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FLOOD08149
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Last modified
1/29/2010 10:12:02 AM
Creation date
10/5/2006 3:25:32 AM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
155
County
Adams
Jefferson
Community
Arvada
Stream Name
Clear Creek
Basin
South Platte
Title
FHAD - Clear Creek, Adams County and Jefferson County, Colorado
Date
11/1/1979
Designation Date
1/1/1980
Floodplain - Doc Type
Floodplain Report/Masterplan
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<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 />I <br />I <br /> <br />and SWMM (Reference 7) runoff models. MITCAT was used to model <br />the 400 square mile mountainous area upstream from Golden, and SWMM <br />was u~ed to model the lower 175 square mile plains basin. Rainfall <br />values used in the models were obtained from the Precipitation-Fre- <br />quency Atlas of the Western United States, Volume III, Colorado, <br />published by the National Oceanic and Atmospheric Administration <br />in 197~ (Reference 8). <br />Discharge-probability profiles for the 10-, 50-, 100-, and <br />50 a-year frequency storms are provided in Figure 3. Peak dis- <br />charges for the lOa-year frequency storm at selected design points <br />are listed in Table I. <br /> <br />The lOa-year flood elevations computed as part of this re- <br />port are different from those published in the Adams County Flood <br />Insurance Study (Reference 2) due to the revised discharges and <br />more detailed topographic mapping. The elevations in this report <br />also differ from those published in the Clear Creek F.P.I. (Refer- <br />ence 3) for the same reasons. <br />A detailed, technical appendix with all hydraulic calculations <br />is available at the offices of the Urban Drainage and Flood Control <br />District. <br /> <br />- FUTURE FLOODS - <br /> <br />Channel - 0.025 to 0.045 <br />Overbank - 0.040 to 0.070 <br /> <br />Flood Frequency and Discharge <br />Floods of the same and greater magnitude as those that have <br />occurred in the past will occur in the future. With development <br />in the basin, flooding will occur more frequently. The discharges <br />reported in Table I for the lOa-year flood frequency represent the <br />relative extent and impact of this flood event. The discharge <br />information is usable not only for floodplain regulation but also <br />for planning, engineering and floodplain management. <br />The 100-year flood event, which can be expected to occur at <br />any time in a given area, based upon recorded historical precipi- <br />tation and other valid data, has a one percent chance of being <br />equalled or exceeded during anyone year. <br />The laO-year flood event is considered by the Urban Drainage <br />and Flood Control District, the Colorado Water Conservation Board <br />and the Federal Emergency ~~nagement Agency as the flood magnitude <br />for which floodplains should be designated for regulatory and im- <br />provement purposes. In Colorado, the 100-year floodplain repre- <br />sents an area of state interest as defined in House Bill 1041. <br />The la-year flood magnitude has also been computed at various <br />design points along Clear Creek. This flood magnitude represents <br />an event which can cause damage but would not inundate the area <br />characteristic of a laO-year event. The peak flows for the la-year <br />event should be used in the planning and engineering of improve- <br />ments where the higher risk of failure or damage is economically <br />feasible and the hazard to life and property is lower or nonexistent. <br /> <br />Hydraulic Analysis <br /> <br />Hydraulic analyses were completed to determine the water-sur- <br />face elevations for the 10- and lOa-year storm events. The eleva- <br />tions were computed using the Corps of Engineers' HEC-II Water Sur- <br />face Profiles computer program (Reference 9). Valley cross sections <br />for the length of Clear Creek in Adams County were measured on <br />1"=100' scale, 2-foot contour interval topographic mapping which <br />was reduced and is included with this report. Valley cross sections <br />for the Jefferson County portion of the study were compiled through <br />photogrammetric methods by Bell Mapping Company, as the topographic <br />maps included with this report were prepared. Estimates of channel <br />and overbank roughness factors were made after a field investiga- <br />tion of the study area. Typical roughness or Manning's "n" values <br />used in this study are: <br /> <br />The computed flood profiles are plotted on the Flood Hazard <br />Area Delineation profile sheets (Sheets 22-29) included in the <br />Drawings section of this report. Also, the computed flood eleva- <br />tions for Clear Creek are tabulated at each reference point in <br />Table I. <br /> <br />-5- <br />
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