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Armorlng TechnlqlHll . e I) those structures whose primary function is to prevent erosion by annoring the eroding bank, Revetments are an example of this type of structure, as are revegetation and soil stabilization. 2) those structures that prevent erosion by deBecting the current away from the bank, Jetties, hardpoints, and petpendicular dikes fall within this category. 3) reducing the erosive capability within the channel. Examples include vanes, chute closures, and clearing of vegetation on islands or selective removal of point bars or snags. Following is a discussion of select structure types. This list is by no means all inclusive. Included is a brief description of the concept, environmental benefits, and experience for each type. Within each of these groups many different erosion contrul methods can be categorized. Since their function is essentially the same they can often be used interchangeably to solve a given problem. Considerations such as economics, constructability, aesthetics, material availability, and environmental impact usually dictate which technique from a functional group is selected for a project. The structures are grouped by fWlction, as described above. Since the impacts vary primarily by function, a discussion of the local effects for each grouping follows the fWlCtion heading. 4.4 Armorlng Techniques - Erosion contrul techniques that function by annoring the bank affect the bank sediment input. roughness coefficient. and local shear velocities. Material type and channel alignment determine the extent of the impacts. The annoring technique is the placement of a protective covering. usually consisting of stone, over part or all of the stream bank. Annoring techniques function by preventing the boundary shear induced by Bowing water from contacting erodible bank material. In general, armor structures cause a scour hole to develop from the toe of the structure riverwanl for 10 - 30 feet. The depth of scour varies with alignment and material type. but averages from I - 3 feet. Velocity increases in the scour region, but there is little or no change in the velocity at points further than 30 feet riverwanl of the structure. If the structure does not encroach on the channel, there is no noticeable change in river stage for a given discharge. Bed sediment movement may increase slightly in the reach if revetting the bank removes a significant source of sediment. Properly constructed annor structures will provide a locally diverse aquatic environment without significant effect on the hydraulic conditions of the adjacent river reaches. Other structure types that fall within this category affect the channel conditions in similar ways, but to varying degrees. e Revetments - Revetments are perhaps the most common of all streambank protection structures. Included within this group are several variations of the general theme of placing quarried stone, broken concrete, cobble, or soil cement parallel to the eroding bankJine. The stone may be placed in a toe section with or without upper bank protection. A thin blanket may be used to armor the entire bank. The revetment may be windrowed, and allowed to launch as erosion undermines the structure. Revetments are often used in conjunction with other bank protection devices. A stone toe section with revegetation of the upper bank is one of the most cost effective solutions to most erosion problems. ColOrado Erosion Control Manual 27