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FLOOD09563
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Last modified
1/26/2010 10:09:42 AM
Creation date
10/5/2006 4:27:58 AM
Metadata
Fields
Template:
Floodplain Documents
County
Dolores
Community
Denver County
Stream Name
Sand Creek, Oxford Creek
Basin
South Platte
Title
Flood Hazard Area Delineation
Date
9/1/1985
Prepared For
Denver County
Prepared By
UDFCD
Contract/PO #
&&
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />"'~~.... - .--- <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 />I <br />I <br /> <br />-8- <br /> <br />the U.S. Army Corps of Engineers HEC-2 water surface profiles computer pro- <br />gram. Valley cross sections were developed using digitized cross sections <br />obtained from aerial photography flown on April 16, 1983. The channel bot- <br />tom elevation shown on these cross sections was actually the water surface <br />elevation on that day, rather than the ,actual stream thalweg. Since there <br />was a significant flow in the South Pl atte River on that day, it was be- <br />l ieved that in certain cases as much as two-feet of difference could exist <br />between the water surface elevation on April 16, 1983 and the actual stream <br />thalweg. In those areas where the water surface prOfiles for the 100-year <br />flood showed that overbank flooding would occur, the thalweg was surveyed in <br />the field or adjusted by assuming a 1.5 foot depth of flow. In addition, in <br />cases where the channel bottom profile obtained from the April 16, 1983 map- <br />ping varied significantly from the channel inverts used by the Army Corps of <br />Engineers in their floodplain analysis in 1977, surveys were performed to <br />confirm the correctness of the cross section information used in this re- <br />port. Sellards and Grigg Engineers have performed a detailed hydraulic <br />study for the reach from Cherry Creek to Colfax Avenue. As part of that de- <br />tai led study, they surveyed river cross sections from approximately Cherry <br />Creek to Interstate 25. These surveyed cross sections were incorporated in- <br />to the digitized cross sections obtained from the April 16, 1983 mapping. <br />In general, the areas where the digitized cross sections were modified are <br />from York Street to Franklin Street, from 23rd Avenue to 15th Street, from <br />Cherry Creek to Interstate 25, at the 3rd Avenue Dam, and at the drop struc- <br />tures near Mississippi and Florida Avenues. <br /> <br />Engineers in their 1977 study. The results of this study were consistent <br />with the results of the Corps study except in areas where there were differ- <br />ences in discharges and stream thalweg elevations. <br /> <br />Starting elevations were obtained from the "Flood Hazard Delineation, South <br />Platte Ri ver, Adams County" completed in September 1 977 . A detail ed de- <br />scription of the hydraulic analysis is included in the Technical Addendum. <br /> <br />A floodway was also defined for the study reach of the South Platte River. <br />A floodway represents the channel and adjacent floodplain which is required <br />to pass the 100-year flood without raising the flood profile more than an <br />acceptable amount. It also represents that part of the floodplain most haz- <br />ardous to personal safety and welfare. In this study, the floodway defines <br />a flow area required to convey the 100-year flood without increasing the <br />100-year energy grade line, and, therefore, the water surface elevation by <br />more than 1.0 feet. Floodway widths, which represent the maximum limits of <br />encroachment into the floodplain, are tabulated in Table 1, <br /> <br />The locations of the field surveyed cross sections, the digitized cross sec- <br />t ions from the topographic maps, and the assumed inverts are shown on the <br />flood hazard area delineation maps. All hydraulic analysis was done on the <br />basis of existing channel conditions, <br /> <br />In Table 1 the rise in the 100-year flood water surface elevation is greater <br />than 1.0 feet at sections 780 to 860. This occurs because under existing <br />conditions part of the 100-year flood flow is carried in a separate channel <br />on the east side of Interstate 25 from Eighth Avenue to Speer 80ulevard. By <br />the time the east channel reaches Colfax Avenue it carries almost 20 percent <br />of the total flow. When this flow is confined to the main channel, the <br />water surface elevation is increased more than 1.0 feet, even without any <br />encroachment. <br /> <br />Estimates of channel and overbank roughness factors were made by field in- <br />spection and use of photographs. Typical roughness, or Manning's "n" val- <br />ues, used in this study are: for the channel .03 to .035, for the overbank <br />.04 to .07. The 100-year flood elevations computed for this study were com- <br />pared to the 100-year flood elevations developed by the U.S. Army Corps of <br />
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