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<br />Armorlng Technique. . <br /> <br />e <br /> <br />needed to insure that the concrete is properly designed. mixed, and c\D"ed. However, this <br />method, over a lifetime, often provides a high degree of reliability when placed. <br /> <br />Costs for concrete pavement are directly related to bank height and slope. Costs for a <br />typical 10 foot high bank may vary from $200-$400 per linear fooL Typical <br />maintenance costs are $10-$20 per foot annually. <br /> <br />Geogrid - Non-woven polyester fabric shaped as hexagons, with sides approximately 8 <br />inches and a depth of 8 inches, are stapled together to fonn a mat to the shape and area <br />of the bank to be protected. The geogrid mat then is placed on the bank and filled with <br />soil. sand, aggregate, or other native materia1s. The raised edges of the geogrid material <br />provide the erosion protection until the vegetation becomes established within the cells. <br /> <br />The geogrid provides virtually permanent erosion control due to the rot-proof nature of <br />the materials they use and the eventual establishment of vegetation which further <br />enhances its structural integrity. Geogrid revetments are inexpensive, quick and easy to <br />build. Turbulent flows at the IDe of the geogrid is the most frequent cause of revebnent <br />failure. For this reason, it should usually be coupled with a low stone IDe structure. A <br />typicallO-foot high bank protected with a geogrid mat would cost around $30-$60 per <br />linear foot, depending primarily upon the types of vegetation used. This cost does not <br />reflect costs for refusals or IDe structures. <br /> <br />Gabions - Gabions are rock-filled wire or synthetic baskets that are wired together to <br />fonn continuous structures. The mesh is typically galvanized or coated with polyvinyl <br />chloride to reduce conosion. Gabions can use lower quality stone than riprap structures <br />and can be placed on steeper slopes. Gabion structures are flexible enough not to he <br />vulnerable to minor bank shifts but need to be placed on a linn foundation. Gabions <br />may also be used to construct deflective structures, and would have the same impacts as <br />jetties or hardpoints when constructed as such. Sediment is usually deposited among the <br />rocks in gabion structures, and vegetation often becomes esmblished so that the <br />structure is obsc\D"ed and the stream has a natural appearance. Unvegetated gabions are <br />similar in appearance to masonry work, which may be visually pleasing in some <br />settings. The steep slopes on which gabions are sometimes placed may hinder wildlife <br />access. Gabion structures can be designed with artificial overhangs, flow deflectors, and <br />other features to enhance fish habitat. Failed baskets may be hazardous to recreationists, <br />especially canoeisls. Gabions have been widely used for streambank protection on <br />streams located in a variety of environments in the US and Europe. Figure 4-7 shows a <br />typical gabion. They are most frequently used in urban areas, particularly on small <br />watersheds where high flood conveyance is desired. <br /> <br />Gabion streambank protection structures have perfonned very well in some settings. <br />The major problem is basket failure. a problem that is aggravated by ice and other <br />debris. gravel bedload movement, vandalism, and corrosive streamflows. Gabions are <br />usually cost prohibitive (5200. to 51000 per linear fool) when compared to riprap <br />structures. However instances may occur when they are a preferred alternative, (ex. <br />narrow channels, very high velocities, etc.) <br /> <br />e <br /> <br />Colol'IIdo Erosion Control Manual <br /> <br />33 <br />