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a capacity, in addition to normal irrigation flow, is provided to intercept runoff from approximately 3.4 square miles of upstream unreservoired drainage area. This additional intercepted flow ;c r";:lrripn wpc:t- hy r...rHTPrnmpnr Highlinp C-;:m<=il .::r.nn ~nil1pCl into IWJ..i.dJl WCl~lJ. 'I'llu;:, the (Ahal provides protection for the irrigated lands immediately below the canal. Of the total watershed area above Government Highline Canal, approximately 9.6 square miles (90 percent) are affected by the IW-l structure and canal. The Indian Wash channel has recently been channelized by the city, from just below North Avenue to Texas Avenue. This project, which also included the replacement of several stream crossings, increased the capacity of the Indian Wash channel. The design discharge for the project was 1000 cfs. Figure 6 shows a typical section of the project. 3.0 ENGINEERING METHODS For the flooding sources studied in detail in the community, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this study. Flood events of a magnitude which are expected to be equalled or exceeded once on the average during any 10-, SO-, 100-, or SOO-year period (recurrence interval) have been selected as having special significance for flood plain management and for flood insurance premium rates. These events, commonly termed the 10-, 50-, 100-, and SOO-year floods, have a 10, 2, I, and 0.2 percent chance, respectively, of being equalled or eX08.eded during any year. Although the recurrence interval represents the long term average period between floods of a specific magnitude, rare floods could occur at short intervals or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood which equals or exceeds the 100-year flood (1 percent chance of annual occurrence) in any SO-year period is approxi- mately 40 percent (4, in 10), and, for any 90-year period, the risk in- creases to approximately 60 percent (6 in 10). The analyses reported here reflect flooding potentials based on conditions existing in the COImlIunity atuthe..-timeof completi.enof this study. ' ,Maps-and flood eleva- tions will be amended periodically to reflect future changes. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak discharge- frequency relationships for floods of the selected recurrence intervals for each flooding source studied in detail affecting the community. For Colorado River, peak discharges for floods of the 10-, SO-, 100-, and SOO-year recurrence intervals were computed by the U.S. Army Corps of Engineers for the Mesa County, Colorado, Flood Insurance Study (Reference 1). For the U.S. Army Corps of Engineers analysis 13