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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.
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 determine which areas potentially qualify for use of the existing land cover condition as the flow control target for new and re-development projects.
The analysis involved the following steps:
• Determine basin scale upon which to do the analysis. Some streams have only one basin designated for their drainage area. Other streams have multiple sub-basins for which TIA and area data are available. The analysis begins just above those points at which a flow control standard does not apply. Usually that is a stream’s discharge to Puget Sound or a large lake system that is exempt from flow control, e.g., Lake Washington/Lake Union/Ship Canal area.
• Using the 1991 data, compute an area-weighted TIA using data for all sub-basins within the larger basin.
• If the basin does not exceed the 40% TIA criterion, none of the sub-basins potentially qualify unless a sub-basin discharges very near to the bottom of the basin drainage. If the basin exceeds 40% TIA at least some area within the basin potentially qualifies. To determine that area, proceed upstream and compute areal weighted %TIA for smaller drainages within the subject basin. A drainage area of an identifiable side-stream or an upper area draining to the main stream channel does not qualify if the respective areal-weighted %TIA of its sub-basins does not exceed the 40% criterion.
• Remove designation of basins which met the 40% criterion, but for which:
1) a basin-specific study suggests the stream channel to be unstable (Thornton Creek – Seattle);
2) an approved basin study identified a target flow regime intended to achieve acceptable natural resource objectives (Des Moines Creek).
Copyright © Washington State Department of Ecology. See http://www.ecy.wa.gov/copyright.html.
