Vulnerability Flood hazard areas in Washington state are located on the floodplains of all rivers, streams, lakes, wetlands, and closed depressions. High water and flooding most commonly occur between October and June, during periods of heavy rainfall or rapid snowmelt. Historically, the Snoqualmie river has been a primary indicator of flooding in King County in that when it floods, other river systems also flood. Effects Floods can cause loss of life and great damage to structures, crops, land resources, flood control structures, roads, and utilities. Flood damages in King County and the State of Washington exceed damages by all other natural hazards.
Inundation is defined as floodwater and debris flowing through an area.
It can cause minor to severe damage, depending on the velocity of flows, the quantity of logs and other debris they carry, and the amount and type of development in the floodwater s path. Bank erosion can threaten areas that are not inundated by floods. For example, a home on a high bank, above flood levels, can be undermined by the flood s erosive flows. The amount of erosion at a site depends on its location on the channel, flow velocities, the pattern of debris and sediment accumulation in the channel, and the erodibility of the bank. Some rivers, such as the Tolt, experience sudden and dramatic patterns of bank erosion that can create major course changes during a single flood event.