Welcome to the Flood Preparedness Week home site!
Flooding in California
- Do you live in California? Do you visit California?
- Do you know when it typically floods in California?
- Do you know that every county in California has been declared a federal flood disaster area at least once in the last 20 years?
Types of Flooding
California is one of the few states to experience all these types of flooding:
- Alluvial Fan
- Debris Flow
- Riverine
- Coastal
- Tsunami
- Flash
- Localized/Stormwater
Alluvial Fan Flooding
<< Aerial view of alluvial fan flood path that became a debris flow flood (photo: 7/08).
An alluvial fan is a fan-shaped buildup of sediment found at the base of steep mountain canyons. Alluvial fan flooding is characterized by relatively shallow depths, high velocity, and moving soil and sediment, creating uncertainty about where rising water and soil will travel. Alluvial fan flooding is a concern primarily in central and southern California. In mid-August 2004, thunderstorms in the Wildomar, Sage, and La Quinta areas resulted in alluvial fan flooding. These thunderstorms brought in monsoonal rains that inundated the areas within minutes.
Debris Flow Flooding
<< This car and boulder became part of the debris in this debris flow flood. Note: the car was completely filled with mud (photo: 7/08).
Debris flow floods are made up of water, mud, and debris; they can form and accelerate quickly, reaching high velocities and traveling great distances. Debris flows are commonly caused by heavy, localized rainfall on hillsides where vegetation has been destroyed by fire. Debris flows can demolish homes and other structures, wash out roads and bridges and sweep away cars. In debris flow flooding, mud remains after a flood has receded, causing additional expense to remove.Riverine Flooding
<< The Eel River flooding across CA Hwy 211, cutting off Ferndale from the north. The river is to the right (photo: 12/05).
Riverine flooding occurs when rivers, streams and lakes overflow their banks. This includes flooding caused by levee failure and channel erosion. Areas adjacent to local streams and creeks can also experience flooding as a result of excessive runoff from heavy rainfall and accumulation of water flowing over broad flat areas. Of particular concern in California are the deep floodplains of the Central Valley, which are subject to periodic riverine flooding. Riverine flooding can be widespread, with floodwaters persisting for several hours to days, weeks or more.
Coastal Flooding
<< Coastal levees protecting Arcata waste water ponds from Humboldt Bay. Without levees protecting them, the waste water ponds flooded frequently (photo: 9/09).
Storms can cause flooding through a process known as "storm surges," which is when ocean waves are significantly larger than normal. If a storm event corresponds with a higher than normal tide, extensive flooding can occur. Winds blowing in an onshore direction (from the sea toward the land) can cause the water to "pile up" against the coast, overtopping natural and manmade flood protection structures like sea walls. Coastal communities, especially low-lying stretches of Southern California, are highly susceptible to this hazard.
Tsunami Flooding
<< Crescent City Harbor after being hit by tsunami (photo: 3/11).
Although many people think of a tsunami as a single, breaking wave, it typically consists of multiple waves that rush ashore like a fast-rising tide with powerful currents. Tsunamis can travel much farther inland than normal waves. Tsunamis are triggered by earthquakes, volcanic eruptions, underwater landslides and onshore landslides in which large volumes of debris fall into the water. Some come from across the globe; for example, the March 2011 earthquake in Japan brought an 8-foot wave into Crescent City harbor, destroying much of the harbor. Some of these triggers happen close to our coastline. If a tsunami-causing disturbance occurs close to the coastline, a resulting tsunami can reach coastal communities within minutes.
Flash Flooding
<< Road washed away by flashing flooding, Barstow, California (photo: 1/05).
Flash floods are the number one weather-related killer in the U.S. because they can roll boulders, tear out trees, and destroy buildings and bridges quickly. A flash flood is a sudden, rapid flooding of low-lying areas typically caused by intense rainfall. Flash floods can also be caused by the collapse of a man-made structure or ice dam. Rapidly rising water can reach heights of 30 feet or more. Although floods can occur throughout the year, California's rainy season usually lasts from November to April. This is when the chance of heavy flooding and flash flood risk is greatest.
Localized/Stormwater Flooding
<< Localized flooding, Sacramento County (Photo: Sacramento County, Date unknown)Localized flooding occurs in both urban and nonurban areas during or after a storm. Any storm, particularly slow-moving, steady rain storms, can overwhelm drainage systems. When the system backs up, pooling water can flood streets, yards and even the lower floors of homes and businesses. Even less intense storms can cause this type of flooding when leaves, sediment and debris plug storm drains.
Localized flooding poses most of the same problems caused by larger floods, but typically impacts fewer people and affects geographically smaller areas. Flooding of this type tends to recur year after year. The aftermath can mean costly damage to homes and property. In many cases, stormwater flooding can easily be avoided by keeping storm drains clear of debris, so the stormwater system can function properly.
Not every part of California experiences every type of flooding, but the results of each is the same: without proper preparedness, lives, homes, infrastructure and agriculture are lost, and damage to the environment and economy is likely. Preparing for devastating floods is critical.
Important Facts About Floods:
- More people die in floods each year than in any other type of natural disaster. Between 1975 and 2005, an average of 127 people died in floods each year in the U.S.
- Six inches of fast-moving water can knock an adult off his or her feet.
- Six inches of water will reach the bottom of most passenger cars, causing loss of control and possible stalling.
- 18 inches of water will float most cars; two feet of water can float SUVs and trucks.
- It is extremely difficult to see how deep flood water is, so don't drive through water on the roadway.
- Just one foot of water can cause more than $54,000 in damages to a $150,000 single-family home.
- A home in the 100-year floodplain is almost twice as likely to suffer damage from flooding than from fire during the life of a 30-year mortgage.
- Statewide, $5.4 billion in crop values are exposed in the 100-year floodplain.
- Statewide, 1.4 million people live and work in the 100-year floodplain.
BE AWARE, BE PREPARED
- Know your risk and take action to reduce your risk.
- Stay away from rising creeks, streams and rivers.
- "Turn Around, Don't Drown"™. Don't drive through water on the roadway; during floods, more people are trapped and die in their vehicles than anywhere else.
- Know how to leave the area quickly if you see water start to rise.
- Be aware of your surroundings and know your evacuation routes.
- Do not attempt to cross flowing water that may be more than six inches deep. If you have doubts, don't cross.
- Have an emergency preparedness kit. (Red Cross PDF for developing a kit)
- Choose a family meeting place and have a plan for how to communicate during an emergency.
- If you live in a flood-prone area, consider buying flood insurance.
- During threatening weather, listen to local radio or TV news channels for watch and warning bulletins:
- Flood Watch means it is possible that flooding will occur in a specified area. Be alert and prepared for a flood emergency.
- Flood Warning means flooding is occurring or is imminent in a specified area. Move to safe ground immediately.
More information about flood types and flood preparedness can be found at the following websites:
- My Hazards: http://myhazards.calema.ca.gov/
- Flood Risk Notification: http://www.water.ca.gov/myfloodrisk/
- Flood SMART: http://www.floodsmart.gov/floodsmart/
- National Weather Service- "Turn Around, Don't Drown"™: http://www.nws.noaa.gov/os/water/tadd/
- American Red Cross: http://www.redcross.org/prepare/location/home-family
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