FLOOD04447 - Laserfiche WebLink

Home Browse Search

Flood Insurance Study Report Data Flood Insuraace Study - Florence, Colorado Page 3 Structures: Existing bridge structures were surveye:d by Valley Surveying of Florence, Colorado. Field notes from the surveys are presented in the Appendix. Water Surface Profile Determination: Water surface profiles were developed using the HEC-2 computer backwater model (Reference 2). Floodplain Discullllm;. South of the D&RGW Railroad (Railroad), at the upstream study limit, 2,940 cfs of the total100-year discharge of 16,000 cfs flow splits from the main channel and flows east along the irrigation ditch alignment. The flow split was determined by HEC-2 flow distribution. This split flow becomes hydraulically disconnected from the main chaanel. The main channel flow is reduced by 2,940 cfs which leaves approximately 13, 060 cfs in the main channel upstream of the Railroad. At the railroad the 13,060 cfs is split as follows: 4,250 cfs is conveyed thtough the bridgl~ opening; 8,310 cfs is conveyed over the railroad tracks, aad 500 cfs splits to the east along the railroad. The flow split to the east (500 cfs) combines with the 2,940 cfs flowing along the 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 A venue. The flow spills continuously over the railroad along this length of track. Downstream of the Railroad, the 100-year floodplain is deline:ated using three HEC-2 models: main channel, right overbank area, and left overbank area. Three models are needed to define 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 feet west of the bridge crossing. This point was selected by aaalysis of the downstream topography. The left overbank flow was estimated using a sloping weir equation aad the wate:r 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 wc:st is defined as the flow path from the Railroad 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 chaane1 is about the same as in the left overbaak area aad the floodplains become hydraulically connected. Thus, it was assumed that the majority of the flow in the left overbank area returns to the main chaanel at cross section E. A jJOrtion of the flow, however, remains in the left overbank area creating an area of shallow flooding to the Arkansas River. The main chaanel flow just downstream of the Railroad 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 c:fs 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 spill!i 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 th,e main chaane1 are hydraulically connected; and, thus, the total flow is used in the main channel model to determine the 100-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 raages 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.