SBCT Final EIS - Govsupport.us

Chapter 3 ⎯ Affected EnvironmentWheeler Army Airfield. WAAF is adjacent to the east side of the SBMP and is underlain by a thick(100 feet or greater) sequence of saprolite, as described above, over which has developed a layer ofclay-rich soil that is approximately 10 feet thick.Schofield Barracks East Range. The geology of SBER is similar to that described above for theSBMP. Stearns and Vaksvik (1935) estimated the thickness of the Koolau deposits (depth to the underlyingWaianae volcanic deposits) to be about 1,500 feet under the east side of SBMR. The thicknessincreases to the east. The eastern side of SBER is part of the Northwest Rift Zone of the KoolauVolcano. This is an area of greater dike intensity. The eruptive center of the Koolau Volcano wasprobably to the east of the ridge of the Koolau Range near Kaneohe Bay.South Range Acquisition Area. The geology underlying the SRAA is dominated by lava flows of thelower and middle members of the Waianae Volcanic Series, which crop out along the uplands on theeast side of the SBMP and underlie WAAF (USAG-HI 2004). The channel of Waikele Stream is incised80 to 120 feet below the surface of the plateau, meaning that the stream channel has erodedthrough softer alluvium, soil, and saprolite deposits and rests near the depth of the underlying weatheredbasalt. The stream channels are covered by sediments eroded from the uplands and from the sideslopes of the channels.SoilsSchofield Barracks Main Post. Four of the seven soil associations found on Oahu occur within SBMP(USAG-HI 2004). Soils of the Tropohumults-Dystrandepts association occur in largely inaccessibledeep V-shaped drainages and narrow ridges on very steep east-facing slopes above an elevation ofabout 1,500 feet amsl, and serve primarily as a watershed (Foote et al. 1972). These are thin, welldrained,light soils derived from volcanic ash that are high in organic matter, can contain more waterthan soil when saturated, and are strongly to extremely acid. The surface layer is generally darkcoloredsilty clay (friable in the case of Dystrandepts), while the subsoil has a strong blocky structureunderlain by saprolite. Because of their occurrence on steep slopes, potential friability at the surface,and fine silty texture, the soil erosion hazard by wind and water is high for this association, especiallyin areas where vegetation has been depleted. Kolekole silty clay loam and Manana silty clay loam aretwo major soil groups found on the lower gently to moderately steep slopes of the range at elevationsranging from 500 to 1,200 feet amsl. Kolekole soils are moderately rapidly permeable to the depth ofabout 2 to 3 feet. Runoff is slow, and erosion hazard is slight.Kunia silty clay is found on the flatter lands of SBMP, on nearly level ground in upland terraces andfans at elevations of 700 to 1,000 feet amsl. Kunia soils are well drained and moderately permeable,exhibiting slow runoff and slight erosion hazard. Soils of the Helemano and Kawaihapai series arefound in the gulches and drainageways on alluvial fans. Helemano soils are well-drained silty claysthat exhibit severe to very severe erosion potential. Kawaihapai soils are well drained, and the erosionhazard is slight.A study conducted for the Army in 1979 (Walter Lum Associates, Inc. 1979) identified soil erosionproblems in the Central and South Ranges of SBMP. The study concluded that erosion of the walls ofgulches, mainly affecting Helemano soils, primarily occurs naturally during heavy rainfall/runoff.The study also identified soil erosion problems associated with unstable or poorly drained road cuts,mainly at gulch crossings and in areas with steep slopes, and bare soil surfaces where vehicle trafficand other military activities caused the removal of vegetative cover. The study found that about1.3 percent of the total study area was undergoing high rates of erosion due to natural conditions,while about 3.5 percent was undergoing a high rate of erosion due to military activities. Erosion ratesin denuded upland soil areas were estimated at between 28 and 80 tons per acre per year, while ero-February 2008 3–4 2/25th SBCT Final EIS