Civil/Structural Engineering, Vashon, Washington, USA 206-463-5311

Floodplain Background

Background (much of this information is taken directly from government websites)

Since standard homeowners’ insurance doesn’t cover flooding, it’s important to have protection from the floods associated with hurricanes, high wind storms, heavy rains, and other conditions that impact the United States.


In 1968, Congress created the National Flood Insurance Program (NFIP) to help provide a means for property owners to financially protect themselves from these events.  The NFIP offers flood insurance to homeowners, renters, and business owners, if their community participates in the NFIP.  Participating communities agree to adopt and enforce ordinances that meet or exceed FEMA requirements to reduce the risk of flooding.


The King County Council decided to participate in the NFIP.  The King County Flood Control District was established in 2007 to protect public health and safety, regional economic centers, public and private properties, and major transportation corridors.  The Army Corps of Engineers and its consultants have worked to establish floodplain maps for King County.  Homeowners need to become familiar with the new revised preliminary mapping and its role as the primary regulatory tool used under the National Flood Insurance Program (NFIP) and the insurance industry to regulate floodplain development and rate flood insurance policies.


Part of the national Map Modernization Program, the new digital maps can be used with geographic information systems. The mapped information is widely used by lending institutions and insurance agents in determining who must purchase flood insurance and the cost of that insurance, should it be necessary. In addition, the maps will be used by the county and local communities for floodplain management and permitting purposes.


The most current revision of these preliminary maps and study, dated November 6, 2010, show flood hazard zones that are subject to inundation by the base flood (100-year flood) as well as the regulatory floodway. 


Sample Floodplain Map, designating Base Flood elevations and Flood Zones on North Vashon Island.

Coastal floodplain mapping was conducted under the auspices of FEMA, assisted by the Army Corps of Engineers, King County, and private consultants.  Much effort was expended to provide good definition through field reconnaissance, surveys from boats, video and photographic information, review of existing USGS maps, underwater surveys and soundings, aerial photography, and extensive computer modeling.  The result of this work was a definition of a100-year Base Flood Elevation (BFE) for coastal property owners.  However, a wide net was cast in terms of the number of homes included in the coastal floodplains, many of which should rightly not be included in the floodplain. 


The computer modeling attempted to define the BFE based upon the geometry of the sea floor, the local land conditions, and other factors; the BFE modeling consisted of three components:

  • Offshore to Nearshore (currents, winds, waves, and low pressure systems)
  • Nearshore to Land (wave runup and overtopping of low bulkheads and banks)
  • Landward of Crest (bore propagation, the travel of the wave inland after overtopping)

Typical Wave Model Sketch

Landward of Crest Wave Model

Nearshore to Land Wave Model