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Last modified
1/25/2010 6:48:39 PM
Creation date
10/5/2006 1:22:05 AM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
518
County
Douglas
Community
Unincorporated Douglas County
Stream Name
Oak Gulch and Lemon Gulch
Basin
South Platte
Title
Flood Hazard Area Delineation - Oak Gulch and Lemon Gulch
Date
2/1/2000
Designation Date
5/1/2000
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br /> Table 3,5 <br /> Comparison of 100,Year Discharge to Other Stndies <br /> 100-year Peak Discharges (cfs) <br /> Cherry <br /> Drainage CWCB Creek <br /> Area USGS Regression Tributaries Paleoflood <br />Stream (sq. mi.) Regression (1997) Draft Estimates <br /> (1987a) Hydrology (1998) <br /> Report <br /> (1998) <br />Oak Gulch at 1.784 2260 1460 2100 1300 (Oak at <br />confluence with (1.8 cfs/acre) Motsen, <br />Cherry Creek bocker Rd.) <br />Unnamed 1.037 1240 1030 1540 900 <br />Tributary at (2.3 cfs/acre) <br />confluence with <br />Cherry Creek <br />Lemon Gulch at 7.530 7480 3650 6570 2150 (Lemon <br />confluence with (1 A cfs/acre) at Crowfoot <br />Cherry Creek Valley Rd.) <br /> <br />Section 4.0 <br />Hydraulic Analysis <br /> <br />4.1 Methodology <br /> <br />A hydraulic analysis was undertaken to estimate water surface elevations for the lO-year and <br />100,year flood events. The analysis was performed using the U.S, Army Corps of <br />Engineers' HEC-RAS Water Surface Profile program. Baseline hydrology peak discharges <br />for future development conditions (documented in Section 3.0) were used for the analysis. <br /> <br />Cross section data for the hydraulic analysis were developed from a digital terrain model <br />representing existing topography along the stream corridors. The terrain model was based on <br />a three-dimensional file of2,foot ground contours provided by Benchmark Mapping, Cross <br />section data at bridges and major culverts were obtained from field measurements provided <br />by Kelly Surveying Associates, Inc. <br /> <br />Discharge estimates along each stream were estimated using the UDSWM2 model. <br />Discharges at HEC,RAS cross section locations were linearly interpolated between <br />UDSWM2 design points. <br /> <br />Approximate 100,year floodplains for Oak Gulch and Lemon Gulch are shown on the <br />Douglas County Flood Insurance Rate Maps. However, the 100-year discharges are not <br />indicated in the FIS (FEMA, 1993), The FEMA Library was contacted and indicated that <br />they could not find any hydrologic data for Oak Gulch or Lemon Gulch. <br /> <br />Manning's roughness coefficients for the channel and overbank areas were estimated by field <br />inspection following procedures outlined in several standard references (Chow, 1959) <br />(USGS, 1987) (Simons and Senturk, 1977). A separate check of the Manning's "n" value <br />was made by checking the flow regime of the conveyance elements during flood conditions. <br />Research of large floods (Jarrett, 1984) (USGS, 1985) (Trieste and Jarrett, 1987) (UDFCD, <br />1995a) indicates that the flow regime for natural channels during floods is close to critical <br />flow, but generally subcriticaL Supercritical flow, except for short distances, generally does <br />not occur in a natural channel. This is due to inherent turbulence created by debris, erosion, <br />and sediment transport that occurs in natural channels during large floods. This turbulence <br />usually prevents long reaches of supercritical flow in natural channel systems. <br /> <br />Stream gage information does not exist on any of the study area streams, precluding the use <br />of a gage analysis to estimate discharges for various return periods. <br /> <br />Based on this research, it is recommended that Manning's "n" values be selected to produce <br />a flow regime near critical flow during extreme floods. Based on the results of the HEC- <br />RAS modeling, it was generally found that a hydraulic roughness value of 0.045 for the main <br />channel and overbanks would provide appropriate flow conditions slightly below critical <br />depth (Froude numbers slightly less than 1.0). <br /> <br />Because the hydrologic analysis for the study area is consistent with standard UDFCD <br />hydrologic criteria and compares reasonably well with the regression equations, the results <br />were used in the subsequent hydraulic analysis. <br /> <br />Culverts throughout the study area were analyzed based on the culvert diameter. Only those <br />culverts greater than 36,inches in diameter were modeled in this study. Culverts less than <br />36,inches in diameter were generally substantially filled with sediment and were assumed to <br />be negligibly small. <br /> <br />Culverts greater than 36,inches and less than 60,inches in diameter were modeled using the <br />HDS,5 nomographs (Hydraulic Design of Highway Culverts. Hydraulic Design Series No.5, <br />Federal Highway Administration, 1985). The culvert discharge was determined from the <br />HDS,5 nomograph based on a headwater elevation equal to the top of the road. The cross <br />section located along the road embankment was then modeled in HEC,RAS using a <br /> <br />6 <br /> <br />Oak Gulch and Lemon Gulch FHAD.DOC <br />
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