FLOOD00479 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I III. HYDROLOGIC AND HYDRAULIC DETERMINATION A. HYDROLOGIC ANALYSIS A hydrologic analysis was completed for Plum Creck and East Plum Creek as part of the Flood Insurance Study (FIS) for Douglas County and Unincorporated Areas (Reference I). As pan of this analysis, the synthetic hydrograph method was used to obtain peak discharges. The analysis was based on a storm distribution of 14 hours and a Type II-A distribution. The rainfall \....as obtained from a Precipitation- Frequency Atlas of the Western United States (Reference ,,0, and areal adjustments were applied to detennine average precipitation o\"er the entire \....atershed. Values o[the 10-.50-,100-, and SOD-year peak discharges were obtained using a Soil Conservation Service computer program (Reference 5) and compared \\ith discharges estimated from the United States Geological Sun'ey Technical Manual No. I (Reference 6). Figure 2 presents the discharge-probability profiles for the 10-. 50-, 100-. and SOD-year Hood events obtained from the FIS hydrologic analysis and used for this study. B. HYDRAULIC ANALYSIS The hydraulic analysis for Plum Creek and East Plum Creek was completed using the U.S. Army Corps of Engineer's step backwater program HEC-RAS (Reference 7). The U.S. Army Corps of Engineer's GEO- RAS (Reference 8) extension \vas used to generate cross-sections from the mapping. t\1anning's roughness coefTicients for the channel and the overbank areas were detennined from field observation using guidelines published in Open Channel lIvdraulics. (Reference 9). Aerial photography was used to assist in detemlining "n" value transitions. Selected channel "n" values ranged from 0.035 to 0.04 to reflect the sand and gravel channel bed with sparse wgetation. Overbank "n" values ranged from 0.045 to 0.06 to rdlect open grass vegetation with scattered brush and scattered woody. tree vegetation. Starting water surface elevations were based on the recently published FHAD for the Plum Creek watershed (Reference 3). The geometry for bridge crossings at Rio Grande Avenue. over Plum Creek. and State Highway 67 and the Burlington. Northern and Santa Fe Railroad. over East Plum Creek. were lield measured and sun'eyed. I I I I I I c. SPECIAL HYDRAULIC ANALYSIS Plum Creek Split Flow (Station 3850 to Station 73(9) A split flow evaluation was used to define the floodplain limits for Plum Creek from Stations 3850 to 7389. Between these sections. the topography indicated that 00\"" could be separated from the main Plum Creek channel in the left. or western. overbank. It was estimated using BEC-RAS. that the split flow approaches -1-,992 cfs during the 100-year c\'cnt and 13.9-1-0 cfs during thc SOD-year cvent. This split results in higher flood elevations along the left overbank than those that occur in the main channel. Discharges conveyed along the flow split will spill back into thc Plum Creek main channel at several locations between cross- sections 3850 and 7389. Therefore, the discharge used for the split flow area \vas not subtracted from the main channel discharge for the purpose of this analysis. Additionally. both the 0.5 foot and 1.0 foot floodways remained confined to the Plum Creck main channel. Rio Grande Avenue Bridge o\'er Plum Creek At the time of this study. the Rio Grande Avcnue Bridge over Plum Creek was under construction. The original. existing bridge was in the process of being replaced with a larger structure. Since the new structure \....as not complete at this time. the existing bridge was used for this study. As indicated by the study, Plum Creek currently overtops the Rio Grande Bridge by approximately 7-feet. The impacts of the new bridge structure ha....e not been analyzed in this study. D. INEffECTIVE fLOW AREAS There are scycral locations within the Plum Creek and East Plum Creck study reaches where a !looded area is ineffective. Ineffective flow areas typically occur at upstream and downstream of hydraulic structures. such as bridges. InefTective flow is a result of backwater areas that deydop as !low is passed through the hydraulic structures. These ineffectiw flow areas are delineated based on the elevation of the adjacent water surface and arc excluded from the hydraulic computations. 4