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<br />Methods For Controlling Erosion <br /> <br />e <br /> <br />4.5 Flow Dltflectlon Technlques- <br /> <br />Structures that deflect erosive cwrent are constructed approximately perpendicular to <br />the flow, and therefore reduce the effective width of the river. LclCally, a scour pocket <br />develops "ff the end of the structure and continues downstream in a teardrop pattern. <br />There is lIsually an increase in the velocity adjacent to the structure. Average cross- <br />channel \ elocity may increase, decrease, or be unaffected Generally, there is an <br />increase ill stage and/or depth for a given flow in the channel adjacent to the structure, <br />particularly if the structure length exceeds 1/6 of the channel width. Material type, <br />length, height, location. and orientation of the structure will affect the degree of impact. <br />These stnlctures are usually constructed with little disruption to the riparian community. <br />Effects Oil wildlife species are usually insignificant. Sediment accretion behind the <br />structures may provide additional access to the river for some species, and provides <br />good sub llrale for benthic organisms. Recreational benefits increase if access is <br />provided to the structures. The primary benefit of deflective structures is the creation of <br />additional habitat for fish species. The cross sectional changes provide diversity and, by <br />using proper materials, suitable cover and substrate increase. <br /> <br />Hardpoints and Jetties - The terms hardpoint and jelly are genera1ly regarded as being <br />synonymcus. However, for this manual, the terms are used to differentiate between <br />differing degrees of the same basic structures. Both structures consist of a stone or soil <br />spur that f xtends riverward of and perpendicular to the bank, and a stone root to prevent <br />flanking lIf the structure. Hardpoints are low stubby structures that are frequently <br />overtopped and extend riverward less than 15 or 20 feet. Jetties are generally <br />constructed to the height of the high bank, and extend riverward more than 20 feel. <br />Hardpoints deflect the cwrent away from the eroding bank for only a short distance, <br />with no attempt to change the general alignment of the river. By contrast, jetties deflect <br />cwrent for a considerable distance, and are often intended to alter the main flow of the <br />river. Dis :ance between structures should be between 3.0 and 4.5 times the length <br />upstream wdpoint width.A typical hardpoint is shown in Figure 4 - 11. <br /> <br />Hardpoinls and jetties are best suited to long straight reaches of river, or on the convex <br />banldine)f meanders. Structures placed on the concave bank are often failed by <br />excessive scour between structures. The main advantage of hardpoints and jetties is the <br />low quantity of material needed to protect a given bank relative to other riprap structural <br />a1temativllS. Construction costs are typically 60 to 80% of comparable revetment <br />protection, with maintenance costs being nearly equal <br /> <br />The envir:Jllmental benefits of this structure type are primarily related to fisheries and <br />recreation. HardpoinlS and jetties create a haJ)itat diversity not found with most other <br />structure lypeS. Scour off the end of the structure creates deep pools and high velocity <br />flows. Scdlop areas of shallow, relatively slow-moving water provide additional habitat <br />diversity llownstream of the structures. <br /> <br />Hardpoinls and jetties have been used extensively on many rivers throughout the U.S. If <br />properly designed, the structures perform extremely well. When too widely spaced, <br />excessive erosion occurs between structures and often results in failure of one or more <br />structures <br /> <br />e <br /> <br />38 <br /> <br />Colorado, Erosion Control Manual <br />