FLOOD04080 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I A. General The 72" x 44" CMPA beneath Wadsworth Boul evard di scharges into an open top concrete vault ("Bubbler"). Exiting from the vault is a smaller 48" RCP storm sewer. The original design intent was that the capacity of the 72" x 44" CMPA would be 200 cfs and that the 48" RCP would have a capacity of 100 cfs. The remaining 100 cfs would spill out of the "Bubbler" and be carried above ground. Hydraulic evaluation of this system indicates the present capacity of the system to be approximately 75 cfs. The system is not able to operate as designed because of the lack of adequate differential head between the maximum upstream water surface elevation and the water surface at the downstream and controlled by the top of the "Bubbler". The flows are therefore choked by the tailwater. CHAPTER IV - HYDRAULIC ANALYSIS The purpose of the hydraulic analysis in the flood hazard area delineation is to define the 100-year floodplain and 10- and 100-year profiles. The methodology used and the results of the analysis are presented in this chapter. The floodplains and profiles were defined for the future basin development condition with the existing drainage facilities. The evaluation of the existing drainage facilities is limited to determining the capacity and general condition and generally does not include evaluating the structural integrity of the facility. The reported capacity of the 76" x 48" HERCP beneath Hampden Avenue was 280 cfs (Reference-8) . Hydraul i c analysi s i ndi cates that the present confi gurati on will pass 260 cfs before overtopping the south frontage road. The hydraulic calculations used in this study are available at the offices of the Urban Drainage and Flood Control District (UD&FCD). B. Evaluation of Existing Drainage Facilities Evaluation of the flood routing through Henry's Lake was made by establishing a stage/discharge relationship for the spillway and outlet works from the construction drawings on file at the State Engineer's Office (SED). The area/capacity relationship was determined by integrating the areas from the 2-foot contour interval maps from construction drawings. Table IV-l presents a summary of facilities evaluated for this study. The evaluation of the existing drainage facil ities was performed using the I" = 100', 2-foot contour interval mapping by Landmark Mapping Company under a separate contract with UD&FCD, In = 200'. 5-foot contour interval mapping provided by the City of Lakewood, and field survey verifications. The facilities that were evaluated included channels, storm sewers, culverts, and detention ponds. C. Floodplain Determination The capacity of the storm sewer systems was determined using the guidel ines presented in the Urban Storm Drainage Criteria Manual (Reference-2). The capacity of the system was estimated by evaluating the pipe and the inlets to determine which controls (limits) the capacity. The full flow capacity of the pipe was calculated using the "minimum" pipe slope and approximate roughness coefficient. The capacity of the in I ets was cal cul ated based upon the type (i.e., grate, curb openi ng, or combination) and whether the inlet was "on-grade" or a "sump" condition, then applyi ng the suggested "all owab I e capacity" reducti on factor (Reference-2). The floodplain for the study area was determined using a computerized backwater program. The program is titled "HEC-2 Water Surface Profiles" (Reference-7). The required input to the program is the channel cross-section coordinates (taken from project mapping), the hydraulic properties of each sub-section within the cross-section, and the discharge value. Estimates of channel and overbank roughness factors ("n") were made after field investigations of the study area. Typical Manning's "n" values used for both the channel and overbank areas ranged from 0.025 to 0.035. The backwater calculations are initiated by inputting a known water surface elevation and discharge value. The longitudinal spacing of the cross-sections varies with channel condition, but generally the spacing was less than 200-feet. The start i ng water surface el evati on was estimated based upon the -21-