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<br />O~k Creek LMMP - lOO-. I Floodplain <br />Page 2 <br /> <br />The limit of the left overbank floodplain on the west ii:: dd'ined as the flow path from the D&RGW <br />tracks to the Arkaasas River. The left overbank area (nodel estimaws the depths of flow. At cross <br />section E, the water surface in the main chaanel is ab,;Jut the same as in the left overbank area and <br />the floodplains become hydraulically connected. Tli:IUS, it was assumed that the majority of the <br />flow in th,~ left overbank area returns to the main charinel at cross section E. A portion of the flow, <br />however, remains in the left overbank area creating !m a:rea of shaUow flooding to the Arkaasas <br />River. <br /> <br />The main channel flow just downstream of the D&RqW is approximately 9840 cfs. Because of <br />the inadequate capacity of the 2nd Street aad 3rd :,';'Ireet bridge crossings to convey the main <br />channel flow, a portion of the flow spills to the el!,st into the right overbank area from cross <br />sections II to K. The spill was quaatified using the i;ilplit flow option in theHEC-2 model. The <br />amount of spill is about 3880 cfs. This split flow corr!!bines with the 3440 cfs which spills over the <br />railroad tracks east of the main chaanel. As the comt:ined flow moves toward the Arkaasas River, <br />a portion Df the flow spills back into the main chaane:t from cross se:ctions C to G. This split flow <br />is quaatified using the split flow option in the HEC-2 ;IlodeI. <br /> <br />At cross ~;ections A and B, the floodplains for the rii::ht overbank area aad the main channel are <br />hydraulicaUy connected; aad, thus, the total flow is I! sed in the main channel model to determine <br />the 100-year floodplain. A general schematic of hydrimlic analysis is shown in Fig-ure 1. <br /> <br />The Manning's roughness coefficient is 0.045 in the l!baill channel and raages from 0.05 to 0.15 in <br />the left overbank arl~a aad 0.09 to 0.15 in the righti overbaak area depending on the degree of <br />urbanization. <br /> <br />If you have aay questions, please do not hesitate to caJ. <br /> <br />Sincerely, <br /> <br />Reviewed by <br /> <br />LOVE (:~ ASSOCIATl3S, INC. <br />.)-:;7:, ."">~ Z - .----:' <br />~::=~ ~-~ <br />Sheila /" Beissel <br />(\) () <br />-A:J!;~~J ~~I1X' <br />David J., LO~~ <br /> <br />SABtDJl :jg <br />atlachrnent <br /> <br />cc: Phillip C. Boawl1 - US ACE, Albuquerque, New :,~[exico <br />