SBCT Final EIS - Govsupport.us

Chapter 3 ⎯ Affected Environmentposed rocks on the north slope of the Waianae Range, south of DMR, are remnants of the dike complexbelonging to the northwest-trending rift zone of the Waianae dome. Along the coast, the volcanicrocks alternately have submerged below and emerged above sea level over recent geologic time. Thecoastline is underlain by an ancient coral reef, which subsequently has been overlain by dune sanddeposits.SoilsSoils at DMR are developed on beach sand deposits, with various mixtures of finer and coarser sediments.Most of the area is underlain by Jaucas sand, which has been disturbed or filled to constructthe airstrip, roads, and building sites. DMR also contains boggy seasonal wetlands, which are underlainby Lualualei clay, and marginal sloping uplands predominantly underlain by Kaena very stonyclay or other stony or rocky soils. The Jaucas sand is highly susceptible to wind erosion. Kaena verystony clay exhibits moderate to severe water erosion (USAG-HI 2004).Dillingham Trail would use existing unpaved farm roads over most of the proposed alignment, someof which would require modification, such as hardening the roads, improving drainage to preventdamage to the road surface, and improvements or modifications to existing stream crossings to ensurepassibility and prevent environmental damage. The soils over which the proposed trail passes aresummarized below with attention paid primarily to soils exhibiting rapid runoff, severe erosion potential,high compaction or shrink-swell potential, or other soil hazards.From the east edge of DMR to just east of Waialua, Dillingham Trail crosses relatively flat lands ofthe coastal plain, underlain by soils of the Kaena-Waialua association, which are deep, poorly drainedto excessively drained with a fine- to coarse-textured subsoil on coastal plains and talus slopes. Exceptfor the clay soils, most of the soils make good road fill. The Pearl Harbor clay, Kaena stony clay,and Waialua clays have a moderate to high shrink-swell potential, poor workability, and high watertable (USAG-HI 2004).The portion of the trail upstream of Kaheaka Reservoir crosses soils belonging to the Helemano-Wahiawa association, which are deep, well-drained soils on uplands. The trail then continues south,leaving the paved road, and skirts the southern edge of the cultivated farmlands along the north rim ofPoamoho Stream, which consists of rock land soils, 25 to 90 percent of the surface of which is exposedrock, transitioning to Helemano soils further upstream. The soil between the rock outcrops isclayey, has a high shrink-swell potential, and is susceptible to sliding. Helemano soils have rapid runoffand very severe erosion hazard. At the rim of the gulch, the trail traverses soils suitable for roadfill, including Wahiawa silty clay, then continues gradually upslope across Manana silty clay andKolekole silty clay loam, skirting the Wahiawa silty clay soils that underlie the adjacent farmlands.The trail crosses Poamoho Stream (near Poamoho Camp) again, in Helemano soils with 30 to90 percent slopes. The portion of the trail between Poamoho Camp and the gate at SBMR on WilikinaDrive crosses Wahiawa silty clays. Wahiawa silty clays were described previously as well drained,with slow runoff and slight erosion hazard.Erosion ManagementAs described previously for SBMR, soil resources management on DMR is mandated by and detailedin the USAG-HI INRMP and ITAM Five-Year Management Plan for Oahu (USARHAW 2001a).Geologic Hazards and SeismicitySteep slopes (greater than 30 percent) within DMR are limited to the southern margin of the installation,which abuts the steep slopes of the northern extension of the Waianae Range (USAG-HI 2004).February 2008 3–10 2/25th SBCT Final EIS