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<br />Struc:ural Practices <br /> <br />Rock Riprap <br /> <br />Rock riprap is an effective method of controlling stream hank erosion on <br />nearLy all streams. Rock can be placed on stream banks which are stralght to <br />severely meandering. Rock for riprap should be material which is de~se, <br />angular, and durable. Rock should be well graded to minimize air voids ln the <br />riprap blanket. The rock should be sized to withstand the force of the water <br />in a stream without displacing the rock. Depending on the type of soil in the <br />bank, a riprap blanket may require a filter of sand and/or gravel. The filter <br />will prevent the piping of soil particles from the bank through the riprap <br />blanket due to groundwater movement. The stream bank is shaped to a slope no <br />steeper than 2.0 horizontal to 1.0 vertical prior to placement of the rock. <br />The toe of a riprap blanket must be deep enough to prevent the stream from <br />undercutting the bank below the riprap and causing the bank to fail. The <br />upstream and downstream ends of the riprap blanket must be tied into the bank <br />at a stable point. <br /> <br />Some stream banks may not require the riprap blanket to extend to the top of <br />the bank. An example of this could be if the bank on one side of the stream <br />is significantly higher than the bank on the other side. The blanket should <br />extend up the bank to a height of one foot above the high water elevation. <br />The area above the rock should be shaped and seeded with appropriate <br />vegetation. <br /> <br />A variation of this practice is to use wire-bound rock. The cross section <br />would be similar to the rock riprap cross section. However, since the rock is <br />held in place with anchored, wire-fencing material, the rock can be a smaller <br />size. This will allow the thickness of the blanket to be reduced. Wire-bound <br />rock is not recommended on channels with high banks or streams with a high <br />flow velocity. <br /> <br />Maintenance requirements for a riprap blanket include replacing or reposi- <br />tioning rock if it is displaced by high flows. The upstream and downstream <br />ends of the blanket should be checked to make sure erosion is not occurring. <br />If scour holes have developed, the eroded areas should be filled and protected <br />with additional riprap. <br /> <br />42 <br />