<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.
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