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Flood Hazard Area Delineation For Lower Box Elder Creek Watershed Future percentage imperviousness is shown in Appendix E and was determined through examination of planning documents for Adams County (Balloffet & Associates, Inc. and Clarion Associates, 1999), DlA and the City of Aurora (City of Aurora, 1998) as well as consultation with the study sponsorsc The future percentage imperviousness is moderate to high in the southern portion of the study area that is close to DlA and Front Range Airport, specific parts of DlA property and the area surrounding Hayesmount Mile Road and 120'h Avenue in the northern portion of the basin. The anticipated future percentage imperviousness is lower in the area surrounding Box Elder Creek, specific portions of DlA property and much of the northern portion of the study areac 2.3 Soils The hydrologic soil grouping for the soils in the study area was determined using the Soil Survey for Adams County (DeS. Department of Agriculture, Soil Conservation Service, 1974). The infiltration characteristics of each soil group are based upon values provided in Table 2-2 of the runoff chapter of the UDFCD manual. The soils within the basin are predominantly Type B and Type C soils (moderate infiltration characteristics) with some Type A (high infiltration rates) around Box Elder Creek and other small isolated areas in the northern portion of the study area. There is little area containing type D soils (low infiltration rates). Soils mapping for hydrologic modeling is described in Section 3.4. The hydrologic soil groups in the study area are shown in Appendix C 2.4 Surface Water Resources 2.4.1 Stream Characteristics The major streams in the study area include the main stem of Box Elder Creek, Hayesmount Creek and Bear Gulchc Bear Gulch is a right bank tributary to Box Elder Creek, with its confluence approximately midway in the study area. It has a drainage area of approximately 20 square miles. Hayesmount Creek lies west of Box Elder Creek and historically was a left bank tributary to the creek with confluence downstream of the study area. During construction of DIA, the upper portion of Hayesmount Creek (a drainage area of approximately 12 square miles) was diverted from its historic flow path directly into Box Elder Creek approximately one-half mile downstream of the Bear Gulch confluencec The remainder of the Hayesmount Creek watershed is approximately 18 square miles in size. (See Figure I for a delineation of the major tributary watersheds.) Additionally, there are numerous small tributaries to the above-mentioned main creeks whose sub-watersheds are delineated in Appendix R The main stem of Box Elder Creek has a wide, shallow, well-defined channel that is, for the most part, relatively well vegetated. The overall slope of Box Elder Creek through the study area is approximately 0.4 percenL The creek meanders through the majority of its length. Hayesmount Creek is ill defined through much of its lengthc The portion of the creek through the middle of the study area has been changed through cultivation of farm fields such that there is no discernible creek channel. The lower portion of the creek is somewhat better definedc The tributaries to the lower portion of Hayesmount Creek are also ill defined. Bear Gulch has two distinct channel types: the creek in the upper portion of the watershed resembles a broad swale, and the lower portion of the watershed has a well-defined low flow channel with a broad overflow channel. 2.4.2 Reservoirs The following description of reservoirs within the study area is adapted from the Upper Box Elder Creek Outfall Systems Planning Study Hydrology Report (CH2M-HiII, 1994). The only reservoir within the study area is Bootleg Reservoir, a state jurisdictional dam owned by the Henrylyn Irrigation District, located on the main stem of Box Elder Creek approximately one mile south of the downstream end of the study areac This dam was breached in the early 1980s because of safety concerns by the Colorado State Engineer's Office (SEa). The U.S. Army Corps of Engineers (USACE) prepared a detailed hydrologic analysis of the Box Elder Creek watershed upstream of Bootleg Reservoir in 1989c This analysis was part of a study examining the feasibility of rehabilitating the reservoir. To date, Bootleg Reservoir has not been rehabilitated. There is a regional detention basin on DlA property that serves to attenuate storm flows from a portion of Lower Hayesmount Creek. This detention basin has a total capacity of 105 acre-feet and a six-foot- diameter reinforced concrete pipe outlet that is uncontrolled. The detention basin is normally dryc 991-134.000 Page 3 Wright Water Engineers, Inc.