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<br />001400 <br /> <br />- 7 - <br /> <br />also used near or at a headcut to prevent its further movement. Chutes <br />and drop inlets are used for the same purpose. Drop structures are fre- <br />quently used in a series. Scour below structures can most effectively be <br />controlled by appropriate spacing in the series. Isolated drop structures <br />in a reach with extensive erosion are not very effective except to control <br />the problem at the specific site. <br /> <br />Channel lining is used to protect the bed and/or banks when it has been <br />determined that excessive erosion will occur without this protection. This <br />measure is usually effective in preventing erosion to the level of the flood <br />frequency for which it is designed. <br /> <br />Sills have little impact on sedimentation except to prevent additional sedi- <br />ment from being derived from channel degradation. Their single purpose is <br />to prevent further degradation or a new cycle of erosion. <br /> <br />Jacks are used roughly parallel to and in front of the bank to direct the <br />flow to a specific width and direction and to furnish protection to the bank. <br />In some instances deposition behind a series of jacks provides a coating to <br />the bank and encouragement to the development of levees. <br /> <br />Jetties, in projecting usually at an angle into the stream flow, are intended <br />to protect only a local segment of the bank. The artificial change in dir- <br />ection of flow may tend to create a similar problem at another place unless <br />it is part of an integrated plan. <br /> <br />Revetments protect the specific site where the installation is made. They <br />are most appropriately used when adjacent banks are stable, such as in a <br />vulnerable bend or where the revetments will provide a comprehensive treat- <br />ment of all banks in the reach. <br /> <br />Structural measures for erosion and sediment control should be evaluated <br />individually on the basis of purpose, site suitability and on the projected <br />benefits as related to costs. <br /> <br />Evaluation of Management and Land Treatment Measures <br /> <br />The elements of a recommended conservation program pertaining to erosion <br />and sediment control may be very broad or fairly specific. Some of the more <br />specific measures which have been defined above and that are included in <br />Table I may need to be combined or modified to match the scope of the recom- <br />mendations. On Table I are given some of the management and land treatment <br />measures considered favorable for application on land with site conditions <br />listed. <br /> <br />In estimating the probable effect of individual or groups of measures on <br />erosion for anyone delineated area, the following steps are recommended: <br /> <br />