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FLOOD07631
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Last modified
1/25/2010 7:12:20 PM
Creation date
10/5/2006 3:07:05 AM
Metadata
Fields
Template:
Floodplain Documents
County
Jefferson
Community
Littleton
Stream Name
South Platte River, Near Bowles Avenue
Basin
South Platte
Title
Floodplain and Floodway Study - South Platte
Date
6/1/1984
Prepared For
Littleton
Prepared By
Wright Water Engineers, Inc.
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />SECTION IV <br />BASIC DATA <br /> <br />determined to be reasonable. The overbank "n" value of <br />previous studies and the Wright Water Engineers study. <br />calibrated, however, it seems reasonable based on aerial <br />vious hydrologic studies of the South Platte River. <br /> <br />.05 was used by the <br />This value was not <br />photographs and pre- <br /> <br />The basic data utilized in the Wright Water Engineers 1984 floodplain and <br />fl oodway study i nc 1 uded: <br />I. Topographic mappin9 prepared for the Urban Drainage and Flood Control <br />Di strict. <br />2. Bridge data prepared for the District from field surveys, plus field <br />measurements of the bridges by Wright Water Engineers. <br />3. Field survey of the water surface profile to determine roughness factors. <br />4. Previous studies done by other consultants. <br /> <br />Ground control and aerial photography for mapping of the study area was per- <br />formed on April 16, 1983 by Delta Aeri al Surveys. The work was performed to <br />provide mapping for the South Platte River Master Drainage Plan. The extent <br />of this mapping was expanded by Delta to provide coverage of the entire study <br />area. <br /> <br />Delta Aerial Surveys provided 23 digitized cross sections of the South Platte <br />River between Belleview Avenue and 2,000 feet upstream of the Bowles Avenue <br />Bridge. The first 10 cross sections are in the same locations as those used <br />by Centennial Downs. Since there was a flow of 210 cfs in the South Platte <br />River on April 16, 1983 it was necessary to adjust the digitized cross sec- <br />tions to reflect the true channel thalweg. A normal depth calculation was <br />done to determine that the water would have been one foot deep on that day. <br />Therefore, one foot was subtracted from the water surface elevations in the <br />digitized cross sections. <br /> <br />Information on the Belleview Avenue, Crestline, and Bowles Avenue bridges was <br />obtained from Delta Aerial Surveys as part of the South Platte Master Drainage <br />Plan. Bridge data include elevations of the low chord, top of road, and the <br />top of railings; width and number of piers; bridge opening widths; and embank- <br />ment side slopes. To supplement this survey information Wright Water Engi- <br />neers conducted field surveys to confirm elevations, area, widths, pier shape, <br />etc. <br /> <br />A rating curve for the Crestline Bridge was obtained from the U.S. Geological <br />Survey which maintains a gage near the bridge. This gage has been rated only <br />to a flow of 4,000 cfs and the datum of the gate is an arbitrary datum which <br />has not been tied into the datum used by Delta Aerial Surveys. For these <br />reasons, the rating curve from the U.S. Geological Survey was not used. The <br />USGS did state that the rating curve has shifted to the left over the years, <br />i.e., there is now more flow at a given elevation. <br /> <br />A channel roughness coefficient ("n" value) for use in the HEC-2 hydraulic <br />model of 0.03 was used in this study. This "n" value is consistent with pre- <br />vious studies. To confirm this value Wright Water Engieners requested a field <br />survey from TriConsultants, Inc. of the water surface profile. This survey <br />was performed on May 23, 1984, when the recorded flow in the South Platte <br />River was 1,700 cfs at the Littleton U.S.G.S. gage. The "n" values obtained <br />by inputting the known flow and water surface profile in the HEC-2 model <br />ranged from 0.025 to 0.35 in the study area. An average value of 0.03 was <br />
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