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Last modified
1/25/2010 6:45:14 PM
Creation date
10/5/2006 12:19:31 AM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
226
County
Jefferson
Community
Lakewood
Stream Name
Henry's Lake Drainageway
Basin
South Platte
Title
Flood Hazard Area Delineation - Henry's Lake Drainageway
Date
7/1/1983
Designation Date
1/1/1984
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />I <br />'. <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />A. General <br /> <br />Input for the SWMM model is shown on Figure 111-2 and Table 111-2, which also <br />includes the basin characteristics. The model describes the study area <br />mathematically as a system of idealized hydraulic elements. Sub-basins are <br />described as rectangular subcatcl1nent areas with a length, width, slope, <br />imperviousness, and infiltration rate. The streets, storm sewers, and channel ways <br />are described in terms of gutters and pipes. The hydraulic properties of each <br />element are defined by size, slope, and roughness coefficient. <br /> <br />CHAPTER III - HYDROLOGIC ANALYSIS <br /> <br />The purpose of the hydrologic analysis in the flood hazard area delineation <br />(FHAD) study is to define the storm runoff peak flows and volumes for the hydrologic <br />points of interest along the drainageway. The first step in the hydrologic analysis <br />is to define the sub-basin boundaries and characteristics and then determine the <br />flood hydrographs. The runoff information is subsequently used to delineate 10- and <br />IOO-year floodplain with existing drainage facilities for future basin development <br />conditions. <br /> <br />The watershed was divided into 17 sub-basins to define the runoff peak flows <br />and volumes using the current Urban Drainage and Flood Control District (UD&FCD) <br />procedures. The details of the analysis used to define the design flood peaks and <br />volumes are presented in this chapter. <br /> <br />(2) - Design Rainfall <br />The design rainfall information was obtained from the UD&FCD as a percent time <br />distribution of the 1 hour depths from the NOAA atlas (Reference-4). The percent <br />distribution was converted to incremental rainfall depths, for input to the CUHP <br />model, and incremental rainfall intensities for input to the SWMM model. The design <br />rainfall information used for this study is presented in Table 111-4. <br /> <br />B. Basin ~drology <br /> <br />(3) - Model Calibration <br />After the CUHP analysis was performed at the three design points for the <br />IO-year frequency storm, subsequent SWMM model runs were made for the same design <br />points. By varying the subcatchment widths in the SWMM model, the response time of <br />the model could be changed resulting in an increase/decrease in the peak runoff <br />values. These values were compared to the CUHP peak runoff values. The models were <br />considered cal ibrated (equivalent) when the flood peaks for IO-year storm (future <br />development conditions) were within 20%, which is considered to be within acceptable <br />limits. Comparison was also made for the IOO-year storm. The results of the com- <br />parison are listed below: <br /> <br />(1) - Runoff Model <br />The runoff model used to determine the peak flows and volumes for Henry's lake <br />Ba sin d ra i nageway was the Storm Water Management Model (SWMM) (Reference-6). The <br />SWMM version used contains Corps of Engineer's modification, which permits <br />evaluation of detention storage. The version is available through the UD&FCD at the <br />Boeing Computer Center located in Englewood, Colorado. The SWMM model was <br />calibrated to the results of a Colorado Urban Hydrograph Procedure (CUHP) model at <br />three design points within the study area: inflow to Henry's lake, routed outflow <br />from Henry's lake, and Wadsworth Boulevard. The calibration was performed for the <br />IO-year storm reflecting future development conditions. Input for the CUHP model is <br />shown on Figure 111-1 and Table 111-1. Also shown on the Figure and Table are the <br />CUHP data for the flow split/direct flow basin, which were used to define the peaks <br />for this area. <br /> <br />PEAK FLOW COMPARISON (CFS) <br /> <br />Henry's lake <br />( Inflow) <br />Henry's lake 55 50 315 260 <br />(Routed Outflow) <br />Wadsworth 245 265 620 635 <br />Boulevard <br /> -7- <br /> <br />
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