Volume 2: Draft Gorst Planned Action Environmental Impact Statement

GORST PLANNED ACTION EIS | AFFECTED ENVIRONMENT, SIGNIFICANT IMPACTS, AND MITIGATION MEASURESGEOLOGIC HAZARDSWatershedGeologically Hazardous Areas, as defined by the Kitsap County CAO, and the City of Bremerton CAO, are areas thatbecause of their susceptibility to erosion, sliding, earthquake, or other geological events are difficult locations tosite commercial, residential, or industrial development, consistent with public health or safety concerns. Withinthe Gorst watershed, mapped Geologically Hazardous Areas include numerous moderate hazard areas, which haveslopes of 15 to 30 percent and/or other geologic issues, and a few high hazard areas, which have slopes of greaterthan 30 percent and are unstable (Kitsap County 2007). Characteristics of instability may include unstable soils,groundwater seeps or springs, erosion hazards, or seismic hazards. The moderate hazard areas, or areas ofconcern, occur predominantly in the northern portions of the watershed, as well as along stream channels in thesouthern portion of the watershed. The high hazard areas all occur in the southern two-thirds of the watershed,and are associated with steep slopes along Parish Creek, a portion of Gorst Creek, and several unnamed streamchannels.Gorst UGAModerate geologic hazard areas cover much of the northeast portion of the Gorst UGA. No high hazard areas havebeen mapped within the UGA.SOILSWatershedA breakdown of soil types within the study area is presented in Table 3.1-1 Soil Types in the Gorst Creek Watershedand Gorst UGA, and Their Associated Construction Limitations and Erosion Hazard Potential. Within the watershed,gravelly loams and sandy loams predominate. Alderwood very gravelly sandy loams and Harstine gravelly sandyloams, which occur on glacial till plains and moraines, are prevalent in the southern half of the watershed. Kilchisvery gravelly sandy loams occur over basalt on mountain slopes and Ragnar fine sandy loam occurs on terraces inthe northern half of the watershed.Flooding of streams and associated export of sediment as result of water erosion during rain events have beenidentified as problems within the watershed that may be exacerbated by future cutting of trees and creation ofadditional impervious surface. Table 3.1-1 Soil Types in the Gorst Creek Watershed and Gorst UGA, and TheirAssociated Construction Limitations and Erosion Hazard Potential summarizes the erodibility of soils within thewatershed. On slopes of less than 15 percent, runoff from Alderwood soils is slight, and the erosion hazard is low.On slopes of 15 to 30 percent, the runoff increases to medium, and the erosion hazard is moderate. Althoughpermeability of Kilchis gravelly sandy loams is moderately rapid, these soils occur on slopes from 15 to 70 percent,therefore runoff is rapid and the erosion hazard is moderate to severe. For Ragnar fine sandy loams, the erosionhazard in flat areas is moderate, increasing to severe at slopes greater than six percent. For Harstine gravelly sandyloams, erosion hazard ranges from slight to moderate, depending on the slope.Gorst UGAWithin the UGA, previously developed urban land, and Alderwood (urban land-Alderwood complex), Indianola,Kilchis, Ragnar, and Norma soils are most prevalent.• Alderwood soils occur primarily in relatively flat areas near the Sinclair Inlet along State Route 3 and StateRoute 16, which already support extensive development. These soils are suited to urban development ifcommunity sewer systems are present. The main limitations are the silica-cemented hardpan at a depth of 20to 40 inches and a seasonal perched water table at a depth of 2.5 to 3 feet.Draft | June 2013 3-5