Get Ready & Prepare

Overview

A dam is an artificial barrier that stops or restricts the flow of water. Dams are built for a variety of uses including flood protection, power generation, agriculture, water supply, and recreation. A dam failure is the structural collapse of a dam that releases the water stored in the reservoir behind the dam. A dam failure is usually the result of the age of the structure, inadequate spillway capacity, or structural damage caused by an earthquake or flood. The sudden release of water has the potential to cause human casualties, economic loss, and environmental damage. This type of disaster is dangerous because it can occur rapidly, providing little warning and evacuation time for people living downstream. The flows resulting from dam failure generally are much larger than the capacity of downstream channels and can therefore lead to extensive flooding. Flood damage occurs as a result of the momentum of the flood caused by the sediment-laden water, flooding over the channel banks, and impact of debris carried by the flow.

Three major dams and reservoirs, as well as several small dams, are located in and within the vicinity of Monterey County. The three largest dams—the Nacimiento Dam, San Antonio Dam, and Los Padres Dam—have never failed or been subject to significant damage.

History

There is no record of any damages, fatalities, or injuries associated with dam failure in the Monterey County. However, water has been released over the Nacimiento Dam spillway crest five times (1958, 1969, 1983, 2011, and 2017) over the last 63 years, and the San Antonio Dam spillway crest three times (1982, 1983, and 2006) over the past 55 years.

The state’s most recent dam emergency occurred in February 2017 when the Oroville Dam in Butte County was on the verge of overflow. The dam’s concrete spillway was damaged by erosion and a massive hole developed. The auxiliary spillway was used to prevent overtopping of the dam, and it experienced erosion problems also. Evacuation orders were issued in advance of a potential large uncontrolled release of water from Lake Oroville, but such a release did not occur. After this incident, state officials ordered that the flood-control spillways of 93 dams be re-inspected for potential geologic, structural, or performance issues that could jeopardize their ability to safely pass a flood event.

The San Clemente Dam was a large concrete dam located about 18 miles from the Ocean on the Carmel River. Over time the reservoir became increasingly sedimented and no longer served as a water source. The dam posed a safety concern and could have failed in the event of a large earthquake or flood. Due to safety and environmental concerns, the CA Coastal Conservancy, NOAA, and California American Water decided to re-route the river and remove the dam. As of 2019, the Carmel River is returning to its wild state and the fish population is rebounding. At the time, this was the largest dam removal project in California history.

Dam Failure Risk

A dam failure is possible at any dam in Monterey County. The most likely location of a damaging levee failure would be at dams classified as having “Extremely High” downstream hazard potential dams by the Division of Safety of Dams of the California Department of Water Resources. This includes the Nacimiento, San Antonio, Forest Lake, and Los Padres Dams. The Nacimiento, San Antonio, and Los Padres Dams are older and are the most likely locations of a dam failure.

A 2017 State law (Calif. Water Code Sections 6160-61) requires that an Emergency Action Plan and dam failure inundation maps be prepared for all dams so classified. The Monterey County Water Resources Agency has prepared the following maps to fulfill this requirement. These flood inundation maps are based on the latest flood simulation models and are estimates of the maximum flood depth, the flood arrival time, and the flood recession time that would occur in four different scenarios: an embankment (dam) failure at San Antonio Dam, a spillway failure at San Antonio Dam, an embankment failure at Nacimiento Dam, and a spillway failure at Nacimiento Dam.

The main assumptions common to all four simulation models are:

1. No flows downstream of the dam prior to the breach
2. Existing levees on all river channels will not fail
3. Channel geometry will remain fixed during the flooding
4. The failure would occur on a sunny day, with no rainfall or runoff into the reservoir
5. The failure would occur when the reservoir was full

Dam failure events are infrequent and usually coincide with events that cause them, such as earthquakes, landslides, and excessive rainfall and snowmelt. Although the recent Oroville event raised public concern about dam failure, the probability of such failures remains low in today’s regulatory environment.

Prepare for Dam Failure

1. Sign up for Alerts

Make sure to Sign up for Alerts. Dam failure can sometimes occur with little warning time. Signing up for alerts is important to ensure to receive quick and accurate information.

2. Make a Plan

The number of people to be alerted and evacuated in the event of impending dam failure can vary widely. But when action is required, a plan will give you the confidence to act and the tool to stay safe. Learn more about how to Make a Plan and learn more about how best to prepare:

• How To Prepare For A Flood
• Evacuation Preparedness
• American Red Cross Flood Safety Information
• Download the General Preparedness Toolkits (English, Spanish)

The Department of Emergency Management encourages residents to remain on high alert and be prepared. To monitor rivers near your home or business, link here.

3. Consider Getting Flood Insurance

Consider getting flood insurance. Flooding isn't typically covered under homeowner’s insurance. Flood insurance can cover damage caused by storms, tsunamis, mudflow, flooding including structural damage and associated cleanup costs; but a basic policy doesn’t usually cover building contents. Visit floodsmart.gov or call 1-888-4FLOODS for more information.

4. Build a Kit

Learn about creating an emergency kit so you are ready to go quickly if needed. Here is list of some basic items to have in your disaster preparedness kit:

• Blankets or sleeping bags
• Flashlights with extra batteries
• First Aid Kit
• Non-Perishable Food
• Water
• Local Maps

The items above are a good start for disaster kits. For a full list of go-kit supplies, click here.

In the Event of Dam Failure

Warning time for dam failure varies depending on the cause of the failure, the size of the dam and volume of water retained, and the location of the dam. In events of extreme precipitation or massive snowmelt, evacuations can be planned with sufficient time. In the event of a structural failure due to earthquake, there may be no warning time. A dam’s structural type also affects warning time. All dams of concern in Monterey County are earthen dams, which do not tend to fail completely or instantaneously. Once a breach is initiated, discharging water erodes the breach until either the reservoir water is depleted, or the breach resists further erosion. The time of breach formation ranges from a few minutes to a few hours.

A dam failure event could lead to flooding, but the severity and extent of the flooding can vary widely. A major factor to consider is the initial flow in the river when the failure occurs. The initial flow is normally very low on all the rivers between May and October. During the winter, the initial flow is much higher. This wide variation in initial flow has a significant impact on the areas that must be evacuated.

In the event of flooding keep in mind the following safety tips:

• Evacuate immediately, if told to evacuate. Never drive around barricades. Local responders use them to safely direct traffic out of flooded areas.
• Listen to EAS, NOAA Weather Radio or local alerting systems for current emergency information and instructions regarding flooding.
• Do not walk, swim or drive through flood waters. Turn Around. Don’t Drown!
  • If floodwaters rise around your car, abandon the car and move to higher ground if you can do so safely. You and the vehicle can be quickly swept away. Six inches of water will reach the bottom of most passenger cars, causing loss of control and possible stalling.
• Listen to EAS, NOAA Weather Radio or local alerting systems for current emergency information and instructions regarding flooding.
• Stay off bridges over fast-moving water. Fast-moving water can wash bridges away without warning.
• Stay inside your car if it is trapped in rapidly moving water. Get on the roof if water is rising inside the car.
• Get to the highest level if trapped in a building. Only get on the roof if necessary and once there signal for help. Do not climb into a closed attic to avoid getting trapped by rising floodwater.

Stay Safe After a Flood

After a flood caused by dam failure, keep the following safety information in mind:

• Listen for news reports to learn whether the community’s water supply is safe to drink.
• Avoid floodwaters; water may be contaminated by oil, gasoline, or raw sewage. Water may also be electrically charged from underground or downed power lines.
• Avoid moving water.
• Be aware of areas where floodwaters have receded. Roads may have weakened and could collapse under the weight of a car.
• Stay away from downed power lines, and report them to the power company.
• Return home only when authorities indicate it is safe.
• Stay out of any building if it is surrounded by floodwaters.
• Use extreme caution when entering buildings; there may be hidden damage, particularly in foundations.
• Service damaged septic tanks, cesspools, pits, and leaching systems as soon as possible. Damaged sewage systems are serious health hazards.
• Clean and disinfect everything that became wet. Mud left from floodwater can contain sewage and chemicals.