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Last modified
1/25/2010 6:27:19 PM
Creation date
10/4/2006 11:46:12 PM
Metadata
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Template:
Floodplain Documents
County
Statewide
Community
State of Colorado
Stream Name
All
Basin
Statewide
Title
Colorado Flood Proofing Manual
Date
10/1/1983
Prepared For
State of Colorado
Prepared By
CWCB
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />14 <br /> <br />Vegetation is also important along the banks of a river or a stream, Along the bank, good <br />vegetation adds to the stability of the bank and thus reduces the bank erosion and lateral <br />movement of the channel. <br /> <br />A major concern when developing in the flood plain is removing the natural vegetation and <br />the protection it provides. When natural vegetation is removed, measures should be taken to <br />replace it immediately or provide stability for the resulting unprotected areas. <br /> <br />3.4 Soil Characteristics <br /> <br />Soil characteristics are important in erosion, sedimentation, infiltration and seepage. Discus- <br />sion of erosion and sedimentation is given in the next section, One of the most important <br />soil characteristics is permeability, a measure of the ability of water to flow through a par- <br />ticular soil. Permeability, to some degree, controls the amount and rate of surface water <br />infiltration, and the zone of saturation, A saturated soil condition occurs when water occupys <br />all the available spaces, or voids, between individual soil grains. Soils with high permeability <br />such as sands and gravels have higher infiltration rates then clayey soils. This implies a faster <br />runoff from clay soils, whereas sandy soils may not permit surface runoff until saturated, <br />However, more importantly, ground water and seepage are more active in sandy soils <br />because of their rapid drainage characteristics. <br /> <br />Groundwater and seepage are major concerns when flood proofing and/or development in <br />the flood plain is being considered, Groundwater level is a significant component of the hyd- <br />rostatic forces that develop during flooding and must be considered in the structural design <br />of footings, foundations and basement walls, Structures b~low the ground level must also <br />prevent seepage into the building, thus, the seepage characteristics of the soil are an impor- <br />tant consideration. <br /> <br />Another concern in regard to soil characteristics that should not be overlooked in areas with <br />steep slopes is the possibility of landslides, land slumping or debris flow, As soils become <br />saturated, they can loose their slope stability resulting in landslides or mudflow situations, <br />Mudflow evaluation requires detailed analysis, however, a qualitative evaluation of an area <br />can provide insight into the potential for mud or debris flow. In general, mudflows can occur <br />where an accumulation of loose soil, rock and other debris on steep slopes become <br />entrained (saturated) with water, The first characteristic to look for is a history of mudflows <br />and how often have they occurred in the area. Areas where mudflows are formed are charac- <br />terized by quite definite geological, geomorphological and climatic conditions of erosion pro- <br />cesses and by the presence of mudflow forming centers. Specific characteristics include <br />badly sheared and shattered bedrock which yields fine and poorly sorted micaceous fines <br />upon weathering and some type of narrow rock gorge or ravine where the debris and <br />weathered material can collect. <br /> <br />3.5 Erosion and Sedimentation <br /> <br />Erosion and sedimentation are processes that occur naturally during flooding, As the dis- <br />charge and velocity of surface runoff increase, the erosional forces also increase. All fine <br />material such as silts and sands become suspended in the flow, Larger rocks and boulders <br />are rolled along the bottom of the main channel. The main sources of sediment being <br />transported by the surface runoff are from the channel bottom and bank erosion. Sedimenta- <br />tion or deposition of the sediments being transported begins when the velocity of the sur- <br />face runoff decreases to the point where the sediments can no longer remain in suspension. <br />During flooding, much of the sediment is deposited on the flood plain due to slower <br />velocities as water leaves the main channel. <br /> <br />Sedimentation can cause several problems, Low lying areas in the flood plain can become <br />completely filled over with sediment. Sediment increases abrasive action and adds to the <br />work and cost of cleanup. Sediment can also caUse blockage of existing channels diverting <br />water to new channels, <br />
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