|
|
Streambeds |
Check Dam: A small dam constructed across an influent, intermittent drainageway to reduce channel erosion by restricting flow velocity. Check dams are not meant for live streams. They can serve as emergency or temporary measures in small eroding channels that will be filled or permanently stabilized at a later date, such as in a construction setting. They can also serve as permanent measures that will sediment in over time in gullies, which is a more common usage in range and agricultural settings. In permanent usage, when the impounded area is filled, a relatively level surface or delta is formed over which the water flows at a noneroding gradient. the water then cascades over the dam through a spillway onto a hardened apron. By constructing a series of check dams along the gully, a stream channel of comparatively steep slope or gradient is replaced by a stair-stepped channel consisting of a succession of gently slopes with "cushioned" cascades in between (Gray and Leiser, 1982). For temporary usage, consider the alternatives of protecting the channel bottom with materials such as riprap, geotextile, biodegradable, or other matting, or other linings in combination with vegetation before selecting check dams (Smolen et al., 1988). Dams can be nonporous, such as those constructed from concrete, sheet steel, or wet masonry, or they can be porous, using available materials such as straw bales, rock, brush, wire netting, boards, and posts. Porous dams release part of the flow through the structure, decreasing the head of flow over the spillway and the dynamic and hydrostatic forces against the dam. Nonporous dams are durable, permanent, and more expensive while porous dams are simpler, more economical to construct, and temporary. For construction details on a number of temporary check dam types, see Gray and Leiser (1982).
Deflectors: Hardened structures anchored in the streambank and protruding into the current with an upstream face that is angled downstream at approximately 45 degrees from the flow. A downstream brace is set at approximately 90 degrees from the upstream deflector and is also anchored in the bank. Gravel/rock fill is placed in the interior of this wing structure. Deflectors are often used in series on alternating banks at separation distances of 5 to 7 channel widths to provide a natural sinuosity of flow, with the final placement involving a pair on opposing banks to reorient the flow down the channel center. The primary use of deflectors is for habitat enhancement in shallow, lower gradient streams lacking pools and riffles, but they can also serve to improve water quality by slowing flows, allowing for sediment deposition, and increasing channel sinuosity and hydraulic residence time. Single deflectors also cause the formation of scour pools, shelter pools, and riffles, while double wings lead to the formation of deep scour pools. For details on deflector construction, please see Finley, Gore, and Hamilton (1992).
Grade Stabilization Structure: A structure designed to reduce channel grade in natural or constructed watercourses to prevent erosion of a channel that results from excessive grade in the channel bed or artificially increased channel flows. This practice is used to stop headcut erosion or stabilize gully erosion. Grade stabilization structures may be vertical drop structures, concrete or riprap chutes, gabions, or pipe drop structures. Permanent ponds or lakes may be part of a grade stabilization system. Concrete chutes are often used as outlets for large water impoundments where flows exceed 100 cfs and the drop is greater than 10 ft. Where flows exceed 100 cfs but the drop is less than 10 ft., a vertical drop weir constructed of reinforced concrete or sheet piling with concrete aprons is generally recommended. Small flows allow the use of prefabricated metal drop spillways or pipe overfall structures. Designs can be complex and usually require detailed site investigations. Design of large structures (100 cfs) requires a qualified engineer. The National Engineering Handbook (Drop Spillways, Section 11, and Chute Spillways, Section 14) prepared by the USDA Natural Resources Conservation Service gives detailed information useful in the design of grade stabilization structures (Smolen et al., 1988).
Low-Head Dam (Weir): Essentially the same type of construction as the check dam, built from rocks, logs, or other material, but intended for use in lower order perennial streams for water quality improvement and habitat enhancement. Weirs are most successful in streams with discharge not exceeding 6m3/s. Benefits include formation of pool habitat, collection and holding of spawning gravels, promotion of gravel bar/riffle formation, trapping suspended sediments, reoxygenating water, allowing organic debris deposition, and promotion of invertebrate production. For details on weir construction, please see Finley, Gore, and Hamilton (1992) or Gray and Leiser (1982).
Finley, M.T., J.A. Gore, and S.W. Hamilton, 1992. Proposed Best Management Practices for Improving Water Quality in the West Sandy Watershed. Austin Peay State University, Clarksville, TN.
Gray, D.H., and A.T. Leiser, 1982. Biotechnical Slope Protection and Erosion Control. Van Nostrand Reinhold Company, New York, NY.
Smolen, M.D., D.W. Miller, L.C. Wyatt, J. Lichthardt, A.L. Lanier, W.W. Woodhouse, and S.W. Broome, 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission, NC Dept. of Natural Resources and Community Development, Raleigh, NC.
|
NCSU Water Quality Group |
|