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<br />e <br /> <br />e <br /> <br />Flow Deflection Technlqu. . <br /> <br />the channel from this type of structure is somewhat different than for stone.fill jetties or <br />hardpoints. Since fence dikes are relatively permeable, less scour occurs riverward of <br />the end of the structure. Sediment accretion bebind the structure is often more extensive <br />than for less permeable structures. <br /> <br />Environmental benefits and considerations for this structure type are the same as for <br />hardpoints and jetties. Recreational benefits are less for fence dikes than for hardpoints <br />or jetties because they do not improve access to the river. Fence dikes have been used <br />extensively on rivers throughout the US with mixed success. They are more prone to <br />damage than hardened structures, particularly from ice or debris. Fence dikes require <br />more maintenance than hardpoints or jetties. <br /> <br />Fences - Used in small low gradient streams, fences are constructed parallel to the bank <br />line to promote sedimentation. Fences, shown in Figure 4 - 15, are made of a variety of <br />materials but the prime materials used are wood and wire. On sandy bottoms, fence <br />posts should be spaced 6 to 10 feet apart and driven 15 feet into the ground if stream <br />velocities over 15 feet per second are expected. To provide extra protection, brush, hay <br />bales, used tires, or roc1c can be placed between the fence and the stream bank. Fences <br />can be designed to deflect stream low or to trap debris. Eliminating the problem of <br />constructing a staJ)le foundation, fences are more economical than riprap or matting <br />methods. Since fences are constructed away from the bank, they promote sedimentation <br />but are vulnerable for damage due to ice flows or large debris from heavy floods. The <br />costs of fence structures can vary from $50 to $150 per linear foot. Maintenance costs <br />are highly variable. <br /> <br />Retards - Retards are semi.permeable structures of wood, wire, tires, or trees that are <br />constructed perpendicular to the banJdine to deflect flows for a very short distance. <br />Spacing between structures is much closer than for other structure types. Retards are <br />usually ineffective if erosion is severe. <br /> <br />Retards are typically constructed as a self.help measure as opposed to being designed <br />by river specialists. Therefore, performance of this structure type varies considerably. <br />Environmental benefits vary with placement and material type. <br /> <br />Colorado Erosion Control Manual <br /> <br />43 <br />