SBCT Final EIS - Govsupport.us

Chapter 3 ⎯ Affected Environment3.2 ALTERNATIVE B — PERMANENTLY STATION THE 2/25TH SBCTAT FORT RICHARDSON WHILE CONDUCTING REQUIREDTRAINING AT MILITARY TRAINING SITES IN ALASKA3.2.1 GEOLOGY, SOILS, AND SEISMICITYThe geologic, soil, and seismic conditions of the Fort Richardson area (FRA) and DTA are detailed inthe Transformation of USARAK FEIS (USARAK 2004), the 2007 INRMP (USARAK 2007), and theConstruction and Operation of BAX and a CACTF within U.S. Army Training Lands in Alaska FinalEIS (USARAK 2006), and are summarized below as they relate to existing conditions used to assesspotential effects of activities associated with implementation of Alternative B.Figure 3–2 shows the extents of the ROI for geology, soils, and seismicity in Alaska.In Alaska, climate is the most important soil-forming factor. Permafrost is a condition in which soil,silt, and rock remain frozen year-round, and it is common throughout Alaska. Only a thin layer oftopsoil may thaw in the summer months, while the rest of the soil down to bedrock remains frozenuntil climatic conditions change or overlying insulating vegetative or organic material (peat) is disturbed.Permafrost typically exists in multiple layers, varying in thickness from less than 1 foot tomore than 150 feet (USARAK 2004). The distribution of vegetation and limitations of human activityin Alaska are significantly influenced by the presence of permafrost, which can influence soil processessuch as cryoturbation (contoured and broken horizons resulting from mixing of soil due tofreezing and thawing), rapid surface water runoff, restricted permeability, and ground subsidence.Thermokarst is a process resulting from irregular subsidence of permafrost that creates features suchas mounds, hummocks, water-filled depressions, flooded forests, and mudflows on steeper slopes(USARAK 2004).3.2.1.1 Fort RichardsonPhysiographyThe FRA lies in the Cook Inlet-Susitna Lowland and Kenai-Chugach Mountains physiographic provinceson an alluvial plain called the Anchorage Lowland (Wahrhaftig 1965). The Anchorage Lowlandis fed by the Chugach Mountains to the east and flows into to the Cook Inlet to the north, south, andwest. The topography of the Anchorage Lowland has been primarily influenced by glacial activityand alluvial deposition and erosion by the four major drainages that originate in the Chugach Mountains— Ship Creek, Eagle River, Campbell Creek, and Chester Creek. The Anchorage Lowland is atriangular area located between the Knik and Turnagain Arms below 500 feet amsl in elevation. It ischaracterized by rolling hills with topographic relief ranging from 50 to 250 feet (Hunter et al. 2000).To the east, rolling uplands extend to elevations up to 3,000 feet amsl at the base of the ChugachMountains (Hunter et al. 2000). A small portion of the western section of the Chugach Mountains,which rise abruptly to more than 5,000 feet amsl on the front, is contained within the FRA boundaries.The peaks of the Chugach Mountains are separated by northwest-trending, steep U-shaped valleys,which are occupied by the four major drainages mentioned above. The Anchorage Lowland ischaracterized by rolling hills with up to 250 feel of topographic relief in the eastern portion along theChugach Mountains. The terrain flattens to the west into an alluvial plain that is inundated withbroad, shallow streams and wetlands. FRA contains many landforms that are characteristic of glaciatedterrain, including moraines, esker deposits, outwash plains, and estuarine sediments (USARAK2004).February 2008 3–94 2/25th SBCT Final EIS