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FLOOD03181
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Last modified
1/25/2010 6:26:32 PM
Creation date
10/4/2006 11:30:41 PM
Metadata
Fields
Template:
Floodplain Documents
County
Weld
Community
Greeley
Stream Name
Cache La Poudre River
Basin
South Platte
Title
Master Floodway Plan
Date
8/1/1983
Prepared For
Greeley, CWCB
Prepared By
Simons Li & Associates Inc.
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />I <br /> <br />20 <br /> <br />I <br /> <br />21 <br /> <br />I <br /> <br />feet in-the Greeley reach (reference 6). Scouring problems are increased if <br />debris blocks some part of the bridge. In addition to a reduction in flow <br />area, debris blo9kages also cause a sudden change in flow direction which <br />increases turbulence. The Union Pacific Railroad bridge is especially prone <br />to debris blockage because of the large numbers of piers. <br /> <br />I <br /> <br />I <br /> <br />The aggradation problem can be controlled in one of two ways, namely, <br /> <br />remove sediment from the river upstream of the City, or increase the sediment <br /> <br />transport capacity of the river in the study reach. <br /> <br /> <br />To decrease the upstream supply, sediment must be trapped upstream of <br /> <br /> <br />Greeley. This can be accomplished by routing the river through an existing <br /> <br /> <br />(abandoned) gravel pit. The gravel pit will then act as a sediment trap and <br /> <br />reduce the sediment load downstream of the pit. The sediment trap will <br /> <br /> <br />require dredging on an annual basis to maintain the necessary volume and <br /> <br /> <br />detention time. A pit with a volume of approximately ten acre-feet is ade- <br /> <br /> <br />quate to trap the majority of sediment during low flows, but a volume of about <br /> <br />70 acre-feet would be required to effectively trap sediment during flood <br /> <br /> <br />flows. If sediment is trapped by a pit upstream of Greeley, precautions need <br /> <br />to be made to protect the channel bed and banks from scouring immediately <br /> <br /> <br />downstream of the pit. Scouring would occur because the river would be <br /> <br /> <br />transporting less sediment than its capacity and the river would begin to pick <br /> <br /> <br />up bed and bank materials until the sediment transport capacity is reached. <br /> <br /> <br />This local scour potential is of concern because bank instability can result <br /> <br /> <br />as can scouring at bridges which could undermine piers. <br /> <br />Sediment transport capacity can be increased in the study reach by <br /> <br /> <br />increasing the flow velocity. Velocity can be increased by (1) increasing the <br /> <br /> <br />channel slope, (2) providing a more efficient channel geometry, or (3) <br /> <br /> <br />reducing channel roughness. Increasing the channel slope is difficult to <br /> <br />I <br /> <br />accomplish in the study area because of the following (1) the confluence with <br /> <br />the South Platte River controls the downstream elevation, (2) several diver- <br /> <br /> <br />sion structures are in the vicinity and would require relocation, (3) lowering <br /> <br />the channel bed could undermine bridge supports and channel banks, and (4) the <br /> <br /> <br />channel would require excavation for a considerable distance downstream of the <br /> <br /> <br />corporate limits. Reducing channel roughness would require lining the channel <br /> <br />with concrete at great expense. A more efficient channel geometry can be <br /> <br /> <br />achieved by providing a narrow, deep, low-flow section. Since flow resistance <br /> <br /> <br />is reduced with increased depth of flow, velocity is increased, as is sediment <br /> <br /> <br />transport capacity. Larger floods would be contained in a wide overbank sec- <br /> <br /> <br />tion. Dredging requirements would be reduced but probably not totally elimi- <br /> <br /> <br />nated because the sediment supply and sediment transport cannot be balanced <br /> <br />for all flood frequencies. <br /> <br /> <br />In summary, the erosion and sedimentation problems can be controlled by <br /> <br /> <br />(1) providing riprap or other channel lining at the outside channel bends, <br /> <br /> <br />(2) providing either an upstream sediment trap OR providing a narrow, deep, <br /> <br /> <br />low-flow channel, and (3) limiting side slopes to a maximum of 2.Sh:lv. <br /> <br /> <br />Local scour at bridges can be reduced by using more efficient pier shapes <br /> <br /> <br />such as sharp-nosed piers or by providing connecting diaphragms between piers. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />Controlling Erosion and Sedimentation Problems <br />The qualitative geomorphic analysis indicates two major problems, namely, <br />aggradation and bank erosion. Aggradation in the study reach occurs because <br />the river upstream of the City can carry more sediment than the reach within <br />the City. Bank erosion is generally limited to river bends and can be <br />controlled by lining the bank on the outside of the bends. Suitable lining <br />materials include riprap, gabions (wire mesh baskets containing rock), <br />concrete, or soil cement. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br />
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