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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />~1~~~06~J <br /> <br />2480 West 26th Avenue. Suite 340.8 . Denver, Colorado 80211 . (303) 455-3600 . FAX (303) 45549929 <br /> <br />March 19, 1997 <br /> <br />Mr. Bill DeGroot <br />Chief, Floodplain Management <br />Urban Drainage and Flood Control District <br />2480 West 26th Avenue, Suite 156-B <br />Denver, CO 80211 <br /> <br />RE: Initial Reconnaissance of Drainage Network and South Boulder Creek's <br />Hydrology, Highway 36 to Approximately the Wellman Ditch, Phase II. <br /> <br />Dear Mr. DeGroot: <br /> <br />Our Phase I effort identified that the Flood Hazard Area Delineation Report <br />(FHAD) underestimated the flow leaving South Boulder Creek at Highway 36 and <br />did not portray spillage paths to the west side of the valley, particularly along <br />Foothills Parkway. This Phase II report provides an initial reconnaissance of the <br />overflow routes and approximate flows of the West Valley Overflow north of <br />Highway 36 to the area of the Wellman Ditch. Drawing No, 3 depicts the results <br />of this overflow analysis (Drawings 1 and 2 are with the Phase I report). <br /> <br />We obtained and reviewed additional hydrology information by the U,S. Army <br />Corps of Engineers (Corps). In Phase I we modified the Corps' Storm Water <br />Management Model (SWMM) model to better reflect the Viele watershed and its <br />outfall channel to South Boulder Creek. In this Phase II we have greatly refined <br />the SWMM model detail downstream of Highway 36 in order to identify <br />significant flow paths and approximate discharges. <br /> <br />The 100 year spill from South Boulder Creek just upstream of Highway 36 would <br />flow to the northwest along the south side of the freeway, with an initial portion <br />returning to South Boulder Creek through the Viele outfall, spillage from the Dry <br />Creek Ditch No.2, and the Anderson Ditch. The majority of the spill would <br />overtop Highway 36 west of Foothills and flow northerly to the intersection of <br />Baseline and Foothills, The intersection topography would largely control the <br />hydraulics of the spill distribution and would result in six surface flow paths and a <br />seventh path through the culverts under Foothills between the small detention <br />ponds that are south of Baseline on either side of Foothills. Three of the surface <br />paths would follow Foothills to the north (two travel lanes and the westerly <br />borrow ditch). Two would follow Baseline to the east, and one would overtop <br />Foothills slightly south of the intersection. The last three routes would <br />commingle and split again, following the streets to the north and the Crossover <br /> <br />Hydrology. Hydraulics. Sediment Transport . <br />Civil Engineering for Flood Control, Waterways, Water Supply and Parks. <br />Environmental Mitigation. Forensic Services. <br />