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<br />1- 4 <br /> <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 />PRIOR HYDROLOGY STUDIES <br /> <br />In 1977 the U.S. Army Corps of Engineers (Corps) conducted a hydrology study <br />that included the South Boulder Creek Basin. That study provided the hydrology for <br />the 1986 Urban Drainage and Flood Control District (UDFCD) Flood Hazard Area <br />Delineation (FHAD) by Greenhorne and O'Mara (South Boulder Creek Flood Hazard <br />Area Delineation). and the Federal Emergency Management Agency (FEMA) Flood <br />Insurance Study (FIS) and Flood Insurance Rate Maps (FIRMs) for South Boulder <br />Creek. <br /> <br />Although Gross Reservoir is not a flood control facility, it does affect flood runoff in <br />two ways. First, if the reservoir level is lower than the spillway, it can trap some <br />or all of the flood runoff. Second, even if the reservoir is full, dynamic flood <br />storage can occur above the spillway level, lowering the flood peak discharged from <br />Gross and delaying the flood discharge from the reservoir. <br /> <br />The 1973 R.W. Beck study determined that a release of 300 cfs would occur <br />during the 100-year flood based upon examination of Denver Water Board records <br />and policy. The G&O FHAD was based on Corps of Engineers modeling which <br />assumed that the reservoir was full and that the flood from the 100-year rainfall <br />above the reservoir would be attenuated by the storage volume above the spillway. <br />The peak flow from the reservoir was modeled as 3900 cfs, and was several hours <br />behind the downstream subbasin time of peak flow. The 1973 study is probably <br />more realistic. The 1986 study is conservative, because it follows conventional <br />UDFCD, CWCB, and FEMA practice to assume no storage below the spillway crest <br />where there is no institutional agreement in place to provide and operate a flood <br />storage function. <br /> <br />In 1996, Love and Associates (Flood Plain Analvses of South Boulder Creek at the <br />Flatiron Prooertv) identified that South Boulder Creek could spill greater flows than <br />identified in the FHAD and that these spills could travel to the west side of the <br />valley and overtop Highway 36, largely west of Foothills, and thus posed a hazard <br />to the neighborhood north of Highway 36. <br /> <br />The TEA Phase I effort reviewed the FHAD and Love's investigation, and identified <br />1 OO-year spills on the order of 2700 cfs from South Boulder Creek above Highway <br />36. The spill was shown to commingle with local drainage from the Dry Creek <br />Ditch No.2 and the Viele watershed. The Phase I effort obtained a copy of the <br />Corps' hydrology model to identify the timing of the Viele inflows and allow <br />simplified simulation of its drainage network. Some general problems were noted in <br />the Corps' model, but the effort focused on the determination of how much of the <br />South Boulder Creek spill would return via Viele and how much would continue to <br />the West Valley Overflow route. Phase I analyses indicated that about 1800 cfs <br />would continue to the West Valley Overflow north of Highway 36. <br />