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<br />EM 1110-2-1601 <br />i July '70 <br /> <br />. <br /> <br />. <br /> <br />-, <br />I Co ' by 50 percent when the riprap is placed underwater to provide for uncer- <br />tainties associated with this type of placement. <br />(4) An increase in thickness of 6 to 12 in., accompanied by appropriate <br />increase in stone sizes, should be provided where riprap revetment will be <br />subject to attack by large floating debris or by waves from boat wakes, wind, <br />and bed ripples or dunes. <br />j. Revetment Toe Protection. As the toe of rip rap revetment is sub- <br />jected to greater erosive forces than other areas of bank revetment, the <br />layer thickness should be increased or the toe should be extended below the <br />anticipated scour depth. The following criteria apply to th~ alternative <br />methods of toe protection illustrated iil plate 37. <br />(1) Method A. When toe excavation can be made in the dry, the normal <br />riprap layer should be extended below the existing groundline a distance a <br />equal to the anticipated depth of scour. <br />(2) Method B. When the low-flow or groundwater level is near the <br />existing groundline so that toe excavation is in the wet, a horizontal rock toe <br />.,. ,should be provided to a depth a equal to 3 to 5 ft, a thickness b not less than <br />" , <br />\' the layer thickness T, and a base width c not less than a, except that when <br />the anticipated erosion depth is more than 2 or 3 it below the rock toe, b <br />and/ or c should be increased to provide sufficient stone for adjustment of <br />the toe and protection of the revetment as degradation o'ccurs. <br />(3) Method C. When the riprap is to be placed underwater and design <br />velocities will not cause appreciable streambed erosion, the toe should be <br />placed on the existing channel bottom with a and c equal to 1.5T and 5T, <br />respectively. <br />(4) Method D. When the riprap is to be placed underwater and appre- <br />ciable streambed erosion may occur, a thickened rock toe should be placed <br />in a trench with a and c equal to 3T and 5T, respectively. <br />(5) Method E. When the bottom of the channel is in rock, the normal <br />layer of riprap should be keyed into rock at streambed or berm level. <br />k. Revetment Top and End Protection. Normally, the upper extremity <br /> <br />. <br /> <br />. <br /> <br />f <br /> <br />14k <br /> <br />44 <br /> <br />I <br /> <br />- <br />.. ' ~ ~ ,', > ," ~ . '. ,.,..., -", <br /> <br />.:.-:...~-, <br />