On December 10, 2015, the Department of Ecology approved a locally-calibrated model for four of the five Clark County soil groups (SG1, SG2, SG4, and SG5). For soil group SG 3, one change is currently being implemented, and should be available by the time the Clark County Stormwater Manual 2015 becomes effective on January 8, 2016. Watch for blog updates coming soon that will provide more detail about this exciting development that will ease stormwater requirements for development projects in Clark County.The Western Washington Hydrology Model (WWHM) is the model accepted by the Department of Ecology (DOE) based on watershed studies in the Puget Sound Basin – where soils drain better than the soils in Clark County. To address Clark County’s unique soils, the Clark County WWHM (CCWWHM) was developed, but in 2014 DOE refused to approve it. Since then, many Clark County development projects have been subject to unreasonable stormwater standards based on science that did not account for Clark County soils.
For developers with current projects or projects on the horizon, now is a great time to review the new stormwater standards for Soil Group (SG) 1: excessively drained soils (hydrologic soil groups A and B); 2: well-drained (mostly hydrologic soil group B); 4: poorly drained (slowly infiltrating group C soils); and 5: wetland soils (mucks). New standards are also expected to become effective for SG 3: moderately drained (hydrologic soil groups B and C), when the Clark County Stormwater Manual 2015 becomes effective on January 8, 2016. Watch for our continuing updates on this subject.
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