Flood Plain Management

 

Flooding in Washington State

What is a flood?

Floods are the most common and widespread of all natural disasters - except fire. Most U.S. communities can experience some kind of flooding after spring rains, heavy thunderstorms, or winter snow thaws. Floods can be slow or fast-rising but generally develop over a period of days.

Dam failures are potentially the worst flood events. A dam failure is usually the result of neglect, poor design, or structural damage caused by a major event such as an earthquake. When a dam fails, a gigantic quantity of water is suddenly let loose downstream, destroying anything in its path.

In Washington, the primary mechanisms causing a flood situation are:  

  1. Heavy rainfall, which is the primary mechanism for floods in Western Washington, and may or may not include low-elevation snow melt or saturated soil. These events usually happen in the fall and early winter.

  2. Rainfall on snow, which can rapidly melt the snow pack and result in winter and early spring floods. Rapid snowmelt during a hot spell can produce large floods, typically in late spring. These conditions primarily affect flooding in Eastern and Central Washington.

What is a flash flood?

Flash floods usually result from intense storms dropping large amounts of rain within a brief period. Flash floods occur with little or no warning and can reach full peak in only a few minutes. Central and Eastern Washington are more prone to flash floods than Western Washington.

Danger zones

Floods and flash floods occur in all 50 states. Communities at risk particularly are located in low-lying areas near water, or downstream from a dam.

Where is the greatest flooding risk?

In areas prone to flooding, the Federal Emergency Management Agency (FEMA) provides local communities with flood maps that identify what is known as the “Special Flood Hazard Area” or SFHA. These are areas that experience a 1 percent annual change of flooding. This statistic is somewhat misleading. In Western Washington the actual occurrence of a flood on any river system that drains to the Puget Sound has been roughly every 4.5 years. The Special Flood Hazard Area is regulated under the National Flood Insurance Program (NFIP) for those communities voluntarily joining the program. Local governments implement flood plain management programs that meet or exceed federal and state standards for protecting local residents and businesses from flooding.

United States Geological Survey, Popular Myths about Flooding in Western Washington

Do I live in a flood hazard area?

Ecology’s Coastal Atlas provides a tool that allows people to type their address and find out if they have a home or business that lies in the Special Flood Hazard Area. Note that local communities may have a larger regulated area than the Special Flood Hazard Area. Some communities, for example, may regulate to the highest recorded level of floodwaters for the community, which may exceed the Special Flood Hazard Area.

Washington Flooding History

Washington is one of the most flood-prone states in the nation.

Here are some unique facts:

  • From 1980 through 2011, Washington had 22 Presidentially-declared flood disasters.
  • In 1997, Washington had the highest number of declared flood disasters in the country.
  • Washington ranks high in the number of flood insurance policies - 57,000 policies providing $13 billion in insurance coverage.
  • 35 percent of flood insurance policies are outside the mapped Special Flood Hazard Area also known as the "100 year floodplain".
  • Washington experienced 34 percent growth in the number of flood insurance policies between October 2009 and September 2010, the second fastest growing state in the country. This is indicative of the still-growing western United States and the relatively high area of land in the Special Flood Hazard Area, especially in western Washington.

Floodplain Management in Washington State

Here's some floodplain management history for Washington:

  • In 1935, the Washington State Legislature enacted the nation's first State Flood Plain Management laws in the United States.
  • The Washington Legislature created authority to issue permits for construction in Flood Control Zones.
  • At that time Flood Control Zones only covered about a third of all flood prone communities.
  • In 1968, the United State Congress established the National Flood Insurance Program.
  • In 1969, the state Legislature passed a measure prohibiting  construction of residential structures in floodways.
  • In 1989, the state Legislature, granted Ecology authority to manage state flood plains.
  • In 1989, the Washington Legislature also required the state to establish “minimum state requirements for floodplain management that equal the minimum federal requirements for the national flood insurance program.” The only exception is the state floodway prohibitions.
  • In 2001, the Department of Ecology became a Cooperating Technical Partner in Federal Emergency Management Agency (FEMA) efforts at map modernization and eventually the RISKMap program.

 

Back to top