Flood Plain Management

Washington State Projects

Channel Migration Zone Assessments

Abstract

River floodplains and channel migration zones (CMZs) are ecologically productive areas heavily impacted by development. Their importance is detailed in the National Marine Fisheries Service Biological Opinion declaring the Federal Emergency Management Agency (FEMA) program results in “take” of Puget Sound Chinook salmon and steelhead.

Flood hazards consist of both inundation and erosion (channel migration). Understanding the extent of a CMZ is critical to assessing risks to development as well as habitat. The Shoreline Master Program (SMP) requires identifying CMZs but this has not been done for over 550 miles of Puget Sound shorelines. Moreover, already completed CMZs methods and delineations are not consistent and often do not address future erosion risks and the loss of historic CMZ areas due to development. Current methodologies do not include evaluating future channel response to altered hydrologic and sediment regimes from climate change and development. Tasks include updating the CMZ mapping methodology, map areas for baseline trend analysis, and develop technical assistance for integrating with SMP and floodplain management and restoration/protection strategies including climate change scenarios.

Resources and guidance

Statewide Levee Inventory

Executive Summary

Today, levees reduce flood damage from many flood events for communities and agricultural areas throughout Washington state. The age and construction of these structures vary considerably. Many older levee systems, originally built for a different purpose, are at a lower design standard than is required today. Further, the flood risk to the public increases as development behind levees continues.

The Statewide Levee Inventory and Flood Protection Study was conducted to better understand the status of levees in Washington in two parts. The first part, consisting of this report, summarizes current levee policies and practices. A second part of the study is a statewide inventory of current levees and their protection status. The statewide inventory is summarized below, but delivered separately as a geospatial database and set of state and county maps.

This report discusses the requirements, processes, and costs associated with certification and subsequent accreditation of levees. The report also explains some of the key challenges levee owners have in complying with regulations and securing funding for levee maintenance, improvements, and certification. The report highlights several case studies to illustrate these issues. The report includes a discussion about alternatives to levees, and concludes with a list of next steps that may support flood damage reduction in Washington state.

Any discussion of levee accreditation should include flood risk and public safety. The 100-year standard may be woefully insufficient in some areas (such as highly urbanized environments) and perhaps overly protective in others (such as agricultural lands, undeveloped land, etc), thus FEMA accreditation should include risk and economic analysis.

Washington State Inventory GIS data