FLOOD06514 - Laserfiche WebLink

Home Browse Search

Erosion Reduction Methods. e dramatic effect upon the morphology of the river. Clearing and snagging reduces stages, changes the velocity distribution at a section, and can increase sediment transport through the reach. Selective clearing of bars and islands can cause realignment of the main channel of the river. Chute closures or channel blocks increase the flow in the main channel and reduce or deplete flows in the chute. The stage of the river will increase upstream of the structure, particularly during high flows. Both velocity and sediment movement may increase slightly in the channel. If flow is eliminated in the chute, sediment deposition will eventually fill it. Vegetation encroachment will occur in the chute, further reducing the flood capacity of the section. Vanes - Vanes are structures placed within the channel at a slight angle to the normal flow that reduce the secondary currents and thus reduce the erosive capacity of the river. The three most common types of vanes are 1) Iowa Vanes, 2) baftle vanes, and 3) stone vanes. Iowa vanes are small flow-training structures (foils), designed to modify the near-bed flow pattern and redistribute flow and sediment transport within the channel cross-section (Odgaard and Wang). The structures are typically installed at an angle of 15-20 degrees to the flow, with a height of 0.2 - 0.4 times loca1 water depth at designed stage. The vanes function by generating secondary circulation in the flow. The circulation alters magnitude and direction of the bed shear stresses and cause a change in the distributions of velocity, depth, and sediment transport in the area affected by the vanes. As a result, the river bed topography may be altered by selective layout of the structures. Baffle.type vanes are structures consisting of boards attached to piles that are placed in series in the stream to disrupt the secondary currents that cause erosion on the outside of meander bends. The number, locations, spacing, orientation, size, and height of the vanes are critical to success and must be determined from careful analysis. Stone vanes are low slone structures angled approximately 15 degrees to normal flow. They are overtopped by all but the lowest flows. Because vanes stop erosion by modifying secondary circulation, no bank sloping or treatment is necessary. Aquatic benefits are not destroyed, and once vegetation becomes re~tablished on the eroding bank, riparian haJ)itat and aesthetic benefits are improved. During low water the vanes are not very appealing visually, and there may be some hazard to navigation and to recreationists using the stream. Vanes have not been used extensively. Prototype vane systems have been installed in a couple of midweslem streams, including the East Nishnabotna River in Iowa. However, it is too soon to evaluate the success of the prototype demonstration at this site. These structures are proprietary, therefore, the user must contact the University of Iowa, Institute of Hydraulic Research for specific design guidance and permission. . Clearing and Snagging - For flood control on small streams, conventional clearing and snagging has been used to remove all obstructions from the channel and to clear all significant vegetation within a specific width on both sides of the channel. Key aspects of selective clearing and snagging involve: 1) selective removal of vegetation based on size, condition, species, or location; 2) removal of only those snags that are major flow obstructions; 3) use of hand labor and small equipment when feasible, and rigid access controls when heavy equipment must be used; 4) protection of existing vegetation of disturbed areas; and 5) greater reliance on multidisciplinary teams in all phases of project planning and management. Disturbed areas should be restored to natural contours, and preserved trees should be spaced at irregular intervals. Natural sloughs, Colol'IIdo Erosion Control Manual 45