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<br />plain boundaries were plotted using flood contour elevations and <br />stationing from the plotted flood profiles. This was done at <br />elevation intervals compatible with the map contour intervals. <br />Flood contours are shown as wiggly lines at 2-ft intervals <br />perpendicular to the direction of flow. <br /> <br />U.S. Army Corps of Engineers computer model HEC-2 Water Surface <br />Profiles. The average watershed slope in the study reach is 4 <br />percent to 6 percent: therefore supercritical flow conditions <br />prevail through all study reaches. <br />Discharge values were discussed in the hydrology section of this <br />report. All of the cross section data were handpicked from <br />topographic maps with a scale of 1 inch. 200 ft and contour <br />interval of 2 feet. Dimensions of bridges and hydraulic roughness <br />coefficients (n-values) were determined from field investigations. <br />Water surface profiles, typical cross sections, and maps showing <br />the 100-year and SOO-yr flood boundaries are shown on included <br />exhibits and flood plain maps. Table 1 shows computed flood <br />elevations at specific cross sections. <br />Flood boundaries were located on the set of topographic maps, <br />previously referred to, by transferring flood elevations (at map <br />contour intervals) from plotted profiles (from HEC-2) to the maps <br />using stationing along the main channel as the location reference. <br />These points were connected and smoothed to create the map flood <br />boundaries. <br /> <br />B. BVdroloqy <br />The watersheds that produce runoff to channels in and around <br />Calhan are relatively small in size. They are mostly range land <br />in fair condition and consist of several old terraces. The <br />retarding effect of these terraces on the rainfall runoff was not <br />considered due to their poor condition. Ponding and storage <br />effects also were not considered for the small ponds in the <br />watersheds because those ponds were not built for flood control. <br />Snowmelt was not a contributing factor to runoff frequencies of <br />lO-year or greater, which are the primary concern of this study_ <br />This implies that significant runoff i9 coming from rainfall <br />events. As a consequence, only rainfall generated runoff was <br />considered in developing peak discharge-frequency values included <br />herein. <br /> <br />The NOAA Atlas for Colorado was used as the source for 24-hour <br />rainfall depths. Historical station precipitation data was <br />considered, but it was not adequate to be analyzed as a possible <br />alternative to using NOAA data. No aerial adjustments were made <br />because of the small drainage areas involved. The NRCS 24-hr Type <br />IIA rainfall distribution, instead of the Type II distribution, <br />was used because the Type IIA was more representative of the <br />rainfall pattern in the area. A shorter rainfall duration (2-hrl <br />and its associated rainfall depths also were used in t~e Natural <br />Resources Conservation Service's (NRCS) TR-2C hydrologic program <br />model. This 2-hr rainfall actually yielded less runoffs than the <br />standard SCS 24-hr Type IIA rainfall distribution. The NRCS Type <br />IIA rainfall distribution was selected for use in the TR-20 <br />computer model to generate peak frequency discharges. These <br />values are listed in Table 1. <br /> <br />!'LOOD PLAIN MANAGEKENT <br /> <br />Potential flood damages to existing development and possible loss <br />of life can be alleviated or lessened through non-structural and <br />structural flood hazard mitigation methods. <br />Non-structural methods include, Local flood plain regulations, <br />l~~d treatment. flood warning and forecasting systems, flood <br />insurance, flood proofing, flood fighting and emergency <br />evacuations. <br /> <br />Hydrologic runoff curve numbers (CN's) were developed from the <br />NRCS soil map, land use information, and field observations. CN <br />values for the TR-20 watersheds reflect weighted average CN's for <br />a variety of land uses and soils within each watershed. <br />~ime of concentration values were computed by the method described <br />~n the Natural Resources Conservation Service Technical Release 55 <br />(Urban Hydrology for Small Watersheds). Another method from the <br />NRCS Engineering Field Manual - Chapter 2. intended for small <br />rural ~ate~sheds, was also considered, but the TR-SS was favored <br />in est~mat~ng the Tc's. <br /> <br />A. Local Reaulations <br />The need to minimize property damage due to flooding has been <br />recognized by planners and local community officials. S~dividers <br />and developers are required to submit proposed storm dra~nage <br />plans to the planning commission for approval. In the past, <br />drainage plans have been prepared singularly or on a plat-by-plat <br />basis. Information contained in this report will be useful in <br />developing a master drainage plan for the study area. This report <br />provides the outline of flood hazard areas on large scale maps <br />,specifically for this purpose. <br />The City may provide zoning regulations" To establish, <br />regulate, restrict, and limit such uses on or along any storm or <br />floodwater runoff channel or bas~n, as such storm or floodwater <br />runoff channel or basin has been designated and approved by the <br />Colorado Water Conservacion Board, in order t.o lesl.I.,1I or ...void the <br />hazards to persons and damage to property resulting from the <br />accumulation of sto~ of floodwaters..." as stated in Section 3Q- <br /> <br />C. Evdr~:ulics <br />Hydraulic analyses cond~cted in chis study were done using the <br /> <br />, <br /> <br />7 <br />