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<br />Flood Hazard Area Delineation <br />For Lower Box Elder Creek Watershed <br /> <br />The length and slope of each element were estimated using USGS topographic mapping. Eight channel <br />geometry configurations (described in Appendix J) were used to model all the stream conveyance elements <br />in the study area. These typical channel size categories approximate the range of channel configurations <br />found in the study areac The channel configurations were developed using two-foot contour interval <br />mapping augmented with site observations. <br /> <br />Information regarding the size of pipe elements draining the concourse areas of DIA was not used for this <br />study. A five-foot-diameter pipe with a one-half percent grade was assumed for all pipe elements. A <br />typical overflow channel configuration for pipe elements was determined using ten-foot contour interval <br />mapping and field observations. A sensitivity analysis showed that changing pipe sizes between five feet <br />and eight feet in diameter and pipe slopes between 0.005 ftIft and 0.01 ftIft generally did not have a <br />significant impact on downstream flow ratesc <br /> <br />The roughness coefficient for each element was determined using an equation provided in the UDSWM95 <br />Users Manual. In this equation, the roughness coefficient is dependent on the friction slope and hydraulic <br />radius of each element. For the purposes of developing the Manning's "n," the friction slope was assumed <br />to be equal to the channel slope, and a flow depth of two feet was assumed for calculation of the hydraulic <br />radiusc <br /> <br />3.6 Results <br /> <br />The peak discharge for both existing and future conditions for the 10-,50-, 100- and 500-year storm events <br />is presented in Appendix L for each SWMM conveyance element. A summary of peak flow values at <br />selected locations is presented in Table 3-4 belowc The hydrographs associated with these locations are <br />presented in Appendix Me Graphs showing the peak flow rate versus stream station (longitudinal flow <br />profile) are presented in Appendix N. <br /> <br />3.7 Comparison With Previous Studies <br /> <br />The results of the hydrologic modeling carried out for the study area were compared to previous studies of <br />Box Elder Creek in order to assess the reasonableness of the results. Two comparisons were carried out: <br /> <br />1. <br /> <br />The peak flow rate for the 100-year storm event and existing percentage imperviousness condition <br />for selected design points with drainage area of less than 2,000 acres in the Box Elder and Bear <br />Gulch watersheds were compared with flow rates from the Upper Box Elder Creek Hydrology Study <br />(CH2M-Hill,1994). <br /> <br />2. <br /> <br />The 100-year peak flow for existing imperviousness conditions at Bootleg Reservoir was compared <br />with results of a study performed by the U.S. Army Corps of Engineers (USACE, 1989). <br /> <br />TABLE 3-4 <br />Summary of Peak Flow for Selected Locations <br /> <br />Location Peak Flow (ers)' <br /> 10-Yr SO-Yr 100- Yr SOO-Yr <br /> Existing Conditions <br />Bear Gulch at mouth 1,400 4,400 6,300 10,300 <br />Box Elder Creek <br />At upstream study area boundary' 1,246 5,741 9,140 15,618 <br />Upstream of Bear Gulch 1,000 5,600 8,900 15,300 <br />Between Bear Gulch and Upper Hayesmount 1,100 6,900 10,400 17,600 <br />Downstream of Upper Hayesmount 1,200 7,900 12,100 21,300 <br />At downstream study area boundary 1,100 7,400 11,500 20,600 <br />Lower Hayesmount at downstream study area boundary 530 1,600 2,400 3,900 <br />Upper Hayesmount at mouth 1,200 2,700 3,500 5,200 <br />Bear Gulch at mouth 4,100 7,900 10,200 15,200 <br /> Future Conditions <br />Box Elder Creek <br />At upstream study area boundary' 6,522 11,752 15,227 22,385 <br />Upstream of Bear Gulch 6,300 11,600 15,300 21,800 <br />Between Bear Gulch and Upper Hayesmount 8,200 16,100 21,600 29,900 <br />Downstream of Upper Hayesmount 9,400 18,800 25,200 35,000 <br />At downstream study area boundary 8,800 18,000 23,500 32,400 <br />Lower Hayesmount at downstream study area boundary 1,400 2,700 3,600 5,700 <br />Upper Hayesmount at mouth 1,900 3,600 4,600 6,600 <br /> <br />I From Appendix M of Upper Box Etder Creek Outfall Systems Planning Study Hydrology Report (CH2M-Hill, 1994)c <br /> <br />, Values between II and 1,000 are rounded to the nearest JOe Values over 1,000 are rounded to the nearest 100. The values <br />quoted from the CH2M-Hill study are not roundedc <br /> <br />991-134cOOO <br /> <br />Page 7 <br /> <br />Wright Water Engineers, Inc. <br />