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<br />Chapter 3 <br />Riprap Protection <br /> <br />Section I <br />Introduction <br /> <br />3-1. General <br /> <br />The guidance presented herein applies to riprap design for <br />the following conditions: open channels not immediately <br />downslream of stilling basins or other highly turbulent <br />areas (for stilling basin riprap, use HDC 712-1), and chan. <br />nel slopes less than 2 percent. The ability of riprap slope <br />protection to resist the erosive forces of channel flow <br />depends on the interrelation of the following f:lCtors: <br />stone shape, size, weight. and durability; riprap gradation <br />and layer thickness; and channel alignment. cross.section, <br />gradient, and velocity distribution. The bed malerial and <br />local scour ch:lr.lcteristics determine the design of toe <br />protection which is essential for riprap revelrnent stability. <br />The bank material and groundwater conditions affect the <br />need for fIlters between the riprap and underlying mate- <br />rial. Construction quality conlrOl of both stone production <br />and riprap placement is essential for successful bani: <br />protection. Riprap protection for nood conlrOl channels <br />and appurtenant SlrUClUreS should be designed so that any <br />nood that could reasonably be expected to occur during <br />the service life of the channel or SlrUcture would not <br />cause damage exceeding nominal maintenance or replace. <br />ment (see ER 1110-2-1150). While the procedures pre- <br />sented herein yield defmite stone sizes. results should be <br />used for guidance purposes and revised as deemed nec. <br />essary to provide a practical protection design for the <br />specific project conditions. <br /> <br />3-2. Rlprap Characteristics <br /> <br />The following provides guidance on stone shape, <br />size/weight relationship, unit weight. gradation. and layer <br />thickness. Reference EM 1110-2-2302 for additional <br />guidance on riprap material characteristics and <br />cOnslrUction. <br /> <br />a. Stone sJllJpe. Riprap should be blocky in shape <br />rather than elongated. as more nearly cubical stones "nest" <br />together best and are more resistant to movement. The <br />stone should have sharp. angular. clean edges at the inter- <br />sections of relatively flat f:lCes. Sa-earn rounded stone is <br />less resistant to movement. although the drag force on a <br />rounded stone is less than on angul:lT. cubical stones. As <br />rounded Slone interlock is less than that of equal.sized <br />angular stones. the rounded Slone mass is more likely to <br /> <br />EM 1110-2.1601 <br />1 Jul 91 <br /> <br />be eroded by channel flow. If used. the rounded stone <br />should be placed on flatter side slopes than angular slOne <br />and should be about 2S percent l:lTger in diameter. The <br />following shape limitations should be specified for riprap <br />obtained from quarry Operations; <br /> <br />(I) The stone shall be predominantly angular in <br />shape. <br /> <br />(2) Not more than 30 percent of the stones distri. <br />buted throughout the gradation should have a ratio of alc <br />greater than 2.5. <br /> <br />(3) Not more than 15 percent of the stones distri- <br />buted throughout the gradation should have a ratio of alc <br />greater than 3.0. <br /> <br />(4) No Slone should have a ratio of ale greater than <br />3.5. <br /> <br />To delennine stone dimensions a and c, consider that <br />the stone has a long axis, an intermediate axis. and a short <br />axis. each being perpendicular to the other. Dimension a <br />is the maximwn length' of the stone. which defines the <br />long axis of the stone. The intermediate axis is defined <br />by the maxirnwn width of the SlOne. The remaining axis <br />is the short axis. Dimension c is the maxirnwn dimen- <br />sion parallel to the short axis. These lirnilations apply <br />only 10 the stone within the required riprap gradation and <br />not to quarry spalls and waste that may be allowed. <br /> <br />b. Relation between stone si;e IJlIIi weight, The <br />ability of riprap revelrnent to resist erosion is related to <br />the size and weight of stones. Design guidance is often <br />expressed in terms of the stone size D%, where % <br />denotes the percentage of the tolal weight of the graded <br />material (lolal weight including quarry wastes and spalls) <br />that contains stones of less weight. The relation between <br />size and weight of stone is described herein using a spher- <br />ical shape by the equation <br /> <br />t6Wt J /J <br />Dt= _ <br />ltys <br /> <br />(3-1) <br /> <br />where <br /> <br />D% = equivalent-volume spherical stone diameter. ft <br />W % = weight of individual stone having diameter of D% <br /> <br />3-1 <br />