Volume 2: Draft Gorst Planned Action Environmental Impact Statement

GORST PLANNED ACTION EIS | AFFECTED ENVIRONMENT, SIGNIFICANT IMPACTS, AND MITIGATION MEASURESMap UnitAcres inWatershed 1Acres inUGAConstructability ofUGA Soils(Limitations forBuilding and StreetDevelopment)depending on slopeErodibility/Erosion HazardMcKenna Gravelly Loam 162 0 Severe (ponding) None (water is ponded)Kilchis-Shelton Complex 130 0Severe (shallow depthto rock, wetness, slope)Norma Fine Sandy Loam 119 31 Severe (ponding) SlightUrban Land-AlderwoodComplex(0 to 8 percent slope)55 55Slight to moderate(wetness, cementedpan, slope)Water 49 2 NA NAPits 32 32 NA NAShalcar Muck 14 0Severe (ponding, lowstrength)Tacoma Silt Loam 11 11 Severe (floods, wetness) NoneNote:1 Watershed refers to the entire Gorst Creek Watershed, inclusive of the UGA.Source: USDA Soil Conservation Service 1980; USDA Natural Resources Conservation Service 2013a,bImpactsImpacts Common to All AlternativesWatershedSevere (very rapid runoff)Slight; moderate on slopes(slow runoff)None to slight (water is ponded)Under all alternatives, development would occur throughout the Gorst watershed, to varying degrees as allowedby zoning and applicable regulations. As discussed in Section 2.5 Study Alternatives: Future Growth, it is assumedthat most development outside of city limits and designated UGAs would be associated with residential dwellings,with less than 500 new dwellings constructed over the next 20 years. Creation of impervious surface would resultin a long-term loss of soil functions within affected areas, and could lead to increased surface water runoff anderosion of soils in adjacent areas. Removal of trees and other vegetation within these areas could also lead toreduced infiltration and erosion of exposed soils from affected sites. Water erosion of soils by stormwater wouldbe the biggest risk, resulting in movement of soil downslope and downstream, and potential loss of soil from thewatershed. In all cases, risks of soil erosion associated with development and other land clearing activities wouldbe greatest on slopes and in areas with highly erodible soils. Additionally, use of heavy equipment for clearing andconstruction activities could result in compaction of soils. Risks of compaction would vary depending on thephysical characteristics of the soil. During construction and operation of new dwellings, there would be risks forlocalized contamination of soils through releases of fuels and other hazardous materials associated with vehiclesand development activities. Stormwater plans, BMPs, and Spill Prevention Control and Countermeasure (SPCC)plans would help minimize these impacts to varying degrees.Given that geologic hazards and unstable soil conditions occur throughout the watershed, future developmentwould have the potential to impact slope stability. However, because of the challenges to construction presentedby slopes, it is expected that development in these areas would be minimal.It is assumed that under all alternatives, future development projects would receive the appropriate permits, andthat buffers, development standards, and other mitigation measures pertaining to slope stability and prevention oferosion would be implemented.Draft | June 2013 3-7