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<br />(base flood) and states that the discharge values computed by the rainfall,runoff models are more accurate.
<br />However, the report does not provide any documentation to explain why the later discharge values are more
<br />accurate.
<br />
<br />We have reviewed the data submitted and the hydrologic analysis used to prepare the effective FIRMs and FIS
<br />reports for Adams and Arapahoe Counties and the City of Aurora, Colorado. We have the following
<br />comments regarding Mr. Najjar's request.
<br />
<br />Rainfall,runoff models are often used to develop flood,frequency estimates for the purposes of the National
<br />Flood Insurance Program (NFIP). In developing a rainfall,runoff model, decisions must be made regarding
<br />the appropriate design storm and the appropriate methods for converting rainfall to runoff. The uncertainty
<br />associated with these decisions can result in considerable variation in flood,frequenc~estimates. The
<br />reasonableness of the results of a rainfall,runoff model can be checked. by comparing the discharges computed
<br />by the model to those determined using regional regression equations or analyses of data from nearby stream
<br />gages.
<br />
<br />We reviewed the results of the rainfall,runoff models prepared by CH2M Hill for reasonableness. Because
<br />no stream gage data are available within the Box Elder Creek watershed, we used the regional regression
<br />equation published in Techniques For Estimating Regional Flood Characteristics of Small Rural Watersheds
<br />in the Plains Region of Eastern Colorado, Water Resources Investigations Report 87-4094 (WRJR 87-4094),
<br />U,S. Geological Survey (USGS), 1987, to assess the reasonableness of the submitted discharge values along
<br />Coyote Run. The discharge values computed by the rainfall,runoff model along Coyote Run appear reasonable
<br />when compared to those determined by the regional regression equation in WRlR 87-4094.
<br />
<br />We assessed the reasonableness of the results of the rainfall,runoff model for Box Elder Creek by comparing
<br />the submitted discharge values to those used to develop the effective FIRMs and those determined by USGS
<br />regional regression equations. As shown in the table above, the discharge values along Box Elder Creek
<br />computed by the rainfall,runoff model are much lower than those determined by the regional regression
<br />equations and those used to develop the effective FIRMs. The discharges used to develop the effective FIRMs
<br />Were estimated using the procedures outlined in Water Supply Paper 1680, entitled Magnitude and Frequency
<br />of Floods in the United States,Part 6B, Lower Mississippi River Basin Below Sioux City, Iowa, USGS, 1968,
<br />for flood,frequency region B and hydrologic area 10. Based on these procedures for a drainage basin of
<br />200 square miles along Box Elder Creek, the computed base flood discharge is 26,300 cfs. The Box Elder
<br />Creek watershed is lOcated within the hydrologic region referred to as the Plains Region in the floodflow
<br />frequency report, entitled Manual For Estimating Flood Characteristics of Natural,Flow Streams In Colorado,
<br />Technical Manual No. I, published by the Colorado Water Conservation Board in cooperation with the USGS,
<br />1976. The floodflow frequency report contains regional regression equations for predicting flood magnitudes
<br />at different recurrence intervals. Using the applicable equation for a drainage basin of 202 square miles along
<br />Box Elder Creek, the computed base flood discharge is 27,700 cfs.
<br />
<br />It is not clear why the submitted discharge values along Box Elder Creek are significantly lower than those
<br />used to develop the effective FIRMs and those determined by the regional regression equation. However,
<br />the Urban Storm Drainage Criteria Manual cautions that the peak of the runoff hydrographs computed by the
<br />CUHP model will tend to decrease if the basin length,to,width ratio is greater than 4. In this case,
<br />approximately 50 percent of the subwatersheds in the CUHP model for Watershed 3 along Box Elder Creek
<br />have a basin length,ro-width ratio gfeater than 4. Typically, for basins as large as Box Elder Creek, a 24, or
<br />48,hour storm duration will produce the largest discharge values for a given return interval, rather than the
<br />6,hour storm duration used in the submitted rainfall,runoff model.
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