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<br />Methods For Controlling Erosion <br /> <br />e <br /> <br />drains, aD< i flood-plain depressions should be left in their original c:ondition. Because of <br />the limite:! improvement in flow hydrau1ics (upper flow capacity limit roughly <br />equivalent to bankfull discharge), selective clearing and snagging is most often used to <br />provide re lief from high frequency nuisance flooding, for drainage improvement in <br />agricultural areas, and recreational benefits. Increased hydrnulic: conveyance results <br />from chan ges in the resistance to flow values in uniform flow equations. Vegetation, <br />channel iJregu1arity, obstruction to flow, and design flow conditions should be <br />considerecl in estimating improvements in resistance coefficients. <br /> <br />A variation of clearing and snagging that may have some relevance on certain rivers is <br />the selecti ve clearing of islands and removal of bars. In locations where vegetated <br />islands or hars are deflecting current into eroding banJdines, pressure may be reduced by <br />removing Ihe obstruction. <br /> <br />Compariscos between clearing and snagging and selective clearing and snagging to <br />determine envjn)nmental consequences are incomplete; but the literature suggests that <br />selectively cleared and snagged streams possess better stream morphology, water <br />quality, and stream haJ)itat. Some adverse impacts may be avoided by scheduling work <br />to avoid pJime fish spawning and recreational seasons. Selective clearing and snagging <br />has been used in several CE districts, by the Soil Conservation Service, and by local <br />governments in a wide range of envio)nments. Most of these projects have been located <br />in rural or undeveloped areas where limited improvement in flow capacity was needed. <br />Selective o:1earing and snagging projects appear to provide better aquatic and riparian <br />habitat an,1 water quality benefits than traditional clearing and snagging. Because of <br />regrowth ~nd the nature of flood events, selective clearing and snagging projects require <br />more freqllent inspection and more rigid enforcement of maintenance guidelines than <br />other flood control channel designs to ensure satisfactory performance. <br /> <br />Channellllocks - Used in small to medium streams or on side chutes of larger streams, <br />channel blocks are used to prevent stream flow from forming a new channel by keeping <br />the flow in the desired channel. Construction takes place during low to normal stream <br />flows, and involves placing a rectangular framework of logs (with a minimum diameter <br />of 6 inches) and stone (that can be conveniently handled) in the mouth of the channel. <br />The structure should conlain riprap on the downstream side to prevent scour and be <br />lower thaJl the existing banldine to permit flood water to pass through the secondary <br />channel. <br /> <br />Channel blocks effectively divert streamflow but, are ineffective on large streams with <br />large side channel. If the material is economically available, unskilled workers can be <br />used in th.: construction of channel blocks to cut costs. However, logs must be replaced <br />as the decay. Aquatic habitat is enriched as a result of deep channel flows. <br /> <br />Chute Clc.sures - Chute closures are stone or earth-fill structures placed across active <br />chutes to reduce or eliminate flow through the chute. Erosion along the banks of the <br />chute is n:duced because of the elimination of water, or the reduction of velocity by <br />backwater. Some chute closures have low-flow notches or culverts to maintain a portion <br />of the flow through the chute. <br /> <br />The environmental effects of chute closures vary with the location and type of structure. _ <br />By notchi >g a structure for low flow, most of the effects in the chute will be eliminated. _ <br /> <br />46 <br /> <br />Coloradr) Erosion Control Manual <br />