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n " Discharge values were determined from TR20 analyses which is discussed in the hydrology section of this report. Cross section data was developed from topographic maps with II. scale of Ifl~400 it with 5 ft contour intervals. Some supplemental field surveys were made at specific sites. Dimensions of bridges and hydraulic roughness coefficients (n- values) were determined from field investigations. Treatment Alternatives The following table shows Mannings n-values used in the hydraulic computations: There are a number of possible treatment alternatives for reducing flood damages from Pawnee creek However for purposes of this study investigations were limited to the f~llowing: (1) Flooding under present conditions. (2) Effects of floodwater retarding structures. (J) Effects of flood dikes. -------------------------------------------------------- Section ID FrOIll To n-value Left Overbank Right Overbank Ch~nnel The major part of this report attempts to define present condition flooding. There are flood plain maps, flood profiles, tables, a flood h1story, etc. to provide this information. The following discussion attempts to evaluate the two treatment alternatives. -------------------------------------------------------- 18.1 18.2 .040 .040 .040 18 .2 " .075 .075 .075 " " .060 .060 .060 " " .100 .110 .110 " 33 .035 .035 .035 33 " .110 .110 .110 Atwood Reach 20.3 17.2 .060 .060 .060 Cemetery Reach , " .040 .040 .040 -------------------------------------------------------- Floodwater Retardinq Structures Seven potential floodwater retarding structures were considered for purposes of reducing flooding on Pawnee Creek. They are located throughout the basin, see Figure 4. These structures were considered high hazard dams for design purposes, which ~eans the reservoir storage and emergency spillway of each dam must handle the probable ~aximum flood from the contributing drainage area. The SCS DAMS2 and TR20 computer programs were used to size and analyze the effects of these structures. Table 2 sho..'s struct~ral.data for each dam and Table 3 gives a very preliminary approX1matlon o! cost. The effects of this alternative can best be.illustrate~ by comparing peak discharge - frequency values for thls alternatlve with no-project conditions at select locations. ThlS comparison is Shown in ~he following table: Water surface profiles, typical cross sections, and maps showing the 100 year flood boundaries are shown on included exhibits anc flood plain maps. Table 4 shows computed flood elevations at specific cross sections. Flood boundaries were located on the set of 1990 topographic maps, previously referred to, by transferring flood elevations (at l'Ilap contour intervals) from plctted profiles (from HFC') t.o the maps using stationing along the main channel as the location reference. These points were connected and smoothed to create the map flood boundaries. The split flow option in HEC2 and the DIVERT and DIVIDE features in TR20 were used to help determine the division of flows that occur throughout the study reach. ----------------------------------------------- 10 yr: Discharge - cts 25 yr SO yr 100 yr Location ---------------------------------------------- Highway 14 With Dams County Rd 25 With Oams 4700 '" 4300 ,,, 7300 '" 6700 '" 9600 1100 8900 m 12200 1400 11400 1100 There are other effects, such as economic and environ~ental, that are not analyzed in this study.