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Permit Revision 2 <br /> Consideration has been given to riprap sizing and placement. The current permit <br /> design (Map II-13-7 Plate (b)) shows a layer of small-sized rock placed beneath larger <br /> materials. However, experience on the steep slopes typical of many, if not most, of the <br /> culvert outfalls (certainly those of greatest concern) is a problem in that the small <br /> material acts like ball-bearings, moving the larger material down-the slope. We've also <br /> concluded that rock material of a uniform size and shape is not effective, as the water <br /> flows through and around and under the rock quite easily, thus eroding the ground <br /> beneath. <br /> We have concluded, therefore, that the best design mixes sizes and shapes of rock <br /> material. The design (II-B-7 Att. 2)has been changed accordingly and is submitted as <br /> part of the revision. <br /> The NoV further directed clean -out of several culvert inlets, and removal of ditch <br /> sediment below drain RD-4 of Road F. This work was done last fall. <br /> The second double culverts at 83+00 has been fitted with a 900 angle to avoid the <br /> embankment. <br /> B.. NoV C-91-017 <br /> The design in the permit calls for installation of riprap check dams in the ditches <br /> at 100' intervals on certain roads. The effect of installation of such structures <br /> would be to back up sediment, creating a damming effect and forcing ditch flows <br /> out into the road. Further, as far as installation and maintenance, they are <br /> impractical. Many of the ditches are on bedrock, making installation of such <br /> structures is very difficult <br /> It would be impossible to do regular ditch maintenance without regularly <br /> destroying the structures. We believe that this design was not well thought-out, <br /> and that its implemtation would be counterproductive to the objective of sediment <br /> control. <br /> Accordingly, we propose as part of this revision to change it, allowing instead for <br /> rock and cobbles along the length of the ditch. These would have the effect of <br /> slowing flows without creating the problems of the rock dams.. <br /> We have specifically indicated those places at which we propose berms to <br /> prevent flows going over the outslope of the road. Generally, these are areas in <br /> which the inside ditch is on bedrock, or cannot be excavated due to bedrock. <br /> Water bars directing flows to the inside ditch are generally proposed in <br /> conjunction with berms, although with the reservation that the bedrock material <br /> of the road may prevent installation of water bars in some cases. <br /> 3 <br />