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<br />. <br />I <br />. <br />I <br />I <br />I <br />. <br />I <br />I <br />I <br />. <br />. <br />I <br />I <br />. <br />I <br />I <br />. <br />I <br /> <br />-10- <br /> <br />Canal passes through this reach and creates a dam causing <br />considerable backup to Laredo Street. Depths range from <br />6 to 12 feet in this area. The existing Laredo Street <br />crossing is also inundated by the lOa-year flood. <br /> <br />flow velocity in the channel would range generally between 4 <br />and 10 feet per second for a lOa-year event in the study <br />area. Overbank velocities range between 1 and 4 feet per <br />second. <br /> <br />Flood Plain and Floodway Data <br /> <br />Water flowing at a rate greater than 6 to B feet per second <br />will cause severe erosion of stream banks and is capable of <br />transporting large rocks. velocities in the range of 6 to 8 <br />feet per second could erode fill around bridge abutments. <br />Water flowing at about 2 feet per second or less will deposit <br />debris and silt. <br /> <br />The limits of future flooding for Granby and Sable Ditches <br />are graphically outlined on the Flood Hazard Area Delineation <br />maps and tabulated in Tables V! and VII, Flood Plain and <br />Floodway Reference Data. The tables list the potential <br />flood elevations, flood plain and floodway widths, floodway <br />location, floodway surcharge elevation and thalweg elevation <br />at each cross-section studied. <br /> <br />SUMMARY <br /> <br />A floodway represents a part of the flood plain which is required <br />to pass a lOa-year flood event without raising the flood <br />profile more than an acceptable amount and also represents <br />that part of the flood plain most hazardous to personal <br />safety and welfare. In this study, the floodway was <br />defined as being the limit of encroachment into the flood plain <br />where existing flood profiles are not increased more than <br />1.0 foot. Also, flood plain areas where average velocities <br />exceeded two to three feet per second were considered to be <br />within the floodway. Floodway widths, which represent the <br />maximum limits of encroachment into the flood plain, are <br />tabulated in Tables VI and VII. <br /> <br />This report has identified the probable flooding limits of <br />a potential lOa-year flood event in the Granby and Sable <br />Ditch drainage basins. <br /> <br />Flood Velocities <br /> <br />The flood plain limits are based on (1) planned, future <br />development in the basin, (2) existing and mapped flood plain <br />conditions, (3) no peak flood flow reduction or interception <br />by the Highline Canal, (4) no consideration of the proposed <br />detention pond near Sixth Avenue, and (5) no blockage at the <br />existing culvert and bridge structures. Even with these <br />assumptions, floods of greater magnitude can and will occur in <br />which the limits of flooding will exceed those shown in this <br />report. <br /> <br />Average flow velocities, characteristic of a lOa-year flood <br />event on Sable and Granby Ditches are shown on the profile <br />portion of the Flood Hazard Area Delineation drawings. The <br /> <br />Presently, Granby and Sable Ditches are considered to have a <br />high flood damage potential. The preservation of the flood <br />plain will serve, at least, to maintain the present level <br />