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<br />Box Elder Creek Hydrology <br /> <br />," <br />,., <br /> <br />; .~ <br />Ii <br /> <br />Representative to the Technical Mapping Advisory Council, has also expressed his belief <br />that regression equations are not appropriate for the Box Elder Creek watershed, at least <br />the reach under consideration here, as indicated in Attachment No.2. Finally, Bob <br />Jarrett, USGS hydrologist, has also indicated the difficulty in utilizing regional regression <br />equations in eastern Colorado in Attachment No.3, "Regional Paleoflood Investigation to <br />Assess Topographic Effects of the Palmer Divide on Flooding in Eastern Colorado," <br />(underlined passages on pages I, 3 and 10). <br /> <br />~, <br />~ <br />~ <br />I <br />l <br />~ <br />J, <br /> <br />j <br /> <br />Rainfall- Runoff Modeling <br />FEMA 37 states that "Where USGS regional flood flow frequency reports have not been <br />developed or are not applicable due to flow regulation, storage, rapid watershed <br />development or other unique basin characteristics the SC with Regional PO approval <br />may select to develop a rainfall-runoff model such as HEC-l or TR-20..." For this <br />watershed, the District used its own rainfall-runoff model, the Colorado Urban <br />Hydrograph Procedure (CUHP), the development and calibration of which is described <br />later in this report. <br /> <br />,;>-,.' <br /> <br />r"1 <br />)(, <br />~ <br />I> <br /> <br />~ <br /> <br />'" <br /> <br />FEMA 37 lists several factors to be considered in developing a rainfall-runoff model. <br />Each factor is listed below, followed by the District's justification as to why the CUHP <br />model meets the FEMA 37 factors. <br /> <br />CTC8.doc <br /> <br />5 <br /> <br /> <br />~' <br /> <br />"The unit hydrograph method is preferred when developing hydrographs. However, sub- <br />basin drainage areas should be provided to limit the area included in the unit <br />hydrograph and to better identify watershed response to changing conditions [Emphasis <br />added]." The CUHP is based on the Snyder unit hydro graph method, with modifications <br />based on data collected for watersheds between 0.15 and 3.08 square miles in the <br />urbanized area of Denver. Additionally, the Box Elder Creek watershed was divided into <br />633 sub-basin drainage areas. Both existing and projected future watershed conditions <br />were utilized in the master planning portion of the study specifically to determine the <br />impacts of changing conditions on discharges and to evaluate alternative means of <br /> <br />j'-t <br />,.1 <br />'.'i <br />!, <br />