SBCT Final EIS - Govsupport.us

Chapter 3 ⎯ Affected Environmenton the alluvial plains north of the foothills. South of the DTA, the Alaska Range is characterized byrugged, snowcapped peaks that rise to 10,000 feet amsl, with glaciers up to 5 miles wide and 40 mileslong that flow north from the mountains. The glaciers feed sediment-laden rivers, which create broad,braided stream valleys and alluvial fans covered by thick layers of sediments. The glaciers slope northat a gradient of 20 to 50 feet per mile and eventually flow to the Tanana River. The braided streamsare spaced 5 to 20 miles apart and fan across the plains, while the outwash areas contain porousgravel beds that facilitate surface water infiltration to groundwater aquifers.The terrain of the DTA encompasses a diverse geomorphic landscape, topography, and sediment parentmaterial, but primarily is generally flat to gently sloping, ranging from 1,200 to 1,600 feet aboveamsl. The southwestern portion of DTA West encompasses a small portion of the Alaska Range foothills,which range in elevation from 4,000 to 6,200 feet amsl and contain some valley glaciers that extendonto the installation. The southern half of DTA West is composed of largely unglaciated, flattopped,eastern trending foothill ridges ranging from 2,000 to 4,500 feet amsl and 3 to 7 miles wide.Rolling hills separate the foothills from the alluvial plains and range from 2 to 10 miles wide and 700to 1,500 feet amsl. There are no glaciers on the rolling hills or alluvial plains; however, the wide valleyswere formed by historical glacial advances.SoilsGlacial and alluvial processes, as well as isolated discontinuous patches of permafrost, primarilyformed soils in the DTA. The NRCS has only mapped soils in the SBMR cantonment area, in which12 soil associations have been identified. The USGS maps produced for the 2004 USARAK TransformationFEIS maneuverability model and subsequent USAGAK 2006 BAX and CACTF FEISevaluation show the distribution of a wide variety of soil engineering types on DTA because of thediverse geomorphic landscape (USARAK 2004 and 2006). Glacial moraine areas were classified asgravelly sand with well-drained, well-graded, gravelly sand outwashes. Lowland and floodplain areaswere classified as organic silts with varying wetness. Soil associations in the northern, west-central,and eastern portions of the DTA have been identified as silt-loam, while soils in the DTA East havebeen identified as shallow silt-loam over gravelly sand. Soils on river floodplains in the DTA comprisealternate layers of sand, silt-loam, and gravelly sand. Soils in muskeg areas have a high watertable and are high in moisture and organic material. Soils on the upland foothills are moist and loamycompared to the mountain soils, which are rocky, steep, and unvegetated. Floodplain soils are knownto have moderate erosion potential, while foothill soils have moderate to high erosion potential.Permafrost is found in irregular patches throughout a large portion of the DTA, particularly in morainalareas where slope and aspect change abruptly (Jorgenson et al. 2001). Predicting permafrost inthe DTA is difficult due to heterogeneous soil types, topography, and microclimate variability. Areascontaining existing and abandoned river channels, lakes, wetlands, and other low-lying areas tend tobe free of permafrost. Known isolated patches of permafrost are found from 2 to 40 feet belowground surface (bgs), with thicknesses varying from 10 to 118 feet, underlying sandy gravel in the alluvialplains. Permafrost controls groundwater movement in these areas. Permafrost degradation, evidencedby thaw ponds, is known to occur only in a small portion of the DTA. However, other areascharacterized by loess or other silty sediments may also be vulnerable to permafrost degradation. Disturbanceof the ground surface or continued climatic warming is likely to increase the amount ofthermokarst in the DTA.The 2006 BAX and CACTF FEIS evaluated permafrost by sub-areas identified in the eastern portionof DTA, including the North Texas Range, the Donnelly Drop Zone, and the Eddy Drop Zone, withBAX and CACTF areas within each sub-area (USARAK 2006). Detailed geotechnical explorationprograms conducted in 2002 for the Eddy Drop Zone Area (R&M Consultants 2002, 2004 andFebruary 2008 3–98 2/25th SBCT Final EIS