 2:00 PM and 5:00 PM. These times represent the periods of the day that different sectors of the community are at their peak occupancy loads. The 2:00 AM estimate considers that the residential occupancy load is at its maximum. The 2:00 PM estimate considers that the educational, commercial and industrial sector loads are at their maximum, and has the highest numbers of casualties estimated. The 5:00 PM represents peak commute time. These estimates of casualties are broken down in Table 4-27.

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Table 4-28 Hazus Earthquake Casualty Estimates from Merced County Source: Hazus-MH 4.2
Social Vulnerability
Populations most vulnerable to earthquake and liquefaction activity would be those that rely on specific services or electrical power, which may not be available during or after a quake, or those which are homeless, would have a difficult time evacuating due to age or disability, cannot communicate easily due to speaking English less than well, for example.

Critical Facilities and Infrastructure
Critical Facility Inventory: Hazus breaks critical facilities into two groups: essential facilities and high potential loss facilities. Essential facilities include hospitals, medical clinics, schools, fire stations, police

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stations and emergency operations facilities. High potential loss facilities include dams, levees, military installations, nuclear power plants and hazardous material sites.

Essential Facility Damage: There are four (4) hospitals in the region with a total bed capacity of 246 beds. There are 145 schools, 31 fire stations, 14 police stations and one (1) emergency operation facility.

The inventory also includes 23 hazardous material sites.

On the day of the earthquake, the model estimates that only 125 hospital beds (51%) are available for use by patients already in the hospital and those injured by the earthquake. After one week, 75% of the beds will be back in service. By 30 days, 94% will be operational. The essential facility inventory and expected damages from the earthquake event are provided in Table 4-28.

Transportation Systems Inventory: Within Hazus, the lifeline inventory is divided between transportation and utility lifeline systems. There are seven (7) transportation systems that include highways, railways, light rail, bus, ports, ferry and airports. There are six (6) utility systems that include potable water, wastewater, natural gas, crude & refined oil, electric power and communications. The transportation systems inventory and expected damages from the earthquake, in terms of number of structures and locations affected, are provided in Table 4-29, while losses in millions of dollars are summarized in Table 4-30. The total value of the lifeline inventory is over $9.4 billion. This inventory includes over 274 miles of highways, 532 bridges, and 11,935 miles of pipes.

Utility Lifeline Systems Inventory: The replacement value of the transportation and utility lifeline systems is estimated to be $4.48 billion and $4.97 billion, respectively. The expected utility system facility damages in terms of total structures or systems affected, along with the inventory of this dataset, are summarized in Table 4-31. Economic losses in millions of dollars are found in Table 4-32. Site specific expected utility system pipeline damages (including their inventory) are included in Table 4-33, while the potable water and electric power system performance limitations, damages, and inventory will be in Table 4-34.

Table 4-29 Essential Facility Inventory and Expected Damage Source: Hazus-MH 4.2

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Table 4-30 Transportation Systems Inventory and Expected Damage by Number of Locations Source: Hazus-MH 4.2

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Table 4-31 Transportation System Economic Losses in Millions of Dollars Source: Hazus-MH 4.2

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Table 4-32 Expected Utility System Facility Inventory and Damages Source: Hazus-MH 4.2

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Table 4-33 Utility System Economic Losses in Millions of Dollars Source: Hazus-MH 4.2
Table 4-34 Expected Utility System Pipeline Damage (Site Specific) Source: Hazus-MH 4.2

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Table 4-35 Expected Potable Water and Electric Power System Performance Source: Hazus-MH 4.2
Beyond the modeling estimates of Hazus-MH, there are several other critical facilities which are unique to Merced County and are crucial to the function of the wider region and the state of California. The California Aqueduct, part of the California State Water Project, runs through the western part of Merced County. The State Water Project is a water storage and delivery system of reservoirs, aqueducts, power plants, and pumping plants. Its main purpose is to store water and distribute it to 29 urban and agricultural water suppliers in Northern California, the San Francisco Bay Area, the San Joaquin Valley, the Central Coast, and Southern California. The State Water Project makes deliveries to two-thirds of California's population. An earthquake event, or landslides triggered by one, which damage the aqueduct could disrupt the aqueduct’s ability to deliver water, which would have serious impacts to agriculture in the county and water users in many areas of California. Similarly, Interstate 5 runs through the western portion of the county and serves as a major shipping and transportation corridor for all of California and the west coast of the United States. Any damage or impacts to Interstate 5 in Merced County from an earthquake would result in disruptions to the entire state.

Economy
Hazus estimates the long-term economic impacts to the region. The model quantifies this information in terms of income and employment changes within the region. The total economic loss estimated for the earthquake is $4.6 Billion, which includes building and lifeline related losses based on the region's available inventory. An estimated 14% of losses computed by Hazus were related to the business interruption of the region.

Since the building losses are broken into two categories (of direct building losses compared to business interruption losses), building-related losses, which summarize estimated costs to fix or replace structures and damages to properties and their contents, are discussed in more detail in Table 4-26 of this section.

However, business interruption losses are summarized herein. They included the temporary living expenses for people displaced from their homes because of the earthquake event. These business-related economic losses are included in Table 4-35 below.

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Table 4-36 Building-Related Economic Loss Estimates in Millions of Dollars Source: Hazus-MH 4.2
Historic, Cultural, and Natural Resources
Earthquake effects on the environment, natural resources, and historic and cultural assets could be very destructive depending on the type of seismic activity experienced and secondary/cascading effects from an event (e.g. wildfire). The biggest impact would likely be on older properties such as wooden or masonry buildings, though reinforced masonry structures would be much more resilient during earthquakes.

Future Development
Future development in the county is not anticipated to significantly affect vulnerability to earthquakes when designed according to modern building codes. However future development will result in a slight increase in exposure of the population, building stock, and related infrastructure to earthquakes.

Risk Summary
● Merced County is located in a geologically complex and seismically active region, albeit less so than other areas of California. There are numerous active and potentially active faults in close proximity to the County. The County does not have a history of significant damaging earthquakes. ● The overall significance of earthquakes is Medium but ranked High for communities on the western side of the county including Dos Palos and Gustine and in the more densely populated cities such as Merced.
● A moderate earthquake occurring in or near Merced County could result in deaths, casualties, property damage, agricultural and environmental damage, and disruption of normal government and community services and activities.
● The location of the epicenter as well as the time of day and season of the year would have a profound effect on the number of deaths and casualties, as well as property damage. ● The hazard of earthquakes varies from place to place, dependent upon the regional and local geology.
● Effects on people: Hazus probabilistic scenario modeling of worst case ground shaking results in estimates of 488-1,132 people needing hospitalization and between 10-65 deaths depending on the time of day the earthquake hits.

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● Effects on property: Hazus probabilistic scenario modeling indicates about 17,106 buildings will be at least moderately damaged, with approximately $2.4 billion in losses. ● Effects on economy: The total economic loss estimated for a large earthquake is $4.6 billion, which includes building and lifeline related losses based on the region's available inventory. An estimated 14% of losses computed by Hazus were related to the business interruption of the region. ● Effects on critical facilities and infrastructure: Essential facility damage (police, fire, school, medical) is predicted to be low; only 246 hospital beds (51%) would be available during the earthquake. Utility lifeline systems impacts would be significant particularly to bridges; moderate damage can be expected to water, wastewater, power and communications facilities; loss of water and power would be significant in the days and weeks following the earthquake. ● Cascading and Secondary Effects: Earthquakes can cause many cascading effects such as fires, flooding, hazardous materials spills, utility disruptions, landslides, and transportation emergencies.

Ground shaking may cause tsunamis or seiche, the rhythmic sloshing of water in lakes or bays.

Table 4-37 Hazard Risk Summary - Earthquake
Jurisdiction Geographic Area
Probability of
Future
Occurrence
Magnitude/
Severity
Overall
Significance
Merced County Significant Unlikely Critical Medium
City of Atwater Extensive Occasional Critical Medium
City of Dos Palos Extensive Occasional Critical Medium
City of Gustine Significant Likely Critical Medium
City of Livingston Limited Occasional Limited Medium
City of Los Banos Limited Likely Limited Medium
City of Merced Extensive Occasional Critical Medium

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

Floodplains are defined as the areas immediately adjacent to a channel from a river, stream, or other waterway. Floodplains are illustrated on inundation maps, which show areas of potential flooding and water depths. In its common usage and based on FEMA guidelines, the floodplain most often refers to the area that is inundated by the 100-year flood, or the flood that has a one percent chance occurrence in any given year of being equaled or exceeded. The 100-year flood is the national minimum standard to which communities regulate their floodplains through the FEMA NFIP. The 500-year flood is the flood that has a

0.2 percent chance of being equaled or exceeded in any given year. A 500-year flood event would be slightly deeper and cover a greater area than a 100-year flood event. The potential for flooding can change and increase through various land use changes and changes to land surface, which then may result in a change to the floodplain. A change in environment can create localized flooding problems inside and outside of natural floodplains by altering or confining natural drainage channels. These changes are most often created by human activity.

A levee is a raised area that runs along the banks of a river or canal. Levees reinforce the banks and help prevent flooding. By confining the flow, levees can also increase the speed of the water. Levees can be natural or manmade. A natural levee is formed when sediment settles on the riverbank, raising the level of the land around the river. To construct a manmade levee, workers pile dirt or concrete along the riverbanks, creating an embankment. This embankment is flat at the top, and slopes at an angle down to the water. For added strength, sandbags are sometimes placed over dirt embankments.

Levees provide strong flood protection, but they are not failsafe. Levees are designed to protect against a specific flood level and could be overtopped during severe flood events. Levees reduce, not eliminate, the risk to individuals and structure behind them. A levee system failure or overtopping can create severe flooding and high-water velocities. It’s important to remember that no levee provides protection from events for which it was not designed, and proper operation and maintenance are necessary to reduce the probability of failure.

Riverine flooding - Riverine flooding, defined as the condition when a watercourse (e.g. river or channel) exceeds its “bank-full” capacity, generally occurs as a result of prolonged rainfall, or rainfall that is combined with already saturated soils from previous rain events. This type of flood occurs in river systems whose tributaries may drain large geographic areas and include one or more independent river basins.

The onset and duration of riverine floods may vary from a few hours to many days. Factors that directly affect the amount of flood runoff include precipitation amount, intensity and distribution, the amount of soil moisture, seasonal variation in vegetation, snow depth, and water-resistance of the surface due to urbanization. Intense storms can overwhelm the local waterways as well as the integrity of any flood control structures. The warning time associated with slow rise floods assists in life and property protection

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Localized flooding - Localized flooding problems are often caused by flash flooding, severe weather, or an unusual amount of rainfall. Flooding from these intense weather events usually occurs in areas experiencing an increase in runoff from impervious surfaces associated with development and urbanization as well as inadequate storm drainage systems.

Levee Failure – Flooding from the failure of levees throughout the county or upstream is also of concern.

A catastrophic flood control structural failure could easily overwhelm local response capabilities to save lives and require mass evacuations. Impacts to life safety will depend on the warning time and the resources available to notify and evacuate the public. Loss of life could result, and there could be associated health concerns as well as negative effects to local buildings and infrastructure.

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

Encroachment into areas subject to inundation by floodwaters having rapid flows aggravates the risk of flood damage and heightens potential flood hazards by further increasing velocities. To reduce the risk of property damage in areas where the stream flows are high, the community may wish to restrict development in areas outside the floodway. FEMA defines a regulatory floodway as the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated height. The floodway is illustrated in Figure 4-13 below. The area between the floodway and 1-percent-annual-chance floodplain boundaries is termed the floodway fringe. The floodway fringe encompasses the portion of the floodplain that could be completely obstructed without increasing the water surface elevation of the base flood more than 1 foot at any point. Typical relationships between the floodway and the floodway fringe and their significance to floodplain development are shown in schematic in the following.

Figure 4-13 Floodway Schematic
Source: Guidance for Flood Risk Analysis and Mapping – FEMA, 2019

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Geographic Area
The Merced River, in the central part of California, is a 145-mile (233 km)-long tributary of the San Joaquin River flowing from the Sierra Nevada into the Central Valley. It is most well-known for its swift and steep course through the southern part of Yosemite National Park, and the world-famous Yosemite Valley. The river's character changes dramatically once it reaches the foothills and the lowlands, becoming a slowmoving waterway meandering through irrigated fields. Large-scale irrigated agriculture was introduced to the Central Valley in the late 19th century, and led to the construction of numerous state, federal and privately owned dams.

The San Joaquin River swings northwest, passing through many different channels, some natural and some manmade. Northeast of Dos Palos, it is joined by the Fresno and Chowchilla Rivers. Fifty miles downstream, the Merced River (highlighted below in Figure 4-14) empties into the San Joaquin. Canal and flood control development in the late 1800’s and early 1900’s redirected water flow and managed waterways through a series of canals, water storage and agricultural levies. This led to the conversion on thousands of acres of lake basin land into farmable ground.

Figure 4-14 Merced River
Source: Merced County website
The Tenaya, Yosemite, Bridalveil, and Pigeon Creeks join the Merced River before it breaches the glacial moraine at the valley's end. From there the river picks up Cascade Creek and turns south near El Portal, flowing through Merced River Canyon. State Route 140 follows the river out of the west entrance to the national park, a few miles before the South Fork Merced River, the largest tributary, joins from the left. The river arcs northwest to receive the North Fork, and a few miles after it enters Lake McClure, formed by New Exchequer Dam. Below New Exchequer, the river flows west through a heavily irrigated region of the Central Valley, passing through McSwain and Crocker-Huffman Dams and the cities of Hopeton, Delhi and Livingston. It joins the San Joaquin River at Hills Ferry, a few miles south of Turlock. There are also many lakes in the watershed of the Merced River, including Merced Lake, Tenaya Lake, the Chain Lakes, May Lake, Lake McClure, and Lake McSwain. The Merced River is the third largest tributary of the San Joaquin River.

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The primary cause of local flooding is due to the drainage patterns that flow towards the San Joaquin River Basin and Merced County. The numerous tributaries of the San Joaquin – the Fresno, Chowchilla, Merced, Tuolumne, Mariposa Creek, Calaveras, Mokelumne, and others flowed freely across alluvial flood plains to join the river. All of the major tributaries of the river originate in the Sierra Nevada. Most of the streams that start in the Coast Range are intermittent and contribute little to the flow of the San Joaquin.

During the winter, spring, and early summer, storms and snowmelt swell the river. In 1914 – before the development of major dams and irrigation diversions – the California Department of Engineering estimated the river's flow in full flood at 325,000 cubic feet per second (cfs). In late summer and autumn, there is little water left over to replenish stream flow.

The floodplain of the Merced River is well defined from the eastern county boundary to approximately 10 miles above the mouth of the river. Downstream of this point, the floodplain is undifferentiated from the valley floor and major floods can spread out over large areas. Floodplain storage reduces peak flows in the downstream direction. With the diminishing channel capacity of the Merced River, flood hazards increase. During the 1950 flood event, approximately 19,600 acres of land along the Merced River were inundated, including the towns of Snelling and Merced Falls.

The Merced River's primary human use is irrigation. The Merced Irrigation District (MID) operates most of the irrigation infrastructure, which supplies water to 154,394 acres (624.81 km2) of farmland. The Irrigation District and watershed are shown in Figure 4-15. As a whole, the system includes about 4,000 sets of control gates and 793 miles (1,276 km) of canals. Irrigation has taken most of the water out of the lower river, which now rarely reaches the sea. Water that does return to the river does so in for form of irrigation return flows, which carry pesticides, fertilizer and other pollutants. The MID is federally required to allow at least 15,000-acre feet (19,000,000 m3) of water annually to flow continually down the river, not including flooding. The water has allowed the San Joaquin River below the Merced River confluence to contain water, while above the confluence all its water is usually diverted for irrigation.

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Figure 4-15 Merced Irrigation District Watershed System Source: MID
The physical risks associated with potential flooding and the regulatory requirements for floodplain management are important considerations when decisions are being made regarding future land use throughout the County. Those same risks guide the local and community-level emergency response needs. Economic growth and prosperity in Merced County are dependent upon federal, state, and local agency involvement on regional and local flood management systems. Flood protection regulations within California have been increased over the past few years through legislation. This legislation included the requirement for the California DWR and Central Valley Flood Protection Board to prepare and adopt a Central Valley Flood Protection Plan (CVFPP) by 2012. The legislation also established certain flood protection requirements for local land use decision making based on the CVFPP. The law sets a higher standard for flood protection for the entire San Joaquin Valley. The standard was set for an urban level flood protection necessary to withstand a 1 in 200 chance of a flood event occurring in any given year (200-year flood) for areas developed or planned to have a population of at least 10,000. It also requires impacted counties to collaborate with cities to develop flood emergency response plans. Figure 4-16 shows the extent of this 200-year flood event based on the Best Available Maps (BAM) developed by California DWR.

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Figure 4-16 Merced County 200-year Floodplain
Source: USACE, Wood Planning Team

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In 2009, FEMA completed their DFIRM conversion and updated a number of flood zone areas using 2005 levee certification criteria. In 2007, the California DWR completed their Awareness Floodplain Mapping of Merced County to identify all pertinent flood hazard areas that are not mapped under FEMA’s program, which provides an additional resource for identifying special flood hazard areas within the County.

Extent (Magnitude/Severity)
The FEMA and the Federal Insurance Administration have assessed flood hazards for major streams in Merced County. A large portion of Merced County continues to be subject to inundation during flood events, as approximately 380,010 acres have been determined to be in the FEMA floodplain. The extent of the floodplain is shown in Figure 4-17. Both a 100-year and 500-year flood event scenario are critical events in Merced County as the flooding would cover at least 35 to 50 percent of the County. More detailed maps are in the jurisdictional annexes.

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Figure 4-17 Merced County 100-year and 500-year Floodplain

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The largest flood of record on the Merced River was 46,200 cubic feet per second (cfs) and occurred on December 4, 1950, downstream of Old Exchequer Dam (built in Mariposa County in 1926). The New Exchequer Dam, which has more flood storage capacity, was built in 1967. Based on USGS stream data, the largest discharges along the Merced River since the construction of New Exchequer Dam occurred in 1983 followed by 1997.

During the major flood events in 1969, 1997, 1998, and 2006, Bear Creek overflowed at the southwest end of the City of Merced in the vicinity of Thornton Avenue and State Highway 140. Stream gauge data is not available to quantify the magnitude of these events. Historical records also indicate that Bear Creek overflowed its banks upstream from the G Street Bridge. This overflow was worsened by debris restricting flow under the bridge. The largest flood on the San Joaquin River since the completion of Friant Dam in 1943 occurred in 1997. Along the San Joaquin River during the past century, the five largest flood events include, in order of magnitude, those from 1997, 1938, 1983, 1969, and 2006 based on USGS data. The historical San Joaquin River floodplain widens from approximately 3 to 4 miles in the reach south of Merced County to approximately 19 miles in central Merced County. This large expansion of the floodplain is accompanied by a decrease in channel slope. The floodplain then narrows to approximately 2 miles downstream of the Merced River in the northern part of the county.

Previous Occurrences
Between 1992 and 2002, every county in California was declared a federal disaster area at least once for a flooding event. California has a chronic and destructive flood history. Of the 336 federally declared disasters in California between 1950 and 2020, 37 were flood related. This makes flooding second only to fire in the number of federal disaster declarations in the state. Historically, floods have been the most frequent cause of disaster in Merced County.

Significant flooding occurs in Merced County approximately every five years. Merced County has had two flood events that were federally declared disasters, once in 1969 and again in 2017. FEMA’s Flood Insurance Study also listed flooding events in 1937-38, 1950, 1955, 1969, 1983, 1997, 1998, and 2006.

Additionally, NOAA’s National Centers for Environmental Information (NCEI) database lists 54 flood or flash flood events which have occurred in the county between 1950 and 2020.

Table 4-37 below highlights several notable flood events in the county that resulted in significant damages or involved federal disaster declarations.

Table 4-38 Major Floods in Merced County
Date of Event Incident Description
January-February
1969
A series of storms delivered 12-50 inches of rainfall over an 8-day period throughout Central and Southern California. This affected the San Joaquin Valley and resulted in the first federal disaster for flood in Merced County.

March 25, 1998 A series of thunderstorms brought locally heavy rain to the City of Merced area, resulting in rainfall totals for 24 hours ending by the morning of the 25th from 3.5" to 5.9" in the Merced City area. One gauge in the north part of town showed 6.8" in the 48-hour period from the evening of the 23rd to the evening of the 25th. Over 1000 people were evacuated from Merced City and nearby County areas. Over 65 homes and 19 apartments were flooded in and around Merced City as the result of torrential/record-setting rainfall in the previous 18 hours. According to the NCEI database, $9.6 million in property damage and $1.5 million in crop damages occurred during this event.

October 24, 1998 The cool season's first significant rain pushed through Central California. Merced received

0.75” of rain in embedded frontal thundershowers and locally heavy rain in about 2 hours.

Some local street flooding was reported just south of Hilmar and within Delhi. In Merced a

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Date of Event Incident Description
tree was toppled by the storm. Several traffic accidents occurred on rain-slickened roads.

According to NCEI, this event also resulted in $120,000 in property damage and six injuries.

January 10, 2005 Heavy rain resulted in water overflowing from Bear Creek in the Merced area from heavy rainfall damaged a handful of homes on the 11th, and $200,000 in property damages.

March 22, 2005 Over 4 inches of rain fell in a 24-hour period in the Mariposa County area of Central California, upstream of Merced County. Numerous small creeks flooded in Mariposa, Merced, and Madera Counties. Flooding caused damage to structures within the town of Mariposa and several bridges and roads in and around Hornitos, Merced Falls, and Catheys Valley in Merced and Mariposa Counties. $250,000 in property damage was recorded.

April 4, 2006 Due to continued heavy rain and subsequent high stream flows, a break in Black Rascal Creek flooded homes in north Merced. And Bear Creek raised over its banks in West Merced flooding a residential section in that vicinity. Overall, there were approximately 300 homes flooded around Merced with over 500 people evacuated during the event. $9.72 million in property damage.

March 27, 2011 During the frequently stormy period that began on the 18th and ended on the 28th, as much as 7 inches of rain fell in the San Joaquin Valley and up to 10 inches of rain drenched the adjacent foothills while the higher elevations of the Sierra (above 7000 feet) received snow accumulations of up to 15 feet. Excessive runoff combined with an increase in reservoir releases at the major dams caused higher than normal flows along rivers and streams through the end of the month. The Merced River at Stevinson peaked a half of a foot below flood stage at 4:30 pm on the 21st but otherwise remained well above monitor stage during the last two weeks of the month. Water levels along the San Joaquin River approached levels that had not been observed since January 1997. In Merced County, several water swollen creeks briefly flowed out of their banks on the morning of the 20th and caused minor flooding. The event caused $500,000 in property damage.

November 30, 2012 California highway patrol reported a multi-vehicle accident in heavy rain on Highway 99 at west side boulevard. Involved three cars and a big rig. Number of injuries unknown at this time. $120,000 in damages were also recorded.

February 1-23,
2017
On March 19, 2017, Governor Edmund G. Brown Jr. requested a major disaster declaration due to severe winter storms, flooding, and mudslides during the period of February 1-23, 2017. The Governor requested a declaration for public assistance for 42 counties and Hazard Mitigation statewide. Merced County was included in the declaration.

Source: NCEI Storm Events Database, FEMA

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Figure 4-18 Image of Past Flooding in Merced County
Source: Fox 26 News, Images of February 2017 flooding
Probability of Future Occurrences
Due to the history of past flooding events and the natural drainage pattern of the planning area, flooding in Merced County is likely to continue to occur. The potential for failure of one of the many levees throughout the county could create more risk for flooding. The 100-year flood is the flood that has a one percent chance in any given year of being equaled or exceeded. Based on past events, flooding events less severe than a 100-year flood and those outside of the 100-year floodplain occur frequently during periods of heavy rains. According to the NCEI Storm Events Database, there have been 54 flood events recorded for Merced County between 1990 and 2020. Utilizing the methodology explained in subsection

4.1.1, this means there is a 180% chance of a flood event occurring in a given year. Significant flooding occurs in Merced County approximately every five years.

Climate Change Considerations
California’s Fourth Climate Assessment found that costs associated with direct climate change impacts by 2050 will be dominated by human mortality, coastal damage, and the potential for droughts and megafloods (California Natural Resources Agency 2018). Scientific studies outlined in the same assessment also indicated shifts in California’s precipitation regime, which show more dry days, more dry years, a longer dry season, and increases in occasional heavy precipitation events and floods. Studies also project greater storm intensity with climate change, resulting in more direct runoff and flooding (California Natural Resources Agency 2018). As a result, high frequency flood events in conjunction with heavy precipitation events and extreme storm events will increase with climate change. Certain climate studies noted in the Fourth Climate Assessment also forecast that during periods with heavy rain and frequent storm events there is an increase in flood events, as well as landslide and debris flow (California Natural Resources Agency 2018). For example, during the 2016-2017 winter, the storms resulted in many secondary hazards, including numerous landslides and in some watersheds an increased sediment load. Also, with wildfires already being a problem in California, increasing periods of drought and lack of precipitation are expected

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to exacerbate conditions for fires to occur, and in turn worsen the potential for runoff and flooding associated with burned areas.

The Fourth Climate Assessment also includes nine reports for the nine regions in California. According to the preview for the San Joaquin Valley Region Summary Report, which has not yet been published at the time of this plan update, the frequency of catastrophic floods will increase in the coming years. This in turn will lead to increased stresses to agriculture, natural ecosystems, water resources, land use and community development, transportation, energy, public health, and climate justice.

Vulnerability Assessment
While there are some benefits associated with flooding, such as the replenishment of sediments and nutrients to agricultural lands, it is considered a hazard to development in floodplains. Floods can cause many cascading effects. Fire can break out as a result of dysfunctional electrical equipment. Hazardous materials can also get into floodways, causing health concerns and polluted water supplies. In many instances during a flood, the drinking water supply will be contaminated. During the 2021 update the following vulnerability analysis using GIS was performed to quantify flood risk.

General Property
Historically, Merced County has been at risk to flooding primarily during the winter and spring months when river systems in the county swell with heavier rainfall and runoff from winter snowmelt. Normally, storm floodwaters are kept within defined limits by a variety of storm drainage and flood control measures. But occasionally, extended heavy rains result in floodwaters that exceed normal high-water boundaries and cause damage. Flooding has occurred in the past within the 100-year floodplain and in other localized areas.

A flood vulnerability assessment was performed for Merced County using the following GIS methodology.

The County’s parcel layer and associated assessor’s building improvement valuation data were provided by the County and were used as the basis for the inventory. Merced County’s effective DFIRM was used as the hazard layer. A DFIRM is FEMA’s flood risk data that depicts the 1% annual chance (100-year) and the

0.2% annual chance (500-year) flood events; this data is incorporated into the National Flood Hazard Layer (NFHL). Merced County’s effective FEMA DFIRM, dated December 2, 2009, was determined to be the best available floodplain data. Figure 4-17 summarizes the flood zones included on these maps.

GIS was used to intersect the parcel boundaries with a master address point layer to obtain the number of buildings per parcel. Only parcels with improvement values greater than zero were used in the analysis, this method assumes that improved parcels have a structure of some type. The DFIRM flood zones were overlaid in GIS on the address points to identify structures that would likely be inundated during a 1% annual chance and 0.2% annual chance flood event. This overlay can be seen graphically in the regional maps in Figure 4-17, and in more detail in the jurisdictional annexes.

Building improvement values and counts for those points were then extracted from the parcel/assessor’s data and summed for the unincorporated county and jurisdictions. Results of the overlay analysis area shown in Table 4-38 for the 1% annual chance flood and Table 4-39 for 0.2% annual chance flood. The jurisdictional annexes provide more detailed information based on property type. Property type refers to the land use of the parcel and includes agricultural, commercial, exempt, industrial, mixed use, residential, and vacant. A loss estimate analysis was also performed based on depth damage functions developed by the Army Corp of Engineers and FEMA. The loss curves depict the expected flood losses associated with the depth of flooding at a structure. Contents values were estimated as a percentage of building value based on their occupancy type, using FEMA/HAZUS estimated content replacement values. This includes 100% of the structure value for agricultural, commercial, exempt, and open space structures, 50% for

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multi-residential and residential structures and 150% for industrial structures. Building and contents values were totaled to obtain total exposure.

There are different depth damage curves for structure and content losses. For the purposes of this planning level analysis, an average flood depth of 2 feet is assumed. A depth damage ratio of 25% was used for structural loss, based on the FEMA damage curves, assuming a 2-foot-deep flood. A loss ratio was also calculated to express the estimated losses in relation to the total value of all property in the jurisdiction, including property values not exposed to the floodplain boundaries. The results are shown in the loss estimate columns in Table 4-38 for the 1% annual chance flood.

The result is an inventory of the number and types of improved parcels subject to flooding. Results are presented by unincorporated county and incorporated jurisdictions. Detailed tables show counts of parcels by jurisdictions and land use type (agricultural, commercial, exempt, industrial, multi-residential, open space and residential) within each flood zone. It is important to note that there could be more than one structure or building on an improved parcel (i.e., a condo complex occupies one parcel but might have several structures). This flood loss analysis does not account for business disruption, emergency services, environmental damages, or displacement costs, thus actual losses could exceed the estimate shown. Conversely, this analysis does not differentiate parcels that may have been developed since when the county and cities adopted floodplain regulations, which would be mitigated to the 1% annual chance flood if developed in accordance with local floodplain regulations.

Table 4-39 1% Annual Chance Floodplain Exposure and Loss by Jurisdiction Jurisdiction Property
Type
Improved
Parcel
Count
Building
Count
Improved
Value
Estimated
Content Value
Total Value Estimated Loss
Atwater Commercial 4 8 $446,109 $446,109 $892,218 $223,055 Exempt 7 7 $0 $0 $0 $0
Industrial 1 1 $16,651 $24,977 $41,628 $10,407
Residential 342 344 $87,919,330 $43,959,665 $131,878,995 $32,969,749 Total 354 360 $88,382,090 $44,430,751 $132,812,841 $33,203,210 Livingston Agricultural 1 1 $7,441 $7,441 $14,882 $3,721 Total 1 1 $7,441 $7,441 $14,882 $3,721
Merced Agricultural 17 19 $704,235 $704,235 $1,408,470 $352,118 Commercial 527 1,873 $339,551,353 $339,551,353 $679,102,706 $169,775,677 Exempt 433 960 $9,185,588 $9,185,588 $18,371,176 $4,592,794 Industrial 118 218 $167,406,079 $251,109,119 $418,515,198 $104,628,799 Mixed Use 9 9 $185,869 $185,869 $371,738 $92,935
Residential 9,161 12,140 $1,218,507,813 $609,253,907 $1,827,761,720 $456,940,430 Vacant
Land
30 38 $1,030,699 $1,030,699 $2,061,398 $515,350
Total 10,295 15,257 $1,736,571,636 $1,211,020,769 $2,947,592,405 $736,898,101 Unincorporated Agricultural 2,240 2,920 $571,674,871 $571,674,871 $1,143,349,742 $285,837,436 Commercial 182 516 $45,633,503 $45,633,503 $91,267,006 $22,816,752 Exempt 84 115 $9,565,302 $9,565,302 $19,130,604 $4,782,651 Industrial 57 82 $63,423,427 $95,135,141 $158,558,568 $39,639,642 Mixed Use 15 17 $233,502 $233,502 $467,004 $116,751
Residential 2,697 2,967 $319,577,554 $159,788,777 $479,366,331 $119,841,583

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Jurisdiction Property
Type
Improved
Parcel
Count
Building
Count
Improved
Value
Estimated
Content Value
Total Value Estimated Loss
Vacant
Land
25 26 $131,577 $131,577 $263,154 $65,789
Total 5,300 6,643 $1,010,239,736 $882,162,673 $1,892,402,409 $473,100,602 Grand
Total
15,950 22,261 $2,835,200,903 $2,137,621,633 $4,972,822,536 $1,243,205,634 Source: Merced County Assessor’s Office; NFHL Effective date 12/02/2008, FEMA; GIS analysis Large areas of Merced County are at risk of being inundated by a 100-year flood event. The City of Merced, City of Atwater, and unincorporated areas of the County are predominantly inundated by the 100-year floodplain and have the greatest percentages of total loss from a 100-year flood event. While other jurisdictions are far less at risk during in a 100-year flood event, several jurisdictions including Dos Palos and Gustine are at a greater risk of inundation in the event of a 200-year flood.

Based on this analysis, the Merced County planning area has 15,950 parcels and 22,261 buildings valued over $2.8 billion in the 100-year floodplain. An additional 2,904 parcels and 3,866 buildings valued at roughly $596 million fall within the 500-year floodplain. As a result, total structural exposure is approximately $4.4 billion. When factoring the content values within these areas in addition to the structures the total combined value of exposure is $6.1 billion. Development in the 500-year floodplain is typically not regulated, thus a large flood event could be extremely damaging in the County and Merced.

This information is summarized in Table 4-39 below.

Table 4-40 0.2% Annual Chance Floodplain Exposure and Loss by Jurisdiction Jurisdiction Property
Type
Improved
Parcel
Count
Building
Count
Improved
Value
Estimated
Content
Value
Total Value Estimated
Loss
Merced Commercial 30 31 $62,378,180 $62,378,180 $124,756,360 $31,189,090 Exempt 33 69 $0 $0 $0 $0
Mixed Use 2 3 $41,361 $41,361 $82,722 $20,681
Residential 1,594 2,412 $337,480,454 $337,480,454 $674,960,908 $168,740,227 Total 1,659 2,515 $399,899,995 $399,899,995 $799,799,990 $199,949,998 Unincorporated Agricultural 204 262 $39,295,471 $39,295,471 $78,590,942 $19,647,736 Commercial 6 14 $9,074,287 $9,074,287 $18,148,574 $4,537,144 Exempt 6 6 $3,145,199 $3,145,199 $6,290,398 $1,572,600
Mixed Use 4 4 $148,839 $148,839 $297,678 $74,420
Residential 1,022 1,062 $144,732,798 $144,732,798 $289,465,596 $72,366,399 Vacant
Land
3 3 $6,699 $6,699 $13,398 $3,350
Total 1,245 1,351 $196,403,293 $196,403,293 $392,806,586 $98,201,647 Grand
Total
2,904 3,866 $596,303,288 $596,303,288 $1,192,606,576 $298,151,644 Source: Merced County Assessor’s Office; NFHL Effective date 12/02/2008, FEMA; GIS analysis Another method of assessing flood vulnerability is through the utilization of the California DWR BAM which display a 200-year floodplain. The BAM does not replace existing FEMA regulatory floodplains, but rather identifies potential flood risks that may warrant further studies or analysis. Analyzing this floodplain data in the same way as described above revealed an estimated additional 2,159 buildings exposed to flood hazard throughout the unincorporated portions of Merced County.

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Insurance Coverage, Claims Paid, and Repetitive Losses
Unincorporated Merced County joined the NFIP in 1979. All of the jurisdictions in the county, with the exception of Gustine, are NFIP participants. The County currently does not participate in the optional CRS, nor do any of the jurisdictions.

In the unincorporated county, there are 1,558 policies in force, of which there are 1,285 single family units, 4 2-4 family, three all other residential, and 266 nonresidential. The number of policies in force by jurisdiction and flood zone are shown in Table 4-40 below. The City of Gustine is not included as it is not a NFIP participant.

Table 4-41 Merced County Community Information System Policies in Force by Flood Zone and Jurisdiction
Flood Zone Merced
County
City of
Merced
City of
Los
Banos
City of
Livingston
City of Dos
Palos
City of
Atwater
A01-30 & AE
Zones
33 275 0 0 0 0
A Zones 411 51 0 0 0 0
AO Zones 240 573 0 0 0 0
AH Zones 80 114 0 0 0 0
AR Zones 0 0 0 0 0 0
A99 Zones 0 0 0 0 0 0
V01-30 & VE
Zones
0 0 0 0 0 0
V Zones 0 0 0 0 0 0
D Zones 0 0 0 0 0 0
B, C & X Zone
Standard 51 59 0 3 0 1
Preferred 62 119 16 1 3 13
Total 877 1,191 16 4 3 14
Source: FEMA NFIP Community Information System
NFIP data indicates that there are 4,987 insurance policies in Merced County representing $1,084,286,000 of insurance coverage in force. Since 1979 there have been 281 paid losses, totaling $4,484,055. Most of the losses have been in the City of Merced and the unincorporated county. Table 4-41 provides more details on flood insurance policies for each individual jurisdiction.

FEMA insures properties against flooding losses through the NFIP. As part of the process to reduce or eliminate repetitive flooding to structures across the United States, FEMA has developed an official Repetitive Loss Strategy. The purpose behind the national strategy is to identify, catalog, and propose mitigation measure to reduce flood losses to the relatively few numbers of structures that absorb the majority of the premium dollars from the national flood insurance fund.

A repetitive loss property is defined by FEMA as “a property for which two or more NFIP losses of at least $1,000 each have been paid within any 10-year period since 1978”. A repetitive loss property may or may not be currently insured by the NFIP. There are 13 repetitive loss buildings in the unincorporated county, 3 of which are insured and a total of $367,067 was paid out. There are no Severe Repetitive Loss properties, as defined by FEMA, anywhere in the County.

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Table 4-42 Merced County Flood Insurance Policy Information Jurisdiction Policies Insurance in Force No. of Paid Losses Total Losses Paid Atwater 14 $4,018,000 1 $1,740
Dos Palos 3 $553,000 0 $0
Gustine N/A N/A N/A N/A
Livingston 4 $665,000 0 $0
Los Banos 16 $5,180,000 0 $0
Merced 3,401 $772,521,900 151 $2,230,265
Unincorporated 1,558 $301,348,300 129 $2,252,050
Total 4,996 $1,084,286,000 281 $4,484,055
Source: FEMA NFIP Community Information System
People
The total number of residential properties in each floodplain was multiplied by the average household size of 3.32 persons for the County (ACS 2019 estimates) to estimate resident population within mapped flood hazard areas. Based on this analysis, which accounts for residents only and not workers, there are 49,878 residents living in the 100-year flood zone throughout the County. Of all study areas, the City of Merced has the most residents living in the 1% annual chance flood area, followed by the unincorporated county. Table 4-42 below details population estimates by jurisdiction, followed by similar tables for the 500-year floodplain.

Table 4-43 Population Living in the 1% Annual Chance Flood Hazard Zone Jurisdiction Population
Atwater 1,094
Merced 38,877
Unincorporated County 9,907
Total 49,878
Source: Wood GIS analysis on Merced County Assessor’s Office data and FEMA NFHL, ACS Census Estimates The same analysis was conducted for the 500-year floodplain, indicating that there are 11,267 residents living in the 500-year flood zone throughout Merced County. The majority of people living in this floodplain are residents of the City of Merced, with almost 8,000 people in the 500-year floodplain. This population distribution is shown in the table below.

Table 4-44 Population Living in the 0.2% Annual Chance Flood Hazard Zone Jurisdiction Population
Merced 7,728
Unincorporated County 3,539
Total 11,267
Source: Wood GIS analysis on Merced County Assessor’s Office data and FEMA NFHL, ACS Census Estimates Critical Facilities and Infrastructure
Key support facilities and structures most necessary to withstand the impacts of, and respond to, natural disasters are referred to as critical facilities. Examples of these critical facility types include utilities, transportation infrastructure, and emergency response and services facilities, given failures of components along major lifelines or even closures or inaccessibility to key emergency facilities could limit if not

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completely cut off transmission of commodities, essential services, and other potentially catastrophic repercussions. Floods and levee failure have the ability to disrupt, damage, or destroy these critical facilities, which in turn can impede the ability of Merced County to respond to and recover from a major flood event.

A GIS analysis of exposed critical facilities was conducted, similar to the parcel analysis, using HIFLD. The results of critical facilities throughout the county which are exposed to the various flood hazards are shown in Table 4-444 and Table 4-45 below, and organized by the jurisdiction they are located in and the FEMA Lifeline category into which they are classified.

Table 4-45 Critical Facilities Within the 1% Annual Chance Flood Hazard by FEMA Lifeline and Jurisdiction
Jurisdiction
CommunicationsEnergyFood, Water, ShelterHazardous MaterialHealth and MedicalSafety and SecurityTransportationTotal Atwater 2 2 1 - - 1 4 10
Dos Palos - - - - - - - 0
Gustine - - - - - - - 0
Livingston - - - - - - - 0
Los Banos - - - - - - - 0
Merced 26 3 30 15 6 51 25 156
Unincorporated 13 5 13 11 - 34 180 256
Total 41 10 44 26 6 86 209 422
Source: HIFLD, FEMA NFHL
Table 4-46 Critical Facilities Within the 0.2% Annual Chance Flood Hazard by FEMA Lifeline and Jurisdiction
Jurisdiction
CommunicationsEnergyFood, Water, ShelterHazardous MaterialHealth and MedicalSafety and SecurityTransportationTotal Atwater - - - - - - - 0
Dos Palos - - - - - - - 0
Gustine - - - - - - - 0
Livingston - - - - - - - 0
Los Banos - - - - - - - 0
Merced - - 2 - 2 5 - 9
Unincorporated 1 - - - - 5 1 7
Total 1 0 2 0 2 10 1 16
Source: HIFLD, FEMA NFHL

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Economy
Flooding can have a major economic impact on the economy. Based on the flood loss analysis, there are 2,397 commercial structures worth an estimated $771 million in total value directly at risk to flooding in the 1% annual chance zone. Based on the loss analysis this could result in approximately $193 million in direct losses. This does not account for other indirect losses such as business interruption, lost wages and other downtime costs.

Effects on the agriculture economy can be devastating, and a large amount of area at risk to flooding in the County is agricultural. Flooding can damage crops and livestock. In addition to the obvious impacts on crops and animals, flooding can have deleterious effects on soil and the ability to reinvigorate the agricultural activities impacted once the flood waters recede. Damage to water resources such as underground irrigation systems, water storage reservoirs, springs and other natural water bodies could have a serious effect upon agriculture operations.

Historic, Cultural, and Natural Resources
There are 15 properties throughout the County on the NRHP. Most of these places are located in the City of Merced, which has the highest risk and exposure to flood hazards.

Natural areas within the floodplain often benefit from periodic flooding as a naturally recurring phenomenon. These natural areas often reduce flood impacts by allowing absorption and infiltration of floodwaters. Natural resources are generally resistant to flooding except where natural landscapes and soil compositions have been altered for human development or after periods of previous disasters such as drought and fire. Wetlands, for example, exist because of natural flooding incidents. A large percentage of flood prone area in the County are wetlands that are part of the San Luis and Merced National Wildlife Refuges. These areas provide natural and beneficial functions to hold and absorb floodwaters. Areas recently suffering from wildfire damage may erode because of flooding, which can temporarily alter an ecological system.

Future Development
Flooding and floodplain management are significant issues for Merced County. The potential or likelihood of a flood event in the county increases with the annual onset of heavy rains in winter and spring months.

Much of the historical growth in the problem areas connected with flooding and stormwater runoff include erosion, sedimentation, degradation of water quality, losses of environmental resources, and certain health hazards. For NFIP participating communities, floodplain management practices implemented through local floodplain management ordinances should mitigate the flood risk to new development in the 100-year floodplains. As noted previously, a large amount of development has occurred in the 500-year floodplain and these areas are not regulated or require flood mitigation.

The development trend in the Merced County planning area is steady growth. Much of this growth is occurring in the urban areas, which can increase stormwater runoff. The 2020 Caltrans Economic Forecast at the county-level for all 58 counties in California for 2018-2050, projects that Merced County’s population will continue to increase by 1.3 percent through 2025. Such growth may consume previously undeveloped acres, and the increase in impervious surfaces could affect existing drainage and flood control facilities.

Changes in municipal boundaries such as annexations into the SOI may change the flood risk profile for certain communities. A GIS analysis of 1% annual chance flood exposure within SOI boundaries is summarized in the tables below.

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Table 4-47 Sphere of Influence Risk to FEMA 1% Annual Chance Flood Hazard Sphere of
Influence
Improved
Parcel
Count
Building
Count
Improved
Value
Estimated
Content
Value
Total Value Estimated
Loss
Population Loss
Ratio
Atwater 72 80 $9,559,223 $6,160,259 $15,719,482 $3,929,871 169 1.4% Livingston 1 1 $103,862 $103,862 $207,724 $51,931 0 0.2% Merced 908 1,121 $154,013,086 $111,545,194 $265,558,280 $66,389,570 2,673 7.5% Total 981 1,202 $163,676,171 $117,809,315 $281,485,486 $70,371,372 2,842 Table 4-48 Sphere of Influence Risk to FEMA 0.2% Annual Chance Flood Hazard Sphere
of
Influence
Improved
Parcel
Count
Building
Count
Improved
Value
Estimated
Content
Value
Total Value Estimated
Loss
Population Loss
Ratio
Atwater 1 1 $81,844 $40,922 $122,766 $30,692 3 0.01%
Merced 80 86 $29,663,971 $17,321,531 $46,985,502 $11,746,375 249 1.3% Total 81 87 $29,745,815 $17,362,453 $47,108,268 $11,777,067 252 Risk Summary
The overall significance of the flood hazard is High. The 100-year floodplain in Merced County is vast, and many thousands of residents and millions in property value are within the floodplain. ● Countywide there are 22,261 structures at risk worth $4.9 billion, with a loss estimate of $1.2 billion. ● The City of Merced accounts for 59% of the total losses in the 100-year floodplain. 48% of the City of Merced’s buildings are in the floodplain.
● Commercial structures account for 15.5% of total losses in the 100-year floodplain, with a loss estimate of $193M
● Countywide approximately 49,878 persons live in the 100-year floodplain, of which the vast majority (38,877) are in the City of Merced.
● There are 11,267 persons within the 500-year floodplain and a flood of this magnitude would have severe consequences.
● Levees and flood control dam infrastructure provide some protection, but also pose potential levee and dam failure risks.
● A total of 438 critical facilities are found in 1% annual chance (422 facilities) and 0.2% annual chance (16 facilities) flood zones in the county. Most of them are in the City of Merced or in the unincorporated areas of the county.
● Related hazards: Severe weather: Heavy Rain, Wildfire, Landslide and Debris Flow.

Table 4-49 Hazard Risk Summary – Flood & Levee Failure
Jurisdiction Geographic
Extent
Probability of
Future
Occurrence
Magnitude/
Severity
Overall
Significance
Merced County Extensive Likely Catastrophic High
City of Atwater Extensive Likely Critical Medium
City of Dos Palos Limited Occasional Limited Low
City of Gustine Significant Occasional Critical Medium
City of Livingston Limited Occasional Negligible Medium

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Jurisdiction Geographic
Extent
Probability of
Future
Occurrence
Magnitude/
Severity
Overall
Significance
City of Los Banos Limited Likely Limited Medium
City of Merced Significant Likely Limited Medium

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4.2.5 Landslide
Hazard Problem/Description
A landslide is a geologic hazard where the force of gravity combines with other factors to cause earth material to move or slide down an incline. Some landslides move slowly and cause damage gradually, whereas others move so rapidly that they can destroy property and take lives suddenly and unexpectedly.

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

Debris flows are a mixture of rock fragments, soil, vegetation, water and, in some cases, entrained air that flows downhill as a fluid. Debris flows can range in consistency from that of freshly mixed concrete to running water. Debris flows can be further classified as mudflows and earth flows depending on the ratio of water to soil and rock debris.

Landslide and debris flow problems can be caused by land mismanagement, particularly in mountain, canyon, and coastal regions. In areas burned by forest and brush fires, a lower threshold of precipitation may initiate landslides and debris flows. As human populations expand over more of the land surface, these processes become an increasing concern.

There are predictable relationships between local geology and landslides, rockslides, and debris flows. The downslope movement of earth material, either as a landslide, debris flow, mudslide, or rockslide, is part of the continuous, natural process of erosion. This process, however, can be influenced by a variety of causes that change the stability of the slope. Slope instability may result from natural processes, such as the erosion of the toe of a slope by a stream, or by ground shaking caused by an earthquake. Slopes can also be modified artificially by grading, or by the addition of water or structures to a slope. Development that occurs on a slope can substantially increase the frequency and extent of potential slope stability hazards.

Knowledge of these relationships can improve planning and reduce vulnerability. Slope stability is dependent on many factors and their interrelationships, including rock type, moisture content, slope steepness, and natural or manmade undercutting.

Geographic Area
In Merced County the landscape is largely characterized by the San Joaquin Valley, which is relatively flat and takes up the majority of the central portion of the county. To the eastern edge of the county, lying within the foothills of the Sierra Nevada range, and the western edge, within the Coast Range, there are several geologic formations commonly associated with slope stability problems. Figure 4-19 below provides a summary of landslide hazard potential. As shown in the map, the areas of the county with the greatest potential susceptibility to landslide hazards are in the far southwest of the county, on the western side of Interstate 5. This area is all unincorporated area in the county and is largely rural in character, indicating a lower potential risk to property and infrastructure relative to other counties in California.

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Figure 4-19 Merced County Deep-Seated Landslide Susceptibility

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Extent (Magnitude/Severity)
The extent of landslides and debris flow events within the county range from negligible to significant.

Landslides and rockslides can result in the destruction of infrastructure such as water and sewer lines, electrical and telecommunications utilities and drainage where they are present. There is potentially risk to Interstate 5, near the county border with Fresno County. There is limited potential for complete destruction of buildings and death and injury from landslides and debris flow.

Previous Occurrences
There have not been any disaster declarations associated with landslides in Merced County. According to records from the California Department of Conservation’s Geological Survey, there have been no recent records of landslides in Merced County and no active mapping efforts within the county. Several adjacent counties, such as Madera, Mariposa, and Santa Clara, have recent or active landslides which have been recorded. This historic record would indicate that landslides occasionally occur in Merced County, but largely affect undeveloped areas.

Probability of Future Occurrences
Based on historical data which suggests limited risk of landslides, but given the presence of landslide susceptible geology and steep slopes within the planning area, it is safe to assume that landslides may occur in the future, with damaging landslides occurring much less frequently. Landslides are usually a cascading effect of severe weather or a period of extended wet weather that saturates local soils. The probability for more severe and damaging landslides increases during El Niño years or severe winter storms. The potential for debris flows dramatically increases following a wildfire.

Climate Change Considerations
Landslides can result from intense rainfall and runoff events. Projected climate change-associated variance in rainfall events may result in more high-intensity events, which may increase landslide frequency. In addition, the increased potential of wildfire occurrence also escalates the risk of landslide and debris flows in the period following a fire, when slopes lack vegetation to stabilize soils and burned soil surfaces create more rainfall runoff. As climate change affects the length of the wildfire season, it is possible that a higher frequency of large fires may occur into late fall, when conditions remain dry, and then be followed immediately by intense rains early in the winter, as occurred with the Thomas Fire in December 2017 and subsequent Montecito and Carpinteria debris flows in January 2018 in Santa Barbara County (CA SHMP 2018).

Vulnerability Assessment
General Property
Landslides directly damage engineered structures in two general ways: 1) disruption of structural foundations caused by differential movement and deformation of the ground upon which the structure sits, and 2) physical impact of debris moving downslope against structures located in the travel path.

During the 2021 update of this plan, a GIS analysis of exposure to landslide hazard areas was performed.

GIS analysis indicates approximately $317 million of property improvements exposed, which takes into account improved values of properties. Table 4-49 summarizes landslide exposure by jurisdiction. GIS was used to intersect the parcel boundaries with a master address point layer to obtain the number of buildings per parcel. Only parcels with improvement values greater than zero were used in the analysis, this method assumes that improved parcels have a structure of some type.

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Based on this analysis there is a total of 482 improved parcels and 547 buildings potentially within landslide hazard areas. The majority of this exposure is in the western unincorporated county. There is a high level of uncertainty as to the actual risk to these exposed parcels, thus a more specific loss estimation is not provided. A more detailed, site specific analysis would be needed to assess actual risk within the identified parcels.

Table 4-50 Merced County Improved Properties at Risk to Landslide Jurisdiction Improved
Parcel
Count
Total
Building
Count
Improved
Value
Estimated
Content Value
Total Value Population
Atwater 11 11 $3,479,587 $2,831,312 $6,310,899 32
Livingston 1 1 $2,714,360 $1,357,180 $4,071,540 4
Merced 27 42 $4,762,383 $2,381,192 $7,143,575 112
Unincorporated 443 493 $156,446,776 $142,936,749 $299,383,525 322 Total 482 547 $167,403,106 $149,506,432 $316,909,538 470 Source: Dept. of Conservation, California Geological Survey, Merced County Assessor, Wood GIS Analysis Figure 4-20 below further breaks down the numbers of exposed buildings by landslide class, as shown previously in Figure 4-19; the higher the class the greater the susceptibility.

Figure 4-20 Total Building Counts by Landslide Class
People
People could be susceptible if they are caught in a landslide or debris flow, potentially leading to injury or death. There is also a danger to drivers operating vehicles, as rocks and debris can strike vehicles passing through the hazard area or cause dangerous shifts in roadways. The parcel analysis described previously estimated the number of people potentially residing in landslide susceptible areas (470), but it is not likely that landslides will occur without warning and direct impacts to people are suspected to be minimal. 0 100 200 300 400 500 600
Class 3 Building Count
Class 5 Building Count
Class 6 Building Count
Class 7 Building Count
Class 8 Building Count
Class 9 Building Count
Class 10 Building Count
Total Building Count
Total Building Counts by Landslide Class

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Critical Facilities and Infrastructure
In addition to buildings, utilities and transportation structures are vulnerable to the impact and ground deformation caused by slope failures. They present a particular vulnerability because of their geographic extent and susceptibility to physical distress. Lifelines are generally linear structures that, because of their geographic extent, have a greater chance of being affected by ground failure due to greater hazard exposure.

Extension, bending, and compression caused by ground deformation can break lifelines. Failure of any component along the lifeline can result in failure to deliver service over a large region. Once broken, transmission of the commodity through the lifeline ceases, which can have catastrophic repercussions down the line: loss of power to critical facilities such as hospitals, impaired disposal of sewage, contamination of water supplies, disruption of all forms of transportation, release of flammable fuels, and so on. Therefore, the overall impact of lifeline failures, including secondary failure of systems that depend on lifelines, can be much greater than the impact of individual building failures.

Table 4-50 below summarizes the results of the critical facilities analysis, highlighting the exposure of critical facilities throughout the county to landslide hazards. All of the exposed critical facilities are located in unincorporated areas of the county, largely to the west of Interstate 5.

Table 4-51 Critical Facilities Within the Class 9 Landslide Hazard by FEMA Lifeline Jurisdiction Landslide
Class
Communications Energy
Food,
Water,
Shelter
Hazardous
Material
Health
and
Medical
Safety
and
Security
Transportation Total
Uninc.
9 2 26 - - - - - 28
8 1 48 - - - - - 49
7 - 4 - - - - 46 50
6 1 - - - - - - 1
5 4 80 - - - - 3 87
3 1 - - - - - - 1
Total 9 158 0 0 0 0 49 216
Source: California Geological Survey, HIFLD, Wood GIS Analysis Economy
Economic impacts typically center around transportation routes temporarily closed by debris flow or landslide activity. These roads may be used to transport goods across the county or provide access by visitors and tourists. Depending on the amount of damage, the road may simply need to be cleaned off, or may require some level of reconstruction.

Historic, Cultural, and Natural Resources
As primarily a natural process, landslides and debris flows can have varying impacts to the natural environment; debris flows have the potential to permanently alter the natural landscape.

Future Development
The severity of landslide problems is directly related to the extent of human activity in hazard areas.

Human activities such as property development and road construction can also exacerbate the occurrence of landslides. Future development should take place carefully to prevent landslide damage to property or people. Adverse effects can be mitigated by early recognition and avoiding incompatible land uses in

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these areas or by corrective engineering. Improving mapping and information on landslide hazards and incorporating this information into the development review process could prevent siting of structures and infrastructure in identified hazard areas. The 2030 General Plan has provisions to address this goal, by prohibiting habitable structures atop unconsolidated landslide debris, discouraging grading and construction on slopes greater than 30%, and enforcing grading standards of the International Building Code.

Risk Summary
The overall significance of landslides in Merced County is low. ● The geologic formations commonly associated with slope stability problems in Merced County, are largely concentrated in the far western and eastern edges of the county, which are also the least populated.
● Effects on people: People are most commonly at risk if they are caught in a landslide or debris flow, including when driving through a hazard area.
● Effects on property: Property loss is rare, but GIS analysis shows there is some property which could be exposed to landslides.
● Effects on economy: Landslides in adjacent counties can disrupt major transportation corridors, possibly affecting the local tourist economy and shipment of goods. ● Effects on critical facilities and infrastructure: Landslides and debris flows can result in the destruction of infrastructure such as water and sewer lines, electrical and telecommunications utilities and drainage. Disrupted transportation routes occur occasionally, usually during heavy rainstorms, and cause considerable inconvenience.
● Related Hazards: Earthquake, Adverse Weather, Wildfire.

Table 4-52 Hazard Risk Summary - Landslides
Jurisdiction Geographic Area
Probability of
Future
Occurrence
Magnitude/
Severity
Overall
Significance
Merced County Limited Occasional Limited Low
City of Atwater N/A
City of Dos Palos N/A
City of Gustine N/A
City of Livingston N/A
City of Los Banos N/A
City of Merced N/A

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4.2.6 Severe Weather: General
The climate in the Central Valley is defined generally by hot, dry summers and foggy, rainy winters. Severe weather is generally any destructive weather event, but usually occurs in Merced County as localized thunderstorms that bring heavy rain and strong winds that occur most often during the winter and spring months. The agricultural industry is among the most vulnerable asset to severe weather. Agricultural losses resulting from natural hazards can have dramatic impacts on the economic health of Merced County.

For this plan, adverse weather is broken down as follows:
● Freeze
● Extreme Heat
● Dense Fog
● Heavy Rain
● High Wind/Tornado
The National Oceanic and Atmospheric Administration’s NCEI has been tracking adverse weather since 1950. Their Storm Events Database contains data on the following: all weather events from 1993 to 2017 (except from 6/1993-7/1993); and additional data from the Storm Prediction Center, which includes tornadoes (1950-1992), thunderstorm winds (1955-1992), and hail (1955-1992). This database contains 254 storm events that occurred in Merced County between January 1, 1950, and December 2020. The table below summarizes these events.

Table 4-53 NCEI Hazard Event Reports for Merced County, 1950-2020 Hazard # of Events Property Loss ($) Crop Loss ($) Deaths Injuries Dense Fog* 102 $1,160,00 $0 3 7
Frost/Freeze* 31 $0 $0 0 0
Heavy Rain 23 $2,007,500 $15,660,000 0 0
Strong Wind* 20 $90,800 $513,000 0 0
Funnel Cloud 18 $0 $0 0 0
Tornado 14 $410,000 $50,000 0 1
Heat* 13 $0 $0 0 0
High Wind* 12 $995,000 $2,110,000 0 4
Hail 9 $30,000 $1,233,000 0 4
Lightning 8 $1,000,000 $0 0 1
Thunderstorm Wind 4 $430,000 $3,100,000 0 0
Total 254 $4,963,300 $22,666,000 3 17
Source: NCEI Storm Events Database, www.ncdc.noaa.gov/stormevents/ *Hazards with wide extents reflect larger zones that extend beyond Merced County.

The NCEI table above summarizes adverse weather events that have occurred in Merced County from 1950 to 2020. Only a few of the events actually resulted in state and federal disaster declarations. It is interesting to note that different data sources capture different events during the same time period, and often different information specific to the same events. While the HMPC recognizes these inconsistencies, this data provides value in depicting the County’s “big picture” hazard environment.

Due to size of the County and changes in elevation and climate, weather conditions can vary greatly across the County. The profiles that follow provide information, where possible, from two weather stations in different parts of the County: Merced Municipal Airport Weather Station in the northeastern part of the county (1889-2016) and the Los Banos City weather station in the southwestern part of the county (1906- 2016). The temperature data from these two weather stations are displayed in subsection 4.3.7 below.

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4.2.7 Severe Weather: Extreme Temperatures: Freeze and Extreme Heat Hazard/ Problem Description
Extreme temperature events, both cold and hot, can have severe impacts on human health and mortality, natural ecosystems, and agriculture and other economic sectors.

Freeze
Extreme cold often accompanies a winter storm or is left in its wake. Prolonged exposure to cold can cause frostbite or hypothermia and can be life-threatening. Infants and the elderly are most susceptible.

Pipes may freeze and burst in homes or buildings that are poorly insulated or without heat. Freezing temperatures can cause significant damage to the agricultural industry.

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

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

Geographic Area
Freeze
The entire county is susceptible to extreme temperatures. Unseasonable cold temperatures can have large impacts on crops in Merced County. The growing season is approximately 257 days per year, and the frost-free period usually extends from mid-February to mid-November. The mean frost-free period in the western part of the county is 225-250 days.

Extreme Heat
The climate in Merced County is hot and arid, and the entire county is susceptible to extreme heat. The agriculturally dominated central region of the county is likely to experience the greatest impacts from large or unseasonable temperature variations. The more urbanized areas of the County may experience the Urban Heat Island Effect due to the amount of impervious surfaces in those areas that absorb and keep heat longer. The HMPC noted the cities of Merced and Dos Palos have experienced annual summer heat waves that have taken place in the past five years and have led to opening of cooling centers.

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Extent (Magnitude/Severity)
Freeze
In 2001, the NWS implemented an updated Wind Chill Temperature index. This index was developed to describe the relative discomfort/danger resulting from the combination of wind and temperature. Wind chill is based on the rate of heat loss from exposed skin caused by wind and cold. As the wind increases, it draws heat from the body, driving down skin temperature and eventually the internal body temperature.

Overall, freeze impacts would likely be limited in the planning area, with the greatest impact being on the agricultural industry.

Figure 4-21 National Weather Service Wind Chill Chart
Source: NWS
Extreme Heat
Figure 4-22 illustrates the relationship of temperature and humidity to heat disorders. The heat index describes how hot the heat‐humidity combination makes the air feel. As relative humidity increases, the air seems warmer than it actually is because the body is less able to cool itself via evaporation of perspiration. As the heat index rises, so do health risks. Specifically:
● When the heat index is 90°F, heat exhaustion is possible with prolonged exposure and/or physical activity.
● When it is 90° to 105°F, heat exhaustion is probable with the possibility of sunstroke or heat cramps with prolonged exposure and/or physical activity.
● When it is 105° to 129°F, sunstroke, heat cramps or heat exhaustion is likely, and heatstroke is possible with prolonged exposure and/or physical activity. ● When it is 130°F and higher, heatstroke and sunstroke are extremely likely with continued exposure.

Physical activity and prolonged exposure to the heat increase the risks.

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Figure 4-22 National Weather Service Heat Index Chart
Source: Source: NWS Note: Since heat index values were devised for shady, light wind conditions, exposure to full sunshine can increase heat index values by up to 15°F. Also, strong winds, particularly with very hot, dry air, can be extremely hazardous.

The NWS has a system in place to initiate alert procedures (advisories, watches, and warnings) when high temperatures are expected to have a significant impact on public safety. The expected severity of the heat determines which type of alert is issued. The “California OES Contingency Plan for Excessive Heat Emergencies” (2014) indicates that through the use of historical weather and mortality data, the NWS and the California Department of Public Health have identified five major types of climate regions within California to account for climate differences among regions in order to recognize what constitutes an excessive heat event in each of the regions. When temperatures spike for two or more consecutive days without an adequate drop in nighttime temperature to cool the outdoor and indoor environments, there is a significant increase in the risk to vulnerable populations.

Overall, extreme heat impacts would likely be limited in the planning area, with the central portions of the County being most affected. Extreme heat will have an impact on vulnerable populations as well as impact the agricultural industry if the event occurs during certain times of the year.

Past Occurrences
A search of records from 1950 to 2020 in the NCEI Storm Events Database showed that between 2000 and 2016 there were 31 frost and freeze events and 13 heat events in Merced County. No records for either hazards were found between 1950 and 2000 or after 2016 in the NCEI database. Since 1972 there have been 4 federally declared disaster in the County for freeze events (refer to Table 4-3). In 2007, 2009 and 2011 and 2012 the USDA designated Merced County as a disaster area due to freeze and extreme cold.

During these events, California's San Joaquin Valley farming communities were hit with freezing temperatures that severely affected the region's crops and resulted in presidential disaster declarations (2007). The declarations made federal funds available to supplement unemployment compensation for farm laborers and other farm industry workers put out of work as a direct result of lost seasonal crops.

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Probability of Future Occurrences
Highly Likely - Temperatures at or above 95°F are common most summer days throughout Merced County, and it is highly likely that extreme heat will continue to occur on an annual basis in the future.

In the past, severe freezes have occurred every few years. Based on the NCEI data, 31 frost and freeze events incidents over a 16-year period equates to a major event every 2 years and a 52 percent chance of a major freeze event in any given year.

Climate Change Considerations
Although heat waves will likely become more frequent, there is also the potential for continued cold outbreaks in winter, even in an overall warmer climate. This could have direct impacts on human health in terms of heat related illness. With the general trend of increased warming of average temperatures, extreme high temperatures will likely increase as well. Cascading impacts include increased stress on water quantity and quality, degraded air quality, and increased potential for more severe or catastrophic natural events such as heavy rain, droughts, and wildfire. Another cascading impact includes increased duration and intensity of wildfires with warmer temperatures. According to the 2013 document, “Preparing California for the Extreme Heat”, Cal-Adapt projects that throughout California urban and rural population centers will experience an average of 40 to 53 extreme heat days by 2050 and an average of 40 days by 2099; compared to a historical average of 4 per year (Cal-Adapt 2013).

Extreme heat has also been shown to accelerate wear and tear on the natural gas system and electrical infrastructure (California Natural Resources Agency 2018a). Projected increases in summer demand associated with rising temperatures may increase risks to energy infrastructure and may exceed the capacity of existing substations and distribution line infrastructure and systems.

Vulnerability Assessment
General Property
The risks of extreme temperatures are often profiled as part of larger hazards, such as severe winter storms or drought. However, as temperature variances may occur outside of larger hazards or outside of the expected seasons but still incur large costs, it is important to examine them as stand-alone hazards.

Extreme cold may also lead to higher electricity and natural gas demands to maintain appropriate indoor heating levels combined with damages caused to the delivery infrastructure such as frozen lines and pipes. Cold temperatures may also impact transportation infrastructure and road conditions through increased wear and stresses on asphalt roads and bridges. Exposed populations may be at risk while waiting for public transportation, particularly when combined with wind chill, and some vehicles may not start, which impacts the commute of the workforce and, in worst case scenarios, the movement of emergency services personnel.

Recent research indicates that the impact of extreme heat, particularly on vulnerable (?) populations, has been historically under-represented. The risks of extreme heat are often profiled as part of larger hazards, such drought or wildfire. However, as temperature variances may occur outside of larger hazards or outside of the expected seasons but still incur large costs, it is important to examine them as stand-alone hazards. Extreme heat may overload demands for electricity to run air conditioners in homes and businesses during prolonged periods of exposure and presents health concerns to individuals that work outside in extreme temperatures. While extreme heat rarely damages buildings, like cold temperatures it can cause infrastructure damage to roads.

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People
Traditionally, the very young and very old are considered at higher risk to the effects of extreme temperatures, but any populations working or recreating outdoors during periods of extreme cold or heat are exposed, including otherwise young and healthy adults and homeless populations. While everyone is vulnerable to extreme temperature incidents, some populations are more vulnerable than others. Extreme heat poses the greatest danger for Merced’s thousands of outdoor laborers who support the County’s agriculture economy who are exposed to extreme temperatures and at higher risk of heat related illnesses than other populations of the county.

The elderly, children, people in poor physical health, and the homeless are also vulnerable to exposure.

Arguably, the young-and-otherwise-healthy demographic may also experience a higher vulnerability of exposure, due to the increased likelihood that they will be out in temperatures of extreme heat and cold, whether due to commuting for work or school, conducting property maintenance such as lawn care, or for recreational reasons.

Critical Facilities
Water infrastructure is at risk from freezing, including line breaks and frozen valve gates affecting the distribution system. The county and municipal governments wrap pipes before freezing temperature events to help prevent damage.

Extreme heat can affect road infrastructure, damaging and buckling road surfaces. Other direct impacts to critical infrastructure includes power line sagging and power surges. Critical infrastructure such as water pumping stations that rely on public utility systems that could be overloaded may result in impacts during extreme heat events. Peak demand exceeding the local utility’s capacity for supply can lead to blackout or brownout conditions, or Public Safety Power Shutoffs (PSPS). The loss of utilities or power outages during extreme heat events could also result in adverse secondary impacts to sensitive populations. Electrical power outages may impact response capabilities or care capabilities for hospitals and clinics.

Economy
Prolonged freezing temperatures can damage or destroy crops, affecting the economy and agricultural jobs in Merced County. According to the USDA RMA Indemnity Report, since 2007 there have been 22,308 acres lost to freeze/frost resulting in $12,933,721 indemnity payments due to insured crop loss.

Annualized crop loss is estimated to be $170,181 due to freeze events.

Extreme heat impacts on the economy may be more indirect compared to other hazards. Merced County has a large agriculture economy. As noted previously outdoor laborers who are exposed to extreme heat and at a high risk of heat related illnesses, and a long-term heat event could cause work interruptions.

Crops are also impacted by heat events and could have an impact on the overall economy in the county.

According to the USDA RMA Indemnity Report, since 2007 there have been 20,207 acres lost to heat resulting in $17,612,071 indemnity payment due to insured crop loss. There is an estimated $187,362 of annualized crop loss due to heat.

Historic, Cultural and Natural Resources
Extreme heat may cause temporary drought-like conditions. For example, several weeks of extreme heat increases evapotranspiration and reduces moisture content in vegetation, leading to higher wildfire vulnerability for that time period even if the rest of the season is relatively moist. Extreme cold can cause vegetation to freeze and cause high stress leading to decline and even disappearance of vegetation on or within landscapes.

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Future Development
Since structures are not usually directly impacted by severe temperature fluctuations, continued development is less impacted by this hazard than others in the plan. However, pre-emptive measures such as construction of green buildings that require less energy to heat and cool, use of good insulation on pipes and electric wirings, and smart construction of walkways, parking structures, and pedestrian zones that minimize exposure to severe temperatures may help increase the overall durability of the buildings and the community to temperature variations. Continued development also implies continued population growth, which raises the number of individuals potentially exposed to variations in temperature. Public education efforts should continue to help the population understand the risks and vulnerabilities of outdoor activities, property maintenance, and regular exposures during periods of extreme heat.

Risk Summary
● Average annual maximum temperature is 76°F and the average annual minimum temperatures is 47°F /48°F.
● On average there is between 97 and 98 days per year above 90°F and about 30 days per year where the minimum temperatures is below freezing (32°F).
● Highest recorded temperatures for the northwestern portion of County is 114°F on July 24, 1902.

While the lowest recorded temperature was 13°F on January 13, 2007. ● High recorded temperatures for the southwestern portion of County is 116°F on July 25, 1931. The lowest recorded temperatures is 14°F on January 11, 1949. ● Extreme temperatures can have considerable impacts on human health, the natural environment and the economy.
● The County’s agriculture economy is at risk of extreme temperatures from outdoor laborers being vulnerable to heat illnesses as well as crop losses due to heat and freeze and frost. ● Since 2007 there has been 42,515 acres of crop lost to extreme temperatures and a combined $30,545,793 indemnity payments made for insured crop loss to due freeze/frost and heat events. ● The very young, the very old, people with poor physical health and the homeless are more susceptible to the impacts of extreme temperatures.
● Climate change is projected to increase temperatures in the Central Valley region of the State. ● Related hazards: Drought, Land Subsidence, Wildfire, Agriculture/Pest Infestation/Plant Disease Table 4-54 Hazard Risk Summary – Extreme Temperatures (Freeze) Jurisdiction Geographic Area
Probability of
Future
Occurrence
Magnitude/
Severity
Overall
Significance
Merced County Extensive Highly Likely Critical High
City of Atwater Limited Likely Limited Low
City of Dos Palos Significant Likely Limited Medium
City of Gustine Limited Occasional Negligible Medium
City of Livingston Limited Occasional Negligible Medium City of Los Banos Limited Occasional Negligible Medium
City of Merced Significant Likely Limited Low

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Table 4-55 Hazard Risk Summary – Extreme Temperatures (Extreme Heat) Jurisdiction Geographic Area
Probability of
Future
Occurrence
Magnitude/
Severity
Overall
Significance
Merced County Extensive Highly Likely Critical High
City of Atwater Extensive Likely Limited High
City of Dos Palos Extensive Likely Negligible Low
City of Gustine Extensive Likely Limited High
City of Livingston Extensive Likely Limited High
City of Los Banos Significant Highly Likely Negligible Medium City of Merced Extensive Highly Likely Limited Medium

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

The interior California valleys have a unique fog problem called the tule fogs. The tule fog is a radiation fog, which condenses when there is a high relative humidity, typically after a heavy rain, calm winds, and rapid cooling during the night. The longer nights during the winter months create this rapid ground cooling and results in a pronounced temperature inversion at a low altitude, creating a thick ground fog.

Above the cold, foggy layer, the air is typically warm and dry. Once the fog has formed, turbulent air is necessary to break through the inversion. Daytime heating can also work to evaporate the fog in some areas. The tule fogs get their name from the tule reeds, which grew around the swamps and deltas of the great Tulare Lake that once covered the southern end of the San Joaquin Valley.

The tule fog season in Merced County is typically in the late fall and winter (November through March) but can occur as late as May. Fog typically forms rapidly in the early morning hours. Tule fogs can last for days, sometimes weeks. Fog can have devastating effects on transportation corridors in the County.

Nighttime driving in the fog is dangerous and multi-car pileups have resulted from drivers using excessive speed for the conditions and visibility. Tule fog has also been found to be connected to air pollution and exacerbates the naturally occurring fog. A study published in the Journal of Geophysical Research in 2019 found that air pollution is a key contributor to the seasonal fog, and due to the passage of the Clean Air Act in 1970, it has declined about 75 percent since 1980 (Gray 2019).

Figure 4-23 Tule Fog over Central Valley
Source: NASA, Jeff Schmaltz, January 5, 2005

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Geographic Area
The San Joaquin Valley is hemmed in on three sides by mountain ranges, with resulting inversion layers trapping cooler air on the valley floor. This predisposes the area to severe episodes of fog in winter months, when barometric pressures are high, humidity is increased, and ambient temperatures are low. All areas of the county are vulnerable to dense fog events.

Extent (Magnitude/Severity)
Tule fog forms on clear nights when the ground is moist, and the wind is near calm and can be widespread throughout the San Joaquin Valley. On nights like this, the ground cools rapidly. In turn, the moist air above it cools and causes water vapor to condense. Once it has formed, the air must be heated enough to either evaporate the fog or lift it above the surface so that visibilities improve. Common areas for tule fog to form include foothills and valleys. Visibility in tule fog is usually less than an eighth of a mile (about 600 ft or 200 m) but can be much lower. Visibility can vary rapidly; in only a few feet, visibility can go from 10 feet (3.0 m) to near zero.

Fog contributes to transportation accidents and is a significant life safety hazard. These accidents can cause multiple injuries and deaths and could have serious implications for human health and the environment if a hazardous or nuclear waste shipment were involved. Other disruptions from fog include delayed emergency response vehicles and school closures.

In 2018 the NWS in Hanford, California developed the Experimental Fog Severity Index in conjunction with the NWS issues fog advisories to help give motorists advance warning of fog events. A dense fog advisory is issued when widespread dense fog develops, and visibilities drop to one-quarter of a mile or less. The Fog Severity Index has five levels with level 5 being the most severe. Figure 4-24 describes each level of the Index.

Figure 4-24 National Weather Service Experimental Fog Severity Index Source: NWS

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Past Occurrences
The NCEI records 102 dense fog events in the Central San Joaquin Valley that impacted Merced County between 2000 and 2020 (refer to Table 4-52 above). The following are descriptions of three events of record in the NCEI Storm Events Database:
● November 14 - 20 2002 - Following the very substantial rain for the 3-day period from the 7th through the 9th of the month, high pressure aloft built and resided over Interior Central California.

This weather regime, quite conducive to dense morning fog formation, established itself between the 10th and 13th, and maintained itself through the end of the month. The fog was especially widespread from the 14th through the morning of the 21st. Numerous vehicle accidents occurred due to the extremely poor visibilities on Central and Southern San Joaquin Valley roadways. On the 20th along a 3-mile stretch of Highway 99 just north of the Merced-Madera County Line, 50 trucks and cars were involved in multi-vehicle pileups.
● December 1-5, 2002 - With the large rainfall amounts on the San Joaquin Valley floor by November 9th, haze and fog prevailed subsequently for a 30+-day period while the adjacent foothills, mountains, and Kern Desert dried again in rainless conditions. Widespread dense fog from mid to late-November ceased by the morning of the 1st of December on the Valley floor. But patchy dense fog and haze prevailed from the 1st of the month through the 9th before the major precipitation in December began on the 13th. The morning of the 5th had some very dense, but again patchy fog. A 9-vehicle pile-up occurred near Los Banos in Merced County (CAZ89) on Highway 152 around 8:35 AM PST that killed one and injured three others.
● January 10, 2019 - Visibility below a quarter mile was reported in Fresno, Madera, Merced and Atwater. Several school districts either had delayed openings or delayed morning bus service. A head on crash in dense fog near Atwater at 6:12 PST caused one fatality and three injuries.

Probability of Future Occurrences
Likely --Based on the NCEI data, 102 dense fog incidents over a 20-year period equates to a major fog event every 5.1 years and a 20 percent chance of a major fog event in any given year. It is likely that minor fog events will continue to occur annually in the Merced County planning area.

Climate Change Considerations
California’s winter tule fog has declined dramatically over the past three decades, raising a red flag for the state’s multibillion-dollar agricultural industry, according to researchers at UC Berkeley. Crops such as almonds, pistachios, cherries, apricots and peaches go through a necessary winter dormant period brought on and maintained by colder temperatures. Tule fog that descends upon the state’s Central Valley between late fall and early spring, helps contribute to this winter chill. “The trees need this dormant time to rest so that they can later develop buds, flowers and fruit during the growing season,” said biometeorologist and study lead author Dennis Baldocchi. “An insufficient rest period impairs the ability of farmers to achieve high quality fruit yields.” The findings have implications for the entire country since many of these California crops account for 95 percent of U.S. production, the authors noted. The researchers paired NASA and National Oceanic and Atmospheric Administration satellite records with data from a network of University of California weather stations, covering 32 consecutive winters. There was a great deal of variability from year to year, but on average, the researchers found a 46 percent drop in the number of fog days between the first of November and the end of February. Climate forecasts suggest that the accumulation of winter chill will continue to decrease in the Central Valley. Tule fog was also less prevalent in recent years in part due to the multi-year drought.

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Vulnerability Assessment
General Property
Based on historic information, the primary effect of fog has not resulted in significant damages to property, or the losses are typically covered by insurance.

People
Reduced visibility is the greatest risk to people when heavy fog is prevalent. Particularly when fog is dense, it can be hazardous to drivers, mariners and aviators and contributes to numerous accidents each year. To reduce injury and harm, people should avoid driving when dense fog is prevalent, if possible. If driving is pertinent, emergency services advise driving with lights on low beam, watching for California Highway Patrol (CHP) pace vehicles to guide through fog, avoiding stopping on highways, and avoiding crossing traffic lanes.

Critical Facilities
Fog can have devastating effects on transportation corridors in the County. Multi-car pileups have resulted from drivers using excessive speed for the conditions and visibility.

These accidents can cause multiple injuries and deaths and could have serious implications for human health and the environment if a hazardous or nuclear waste shipment were involved. Other disruptions from fog include delayed emergency response vehicles and school closures.

Economy
Economic impacts due to dense fog are generally related to road closures leading to a decrease in travel to businesses. Other economic impacts relate to property damages to vehicles from accidents. As shown in Table 4-52 above there is a recorded $1,060,000 of property damages as a result of dense fog all of which are from traffic accidents in the Central Valley region and the records mention Merced County.

Based on that amount the Merced County region has an estimated average loss expectancy of $10,495 and annualized loss of $1,070,495 due to dense fog events.

Historic, Cultural and Natural Resources
As referred to in the Climate Change Considerations section, California’s winter tule fog has declined dramatically over the past three decades, raising a red flag for the state’s multibillion-dollar agricultural industry. Crops such as almonds, pistachios, cherries, apricots and peaches go through a necessary winter dormant period brought on and maintained by colder temperatures and tule fog that descends upon the state’s Central Valley between late fall and early spring, helps contribute to this winter chill.

Future Development
Population and commercial growth in the County and outside the County will increase the potential for complications with traffic accidents and commerce interruptions associated with dense fog.

Risk Summary
● Tule Fog season begins in late fall and lasts from winter (November – March). ● Between 2000 and 2020 there were 102 dense fog events that impacted Merced County and have resulted in $1,060,000 in property damages, largely due to traffic accidents. ● Fog can have devastating effects on transportation corridors. On November 4, 2002, a 50-vehicle collision occurred on Highway 99 near Merced.
● Major fog events in the County are estimated to occur every 5.1 years or a 20% chance in any given year.
● Related Hazards: Hazardous Materials Incidents

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Table 4-56 Hazard Risk Summary – Dense Fog
Jurisdiction Geographic Area
Probability of
Future
Occurrence
Magnitude/
Severity
Overall
Significance
Merced County Extensive Highly Likely Critical High
City of Atwater Extensive Highly Likely Critical High
City of Dos Palos Extensive Highly Likely Critical High City of Gustine Extensive Highly Likely Critical High
City of Livingston Extensive Highly Likely Limited Medium City of Los Banos Extensive Highly Likely Limited Medium City of Merced Extensive Highly Likely Limited Medium

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4.2.9 Severe Weather: Heavy Rain
Hazard/ Problem Description
Most of the adverse weather experienced in Merced takes place in the winter months as heavy rain and thunderstorm events sometimes accompanied by high winds, dense fog events. Dense fog is profiled under 4.2.8 and high winds and tornado events are profiled in 4.3.10.

Merced’s weather is influenced by the Pacific Ocean and routine climate patterns such as El Niño. El Niño is the warm phase of the El Niño-Southern Oscillation, a pattern found in the tropical Pacific when there are fluctuations in temperatures between the ocean and atmosphere. During El Niño, the surface winds across the entire tropical Pacific are weaker than normal and the ocean surface is at above-average temperatures in the central and eastern tropical Pacific Ocean (L’Heureux 2014). El Niño typically develops over North America during the winter season causing the severe winter storms the County often experiences. This climate pattern occurs every few years and brings with it above-average rain and snow across the southern region of United States, especially in California.

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

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

Geographic Area
Thunderstorms are generally expansive in size. The entire county is susceptible to any of the effects of a severe thunderstorm, including heavy rain.

Extent (Magnitude/Severity)
The heavy precipitation events that Merced County and all of California experiences is often the result of an atmospheric river. Atmospheric rivers are categorized by a unit of measurement known as the Integrated Water Vapor Transport (IVT), which takes into account the amount of water vapor in the system and the wind that moves it around. For a storm to be classified as an atmospheric river it has to reach an IVT threshold of 250 units; 1,000 IVT or more is considered to be “extreme” (Arcuni 2019). In 2019 a system for categorizing the strength and impacts of atmospheric rivers was developed by the Center for Western Weather and Water Extremes (CW3E), out of the Scripps Institution of Oceanography at UC San Diego. The newly developed scale ranks ARs into five categorizes from weak to exceptional.

Unlike the Fujita Scale for tornadoes that focuses on potential damages, the AR scale accounts for both storms that may be hazardous and storms that can provide benefits to the local water supply. A category

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one AR is considered to be primarily beneficial, generally lasting only 24 hours and produces modest rainfall. While a category five AR is considered “exceptional” and primarily hazardous, lasting for several days and associated with heavy rainfall and runoff that may cause significant damages. Table 4-56 below describes the scale further. The CW3E developed the scale as a tool for officials with an operational need to assess flooding potential in their jurisdictions before the storms makes landfall.

Table 4-57 Atmospheric River Categories
Category Potential Impacts
AR Cat 1: Weak Primarily beneficial. For example, a Feb. 2, 2017 AR hit California, lasted 24 hours at the coast, and produced modest rainfall.

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

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

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

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

Source: Center for Western Weather and Water Extremes, Scripps Institution of Oceanography at UC San Diego. Scale was developed by F. Martin Ralph Director of CW3E in collaboration with Jonathan Rutz of NWS In both February 2018 and 2019 the West Coast experienced six atmospheric rivers. But as the following figure from the Center for Western Weather and Water Extremes shows, California experienced vastly different precipitation totals due to the location of where the atmospheric river made landfall as well as each atmospheric river’s IVT. Using the AR scale developed by CW3E, the ARs in February 2019 were all considered to be moderate to extreme and concentrated more on California, resulting in heavy precipitation.

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Figure 4-25 Atmospheric River Strength and Land Distribution, February 2018 vs. February 2019 Source: Center for Western Weather and Water Extremes, Scripps Institution of Oceanography at UC San Diego

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Past Occurrences
Heavy rains and adverse storms occur in the Merced County primarily during the late fall and winter but have a chance of occurring in every month of the year. According to information obtained from the Western Regional Climate Center (WRCC) the majority of precipitation is produced by storms during January and other winter months. Precipitation during the summer months is in the form of rain showers and is rare.

The NCEI records show heavy rainstorms can cause both widespread flooding which can lead to extensive localized drainage issues. In addition to the flooding that often occurs during these storms, strong winds, when combined with saturated ground conditions, can down very mature trees. Refer to the Flood section for more information related to flooding events in the county. There have been two USDA disaster designations (2016, 2019) for excessive rainfall in Merced County (refer to Table 4-4).

Information from the three representative weather stations introduced in subsection 4.3.6 Severe Weather: General are summarized below and in Figure 4-26 and Figure 4-27.

Northeastern Merced County - Merced Municipal Airport Weather Station (1899-2016) Information from the closest weather station with the most comprehensive data to represent the northeastern county area, the Merced Municipal Airport Weather Station, is summarized below in Figure 4-26 and Figure 4-27. Average annual precipitation recorded at this weather station is 12.27 inches per year. The highest recorded annual precipitation was 21.66 inches in 1998; the highest recorded precipitation for a 24-hour period is 2.20 inches on January 30, 1911. The lowest recorded annual precipitation was 5.50 inches in 1947.

Figure 4-26 Merced Municipal Airport Weather Station Monthly Average Total Precipitation (Period of Record 1899-2016)
Source: WRCC, www.wrcc.dri.edu/

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Figure 4-27 Merced Municipal Airport Weather Station Daily Precipitation Average and Extreme (Period of Record 1899-2016)
Source: WRCC, www.wrcc.dri.edu/
Southwestern Merced County - Los Banos City Weather Station (1906-2016) Information from the closest weather station with the most comprehensive data to represent the southwestern county area the Los Banos Weather Station, is summarized below in Figure 4-28 and Figure 4-29. Average annual precipitation recorded at this weather station is 9.19 inches per year. The highest recorded annual precipitation was 21.08 inches in 1998; the highest recorded precipitation for a 24-hour period is 2.40 inches on January 18, 1914. The lowest recorded annual precipitation was 4.61 inches in 1947.

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Figure 4-28 Los Banos Weather Station Monthly Average Total Precipitation (Period of Record 1906-2016)
Source: WRCC, www.wrcc.dri.edu/
Figure 4-29 Los Banos Weather Station Daily Precipitation Average and Extreme (Period of Record 1906-2016)
Source: WRCC, www.wrcc.dri.edu/

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The NCEI Storm Events Database records 23 heavy rain events in Merced County between 1998 and 2018.

Note no heavy rain events are recorded in the database after 2018. No casualties are recorded for any of the events. As noted above there is a connection between heavy rain events and flooding and is it assumed that the property damages listed in the NCEI database are the results of flooding caused by heavy rain events.

The following table show the records collected from the NCEI Strom Events Database for heavy rain.

Table 4-58 Heavy Rain Events in Merced County, 1998-2018 Location Date Property Damages ($) Crop Damages ($)
Merced 2/1/1998 $2,000,000 $1,400,000
Merced 5/1/1998 $0 $10,200,000
Merced 5/15/2000 $0 $100,000
Merced ARPT 12/13/2002 $0 $0
Merced 4/21/2003 $7,500 $0
Countywide 5/1/2003 $0 $2,000,000
Countywide 12/7/2004 $0 $0
Countywide 12/27/2004 $0 $0
Countywide 1/1/2005 $0 $0
Countywide 1/7/2005 $0 $0
Countywide 1/24/2005 $0 $0
Merced 2/15/2005 $0 $0
Merced ARPT 3/22/2005 $0 $0
Dos Palos/Atwater 9/20/2005 $0 $0
Countywide 1/1/2006 $0 $0
Countywide 3/2/2006 $0 $0
Livingston 3/28/2006 $0 $0
Countywide 4/2/2006 $0 $1,960,000
Merced 2/23/2010 $0 $0
Gustine 12/11/2014 $0 $0
Atwater Muni ARPT 4/27/2016 $0 $0
Santa Nella, Los Banos,
Brito 5/5/2016 $0 $0
Cregan 4/16/2018 $0 $0
Total $2,007,500 $15,660,000
Source: NCEI
Probability of Future Occurrences
Highly Likely – Based on the NCEI data, 23 heavy rain incidents over a 20-year period equates to a major event every 1.2 years and an 87 percent chance of a major heavy rain event in any given year.

Climate Change Considerations
As average temperatures increase over time, this generally will result in higher extreme temperatures and more warming in the atmosphere can trigger climate changes, which could result in more frequent extreme weather events. Pacific Northwest National Laboratory researchers have also found that atmospheric rivers will reach the West Coast more frequently (Gao 2015). Currently, the west receives rain or snow from these atmospheric rivers between 25 and 40 days each year. By the end of this century, days on which the atmospheric rivers reach the coast could increase by a third, or between 35 and 55 days a year.

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Vulnerability Assessment
General Property
The Merced County planning area experiences a rainy season in the winter months through early spring.

These winter storms can include significant precipitation. The primary effect of these storms has not resulted in significant injury or damages to people and property, or the losses are typically covered by insurance. It is the secondary hazards caused by weather, such as floods, that have had the greatest impact on the County. Damage and disaster declarations related to adverse weather have occurred and will continue to occur in the future. Heavy rain and thunderstorms are the most frequent type of severe weather occurrences in the County. Utility outages, downing of trees, debris blocking streets and damage to property can be a direct result of these storm events. Given the nature of these types of storms, the entire County is potentially at risk.

People
Exposure is the greatest danger to people from heavy rain events. People can be caught in rising waters and need to be rescued.

Critical Facilities
Because of the unpredictability of heavy rain events, most critical facilities and infrastructure that is above ground is equally exposed to the storm’s impacts. Due to the random nature of this hazards, a more specific risk assessment was not conducted for this plan.

Economy
Economic impact of a heavy rain event is typically short-term. Generally, long-term economic impacts center more around hazards that cascade from a heavy rain even such as flooding.

In general, all severe weather poses a risk to agriculture economy in the county. Table 4-58 below describes the crops losses related to heavy rain events and associated indemnity amounts or loss payments from the USDA, RMA in the past 13 years. Merced County experiences an estimated $249,076 annualized loss due to crop damages from heavy rain events.

Table 4-59 Crops Loss Due to Heavy Rain, Risk Management Agency Crop Indemnity Reports, 2007-2020
Year Net
Determined
Acres
Indemnity
Amount
2007 354.18 $189,228
2009 1,639.58 $1,464,029
2010 4,173.48 $2,055,617
2011 584 $280,280
2012 1,475.48 $359,758
2013 354.6 $106,905
2014 3,545 $1,916,895
2015 3,303.7 $2,974,961
2016 1,742.3 $2,473,741
2017 4,920.1 $3,936,051
2018 766.47 $682,953
2019 6,741.8 $4,983,748
2020 781.8 $743,567
Total 30,382.49 $22,167,733
Source: USDA RMA

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Historic, Cultural and Natural Resources
As a natural process, the impacts of most heavy rain events by themselves are part of the overall natural cycle and do not cause long-term consequential damage.

Future Development
New critical facilities, such as communication towers should be built to withstand heavy rain damage.

Future development projects should consider adverse weather hazards at the planning, engineering and architectural design stage with the goal of reducing vulnerability. Stormwater master planning and site review should account for buildings to withstand heavy rain events considered for all new development.

Thus, development trends in the County are not expected to increase overall vulnerability to the hazard but all development will be affected by adverse weather and storm events and population growth will increase potential exposure.

Risk Summary
● During the 20-year period of 1998-2018 there were 23 heavy rain events in Merced County. ● Average annual precipitation ranges from 9.19 inches the southern portion of the County to 12.27 inches the northern portion.
● The County experiences an estimated $249,076 annualized crop loss due to heavy rain events. ● Related Hazards: Flooding, Landslide
Table 4-60 Hazard Risk Summary – Heavy Rain
Jurisdiction Geographic Area
Probability of
Future
Occurrence
Magnitude/
Severity
Overall
Significance
Merced County Extensive Highly Likely Critical High
City of Atwater Extensive Highly Likely Critical High
City of Dos Palos Extensive Highly Likely Critical High City of Gustine Extensive Highly Likely Critical High
City of Livingston Extensive Highly Likely Critical High City of Los Banos Extensive Highly Likely Limited Medium City of Merced Extensive Highly Likely Limited Medium

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4.2.10Severe Weather: High Wind/Tornado
Hazard/ Problem Description
High winds, often accompanying severe thunderstorms, can cause significant property and crop damage, threaten public safety, and have adverse economic impacts from business closures and power loss.

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

Tornadoes are another severe weather hazard that can affect the Merced County planning area.

Tornadoes form when cool, dry air sits on top of warm, moist air. Tornadoes are rotating columns of air marked by a funnel-shaped downward extension of a cumulonimbus cloud whirling at destructive speeds of up to 300 mph, usually accompanying a thunderstorm. Tornadoes are the most powerful storms that exist. They can have the same pressure differential that fuels 300-mile-wide hurricanes across a path only 300-yards wide or less.

Geographic Area
Wind and tornadoes have the potential to happen anywhere in the County. The resulting damage from wind and tornado events may be most severe in the downtown areas of incorporate communities where there are more large trees, infrastructure, and higher density development.

The National Renewable Energy Laboratory (NREL) collects data on wind resources in the United States to help determine the location of new wind energy sites. Figure 4-30 from the NREL Wind Prospector webviewer shows the average annual wind speed in Merced County at the height of 100 meters. The eastern and central portions of the County have average wind speeds between 4.50 meters/second and

5.50 meters/second. The wind speeds in the western portion of the County vary greater than the eastern portions. Average wind speeds in western Merced County range from less than 4.00 meters/second to

7.00 meter/second in northwest.

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Figure 4-30 Merced County Average Annual Wind Speed at 100 meters Source: NREL Wind Prospector
Extent (Magnitude/Severity)
High winds and tornadoes can cause damage to property and loss of life. While most tornado damage is caused by violent winds, most injuries and deaths result from flying debris. Property damage can include damage to buildings, fallen trees and power lines, broken gas lines, broken sewer and water mains, and the outbreak of fires. Agricultural crops and industries may also be damaged or destroyed. Access roads and streets may be blocked by debris, delaying necessary emergency response.

In 2007, the NWS began rating tornadoes using the Enhanced Fujita Scale (EF-scale). The EF-scale is a set of wind estimates (not measurements) based on damage. Its uses three-second gusts estimated at the point of damage based on a judgment of eight levels of damage to the 28 indicators listed in Table 4-60.

These estimates vary with height and exposure. Standard measurements are taken by weather stations in open exposures.

Table 4-60 describes the EF-scale ratings versus the previous Fujita Scale used prior to 2007 (NOAA 2007).

Table 4-61 The Fujita Scale and Enhanced Fujita Scale
Fujita Scale Derived Operational EF-Scale
F Number
Fastest ¼
mile (mph)
3-second
gust (mph) EF Number
3-second
gust (mph) EF Number
3-second gusts
(mph)
0 40-72 45-78 0 65-85 0 65-85
1 73-112 79-117 1 86-109 1 86-110

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Fujita Scale Derived Operational EF-Scale
F Number
Fastest ¼
mile (mph)
3-second
gust (mph) EF Number
3-second
gust (mph) EF Number
3-second gusts
(mph)
2 113-157 118-161 2 110-137 2 111-135
3 158-207 162-209 3 138-167 3 136-165
4 208-260 210-261 4 168-199 4 166-200
5 261-318 262-317 5 200-234 5 Over 200
Source: NWS. Notes: EF - Enhanced Fujita F – Fujita mph - Miles per Hour Figure 4-31 illustrates the potential impact and damage from a tornado.

Figure 4-31 Potential Impact and Damage from a Tornado
Source: NOAA NWS, Storm Prediction Center
The damaging effects of wind speed are measured using the Beaufort Wind Scale as shown in Table 4-61 below. This scale only reflects land-based effects and does not take into consideration the effects of wind over water.

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Table 4-62 Beaufort Wind Scale
Wind Speed
(mph) Description – Visible Condition
0 Calm; smoke rises vertically
1-4 Light air; direction of wind shown by smoke but not by wind vanes 4-7 Light breeze; wind felt on face; leaves rustle; ordinary wind vane moved by wind 8-12 Gentle breeze; leaves and small twigs in constant motion; wind extends light flag 13-18 Moderate breeze; raises dust and loose paper; small branches are moved 19-24 Fresh breeze; small trees in leaf begin to sway; crested wavelets form on inland water 25-31 Strong breeze; large branches in motion; telephone wires whistle; umbrellas used with difficulty 32-38 Moderate gale whole trees in motion; inconvenience in walking against wind 39-46 Fresh gale breaks twigs off trees; generally, impedes progress 47-54 Strong gale slight structural damage occurs; chimney pots and slates removed 55-63 Whole gale trees uprooted; considerable structural damage occurs 64-72 Storm very rarely experienced; accompanied by widespread damage 73+ Hurricane devastation occurs
Source: NWS
Based on NCEI records between 1955 and 2019 there have been a combined 36 strong wind/high wind/thunderstorm winds and 32 tornado/funnel cloud events in Merced County which has resulted in a total of $1,925,800 in property damage. The most damaging event took place on January 20, 2010 and was a 59-mph wind event that resulted in $750,000 in property damages. Overall, high wind event impacts would likely be limited, with a majority of impacts being related to property damages caused my downed trees as well as power outages.

In the past 65 years all the tornado events that have taken place in Merced County have been between F0-F1 or EF0 tornadoes. However, it should be noted that, although unlikely, larger tornadoes could occur.

Should the County be hit by an EF-3 or higher tornado, it can be extrapolated that because of its relative size and the potential size and length of a tornado’s path a significant portion of the County could be impacted, resulting in property and crop damage and loss of life.

Tornado impacts to the County would likely be negligible, with less than 10 percent of the planning area affected by events in the EF0-2 range, though stronger tornadoes are possible. The impact to quality of life or critical facilities and functions in the affected area would depend on where the tornado occurred.

Injuries or deaths are possible due to wind thrown trees or property damage caused by wind events.

Overall, impacts from high wind and tornado events would likely be negligible, with less than 10 percent of property severely damaged and shutdown of facilities due to loss of power for 24 hours or less.

Past Occurrences
During the rainy season, the Merced planning area is prone to relatively strong thunderstorms, sometimes accompanied by high winds and tornadoes. While tornadoes do occur occasionally, most often they are of F0/EF0 intensity. The NCEI Storm Events Database does not record any F2, or F3 or anything events greater than EF0 events that have occurred in the planning area in the past. Table 4-62 Past High Wind and Tornado Events contains incident descriptions for significant historic events. In addition to the events listed in the NCEI database, there was a USDA disaster designation declared for Merced County in 2016 for “severe weather including excessive rainfall and high winds” (refer to Table 4-4).

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Table 4-63 Past High Wind and Tornado Events in Merced County, 1955-2019 Hazard Type Date Location Magnitude
Property
Damages
($)
Crop
Damages
($)
Deaths Injuries
Tornado 4/18/1955 Merced County Unknown $0 $0 0 0
Thunderstorm
Wind 7/6/1957 Merced County 70mph $0 $0 0 0
Tornado 4/22/1967 Merced County F1 $0 $0 0 0
Tornado 4/5/1975 Merced County F0 $25,000 $0 0 0
Tornado 3/4/1978 Merced County F1 $250,000 $0 0 1
Tornado 2/19/1986 Merced County F0 $25,000 $0 0 0
Tornado 3/26/1991 Merced County F0 $0 $0 0 0
Thunderstorm
Wind 6/15/1995 Merced/Dos Palos Unknown $50,000 $3,000,000 0 0 Thunderstorm
Wind 4/1/1996 Los Banos/Gustine 80mph $320,000 $100,000 0 0 Tornado 4/1/1996 Merced F0 $10,000 $0 0 0
Tornado 11/22/1996 Merced F1 $45,000 $0 0 0
Tornado 11/13/1997 Los Banos F0 $5,000 $0 0 0
Tornado 2/14/1998 Los Banos F0 $0 $0 0 0
Funnel Cloud 3/25/1998 Atwater/Livingston/
Merced Unknown $0 $0 0 0
Funnel Cloud 3/28/1998 Plainsburg Unknown $0 $0 0 0
Funnel Cloud 4/5/1998 Dos Palos/Los
Banos Unknown $0 $0 0 0
Funnel Cloud 1/31/1999 Merced Unknown $0 $0 0 0
High Wind 1/10/2001 Countywide Unknown $20,000 $0 0 4
High Wind 5/2/2001 Countywide 57mph $75,000 $1,960,000 0 0 High Wind 8/29/2001 Santa Nella Unknown $0 $0 0 0
High Wind 11/24/2001 Countywide Unknown $0 $0 0 0
High Wind 12/2/2001 Countywide Unknown $0 $0 0 0
High Wind 3/13/2002 Countywide Unknown $0 $0 0 0
High Wind 6/8/2002 Countywide Unknown $0 $0 0 0
High Wind 6/9/2002 Countrywide Unknown $0 $0 0 0
High Wind 8/3/2002 Los Banos Muni
ARPT Unknown $0 $0 0 0
Strong Wind 12/14/2002 Countywide 49mph $0 $0 0 0
Strong Wind 12/16/2002 Countywide 36mph $5,000 $0 0 0
Tornado 12/16/2002 Merced ARPT F1 $400,00 $0 0 0
Strong Wind 12/19/2002 Countywide 46mph $5,000 $0 0 0
Strong Wind 12/28/2002 Countywide 34mph $4,000 $0 0 0
Funnel Cloud 5/3/2003 Atwater/Merced/
Livingston Unknown $0 $0 0 0
Strong Wind 2/25/2004 Countywide Unknown $0 $0 0 0
Funnel Cloud 10/17/2004 Countywide 40mph $0 $0 0 0
Strong Wind 10/19/2004 Countywide 41 $1,000 $0 0 0
Strong Wind 1/7/2005 Countywide 36 mph $10,000 $0 0 0
High Wind 1/7/2005 Countywide 57.4 mph $150,000 $150,000 0 0 Funnel Cloud 3/20/2005 Los Banos Unknown $0 $0 0 0
Strong Wind 3/22/2005 Countywide 40 mph $5,000 $0 0 0

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Hazard Type Date Location Magnitude
Property
Damages
($)
Crop
Damages
($)
Deaths Injuries
Tornado 4/8/2005 Ballico F0 $25,000 $50,000 0 0
Funnel Cloud 4/8/2005 Gustine Unknown $0 $0 0 0
Funnel Cloud 1/18/2006 Gustine/El Nido Unknown $0 $0 0 0 Funnel Cloud 3/28/2006 Merced Unknown $0 $0 0 0
Tornado 3/28/2006 Atwater/Merced/
Merced ARPT F0 $25,000 $0 0 0
Funnel Cloud 4/14/2006 Atwater Unknown $0 $0 0 0
Strong Wind 9/8/2006 Countywide 40 mph $1,000 $0 0 0
Thunderstorm
Wind 10/29/2007 Cregan 55 mph $60,000 $0 0 0
Strong Wind 2/23/2008 Countywide 39 mph $0 $12,000 0 0
Strong Wind 5/31/2008 Countywide 30 mph $500 $0 0 0
Strong Wind 6/10/2008 Countywide 39 mph $200 $1,000 0 0 Strong Wind 6/26/2008 Countywide 33 mph $1,000 $0 0 0
Funnel Cloud 1/25/2009 Stevinson Unknown $0 $0 0 0
Tornado 2/9/2009 Merced ARPT EF0 $0 $0 0 0
High Wind 1/20/2010 Countywide 59 mph $750,000 $0 0 0
Strong Wind 3/6/2012 Countywide 36 mph $35,000 $0 0 0
Funnel Cloud 4/12/2012 Atwater Unknown $0 $0 0 0
Funnel Cloud 3/2/2014 Dos Palos Unknown $0 $0 0 0
Strong Wind 12/11/2014 Countywide 48 mph $0 $500,000 0 0 Strong Wind 3/5/2016 Countywide 35mph $100 $0 0 0
Strong Wind 2/1/2017 Countywide 30 mph $1,000 $0 0 0
Funnel Cloud 3/21/2017 Castle AFB Unknown $0 $0 0 0
High Wind 12/16/2017 Countywide 51 mph $0 $0 0 0
Strong Wind 1/6/2019 Countywide 43 mph $16,000 $0 0 0
Strong Wind 1/17/2019 Countywide 30 mph $1,000 $0 0 0
Funnel Cloud 3/27/2019 Livingston Unknown $0 $0 0 0
Strong Wind 4/9/2019 Countywide 39 mph $5,000 $0 0 0
Funnel Cloud 5/21/2019 Los Banos Unknown $0 $0 0 0
Funnel Cloud 11/28/2019 El Nido Unknown $0 $0 0 0
Total $1,925,800 $5,773,000 0 5
Source: NCEI Storm Events Database, Notes: mph – Miles per Hour, ARPT – Airport, AFB – Airforce Base Figure 4-32 and Figure 4-33 show the location of the past wind and tornado events in Merced County based on data from NOAA’s NWS Storm Prediction Center.

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Figure 4-32 Merced County Wind Events (1950 -2019)

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Figure 4-33 Tornado Events in Merced County, 1950 -2019

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Probability of Future Occurrences
Highly Likely – Sixty-eight high wind and tornado events have occurred in Merced County over 64 years of record keeping, which equates to one high wind or tornado event every 1.1 years, on average, and a 94 percent chance of a high wind or tornado event occurring in any given year. Historical wind activity within the planning area indicates that the area will likely continue to experience high wind during thunderstorm events with a potential of the formation of funnel clouds and low intensity tornadoes during adverse weather conditions. The actual risk to the County is dependent on the nature and location of any given thunderstorm or tornado event.

Climate Change Considerations
There presently is not enough data or research to quantify the magnitude of change that climate change may have related to tornado frequency and intensity. NASA’s Earth Observatory has conducted studies that aim to understand the interaction between climate change and tornadoes. Based on these studies meteorologists are unsure why some thunderstorms generate tornadoes and others don’t, beyond knowing that they require a certain type of wind shear. Tornadoes spawn from approximately one percent of thunderstorms, usually supercell thunderstorms that are in a wind shear environment that promotes rotation. Some studies show a potential for a decrease in wind shear in mid-latitude areas. Because of uncertainty with the influence of climate change on tornadoes, future updates to the mitigation plan should include the latest research on how the tornado hazard frequency and severity could change. The level of significance of this hazard should be revisited over time.

As for wind, studies referenced in California’s Fourth Climate Assessment indicated that extreme fire weather, particularly in the form of hot and dry winds, can strongly influence shrub-land fire regimes.

Strong winds have also been now associated with severe forest fires in California meaning climate change impacts on wind patterns may also affect forest health and wildfire susceptibility. Lastly, other ongoing research compiled in the recent climate assessment has resulted in different conclusions on the effect of climate change on wind regimes, particularly extreme wind events, such as the Santa Ana and Diablo winds that created some of the most devastating wildfires (California Natural Resources Agency 2018a). At this time, these changing factors are not well understood and are still being incorporated into state and regional research and risk analysis.

Vulnerability Assessment
General Property
General damages are both direct (what the wind event physically destroys) and indirect, which focuses on additional costs, damages and losses attributed to secondary hazards spawned by the event, or due to the damages caused by the wind event. Depending on the magnitude of the wind events as well as the size of the tornado and its path, a tornado is capable of damaging and eventually destroying almost anything.

Construction practices and building codes can help maximize the resistance of the structures to damage.

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

Downed trees caused by a wind event are a common occurrence in the County. Falling trees can cause significant damage to property and put people at risk. Due to multiple years of drought in the County, many trees in the area have been impacted making them more susceptible to blow-down during wind events.

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People
Community members are the most vulnerable to high wind and tornado events. The availability of sheltered locations such as basements, buildings constructed using tornado-resistant materials and methods, and public storm shelters, all reduce the exposure of the population. However, there are also segments of the population that are especially exposed to the indirect impacts of high winds and tornadoes, particularly the loss of electrical power. According to the data obtained from emPower.com, a website maintained by the U.S. Department of Health and Human Services, 4%, or 1,412 of the 36,689 of the Medicare beneficiaries in the County rely on medical equipment that is dependent on electricity in order to live independently. These populations include the elderly or disabled, especially those with medical needs and treatments dependent on electricity. Nursing homes, community-based residential facilities, special needs housing facilities, and isolated communities are also vulnerable because of life support needs if electrical outages are prolonged, since backup power generally operates only minimal functions for a short period of time.

Following the unprecedented 2018 wildfire season in California, PG&E announced it will be conducting PSPS when there are high winds and dry conditions and generally a heightened fire risk forecasted. The outages could last several days, and PG&E has suggested customers be prepared for outages that could last longer than 48 hours. A majority of Merced County could be affected by the power outages.

Critical Facilities
Both winds and tornadoes may impact exposed critical infrastructure such as power lines; depending on the impact and the function, this could cause a short-term economic disruption. The most common problems associated with tornadoes and high winds are loss of utilities. Downed power lines can cause power outages, leaving large parts of the county isolated, and without electricity, water, and communication. Damage may also limit timely emergency response and the number of evacuation routes.

Downed electrical lines following a storm can also increase the potential for lethal electrical shock.

Damaging winds can also cause wildfires; refer to the Wildfire section for more information on the County’s vulnerability to wildfires.

In an effort to mitigate the impacts of PSPS, the State of California has allocated PSPS Resiliency funds for counties, cities, tribes and state agencies to “procure fixed, long-term emergency electrical generation equipment, develop continuity plans, risk assessments for critical infrastructure, post event reports, and public education material or supplies to prepare of electric disruption” (Cal OES 2021). In fiscal year 2019- 20 Merced County was allocated $422,987 from the PSPS Resiliency Allocation to counties, to purchase and install generators at County fire stations, backup generators and lighting infrastructure for the Merced County Operations Areas and purchase a battery-powered backup Public Safety Answering Point for all dispatch centers within the Merced County Operational Area.

Economy
Winds typically don’t have long-term impacts on the economy, although wind does have an impact on the agriculture economy in the county. As shown in Table 4-63, wind events have been a leading cause of crop loss in the past three years (2007 – 2019), resulting in over $2 million in loss payments from the USDA and more than 3,000 acres lost to high wind events. Merced County experiences an estimated $187,255 annualized loss due to crop damages from excessive wind events.

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Table 4-64 Crops Loss Due to High Wind, Risk Management Agency Crop Indemnity Reports, 2007-2019
Year Net
Determined
Acres
Indemnity
Amount
2007 372.4 $97,945
2008 327.02 $130,399
2012 32.4 $2,159
2013 1,624.8 $1,267,860
2014 176 $171,649
2015 301.9 $75,625
2016 7.4 $11,792
2017 398.68 $303,600
2019 298 $186,034
Total 3,539 $2,247,063
Source: USDA RMA
Historic, Cultural and Natural Resources
High winds and tornadoes can cause massive damage to the natural environment, uprooting trees and other debris. This is part of a natural process, however, and the environment will return to its original state in time.

Future Development
As the County continues increase in population, the number of people and housing developments exposed to the hazard increases. Adherence to current building codes, coupled with proper education on building techniques and the use of sturdy building materials, attached foundations, and other structural techniques may minimize the property vulnerabilities. Public shelters at parks and open spaces may help reduce the impacts of tornadoes and high wind events on the recreational populations exposed to storms.

Risk Summary
● Between 1955 and 2019 there were a combined 68 high wind (36) and tornado (32) events in the County.
● Wind and tornado events have resulted in $1,925,800 in property damages. The most damaging event took placed on January 20, 2010 with 59 mph wind causing $750,000 in property damages. ● Excessive wind has caused 3,539 acres to be lost and $2,247,063 crop loss payment made by the USDA to farmers in the County.
● PSPS pose a risk to individuals in the County who depend on electricity to live independently. Four percent of Medicare Beneficiaries in the County are electricity dependent. ● Related Hazards: Extreme heat, Heavy Rain, Agriculture/Pest and Crop Disease, Wildfire

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Table 4-65 Hazard Risk Summary – High Wind/Tornado
Jurisdiction Geographic Area
Probability of
Future
Occurrence
Magnitude/
Severity
Overall
Significance
Merced County Extensive Highly Likely Critical High
City of Atwater Extensive Highly Likely Critical High
City of Dos Palos Extensive Highly Likely Critical High City of Gustine Extensive Highly Likely Limited Medium
City of Livingston Extensive Highly Likely Limited Medium City of Los Banos Extensive Highly Likely Limited Medium City of Merced Extensive Highly Likely Limited Medium

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4.2.11Subsidence
Hazard/Problem Description
Land subsidence is defined as the vertical sinking of the land over natural or manmade underground voids. Subsidence is common in several areas of California, usually as a result of groundwater pumping, peat loss, or oil and gas extraction. Fluctuations in the level of underground water caused by pumping or by injecting fluids into the earth can initiate sinking to fill the empty space previously occupied by water or soluble minerals. Weight, including surface developments such as roads, reservoirs, and buildings, and manmade vibrations from such activities as blasting and heavy truck or train traffic can accelerate the natural processes of subsidence, or induce subsidence over manmade voids.

The 2018 California SHMP profiles subsidence as a secondary impact of drought. Through reduced availability of surface water, groundwater pumping in order to meet water needs. According to a study published by Yale Environmental Review, approximately 1 million people in the San Joaquin Valley rely on groundwater pumping for drinking water, and thousands of farmers rely on groundwater to irrigate crops.

While groundwater pumping occurs in both drought and non-drought years to support urban, rural, and agricultural water needs, the 2018 California State HMP notes that it is greatly increased during dry years.

Land subsidence due to groundwater pumping can permanently damage or collapse underground aquifers, increase flood risk in low-lying areas, and pose hazards to buildings, infrastructure, and water storage facilities.

Subsidence can result in serious, localized structural damage to buildings, roads, irrigation ditches, canals, streams, underground utilities, and pipelines. It can disrupt and alter the flow of surface or underground water. The consequences of improper use of land subject to ground subsidence can be excessive economic losses, including the high costs of repair and maintenance for buildings, irrigation works, highways, utilities, and other structures. This results in direct economic losses to citizens as well as indirect economic losses through increased taxes and decreased property values.

Geographic Area
Figure 4-34 below shows areas statewide that have a recent history of subsidence or are known to be at an increased risk of subsidence. Most studies and concern about land subsidence in California have focused on the Santa Clara Valley, San Joaquin Valley, Sacramento Valley, Antelope Valley, Coachella Valley, and Mojave River Basin Areas.

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Figure 4-34 Historic Land Subsidence in California
Source: California DWR, November 2014 Public Update for Drought Response The Merced Subbasin GSA has a focused map of subsidence in the vicinity of Merced County, shown in Figure 4-35 below. This agency was created for the purpose of forming a joint powers agency that will be the GSA for a portion of the Merced Groundwater Subbasin. The Merced Subbasin GSA developed and adopted the Merced Groundwater Sustainability Plan in December 2019. The implementation of plan will address the SGMA requirements and achieve the sustainability goals outlined in SGMA, in addition to positive impacts to future subsidence in the County.

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Figure 4-35 Merced County Area Subsidence
Source: Merced County
Extent (Magnitude/Severity)
A lack of data makes it difficult to predict the magnitude of future land subsidence incidents in Merced County. As shown in Figure 4-35 above, most of the documented land subsidence in the county has been of 0.5 feet or less per year. Much of this subsidence is continuous and occurs gradually over a vast area of the county. However, that can still be enough to cause structural issues. Sudden and localized subsidence events, such as sinkholes, can cause damage to building foundations, roadways, and utilities. Such events have been documented in nearby Fresno and Madera counties, where sinkholes or sudden subsidence has damaged and closed roads. Ongoing displacements of as much as 12” per year have been documented in other parts of the state. As areas of the county continue to gradually sink, the impacts on infrastructure may become more apparent.

Previous Occurrences
While actual effects of subsidence are visible throughout the County, research on the hazard produced thus far has not produced records of any specific events. Some areas of the County have subsided 10-15 feet over the past 50 years according to the HMPT.

Probability of Future Occurrences
Land subsidence has been a recurring issue in Merced County for decades and can be expected to continue in the future. The frequency of future land subsidence incidents in the county will largely be dependent on the mitigation actions and pumping regulations initiated by the state, the county, and local regulations. Based on past trends of gradual subsidence throughout the entire San Joaquin Valley and the

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continued dependence on groundwater pumping, it can be assumed that subsidence will continue in the future, with increased rates of subsidence coinciding with drought years.

Climate Change Considerations
The most likely impact that climate change will have on land subsidence risk is the potential for extended and severe drought, which could likely result in more groundwater pumping and human-induced subsidence. During periods of drought, water levels may be drawn too low, which results in an irreversible compaction of aquifers. The water cannot recharge the layers, causing permanent subsidence and diminishment of groundwater storage capacity.

Vulnerability Assessment
General Property
The lack of detailed data on land subsidence in Merced County makes it difficult to quantify past or potential future losses. Most subsidence instances result in relatively minor damage and settling of buildings. But in some cases, subsidence can result in serious structural damage to buildings, roads, irrigation ditches, underground utilities, and pipelines, potentially resulting in the complete loss of the structure. Subsidence can also disrupt and alter the flow of surface or underground water, as well as reducing the future capacity of aquifers.

People
This hazard typically results in property damage, not risk to human life. In the event of a sudden sinkhole, it would be possible for people to sustain injuries if they were walking or driving and were caught in the sinkhole.

Critical Facilities and Infrastructure
Linear infrastructure (roads, buried pipelines) tends to have the most risk to land subsidence. Statewide, subsidence has caused damage to dams and levees, canals, roads and bridges, water and sewer lines, pipelines, well casings, and aircraft runways, in addition to a variety of buildings and other structures.

Highways 59 and 152 traverse areas of historic subsidence in the County. The threat to the California Aqueduct has been identified as a significant concern for the state. Evidence of the gradual impacts of subsidence on infrastructure in the San Joaquin Valley can be seen in Figure 4-36 below. This image was taken from the California Water Foundations Summary Report on Land Subsidence from Groundwater Use in California. The bridge shown is over the Outside Canal north of Mendota, California. While it is partially submerged in this image, the bridge used to have canal inspections by the Central California Irrigation District staff conducted from a boat passing under the bridge.

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Figure 4-36 Evidence of Gradual Land Subsidence, Partially Submerged Russell Avenue Bridge, Mendota, CA
Economy
Damage resulting from land subsidence can cause direct economic losses in the form of needed structural repairs to affected buildings and facilities. It can also result in indirect losses, such as from increased taxes and decreased property values. Subsidence may impact tourism and shipping industries if transportation routes are affected.

Historic, Cultural, and Natural Resources
Historic and cultural facilities are just as susceptible to subsidence related damage as other structures.

However, there is not sufficient data to identify specific historical or cultural facilities at increased risk from this hazard.

Groundwater pumping can cause significant impacts to underground water resources, and the resulting subsidence events may further disrupt and alter the flow of surface or underground water, in a cyclical pattern of environmental degradation. Furthermore, soil compaction resulting from subsidence can permanently reduce aquifer capacity, impacting water supplies long into the future.

Future Development
The areas of the county which have experience the greatest historic rates of subsidence are largely in the middle of the county, which is occupied by the Merced National Wildlife Refuge, San Luis National Wildlife Refuge, and Great Valley Grasslands State Park. As such, future development of these areas will be significantly restricted, and growth will be limited. While vulnerability to this hazard is not anticipated to increase with new development, increased water pumping resulting from the demand of new development has the potential to increase the frequency and severity of subsidence. This could be

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especially important in Los Banos, given that area has higher subsidence potential. Increased efforts to monitor and manage groundwater pumping, increased accuracy of mapping, and emphasis on appropriate grading and ground compaction during development will help alleviate vulnerability for future development in unknown areas of risk.

Risk Summary
While a significant portion of Merced County is at risk from ongoing land subsidence, the gradual nature of this hazard makes the overall significance of this hazard low. ● The gradual nature of land subsidence incidents limits their overall impact, in terms of property damage, at the county level.
● The entire San Joaquin Valley is vulnerable to land subsidence, and Merced County is vulnerable due to its dependence on agriculture and groundwater pumping. ● There is a shortage of data on specific occurrence of this hazard in Merced County. Most studies on land subsidence in California have focused on the Santa Clara Valley, San Joaquin Valley, Sacramento Valley, Antelope Valley, Coachella Valley, and Mojave River Basin Areas, however there is not extensive information on individual subsidence events.
● Increased drought conditions resulting from climate change has the potential to result in more groundwater pumping, which could lead to more human-induced subsidence; groundwater sustainability planning and management could help offset future subsidence. ● Land subsidence can alter the flow of surface or underground water, an impact that may not be noticed until long after the fact.
● Related Hazards: Drought
Table 4-66 Hazard Risk Summary – Subsidence
Jurisdiction Geographic Area
Probability of
Future
Occurrence
Magnitude/
Severity
Overall
Significance
Merced County Extensive Likely Negligible Low
City of Atwater Limited Occasional Negligible Low
City of Dos Palos Limited Occasional Negligible Low
City of Gustine Limited Unlikely Negligible Low
City of Livingston Limited Occasional Negligible Low
City of Los Banos Limited Likely Negligible Low
City of Merced Limited Occasional Negligible Low

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4.2.12Wildfire
Hazard/Problem Description
A wildfire is an uncontrolled fire spreading through vegetative fuels, posing danger and destruction to property and watersheds. While wildfires are often the direct result of lightning strikes, they can be caused by downed powerlines or mechanical equipment or are the result of human activities like landscape debris burns, carelessness, or arson. Wildfires often start in undeveloped areas and public land areas, such as state and federal lands, but can spread to urban areas where structures and other human development are more concentrated. The predominant dangers from wildfires are:
● Injury or loss of life to people in the affected area; and ● The destruction of vegetation, property, wildlife.

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

Merced County is exposed to a variety of wildfire hazard conditions that varies based on fuels, topography, weather, and human behavior. CAL FIRE, as required by government code 51181, has undertaken a statewide program to map areas of potential wildfire severity, and to describe the potential for wildfires to occur in a given area; the resulting Fire Hazard Severity Zones are shown in Figure 4-37 below.

It should be noted that fires are also an important natural component of Merced County’s ecosystem.

Wildlands need to burn periodically to naturally maintain viable environments. Fuel maintenance (controlled burns, mowing, cattle grazing, and other means) is a necessary replacement to uncontrolled wildland fires because of threats to human habitation. Development patterns in rural lands can reduce the ability to manage fuel and defend “values at risk”.

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

These factors are fuel, topography, and weather.
● Fuel—Fuel is the material that feeds a fire and is a key factor in wildfire behavior. Fuel is generally classified by type and by volume. Fuel sources are diverse and include everything from dead tree leaves, twigs, and branches to dead standing trees, live trees, brush, and cured grasses. Also, to be considered as a fuel source are manmade structures, such as homes and other associated combustibles. The type of prevalent fuel directly influences the behavior of wildfire. Fuel is the only factor that can be modified by humans. The main fuel types in Merced County are crops and grasses, while there are also brush and pine fuels present in some areas in the southwest of the county. ● Topography—An area’s terrain and slopes affect its susceptibility to wildfire spread. Both fire intensity and rate of spread increase as slope increases due to the tendency of heat from a fire to rise via convection. The arrangement and types of vegetation throughout a hillside can also contribute to increased fire activity on slopes.
● Weather—Weather components such as temperature, relative humidity, wind, and lightning also affect the potential for wildfire. High temperatures and low relative humidity dry out fuels that feed wildfires, creating a situation where fuel will more readily ignite and burn more intensely. Thus, during periods of drought, the threat of wildfire increases. Wind is the most influential weather factor of the three and its influence can increase rates of spread regardless of temperature and relative humidity.

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Geographic Area
Both urban and wildland fires conditions exist in Merced County which increases the chances for damage to property, loss of life and/or injury. In the wildland-urban interface, where development has expanded into rural, higher risk areas, fires can result in major losses of property and structures. In most of Merced County, the California Department of Forestry and Fire Protection (CAL FIRE) ranks fuel loading as low.

Fuels are mainly crops and grasses. In the southwest corner, there are some brush, pine, and grass fuels, which are ranked as moderate fuel hazards, primarily in the area west of Interstate 5.

Generally, fire season in Merced County extends from early spring to late fall. Onset can happen suddenly due to lightning or human caused factors and wildfires can last from a few hours to a few months, but the likely hood of a large and damaging fire lasting for months in Merced County is not likely. Secondary effects from wildfire include increased erosion, destabilized slopes, degraded air and water quality, and economic impacts from burned landscapes. Urban fires primarily involve the uncontrolled burning of residential, commercial and/or industrial structures generally caused by human activities.

The California Department of Forestry and Fire Protection’s Fire and Resource Assessment Program (FRAP) assesses the amount and extent of California’s forests and rangelands, analyzes their conditions and identifies alternative management and policy guidelines.

CAL FIRE identifies areas that are at high risk of damage from wildfire based primarily on three factors:
● Ranking Fuel Hazards = ranking vegetation types by their potential fire behavior during a wildfire. ● Assessing the Probability of Fire = the annual likelihood that a large damaging wildfire would occur in a particular vegetation type.
● Defining Areas of Suitable Housing Density that Would Create Wildland-Urban Interface Fire Protection Strategy Situations = areas of intermingled wildland fuels and urban environments that are in the vicinity of fire threats.

Population density and the presence of structures are not currently used to determine the fire hazard severity for a particular region, although they do have a significant impact on fire behavior.

Based on the above criteria, CAL FIRE maps Fire Hazard Severity Zones for each county as “Very High,” “High,” or “Moderate”. As shown below in Figure 4-37, there are no areas in Merced County classified as Very High and the areas ranked High are primarily located west of Interstate 5. These areas exhibit the combination of vegetative fuel and topography that contribute to an increased fire hazard potential. The fact that an area is in a Moderate Hazard designation does not mean it cannot experience a damaging fire. It only means that the probability is reduced, generally because the number of days a year that the area has “fire weather” is less.

The Fire Hazard Severity Zones (FHSZ) spatial dataset was explicitly developed for adopting new ignitionresistant building code standards and adopted by the California Building Commission in 2007. It is constructed to describe the nature and probability of fire exposure to structures, including those lands that are highly urbanized, but in close proximity to open wildlands. It is broken into Federal Responsibility Areas, State Responsibility Areas, and Local Responsibility Areas shown below.

Merced County Multi-Jurisdictional Hazard Mitigation Plan Hazard Identification and Risk Assessment
2021-2026 Update Page 4-135
Figure 4-37 Merced County Federal, State, and Local Responsibility Severity Zones CAL FIRE has also designated the following Merced County communities as being at increased risk from wildfires (https://osfm.fire.ca.gov/divisions/wildfire-planning-engineering/fire-plan/communities-at risk/):
● Dos Palos
● Gustine
● Los Banos
● South Dos Palos
Some unincorporated communities located within the county are not confronted with a high wildfire risk, due primarily to the dominant type of vegetation in those areas. The low-growing native grasses and shrubs found in these communities present a minimal vegetative fuel source and a corresponding low wildfire risk. In addition, the topography of those areas is mainly level and well developed in both residential and agricultural land uses.

Extent (Magnitude/Severity)
Vegetation (or fuel) plays a major role in fire behavior and shaping fire hazard potential. Vegetation distribution throughout the county varies by location and topography, with dramatic differences observed between the western, central, and eastern portions of the county. For instance, fire behavior in brush fuel