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Last modified
1/26/2010 10:08:49 AM
Creation date
10/5/2006 4:11:19 AM
Metadata
Fields
Template:
Floodplain Documents
County
Denver
Community
Denver
Stream Name
Marston Lake North
Basin
South Platte
Title
Major Drainageway Planning
Date
10/1/1979
Prepared For
Denver
Prepared By
UDFCD
Contract/PO #
&&
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 />10 <br /> <br />V. BASIS OF DESIGN <br /> <br />Underground Conduits <br /> <br />The Marston Lake North Phase A report "Development of Alternative <br />Plans" was prepared by Camp Dresser & McKee Inc. in February, 1979. <br />The report develops the basic data and the potential drainage alter- <br />natives for the drainageway planning so that the City and County of <br />Denver, the Denver Water Department and the Urban Drainage and Flood <br />Control District can select the most desirable alternative. The <br />recommended and selected alternative is construction of channel impro- <br />vements including open channels, closed conduits, street crossings, <br />and a sediment settling pond. The following paragraphs describe the <br />design criteria used in this report. <br /> <br />Underground conduits were used for drainage improvements for reaches <br />where channels were not practical, such as street crossings. Culverts <br />were designed to convey the design flow with either free-flow or <br />surcharged conditions. Wing walls were provided at culvert entrances <br />and exits to ensure smooth transitions. Manning's n values used were <br />0.015 for cast-in-place concrete conduits and 0.024 for corrugated <br />metal pipes. In streets where low flow pipe was used, the overflow <br />depth was kept to 1 ess than one foot for maj or storms. <br /> <br />Sedimentation Pond <br /> <br />Open Channels <br /> <br />A sedimentation pond was designed to contain the future runoff volume <br />from the tributary drainage area for a 2-year I-hour storm event with <br />no embankment or spillway overflow. It is assumed that sediment in <br />the tributary drainage area will be washed out during a storm of such <br />magnitude. The sedimentation pond was designed to reduce the sediment <br />load that would be delivered by the drainage improvements and urbani- <br />zation in the basin. <br /> <br />Open channels were used for the Marston Lake North drainage improve- <br />ments wherever possible because of their relatively low cost, <br />large capacity and potential for recreational use and detention <br />storage. Grass-lined channels were chosen for most of the improvement <br />reaches for the following reasons: (1) they have a relatively low <br />construction cost; and (2) they provide a desirable green belt and <br />recreational area adding significant social benefits. Concrete-lined <br />channels were used in some reaches because of the limited available <br />space. <br /> <br />The emergency spillway was designed to pass a major storm without <br />embankment overflow. Outlet work was designed to drain the ponding <br />water below the riser slowly so that the finer silt particles could be <br />settl ed. <br /> <br />The Urban Storm Drainage Criteria Manual of the Urban Drainage and <br />Flood Control District was used as a guide for channel design. A <br />trickle channel was used in conjunction with a grass-lined channel to <br />improve channel efficiency and maintenance during low flow periods. A <br />minimum of one foot free board from the design water surface to the <br />top of all channels was provided as a safety factor. Manning's n <br />values used were 0.03 for grass-lined channel and 0.015 for cast-in- <br />place concrete channel. <br /> <br />Cost Estimate <br /> <br />Supercritical flow in an open channel in an urban area creates certain <br />hazards. The open channels in this report were designed to have <br />subcritical flow with drop structures located where necessary. <br /> <br />The development of construction costs for the recommended alternative <br />was based on June, 1979 costs. Tables 5 and 6 present unit costs and <br />unit operation and maintenance costs used in the development of pro- <br />ject costs. All unit costs include labor and overhead for a par- <br />ticular construction item. Detailed construction costs were presented <br />with each of the plan and profile drawings at the end of this report, <br />including cost of channel improvements, street crossing, utility relo- <br />cation, property acquisition, present worth of operation and/or main- <br />tenance costs, and contingencies. <br />
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