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<br />;. <br /> <br />\ <br /> <br />ce <br /> <br />Ie <br /> <br />2 <br /> <br />II. HYDROLOGIC ANALYSIS <br />A hydrologic analysis was conducted to determine runoff hydrographs from <br />one-hour storm events of 2-, 10-, 25-, 50-, 100- and SOo-year return periods <br />for Boxelder Creek and Cooper Slough. The .Storm Water Management Model. <br />(SWMM) (reference 3) as developed by the Environmental Protection Agency (EPA) <br />and later modified by the Missouri River Division (MRD) of the u.s. Army Corps <br />~-'" <br />of Engineers was used to estimate the peak flows and runoff volumes. A storm <br />event of a given return period was assumed to generate a flood hydro graph and <br />peak of the same return period. The parameters used in the analysis were con- <br />sistent with previous studies in the area, such as the "Fossil Creek <br />Hydrology, Larimer County, Colorado. (Fossil Creek) and "Major Drainageway <br />Planning, Dry Creek, Fort Collins, Larimer County, Colorado" (Dry Creek) stu- <br />di~s (reference 4 and S). <br />The entire drainage basins for Boxelder Creek and Cooper Slough are shown <br />in Figure 1. The shaded area represents nontributary area. The total <br />Boxelder Creek watershed covers 2S1 squre miles of which 60.3 is tributary <br />area. The Cooper Slough basin has a trl-butary area of 10.2 square miles. <br />Figure 2 shows the study area which consists of the lower 6.4 miles of <br />Boxelder Creek and approximately two miles of Cooper Slough. The drainage <br />basins were subdivided into subcatchments and channel units. The Boxelder <br />drainage was divided into 99 subcatchments and 64 channel units. The Cooper <br />Slough basin was divided into 40 subcatchments and 32 channels. Figure 3 <br />shows the subcatchment and channel locations. Subcatchment delineations were <br />based on the assumption that all irrigation ditches are full during flood <br /> <br />events. <br /> <br />2.1 Rainfall Data <br />Rainfall depths for the 1-hour, 2-, 10-, 25-, 50-, and 100-year <br />recurrence interval events were obtained from the .Larimer County Storm-Water <br />Drainage Manual" (reference 6) which is based on data and procedures described <br />in the Precipitation Frequency Atlas of the Western United States, Atlas 2, <br />Volume III, Colorado (reference 7). The SOO-year rainfall value was estimated <br />by extrapolation of the 1-hour values of the other recurrence intervals on <br />Extreme Value paper. The distribution of the rainfall volume was based on the <br />distribution used in the previously mentioned studies in the area and is shown <br />