SBCT Final EIS - Govsupport.us

Chapter 3 ⎯ Affected Environment800 feet amsl. The gulch is underlain by Helemano silty clay. The remainder of the route to HMRtraverses similar soils, alternating between rock land, Helemano silty clay in gulches, and eitherLeilehua silty clay soils or Paaloa soils on ridges.Erosion ManagementAs described previously for SBMR, soil resources management on KTA is mandated by and detailedin the USAG-HI INRMP and ITAM Five-Year Management Plan for Oahu (USARHAW 2001a).Geologic Hazards and SeismicityKahuku Training Area. The high rainfall and runoff from the Koolau Mountains has created manydeep, nearly vertical gulches that are susceptible to rockslides, which are separated by long narrowridges that radiate from the Koolau Mountains toward the sea. KTA is dominated by slopes that aregreater than 30 percent, with the exception of interspersed, relatively flat coastal plains in the northernportion of the training area (USAG-HI 2004). Soils do not accumulate on the upper slopes; rather, therock itself becomes weakened by weathering and sloughs off, often unexpectedly with no identifiabletrigger aside from weathering. Earthquakes or vibrations from sonic booms may also trigger theseslides (Jibson and Baum 1999). The rock rubble from these failures accumulates on the floors of thegulches and is ultimately carried downstream by runoff.The probability of earthquakes in KTA is similar to that of the other installations on Oahu becausemost earthquakes are centered in the active volcanic areas beneath the Island of Hawaii. The intensityof ground shaking, which is influenced by the underlying geologic materials, would be lowest inrocky upland areas and would probably increase somewhat on the lower slopes, where the alluvialdeposits are thickest.Kawailoa Training Area. The route of Drum Road is mainly along ridges within KTA and alternatesbetween ridges and gulches along the western boundary of the KLOA. The potential for slope failureis probably high on slopes underlain by saprolite (deeply weathered basalt that retains the appearanceof the original rock but that does not have the strength of the rock). The saprolite forms steep slopesin stream gulches, but the slopes may be weakened if undercut at the base or if overloaded on top.3.1.1.4 Pohakuloa Training AreaPhysiographyPTA is in the Humuula Saddle between the two major peaks on the Island of Hawaii; Mauna Kea liesto the northeast, and Mauna Loa lies to the south. Elevations within PTA range from 4,030 to8,650 feet amsl. The slope of the Mauna Kea volcano rises steeply (about 26 percent) from SaddleRoad to an elevation of 13,796 feet amsl over a distance of about 6 miles. The slope of Mauna Loa,by contrast, rises to the southwest at about a 4 percent slope to an elevation of 13,678 feet amsl over adistance of about 20 miles. To the west of these two peaks is the Hualalai volcano (about 8,690 feetamsl).The PTA trail from Kawaihae Harbor to PTA runs south inland of Highway 270 and rises to an elevationof about 250 feet amsl near the junction with Highway 19. The trail continues east, parallelingHighway 19 along the foot of the Kohala Mountains to the western edge of Waimea (Kamuela). Thissegment rises to an elevation of about 2,500 feet amsl over a distance of about 10 miles. Near Waiaka,on the outskirts of Waimea, the trail turns south and runs west of Highway 190, approximatelyfollowing the 2,400-foot amsl elevation contour to the junction with Saddle Road (Route 200). TheFebruary 2008 3–13 2/25th SBCT Final EIS