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<br />Erosion Reduction Methods. <br /> <br />e <br /> <br />There is usually no disruption of riparian habitat in the construction of a closure, and <br />effects in the main channel are minimal if the chute is not oversized. Chute closures <br />without low-flow provisions ultimately lead to a deterioration of the quality and <br />availability of aquatic habitat. Initially, backwater areas are created but these soon fill <br />with sedimenl. <br /> <br />Chute closures have been used extensively on rivers throughout the US. They are an <br />integral part of most navigation projects. Successful design of chute closure structures is <br />dependent upoo accurate hydraulic determinations. The structures must be designed so <br />that they will not fail by overflow or piping. Environmental concerns should be <br />addressed during the design phase to insure maintenance of important habital. <br /> <br />Grade Control Structures - Maintaining bank stability, grade control structures prevent <br />undermining of degrading streams. This can be achieved by the use of log steps, trash <br />collectors, and check dams. Grade control structures, shown in Figure 4 - 16, are placed <br />bank to bank and constructed pelpendicular to the direction of flow. Special attention is <br />needed to insure that the structure is properly keyed into the bank to prevent scour. Log <br />steps are constructed by log place across the channel or by naturally fallen logs. Trash <br />collectors rely on debris in the river. Posts and wire are placed across the river as <br />described above, and the debris is collected by the wire. This debris, or trash, then <br />reduces flow energy. If sufficient debris is not available, reinforced fabric can be used as <br />a substitute. Check dams are a series of small dams used similar to the log steps. <br /> <br />Grade control structures reduce stream velocity thus retaining water longer. This <br />increases sediment deposits and increased plant life. Check dams, however, promote <br />bank erosion downstream and are usually quite expensive. Design of channel blocks, <br />chute closures, grade control structures, and check dams require detailed hydraulic <br />analysis and should be done by an experienced engineer. <br /> <br />FIGURE 4 -16 <br /> <br />Drop grade control slructu1'9 <br /> <br />Flow <br /> <br />r---.. <br /> <br />s_J <br /> <br /> <br />Water Surface <br /> <br /> <br />~ <br /> <br />Concrete <br /> <br />e <br /> <br />COlorado ErosIon Control Manual <br /> <br />47 <br />