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<br />~ <br /> <br /> <br />CHAPTER V <br />HYDRAULiC ANALY5!S <br /> <br />.. <br /> <br />Based upon the hydr~logic report, a study was made to estimate water surface <br />profiles for the 10-, 50- 100- and 500-year floods and to cetermine a one-foot <br />rise floodway for the lOO-year flood. Data supplied by field surveying was used <br />to construct d river model of the study reach. The basis for this model was the <br />HEC 11, ~ater Surface Modeling Program of the U.S. Army Corps of Engineers <br />(reference 3). location of cross sections used in the construction of the model <br />are shown ontne flooded area maps, Plates 3 through 8. Water surface profiles <br />generated by the model for the selected floods have been plotted on the flood <br />profiles, Plates 9 through 15, and the areal extent of flooding by the 100- and <br />SOD-year eyents are shown on the flooded area maps, Plates 3 through 8. <br /> <br />. <br />0, <br /> <br />Roughness coefficients, Manning's "n"values,wereestimatedbyf ield <br />observation at each cross section. The locations and values obtained are shown <br />in ~he Technical Addendum. <br /> <br />~ <br /> <br />. <br /> <br />Tables 5 and 6 show the water surface elevations at selected reference <br />points for the 10-, 50., 100-andSOO-yearfloods, along with the discharges <br />in cubic feet per second (CFS) used in the hydraulic analysis. water surface <br />elevations shown are for clear water conditions. Debris accumulations on narrow <br />s~reams such as these could cause severe blockage, especially near bridges where <br />the channel narrows and thereby raise the surfacede"Jtions signifiCJI1tly above <br />those shown. <br /> <br />" <br />'Iear <br /> <br />, <br />50 <br />Y€'ar <br /> <br />100 <br />Year <br /> <br />A one. foot rise floodway was calculated for the lOO-year flood. In accordance <br />wit~ the Naticnal Flood Insurance Program, the floodway is the portion of channel <br />and floodplain required to pass the 100-year discharge without causing the flood <br /><,levationtoincreasemorethanonefoot. The flooctwaY'Ids calculated assuming <br />encroachment into the floodplain was equal from bot~ sides. Tables 7 and 8 ShOW <br />the width of t~e floadway at eaCh reference point as well as t~e elevation resulting <br />frem encroachment as compared to t~e 100.year flood WithOut encroaChment. <br />A detailed copy of the HEC-Il analYSls may be seen in t~e TeChnical Adaendum. <br /> <br />, <br />, <br /> <br />~ <br />. <br /> <br />ReturnPerio:::! <br />(years) <br />FIGURE 4 <br />Di~CHARGE ~ FREQUENCY CURVES <br /> <br />18. <br /> <br />19 <br />