SBCT Final EIS - Govsupport.us

Chapter 3 ⎯ Affected EnvironmentThe intensity and magnitude of an earthquake are other measures of earthquake severity. Intensity isestimated at points where the shaking is felt, while magnitude is measured at the source of the earthquake.Ground accelerations of more than 12 percent of gravity roughly correlate with earthquakes ofmagnitude 5.0 to 5.9 on the Richter Scale and intensity of VI to VII on the Modified Mercalli Scale,which can cause negligible to slight damage in specially designed structures, slight to considerabledamage in well-built ordinary structures, and considerable damage to poorly built structures. Theyalso and can potentially cause slope failure or landslides.Wheeler Army Airfield. Geologic hazards at WAAF are similar to those described above for SBMP,and the potential for earthquakes and ground motion is the same. The steep slopes of Waikele Gulchare underlain by erodible soils and soft saprolite deposits, which are vulnerable to slope failure.Schofield Barracks East Range. SBER contains many areas of steep slopes and deeply weatheredrock, particularly in the eastern portion of the range, on the leeward slope of the Koolau Range(USAG-HI 2004). Erosion tends to prevent the accumulation of alluvium and colluvium, but slopefailure remains a potential hazard in many areas. Similar to SBMP, there is little risk of strong groundshaking in areas underlain by firm rock in SBER. However, site-specific conditions, such as thethickness of loose geologic deposits and the depth of the water table, may intensify ground shaking.Earthquakes may also trigger landslides in areas of unstable slopes, although the risk of seismicallyinduced slope failure and landslides is significantly lower than slope failure and landslides resultingfrom water and erosion.South Range Acquisition Area. The SRAA is dissected by the channels of Waikele Stream and itstributaries. The streams have incised steep-sided gullies, 80 to 120 feet deep, into the relatively gentlynortheast-sloping surface of the plateau. The floors of the gullies are relatively wide and flat, and theWaikele Stream meanders within this incised channel. The slopes of the plateau surface are stable,while the walls of the gullies are subject to collapse due to erosion at the base of the slopes from migrationof the streams within their channels. This situation is similar to what occurs in stream channelson SBMP. Seismic hazards are the same as those described above for SBMP.3.1.1.2 Dillingham Military ReservationPhysiographyDMR is on Oahu’s Waialua Plain and extends inland to the foot of the Waianae Range. Elevationranges from near sea level on the northern boundary to 200 feet amsl near the southern boundary. Theproposed Dillingham Trail would connect DMR and SBMR. From DMR to Ranch Camp at Waialua,the proposed trail would be on the coastal plain inland of the Farrington Highway. The proposed routewould cross several small streams, the largest of which is Makaleha Stream, near Dillingham Ranch.At Ranch Camp, the trail would head south up to an elevation of about 250 feet amsl, where it wouldcross a tributary of Kaukonahua Stream. Here it would head east, below the Ito Ditch, which runs approximatelyalong the contour of the hillslope for about 0.25 mile. Then the trail would turn upslopeand follow a ridge up to an elevation of about 1,800 feet amsl. The proposed trail would turn eastagain and descend gradually along the contour of the mountain to an elevation of about 1,500 feetamsl, where it would round the shoulder of a prominence called Maili. The trail would continue southalong the contour of the mountain at an elevation of about 1,200 feet amsl and then descend steeply toSBMR, crossing both Haleanau Gulch and Mohiakea Gulch.GeologyDMR is on the north slope of the Waianae Range and is underlain by chronologically alternating basalticflows and eroded volcanic rocks of the Waianae and Koolau volcanoes (USAG-HI 2004). Ex-February 2008 3–9 2/25th SBCT Final EIS