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<br />Peak discharge-drainage area relationships for the Tri-Area Drainage- <br />way area shown in Table 1. <br /> <br />Table 1. Summary of Discharges <br /> <br />Flooding Source <br />and Location <br /> <br />Drainage Area <br />(Square Miles) <br /> <br />Peak Discharges <br />(Cubic Feet per Second) <br />10-year 50-year 100-year 500-year <br /> <br />Tri-Area Drainageway <br />At County Road 13 <br />At McClure Avenue <br />At 1st Street <br /> <br />3.8 <br />3.1 <br />2.7 <br /> <br />435 836 <br />269 472 <br />106 205 <br /> <br />REVISED DATA / <br /> <br />1,030 <br />561 <br />249 <br /> <br />1,393 <br />884 <br />711 <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of streams in the ~U- <br />nity were carried out to provide estimates~f.'~. N\~r,;~ns! <br />~ -.(, ~ '1 J ..,.~ 'i.&. <br />floods of the selected recurrence intervalt.'.ai-o - ~q, ea <br />studied in the community. 't. '.~ . 01\ /I R <br />'~-."" r-' r: . Ii. <br />Cross section data for the Town of Firesto1ei~4;bj;JGCT fr\.J 1\1 I \ <br />topographic mapping at a scale of 1: 1200, witha..;~E!tour W'ifV:r120~005 <br />1 foot, produced as part of a 1975 drainag~~t,~d~ ~qDeld~~~nl~ L <br />(Reference 8). Bridges were measured to obt~1.~~r~' a1 geometry. <br /> <br />Location of selected cross sections used in the hydraulic analyses <br />are shown on the Flood Profiles (Exhibit 1). For stream segments <br />for which a floodway is computed (Section 4.2), selected cross <br />section locations are also shown on the Flood Boundary and Floodway <br />Map (Exhibit 2). <br /> <br />The water-surface profiles for the 10-, 50-, 100-, and 500-year <br />floods were computed using the U.S. Army Corps of Engineers' HEC-2 <br />step-backwater Computer Program (Reference 9). Starting water- <br />surface elevations were obtained by estimating water-surface elevations <br />at Firestone Lake and calculating a backwater profile 800 feet <br />upstream to the corporate limits. <br /> <br />Flood profiles were drawn showing computed water-surface elevations <br />to an accuracy of 0.5 foot for floods of the selected recurrence <br />intervals (exhibit 1). <br /> <br />Roughness coefficients (Manning's "nUl were estimated by field <br />inspection. The roughness coefficients for the Tri-Area Drainageway <br />and overbank areas generally ranged between 0.45 and 0.60. In <br />some overbank areas, "nil values up to 0.090 were appropriate. <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929 (NGVD). Elevation reference marks used in the study <br />are shown on the maps. <br /> <br />6 <br />