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<br />. <br /> <br />, <br /> <br />'" <br />..~ <br />. <br />. <br />I <br />. <br /> <br /> <br />.... <br /> <br />, <br />,II <br />.. <br />! <br /> <br />, <br />, <br /> <br />.. <br /> <br />,. <br />. <br /> <br />CHAPTER V <br />HYDRAULIC ANALYSIS <br /> <br />'7 <br /> <br />Based upon the hydrologic report. a study was made to estimate water surface <br />profiles for the 10-, 50- 100- and 500-year floods and to determine a one-foot <br />rise floodway for the 100-year flood. Data supplied by field surveying was used <br />to construct a river model of the study reach. The basis for this model was the <br />HEC II, Water 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 on the 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 />500-year events are shown on the flooded area maps, Plates 3 through 8. <br /> <br />Roughness coefficients, Manning's "n" values. were estimated by field <br />observation at each cross section. The locations and values obtained are shown <br />in the Technical Addendum. <br /> <br />Tables 5 and 6 show the water surface elevations at selected reference <br />points for the 10-, 50-, 100- and SOD-year floods, 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 />streams such as these could cause severe blockage, especially near bridges where <br />the channel narrows and thereby raise the surface elevations significantly above <br />those shown. <br /> <br />A one-foot rise floodway was calculated for the 100-year flood. In accordance <br />with the National 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 />elevation to increase more than one foot. The floodway was calculated assuming <br />encroachment into the floodplain was equal from both sides. Tables 7 and 8 show <br />the width of the floodway at each reference point as well as the elevation resulting <br />from encroachment as compared to the 100-year flood without encroachment. <br /> <br />A detailed copy of the HEC-II analysis may be seen in the Technical Addendum. <br /> <br />19 <br />