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Tom Smith Oak Creek Floodplain and Floodway - Florence, Colorado FEMA LMMP Contract No. EMW-90-C-3132 Page 2 irrigation canal alignment. This combined flow of 3,440 cfs continues east along the railroad to the low point in the tracks at about Frazier Avenue. The flow spills continuously over the railroad along this length of track. Downstream of the D&RGW, the lOO-year floodplain is delineated using three HEC-2 models: main channel, right overbank area, and left overbank area. wee models are needed to defme the floodplain due to the split flow nature of the floodplain and the topography in the area. The flow in the left overbank area is estimated as that portion of the flow over the railroad from a point 200' west of the bridge crossing. This point was selected by analysis of the downstream topography. The left overbank flow was estimated using a sloping weir equation and the water surface elevation at cross section L given in the main channel model (see Figure 1). The flow in the left overbank area is approximately 2,720 cfs. The limit of the left overbank floodplain on the west is defmed as the flow path from the D&RGW tracks to the Arkansas River. The left overbank area model estimates the depths of flow. At cross section E, the water surface in the main channel is about the same as in the left overbank area and the floodplains become hydraulically connected. Thus, it was assumed that the majority of the flow in the left overbank area returns to the main channel at cross section E. A portion of the flow, however, remains in the left overbank area creating an area of shallow flooding to the Arkansas River. The main channel flow just downstream of the D&RGW is approximately 9,840 cfs. Because of the inadequate capacity of the 2nd Street and 3rd Street bridge crossings to convey the main channel flow, a portion of the flow spills to the east into the right overbank area from cross sections H to K. The spill was quantified using the split flow option in the HEC-2 model. The amount of spill is about 3,880 cfs. This split flow combines with the 3,440 cfs which spills over the railroad tracks east of the main channel. As the combined flow in the right overbank of 7,320 cfs moves toward the Arkansas River, a portion of the combined flow spills back into the main channel from cross sections C to G. This split flow is quantified using the split flow option in the HEC-2 model. At cross sections A and B, the floodplains for the right overbank area and the main channel are hydraulically connected; and, thus, the total flow is used in the main channel model to determine the lOO-year floodplain. A general schematic of the hydraulic analysis is shown in Figure 1. The Manning's roughness coefficient is 0.045 in the main channel and ranges from 0.05 to 0.15 in the left overbank area and 0.09 to 0.15 in the right overbank area depending on the degree of urbanization. .' If the Oak Creek floodplain were to be defined using one model which includes the main channel and overbanks as opposed to three separate models, the HEC-2 program shifts the flow in the overbanks and main channel from one overbank to the other. From examining the topography in the area, the flow cannot physically shift from one overbank to another; thus, a single model would not realistically defme the floodplain. Floodway The flood way limits will be defined based on FEMA's one foot rise criteria. If the floodway is defined using one HEC-2 model and the floodplain is defmed using the hydraulic analysis for three separate HEC-2 models, the water surface elevations will not be directly comparable to determine the one foot rise. Also, for a floodway defmed using one HEC-2 model, the HEC-2