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<br /> <br />'. <br /> <br />f\) <br /><.0 <br />N <br />Q <br /> <br /> <br />The following page lists lateral recession rates, categories of erosion <br />severity, and corresponding narrative descriptions for each category. <br />Roadbank erosion is separate because flows in road ditches are not, as severe <br />as in streams and gullies. The placement of road cuts across slopes generally <br />invites erosion problems, however, either due to mass movements or changes in <br />drainage. <br /> <br />When using the direct volume method for gullies, different lateral receSSion <br />rates may be required for the eroding walls than for the eroding headcut or <br />nick point area. If there is field evidence that the bed of the channel is <br />being eroded the direct volume method can be used to estimate that erosion. A <br />recession rate for the downcutting would be required. <br /> <br />To estimate chanhe1,erosion, first detennine the slope height and length of <br />the eroding banks. By field observation, match the appearance ,of the eroding <br />areas with the narratives shown to identify what category t~e erosion is in. <br />Once you have categorized the erosion, note whether all th( symptoms discussed <br />in the narrati,ve are present or if only a few symptoms occu. If only a few <br />of the symptoms in the narrative characterize the eroding area, you may want <br />to use the low end of the range of recession rates shown for that category, <br /> <br />When you are actually obserYing sample areas in the field, you will probably <br />note that eroding areas are mixed in severity and in frequency of occurrence. <br />As an example, a 500 foot long streambank may generally be in the moderate <br />erosion category (0.06 feet/year). A few 50 foot reaches, within that 500 foot <br />reach may be eroding very seyere1y (0.5+ feet/year). Ify'ou are only, <br />interested in the total tons of erosion, you could increase the recession rate <br />to 0.1 feet/year and use that for the entire 500 foot reach. This simplifies <br />data collection and decreases time in the field, without jeopardizing the <br />level of accuracy of your study. <br /> <br />Continuing the example above; if you wish to locate erosion problems on a map <br />and give recommendations for treating the problems, it is then necessary to <br />record each and every segment of bank where changes in the cdtegory of erosion <br />occur. Thi s a 1.1 ows you to, recommend how many mil es of rock riprap or " <br />vegetatiye stabilization will be required to remedy the sev~re1y and very <br />seyere1y eroding banks. <br /> <br />Volume-Weight Conversions <br /> <br />By multiplying eroding areas by a lateral recession rate, cubic feet of eroded <br />material is obtained. To convert this volume of erosion to a weight, the dry <br />density of the soil must be known. Table 1 lists soils by texture with <br />corresponding volume weights. These can be used or soil samples can be <br />collected in the field. The samples must be undisturbed, moist, and of known <br />yolume. Dry weights ,are measured and the volume of the sample is used to <br />detennine the dry denSity of the sample. The samples must be moist because <br />that will probably be their condition at the time of erosion. <br /> <br />20 <br /> <br />