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e e e DRAINAGE CRITERIA MANUAL (V. 2) HYDRAULIC STRUCTURES Wetland check structures involve construction across the entire low-flow channel because wetland channels do not have a trickle channel. Necessary care during construction includes insuring the extension of the cutoff wall into the main channel a minimum of 10 feet to avoid side cutting. The cutoff wall is important to help sustain a water table upstream of the sill. Riprap or large rock downstream of the cutoff wall should be properly shaped to dissipate the kinetic energy of the force of the drop when un- submerged so as to avoid erosion downstream of the sill that would inhibit wetland vegetation and appear unsightly. One type of check structure that can be used to stabilize low-flow channels within wide, relatively stable floodplains is the control check (see Figure HS-13). The check can be constructed by filling an excavated trench with concrete if soil conditions are acceptable for trenching, or by forming a simple wall if a trench will not work. The check structure, which crosses the low-flow channel, should extend a significant distance into the adjacent floodplain on both sides. The top of the check should conform to the top of the ground at all points along its length. Riprap or other erosion control methods can then be added as erosion occurs. The basic design steps are to (1) determine a stable slope as described earlier in this chapter and in the MAJOR DRAINAGE chapter and (2) determine spacing of the checks based on the difference in slope between the natural and projected stable slope and the amount of future drop to be allowed (not to exceed 3 feet). The MAJOR DRAINAGE chapter provides excellent examples of control check structures. 06/2001 Urban Drainage & Flood Control District HS-41