What is Flooding?

Flooding is an overflow of water onto land that is normally dry. Floods can happen during heavy rains, when ocean waves come on shore, when snow melts quickly, or when dams or levees break. Floods can result in injuries and death, damaged or destruction of property and damaged major infrastructure.

Three causes of flooding in Los Angeles County are: (1) coastal flooding (sea level rise or storm surges), (2) heavy rains, and (3) damaged flood control infrastructure resulting from heavy rains or earthquakes.

Which parts of Los Angeles County are affected by flooding?

Low-lying areas near the coast are prone to flooding. In Los Angeles County, this includes the ports of Los Angeles and Long Beach, as well as Marina del Rey. Other low-lying areas prone to flooding from heavy rains include communities in the Santa Monica Mountains Coastal Zone and in the Antelope Valley. Communities in the lower reaches of rivers are also at risk of flooding from breaches in upstream levees during heavy storms. This includes communities near the Los Angeles and San Gabriel Rivers.

The Federal Emergency Management Agency (FEMA) has created Flood Insurance Rate Maps (FIRMs) to show areas at high risk of flooding. High risk areas have at least a 1% annual chance of flooding. These areas are also called “a 100-year floodplain”. Similarly, an area that has at least a 0.2% annual chance of flooding is also called “a 500-year floodplain.” Properties within the 100-year floodplain are required to have flood insurance as a prerequisite for federally backed mortgages; properties within the 500-year floodplain are not required to have flood insurance to qualify for federally backed mortgages.

Cascading effects: wildfires + heavy rains = mudflows

Heavy rains in areas affected by wildfire can result in mud and debris flows. As wildfire season lengthens and extends into California’s wet season when begins in October, heavy rains that follow wildfires are becoming more common. These post-wildfire rains may create a new hazard as they wash debris from damaged and destroyed properties, burnt vegetation, and soil from landscapes down canyons.

These mudflows are more likely to occur after wildfires because hillsides are more likely to erode; vegetation roots that held the soil on hillsides in place are no longer present.

How do I find out if I am in a flood hazard area?

The Federal Emergency Management Agency (FEMA) identifies areas prone to flooding. Visit the Flood Hazard Zones map to find out whether your property is at risk of flooding.

In addition, FEMA identifies properties which have incurred two or more National Flood Insurance Program losses of at least $1000 within any 10-year rolling period since 1978. The Los Angeles Department of Public Works recommends mitigation actions for these properties in a report called the Repetitive Loss Analysis.

Communities downstream from reservoirs and dams can be at risk in the event of a breach. Flood control infrastructure is designed to prevent flooding during wet winter and spring months, and to store water for use during the dry summer and fall months. In California, earthquakes can compromise dam structures and result in flooding. Additionally, climate change is causing more intense rains and mudflows which can stress and damage the flood control infrastructure, compromising its ability to protect downslope communities.

How do floods affect residents and communities?

Flood hazards can result in injuries and death, property damage, isolation of communities, damage to essential public facilities, interruption of essential services, and disruption to economic activity.

Flooding can also initiate a cascade of hazards. After the heavy rainstorm, buildings that were flooded may grow mold which is hazardous to human health. Flooding can also damage critical facilities in low-lying areas such as wastewater treatment plants. If sediment enters the pumps, untreated sewage can flood into and contaminate wells that provide residents with drinking water.

Sea level rise can also cause harm to coastal habitats. When salt water inundates fresh water habitats at a fast rate, plants and animals may be unable to adequately adapt.

How does climate change affect floods?

Climate change is changing the pattern of precipitation events. More intense rains punctuated by longer periods of drought are becoming the new norm. When the rate of rainfall exceeds the amount of water the soil can absorb, water moves into stream channels, levees and reservoirs. If the water exceeds the capacity of flood control infrastructure, flooding can occur.

Cascading effects: mudflows + reservoirs = potential flooding

Mudflows that come from heavy rains after wildfires can collect in reservoirs, compromising the flood control infrastructure in two ways. First, sediment build-up reduces the capacity of reservoirs to hold water. Second, debris can clog dam safety valves that are designed to remove pressure from the dam face during heavy rains. These cascading effects can compromise the flood infrastructure’s capacity to protect communities from flooding during heavy rains.

What is Los Angeles County doing to address flooding?
Department Requirements
Public Works Sediment Management Strategic Plan
Comprehensive Floodplain Management Plan
Repetitive Loss Area Analysis Report
Community Rating System
Debris Management Plan
At-Risk Properties Hazard Fund and Strategies
What can I do to prepare for floods?

Residents can prepare for future flood events by learning whether they live in a flood hazard zone. To prepare, communities can create an evacuation plan and support efforts to remove sediment from flood control reservoirs located upslope.

Check out these resources:

My Hazards Interactive Map – Enter your address to discover what hazards are in your area (earthquake, flood, fire, and tsunami (California State Office of Emergency Services)

Los Angeles County Emergency Survival Guide – How to prepare for, respond to, and recover from disasters in Los Angeles County

Floods, Debris Flows, Landslides Terms and Definitions – Common terms (Los Angeles County Chief Executive Office)

History of Floods, Mudslides, Debris Flows, Landslides in Los Angeles County– Notable events since 1770 (Los Angeles County Chief Executive Office)

Los Angeles County Tsunami Inundation Maps (2009)

All-Hazard Mitigation Plan (Oct 1, 2019 Public Draft)

Dam Information

Map of Los Angeles County’s fourteen dams

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