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<br />. <br /> <br />. <br /> <br />. <br /> <br />OM 500-1-6 <br />January 1991 <br /> <br />g. Levee Erosion and Bank Scour. <br /> <br />(1) General. Careful observations should be made of the streamside <br />of the levee along all localities where a current of more than two feet per <br />second is observed adjacent to the levee or where the river gradients show a <br />high-water slope of two feet per mile or greater. Trouble may be expected at <br />the ends of old levee dikes, road-crossing ramps, and old stream channels. If <br />any sign of levee erosion is observed, soundings should be taken to observe the <br />amount and progress of the scour. One approved method of construction to check <br />scour is to construct deflection dikes using rock, brush, treetips, and lumber. <br />Two rows of stakes are driven, then wired together with wire fencing. The space <br />between the rows is then filled with brush. Anchored rubber tires, cabled <br />together and/or anchored trees are effective for stopping scour and bank caving <br />but may be considered an eyesore when the flood recedes. Currents on mountain <br />streams frequently exceed 10 feet per second, and rock used for bank protection <br />must be heavy enough to resist displacement from the force of the flowing water, <br />Rocks weighing about 250 pounds are needed to provide a margin of safety for <br />current velocity of 10 feet per second. Heavier rocks are needed for higher <br />velocity flows. Such rocks should be angular rather than round to resist <br />rolling, In ~eneral. sandba~s should be filled about two-thirds full if tied, <br />one-half full if not tied and placed bv lappin&. <br /> <br />(2) Groins. Scour, bank caving and levee erosion can be stopped <br />by constructing earthen groins extending 50 feet or more into the channel from <br />the bank or the levee. The elevation of the top of the groin must be lower than <br />the levee top so that the groin will be overtopped before it backs water over <br />the levee. The upstream face and outer end of the groin are subject to rapid <br />erosion and should be protected with wire wrapped rock, large rock, sandbags or <br />rock filled gabions. In 1973, the Albuquerque District and Santa Clara Pueblo <br />conducted a successful flood fight to save a concrete lined main irrigation canal <br />by constructing a groin into the Rio Grande from the location where the bank <br />cutting was occurring. The earthen groin was constructed with a front end <br />loader. Santa Clara residents filled sandbags (about two-thirds full) with a <br />soil cement mixture and used these bags for erosion protection. At one time, <br />undercutting of the groin was threatened and several hundred filled sandbags were <br />required to fiil the cavity. <br /> <br />(3) Anchored Trees. Another method of erosion control is by use <br />of trees tied to an anchored cable. In this method, trees are cut in the <br />vicinity of the trouble and the butt ends of several trees are tied to a cable <br />which in turn is securely anchored to a standing tree or dead man. The trees <br />are then pushed over the bank. They act as a deflector to divert the water away <br />from the danger area and the tree branches tend to trap sediment to repair the <br />eroded condition. Tree should be oriented butt upstream. (See Plate B-II) <br /> <br />(4) Flexible steel jetties (jacks) may also be used to control <br />erosion during a flood fight, These are readily transported to any area where <br />they are quickly assembled on the bank and tied to a length of cable which in <br />turn is securely anchored. When the required number of jacks have been tied to <br />the cable they ate tumbled into the danger spot. The jacks have the advantage <br />of being easy to handle and can be assembled and placed in areas where use of <br /> <br />5 <br />