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FLOOD00158
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Last modified
1/29/2010 10:15:20 AM
Creation date
10/4/2006 9:07:28 PM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
123
County
Adams
Arapahoe
Douglas
Community
Denver Metro Region
Stream Name
Little Dry Creek
Title
Flood Hazard Area Delineation - Little Dry Creek, Adams & Jefferson Counties, Arvada & Westminster, CO
Date
6/1/1978
Designation Date
11/1/1978
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 />HYDRAULIC DETERMINATIONS <br /> <br />Analysis of the hydraulic characteristics of Little Dry Creek and its tributaries was <br />performed to provide estimates of the water surface elevations of each of the design <br />floods. A total of approximately 15.4 miles of channel was included in the analysis. <br />Water surface profiles were computed using the U. S. Army Corps of Engineers HEC-2 <br />computer program. Cross-sections were digitized when the topographic maps were <br />compiled and have been augmented by field surveys at culverts. Locations of the cross- <br />sections are shown on the Flood Hazard Area Delineation Plan and Profile Sheets. <br />Channel roughness coefficients (Manning's "n" values) for these computations were <br />assigned on the basis of field inspection of the floodplain areas. Typical values for <br />Manning's 'n' used in this study are: <br /> <br />natural channel sections <br />natural overbank areas <br />obstructed overbank areas <br /> <br />0.04 to 0.066 <br />.045 to 0.055 <br />.055 to 0.115 <br /> <br />0.03 to 0.04 <br /> <br />0.015 to 0.025 <br /> <br />maintained grass channel sections- <br />concrete or asphalt channel or <br />overbank areas <br /> <br />Hydraulic losses through culverts on Little Dry Creek have been determined with <br />the use of nomographs published by the Bureau of Public Roads, Reference I, and input <br />into HEC-2 program. In developing rating curves for the culverts in the study reaches, <br />zero blockage was assumed for culverts greater than 3 feet in diameter. The normal <br />bridge routine of the HEC-2 program was used to calculate losses through non-standard <br />culverts. Culvert data is listed in Table 3. <br />Computed flood profiles for the 10- and 100- year floods and the floodplain <br />delineation for the 100-year flood for future basin conditions are plotted on the Flood <br />Hazard Area Delineation Drawings for the main stem and three tributaries. Floodplain <br />and flOOdway reference data are tabulated in Tables 4, 5, 6 and 7. More detailed <br />information on the hydraulic calculations is available at the office of the Urban Drainage <br />and Flood Control District. <br /> <br />-18- <br /> <br />FLOOD HAZARD AREAS <br /> <br />The 100-year flood has been selected by the Federal Government, the State of <br />Colorado, the Urban Drainage and Flood Control District, and local governments as the <br />flood event to be used for flOOdplain delineation and regulation. Thus, the Flood Hazard <br />Area maps depict the profile and floodplain limits of the 100-year flood within the study <br />reaches. Floodplains for the 2-, 5-, 10-, 25-, and 50-year events determined as part of the <br />concurrent major drainageway planning study indicate a number of areas of Little Dry <br />Creek exposed to severe flood hazards even in lesser events. In addition to the 100-year <br />flood, the 10-year flood profile is shown on the drawings. <br />Floodplain and floodway reference data including potential100-year flood discharge, <br />thalweg elevation, flood elevations and widths, are listed by cross-section in Tables 4, 5, 6 <br />and 7. Floodplain reference data corresponds to the 100-year floodplain as shown on the <br />Flood Hazard Area Delineation Maps. <br />The floodway is defined as the minimum channel required to pass the 100-year flood <br />without increasing flood depths or velocities more than a specified amount over the <br />natural floodplain conditions. The floodway data for Little Dry Creek listed in the <br />reference tables was developed using the more restrictive of either the Urban Drainage <br />District or local criteria. The following floodway criteria have been used for the <br />respective jurisdictions: <br /> <br />Adams County: Velocities of 2 feet per second or depths greater than 18", and/or <br />1/2 ft. rise in the water surface profile based on equal encroachment. <br /> <br />Westminster: the UD&FCD criteria of 1/2 ft. rise in the water surface based on <br />equal encroachment. <br /> <br />Arvada: the UD&FCD criteria. <br /> <br />Jefferson County: Velocities of 3 feet per second or depths greater than 1 foot. <br />
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