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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Specific flooding problems along Tributary 4 are described following: <br /> <br />Floodplain and Floodway Data <br /> <br />1. All four of the road crossings in this study reach are <br />overtopped. These include Quebec Street (2 feet), East <br />144th Avenue (1.5 feet), the private road (2.5 feet), <br />and Yosemite Street (3 feet). Overtopping results from <br />inadequate cul vert capacity, or in the case of East <br />144th Avenue and Yosemite Streets, absence of culverts. <br />Debris blockage of the existing culverts is probably of <br />greater concern for smaller events because the major <br />event runoff greatly exceeds the cul vert capaciti es. <br />All stream crossings are gravel or dirt roads, and high <br />erosion and road washout potential can be expected. <br />Refer to Sheets 18 through 22. <br /> <br />The Flood Hazard Area Delineation plan and profile drawings show the <br />outline of the 100-year flood event for present channel and floodplain <br />conditions. The drawings are supplemented by Table V, "Floodplain and <br />Fl oodway Reference Data. n <br /> <br />The table contai ns a 1 i sti ng of descri pti ve i nformat i on, 100-year <br />flood discharges, stream stationing, thalweg elevations, 100-year <br />flood elevations, 100-year floodplain widths, and average channel <br />velocities at cross sections used in the study. <br /> <br />2. Two old reservoir embankments exist about one half mile <br />downstream at Quebec Street. The upstream embankment <br />has been breached and does not overtop. The downstream <br />embankment overtops by about three feet. In either <br />case, erosion potential is high, although neither of <br />the embankments create a significant backwater effect. <br />Both reservoirs are normally dry. Refer to Sheet 19. <br /> <br />The floodway data in this table define an area required to convey the <br />100-year flood without causing significant increase in stage or <br />velocity due to future development outside the established floodway. <br />The floodway was determined based on criteria speCified by the Urban <br />Drainage and Flood Control District which allows a maximum rise of 0.5 <br />feet on the energy grade line, assuming equal conveyance reductions on <br />both sides of the floodplain. The floodway is a high hazard area <br />characterized by relatively deep water and high flow velocities and <br />should remain free of any development which would obstruct flow. <br /> <br />3. An on 1 i ne reservoi r with a permanent pool is located <br />approximately one-half mile downstream of Yosemite <br />Street. The entire embankment becomes overtopped <br />du ri ng the 100-year event due to inadequate spi 11 way <br />capacity. Erosion potential is very high. The water <br />surface elevation increases about five feet over that <br />level shown on the date of survey, but no residential <br />flooding results from this. Refer to Sheet 22. <br /> <br />The floodway data table presents floodway widths to the right and left <br />of the channel centerline, the total floodway width, the mean velocity <br />in the floodway, and the 100-year flood elevations resulting from <br />flows being confined to the floodway. <br /> <br />Flood Velocities <br /> <br />Maximum flow velocities characteristic of a 100-year flood event are <br />tabul ated at cross secti ons on the water surface profi le sheets. <br />Average channel and floodway velocities are tabulated in Table V for <br />each cross section. <br /> <br />18 <br /> <br />