Flow control guidance for highly urbanized basins

Introduction:

The map below shows those basins that qualify for use of a flow duration standard that would require matching high flow durations of a project to the durations produced by the existing land cover condition. To qualify, a basin must have been at or above 40% total impervious area (TIA) since 1985. The map depicts basins that exceeded 40% total impervious area as of 1986. The Department of Ecology has used 1986 land covers as estimated from satellite images as the best available information upon which to make these designations.

Project planners must check with the local government to know what flow control standard applies to their project site. Ecology cautions project planners not to assume that the local government is using the above-mentioned flow control standard in a basin depicted as exceeding 40% total impervious area.

The map, in GIS format, and all associated metadata are available to local governments from the Department of Ecology at Ecology's GIS Web site:

Western Washington Land Cover Change

Map

Impervious Surfaces in Western Washington, 1985
Printable map (pdf)

Background:

The 2005 update to the Stormwater Management Manual for Western Washington includes the following Minimum Requirement in regard to control of stormwater release rates:

“Stormwater discharges shall match developed discharge durations to pre-developed durations for the range of pre-developed discharge rates from 50% of the 2-year peak flow up to the full 50-year peak flow. The pre-developed condition to be matched shall be a forested land cover unless:

1) reasonable, historic information is provided that indicates the site was prairie prior to settlement (modeled as “pasture” in the Western Washington Hydrology Model); or

2) the drainage area of the immediate stream and all subsequent downstream basins have had at least 40% total impervious area since 1985. In this case, the pre-developed condition to be matched shall be the existing land cover conditions. Where basin-specific studies determine a stream channel to be unstable, even though the above criterion is met, the pre-developed condition assumption shall be the “historic” land cover condition, or a land cover condition commensurate with achieving a target flow regime identified by an approved basin study.” (emphasis added)

Ecology published a discussion paper that explains the basis for the less stringent flow control standard highlighted in italics above.

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Map basis:

Ecology contracted with Sanborn, Inc. to provide land cover data for Western Washington for 1991, and an analysis of change in land cover, impervious surface, and forest canopy for all of Western Washington between 1991 and 2001. The project built upon land cover data classified under the NOAA Coastal Change Analysis Program (C-CAP) for 1996 and 2001.

A report, “Western Washington Land Cover Change Analysis,” is available from the Dept. of Ecology at Ecology’s GIS website.

Ecology used one of the report’s outputs, total impervious area by basin in 1991, to create and publish a map of areas that potentially qualified for use of the existing land cover condition as the flow control target for new and re-development projects. That map previously appeared at this website. Now that 1986 land cover data using similar estimating techniques has been made available by NOAA, Ecology has produced the above map that supersedes the previous map. The map identifies areas that Ecology considers as qualifying for use of the existing land cover condition as the flow control target.

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Ecology’s analysis:

The analysis involved the following steps:

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