FLOOD02072 - Laserfiche WebLink

Home Browse Search

" '" 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 a scale of 1""400 it with 5 it 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. The following table shows Mannings n-values used in the hydraulic computations: Treatment Alternatives There are a number of possible treatment alternatives for reducing flocd damages from Pawnee Creek. However, for purposes of this study investigations were limited to the following: Section ID From To n-value Left Overbank Right Overbank Channel (1) Flooding under present con~itions. (2) Effects of floo~water retarding structures. (3) Effects of flood dikes. The major part of this report attempts to define present condition flooding. There are flood plain maps, flood profiles, tables, a floo~ history, etc. to provide this information. The following discussion attempts to evaluate the two treatmont alternatives. -------------------------------------------------------- 18.1 18.2 .040 .040 .040 18.2 22 .075 .075 .075 22 " .060 .060 .060 " " .100 .110 .110 " " .035 .035 .035 " " .110 .110 .110 AtWOOd Reach 20.3 17.2 .060 .060 .060 Cemetery Reach , " .040 .040 .040 Floodwater Retardina 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 means the reservoir storage and emergency spillway of each dam must handle the probable maxi~um floo~ 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 shows structural data for each dam and Table 3 gives a very preliminary approximation of cost. The effects of this alternative can best be illustrated by comparing peak discharge - frequency values for this alternative with no-project conditions at select locations. This comparison is shown in the following table: Water surface profiles, typical cross sections, and maps showing the 100 year flood boundaries are shown on included exhibits and flood plain maps. Table 4 shows computed flood elevations at specific cross sections. Flood boundaries ware located on the set of 1990 topographic maps, previously referred to, by transferring flood elevations (at map contour intervals) frop plotted profiles (from HEC2) to the maps using stationing along the main channel as the location reference. These points were connected an~ sffioothed to create the map flood boundaries. The split flow option in HEe2 and the DIVERT and DIVIDE features in TR20 were used to help determine the division of flows that occur throughout the study reach. 10 YIC Discharge - cfs 25 yr 50 yr 100 yr Location High..'ay 14 With Dams County Rd 25 with Dams 4700 '" 4300 550 7300 '" 6700 '" 9600 1100 8900 000 12200 1400 11400 1100 There are other effects, such as economic and environmental, that are not analyzed in this study.