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<br />, ~ <br />" <br />I <br />, <br /> <br />i', <br /> <br />Box Elder Creek Hydrology <br /> <br />, <br />.-! <br /> <br />Reservoir is 15 miles downstream from the downstream end of the current study where <br />the existing watershed conditions 100-year discharge is estimated at 9090 cfs. <br /> <br />, <br />1<:.' <br />i< <br />1 <br />I:' <br />i <br />'i <br />" <br /> <br />~. <br />I, <br />~,~ <br /> <br />The CaE conducted a detailed rainfall-runoff modeling study of the Box Elder Creek <br />watershed in 1989, using a modified version of the Stormwater Management Model. The <br />results at 1-70 are shown in the above table. A comparison of existing conditions <br />discharge profiles along the Box Elder Creek channel is shown in Figure 2. <br /> <br />c <br />~,f <br /> <br />Bob Jarrett, hydrologist with the regional office of the U. S. Geological Survey, has <br />conducted paleohydrology studies of several streams in eastern Colorado, Attachment 3. <br />His investigations, while not able to pinpoint discharge-frequency relationships, do <br />provide discharges within a range of probable frequencies. Figure 9 from Attachment 3 <br />shows that at 1-70, the District discharge profile, the CaE's profile lmd the <br />paleohydrology discharges all compare favorably for existing conditions. Only the <br />revised FEMA (labeled "Thomas") regression equation does not compare favorably with <br />the other discharges. <br /> <br />I <br />If <br />\I' <br />-:~:. <br /> <br />, <br />~ <br />~~ <br />Ii< <br />.It <br />, <br />f. <br />t, <br />iii; <br /> <br />State of Colorado acceptance of Box Elder Creek hydrology methods and results. <br />The CWCB has reviewed the Box Elder Creek hydrologic analysis, and has designated <br />and approved the future conditions 100-year floodplains which resl.:lted from the <br />completion of the Box Elder Creek study. A letter from the CWCB to the District dated <br />March 7,1997, is included as Attachment 5. <br /> <br />r <br />} <br />~ <br /> <br />Conclusions <br /> <br />This report has demonstrated that all parties to the District's Box EI der Creek watershed <br />hydrologic analysis agree that the published regional regression equations yield results <br />that are unrealistically high when applied to the Box Elder Creek watershed. In such <br />instances FEMA 37 allows for the utilization of rainfall-runoff models. The District <br />utilized its rainfall-runoff model, CUHP, and its routing model, UDSWM, to develop <br />hydrology for the Box Elder Creek watershed. The results of that hydrologic analysis, for <br /> <br />CTC8.doc <br /> <br />8 <br />