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<br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of streams in the commu- <br />nity were carried out to provide estimates of the elevations of <br />floods of the selected recurrence intervals along each stream <br />studied in the community. <br /> <br />Surveying for cross section data for Spring Creek in Pierce was done <br />by the study contractor in June 1977. Locations of selected cross <br />sections used in the hydraulic analyses are shown on the Flood <br />Profiles (Exhibit 1). For stream segments for which a floodway is <br />computed (Section 4.2), selected cross section locations are also <br />shown on the Flood Boundary and Floodway Map (Exhibit 2). <br /> <br />All culverts were surveyed to obtain elevation data and structural <br />geometry. <br /> <br />Roughness coefficients (Manning1s tin") were estimated by field <br />inspection. Manning's IIn" coefficients varied from 0.020 to 0.040 <br />in the channel and from 0.040 to 0.080 in the overbank areas. <br /> <br />Water-surface elevations for the 10-, SO-, 100-# and SOG-year floods <br />were computed using the U.54 Army Corps of Engineers HEC-2 step- <br />backwater computer program (Reference 10), and supplemented with <br />other manual computations. <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). Starting water-surface elevations were <br />computed using the slope-area option of the HEC-2 program (Reference 10) <br />Two sets of flood profiles were produced, one following an artificial <br />thalweg on the west side of the Union Pacific Railroad tracks, the <br />Spring Creek Overflow path, and one following the Spring Creek <br />natural channel on the east side of the tracks. Both profiles were <br />computed taking into account the split inflow occurring at the <br />intersection of Rowe Avenue and u.s. Highway 85 and the weir flow <br />along the Union Pacific Railroad tracks south of Rowe Avenue. <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 />4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS <br /> <br />A prime purpose of the National Flood Insurance Program is to encourage <br />State and local governments to adopt sound flood plain management pro- <br />grams. Each Flood Insurance Study, therefore, includes a flood boundary <br />map designed to assist communities in developing sound flood plain manage- <br />ment measures. <br /> <br />7 <br />