SBCT Final EIS - Govsupport.us

Chapter 3 ⎯ Affected Environmentthe state that have not been extensively mapped or studied; therefore, predicting the timing or locationof potentially dangerous earthquakes is not possible (CGS 1999).The PCMS is located within the low-risk Seismic Zone 1 (DECAM 2002a). Several seismic faults arelocated within the vicinity of the PCMS, although none cross through the installation (USGS 2005,Widmann et al. 2002). Small faults potentially associated with the Apishapa Uplift are found in thenorthern edge of the PCMS (DECAM 2002a). As described for Fort Carson, small earthquakes areknown to occur in the southeastern portion of Colorado, with generally undetectable effects (DECAM2002a). Since 1973, most earthquakes within 60 miles of the PCMS registered a magnitude of lessthan 4.0. The largest earthquake in the area recorded a magnitude of 5.0 approximately 50 miles fromthe center of the PCMS (CGS 1997). There is low potential for significant seismic activity near thePCMS.A major landslide occurs every 20 to 40 years at the PCMS, affecting soils with slopes that are greaterthan 30 percent. Landslides tend to occur at the PCMS from approximately the middle of the westernboundary, southwest to Dillingham Ridge (USACE 2007a).3.3.2 WATER RESOURCESThe ROI for surface water resources includes portions of watersheds containing FTC and PCMS.More specifically, the ROI includes the areas from the upstream military boundary to the downstreamwatershed boundary of Fountain Creek, Upper Arkansas, and Purgatoire watersheds. The ROI for surfacewater is not necessarily the same as the ROI for groundwater. Because groundwater often crossestopographic (watershed) boundaries, the ROI for surface water is expanded to include the aquifersunderlying these watersheds and any aquifers downgradient (in the direction of groundwater flow)from these areas.3.3.2.1 Fort CarsonSurface WaterFTC is located on the eastern side of the Rocky Mountains adjacent to and south of Colorado Springs.The northern and eastern portions of the installation are located within the Fountain Creek watershedof the Arkansas River Basin and drain eastward into Fountain Creek. The southern and western portionsof the installation are located in the Upper Arkansas watershed of the Arkansas River Basin anddrain into the Arkansas River to the south (USACE 2005).Annual precipitation on the installation ranges from approximately 12 inches in the south to 15 inchesin the north. Approximately 80 percent of precipitation occurs between early April and late September.Short, intense thunderstorms, typically 2 to 6 hours in duration, are the principal source of floodingon post. Snowfall at FTC averages 42.4 inches annually (DECAM 2002a), and spring snowmeltrunoff is generally not a source of flooding.All of the streams entering and originating on FTC are intermittent. Most of the stream flow consistsof runoff from precipitation, although groundwater seepage to streams occurs in some areas. No flowoccurs in most reaches of the streams for long periods during the year, with most stream flow occurringbetween April and June (USACE 2005). Streams generally flow from the northwest to the southeast.Stormwater in the cantonment area flows into one of three main ditches. These include “B” Ditch, “I”Ditch (Clover Ditch), and the unnamed “U” Ditch. All three ditches are tributaries to Fountain Creek.A FEMA-regulated 100-year floodplain is associated with these three ditches (USACE 2000). FlashFebruary 2008 3–143 2/25th SBCT Final EIS