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<br />Hydraul ics <br /> <br />The U.S. Army Engineers HEC-2 computer program was used to <br />perform water surface prof i Ie computati ons. The program <br />uses the standard step back water method. Cross section <br />data from the 1981 Flood Insurance Study was used where the <br />data was avai lable. This study extE'nds further upstream and <br />further downstream than the flood insurance study, <br />therefore, field surveys were made to supplement the <br />available data. Dimensions for bridges were determined fl'om <br />field investigations for those areas not covered by the 1981 <br />Flood Insurance Study. <br /> <br />Hydrau Ii c roughness coef i c i ents (Mann i ngs n-'va I ue,;) were <br />also determined from field investigations for those stream <br />reaches not covered by the 1981 Flood Insurance Study. In <br />some locations private residences with driveways and bridges <br />along the stream were not i nc I uded in the mode I as seperate <br />cross sections but were ~Iccounted for as extra roughness In <br />the mannings n-value. A tabulation of roughness <br />coefficients is included in the technical documentation and <br />in the HEC-2 1 isting. <br /> <br />Water surface profi les, typical cross sections and maps <br />showing the 100 year flocd boundaries are shown on included <br />exhibits and flood plain maps. Table 1 shows computed flood <br />elevations at specific Cf'OSS sections. <br /> <br />Flood boundar i es were located on a set of March 1,382 <br />topographic maps by transferring flood elevations (at map <br />contour intervals) from plotted profi les (from HEC-2) to the <br />maps, using stationing along the main channel as the <br />location reference. These points were connected and <br />smoothed to create the map flood boundaries. This set of <br />maps does not show recent changes in the Plateau Creek <br />channe I, therefore, a set of aer i a I photographs taken in <br />July 1984, which reflect recent changes, was used to update <br />the 1982 topographic maps. <br /> <br />The main channel of Plateau Creek has changed at two <br />locations since the 1981 Flood Insurance Study was made. <br />Their location is noted on the flood plain maps as channel <br />changes. New bridges were built ,-ece,ntly on Plateau Creek <br />and on Grove Creek. These changes, along with some <br />rea Ii gnment of cross sect ions used in the HEC-2 Mode I, has <br />made some differences in water surface elevations from the <br />1981 Flood Insurance Study. Data fr'om table 1 in this <br />report can be used to compare with data from the pub I i shed <br />flood insurance report. <br /> <br />1 1 <br />