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<br /> <br />3.0 HYDROLOGIC ANALYSIS <br /> <br />This study includes a detailed hydrologic analysis of Indian <br />Wash within the corporate limits of the City of Grand <br />Junction. Peak discharges for floods of the 10-, SO-, 100-, <br />and SOO-year recurrence intervals were computed for use in the <br />Grand Junction Flood Insurance Study. <br /> <br />3.1 General Analysis <br /> <br />Historically, the principal cause of flooding on Indian <br />Wash has been intense cloudburst storms. Snowmelt <br />flooding is relatively insignificant due to the low <br />altitude. Since no stream gauge date is available for <br />Indian Wash and little is available for the general area, <br />. and due to the presence of the flood retention structure, <br />a rainfall-runoff analysis was conducted on the watershed <br />to determine the flood discharges. This was accomplished <br />using the U. S. Army Corps of Engineers HEC-l rainfall- <br />runoff computer program (Reference 4). For this <br />analysis, basin characteristics which define the size, <br />shape and runoff characteristics of the watershed as well <br />as rainfall amounts based on the selected' recurrence <br />intervals are used to compute flood hydrographs for <br />various design points in the basin. All stream routings <br />were accomplished using the Tatum routing method. For <br />the reservoir routing through the SCS IW-l structure, <br />rating curves obtained from the SCS (Reference 1) were <br />used in conjunction with the Modified PuIs method. <br /> <br />A more detailed description of the input variables for <br />the HEC-l rainfall-runoff analysis are listed in the <br />following paragraphs. The final HEC-l computer output is <br />listed in the appendix of this report. <br /> <br />Sub-basin Locations <br /> <br />3.2 <br /> <br />. <br /> <br />The entire Indian Wash watershed was divided into 12 sub- <br />basins, representing key points where flood hydrographs <br />were desired. Table 1 lists a brief description of the <br />downstream points on the sub-basins, Figure 6 shows a <br />schematic drawing of the stream network, and Figure 7 <br />shows the locations of the sub-basins. It should be <br />noted that for the sub-basin which lies upstream of the <br />llighline Canal and to the east of Indian Wash, and for <br />the sub-basin drained by the 29~ Road Channel, each area <br />was treated as one basin. This was done because runoff <br />hydrographs were desired at the downstream limits of the <br />areas only, and the lack of data on basin runoff <br />characteristics made it impractical to further subdivide <br />the areas. <br /> <br />. <br /> <br />. <br /> <br />10 <br /> <br /> <br />\ <br />