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The following is an excerpt from hydrologist Jerry Fifield's upcoming textbook, Designing for Effective Sediment and Erosion Control on Construction Sites. By Jerald S. Fifield |
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Check dams are small temporary barriers constructed across a swale, a gully, or small drainageways to "check" or reduce gully erosion until stabilization occurs or to capture small amounts of sediment in runoff waters. They are intended to reduce flow velocities, trap and store larger-sized sediment, and provide stabilized drops. Limitations on use of check structures include:
Historically, check structures have been constructed out of straw bales, rock, or silt fence material. Since the force of concentrated flows is large, however, it is recommended that silt fence material not be used as a check structure unless properly installed, supported, and anchored. If check structures are to reduce channel erosion until stabilization occurs, it is important that they be installed in a manner that allows for the energy dispersion of runoff. Other than providing some armoring of the channels, the energy of water energy will have to be dispersed by creation of stilling ponds (see Figure 1).
The lower two structures in Figure 1 provide an illustration of how check structures must be placed to ensure channel bottom stability. If the channel has a slope of Z percent (e.g., 4.3%), then by similar triangles, D = (100 x H) ÷ Z Figure 2 illustrates the spacing of check structures having various heights for different channel slopes as calculated by the above equation.
Notice the short distance of structure spacing for relatively flat slopes. For example, a 30-cm- (12-in.-) high check structure (measured at the flow line) can be placed about 30 m (107 ft.) apart as long as the channel slope does not exceed 1.0%. Channel degradation can be anticipated, however, if the 30-cm-high structures are placed 30 m apart and the slope is 3.0%. Perhaps the most widely used (and abused) check structures are constructed from bale barriers. While the intent of these structures is to stabilize a drainage channel and/or trap sediment, often the opposite occurs. Evaluating Bale Check Structures in Channels If bale check structures are to be used in a drainage channel, they must be installed in a trench, staked, and backfilled if they are to be effective in reducing flow velocity. In addition, the top of the downstream bale should be approximately level with the lowest ground elevation of the upstream bale. Bales must be installed so the top of the flow-line bale is significantly below ground level of the outside and lowest sideslope bale edge. When channel sideslopes are flatter than about 7:1, other methods for protecting channels should be considered. Figure 3 illustrates proper placement of bale check structures.
If bale barriers must be used, it is important to heed the following:
Maintenance Bale check structures should be inspected after every storm event that results in runoff. Structures displaying erosion around or under the bales must be repaired immediately; they should not remain in place once significant deterioration is evident. In summary, bale check structures (1) must be properly installed, (2) must be maintained, and (3) must be removed. Rock Check Structures Rock check structures are temporary barriers constructed across a swale or drainage ditch (see Figure 4). They can lower the velocity of concentrated stormwater flows and reduce channel degradation. In addition, they have much less of a tendency to fail when compared to use of bales.
It is important that sufficient-sized rocks be used as a mixture to ensure stability of the structure. When low flows occur, the rock size must have diameters that range from 8 to 15 cm (3 to 6 in.). Larger-diameter rock must be used for high flows and will require a design by qualified professionals.
Rock check structures must be installed with the flow line 15 to 45 cm (6 to 18 in.) lower than the sides. In this manner, water will flow over the structure rather than around it. Regular inspections should be made to ensure that the center remains lower than the edges. Erosion caused by high flows around the edges must be repaired immediately. Many new products exist that can be used in place of bale or rock check structures. It is important to remember that the use of any check structure is usually a temporary measure while construction activities occur. Unless designers seek a series of terraces within a drainage channel using permanent check structures (e.g., rock, concrete blocks), using erosion control measures in drainage channels must occur once construction activities have stopped. Such measures include establishing vegetation, installing rolled erosion control products, or placing hard armoring, such as riprap or articulated blocks. Jerald S. Fifield, CPESC, Ph.D., is a hydrologist with HydroDynamics Inc. in Parker, CO.
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