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<br />8 <br /> <br />'in bridge analysis were consistent with loss coefficients suggested in the <br />HEC-2 manual (Reference 3). <br />Manni ngs roughness coeffi cients were sel ected after fi el d observati on. <br />Photos are given in Appendix A. Photos of bridges are also given in Appendix <br />A. River flows are given in Table 1. Complete input listings for the HEC-2 <br />model are contained in Appendix B. <br /> <br />Modeling Procedures <br />Once the input data was prepared, a, prel iminary computer model was <br />generated. The reasonableness of the hydraul ic modE!l was checked by hand <br />methods. Bridge hydraul ics \~ere compared by methods given in Reference 3. <br />Cul vert hydraul i cs were checked by usi I1g appropri ate nomographs provi ded by <br />culvert manufacturers (Reference 5). Preliminary floodplains wet"e drawn to <br />check the adequacy of cross sections. Cross sections were added at locations <br />\~here the floodplain continuity were questioned. At one location a flow split <br />occurred. Thi s happens when overbank fl o'""s are di sconnected hydraul i ca lly <br />with mai n channel fl ows. A separate analysi s was performed at thi s 1 ocati on <br />to balance the water surface elevation and determine the quantity of flol'l in <br />the overbank area. Separate cross sections. were then map interpolated and a <br />separate model developed. The flow split model is contained in Appendix B. <br />Once the model was calibrated and checked, flood profiles I.ere drawn. <br />The profiles are contained in the map pocket. Complete input and output HEC-2 <br />model listings for the 10-, 50-, 100- and 500-year flood profiles are given in <br />Appendix B. <br /> <br />Floodway Analysis <br />A floodway analysis was performed to define the minimum area required to <br />safely convey the lOa-year flood without causing a rise in watel" surface ele- <br />vation of more than one foot. The HEC-2 mode'l was used with the encroachment <br />opti on that reduces conveyance of the ovel"banks in proporti on to the con- <br />veyance of the natural condition (sE!e HEC-2 manual section ET, -X.4). <br />Floodway boundaries are the Qasi s for n oodpl ai n encroachment enforcement and <br />do not exceed a one-foot rise in water surface elevation at each cross section. <br />