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<br />N <br />co <br />..... <br />co <br /> <br />Average Annual Lateral Recession Rate <br /> <br />The average annual lateral recession rate is the thickness of soil eroded from <br />a bank surface (perpendicular to the face) in an average year. Recession <br />rates are measured in feet per year. Channel erosion often occurs as "chunk" <br />or "blowout" type erosion. A channel bank may not erode for a period of years <br />when no major runoff events occur. When a major storm does occur, the bank <br />may be cut back tens of feet for short distances. It is necessary to assign <br />recession rates to banks with such a process in mind. When a bank is observed <br />after a flood and ten feet of bank has been eroded, that ten feet must be <br />-averaged with the years when no erosion occurred. This will result in a much <br />lower average annual lateral recession rate than a recession rate for one <br />storm. <br /> <br />,. <br /> <br />Selecting the average annual lateral recession rate is the most critical step <br />in estimating channel erosion using the direct volume method. A historical <br />perspective is required in many instances. Old photographs, old survey <br />records, and any other information that tells you what a bank looked like at <br />known times in the past are very useful data. In most instances, such <br />information is lacking and field observations and jc gement are needed to <br />estimate recession rates. <br /> <br />Exposed bridge piers, suspended outfalls or culyerts, suspended fence lines, <br />and exposed tree roots are all good indications of lateral recession. <br />Discoloration on the bridge piers may show the original channel bottom <br />elevation. Giyen the date of bridge installation, a recession rate can be <br />calculated for that reach of stream. Culverts are generally installed flush <br />w.ith a bank surface. The amount of culyert exposed and age of the culvert <br />will allow you to calculate a recession rate. <br /> <br />Exposed tree roots are probably the most common field eyidence of lateral <br />recession. Consult references, to famil iarize yourself with tree height and <br />appearance as related to tree age. Roots will not grow towards a well , <br />drained. exposed, eroding channel bank. The amount of root exposed should be <br />increased by at least a factor of 2x to account for soil that was in the bank <br />and that the root was growing in. By dividing the length of root exposed and <br />the thickness of soil around the, root by the age of . letree, an estimated <br />recession rate can be obtained. <br /> <br />Textbooks on photogrammetry should be consulted to ypview techniques for <br />estimating bank erosion from aerial photos. There are techniques available to <br />measure bank height from a stereo pair of air photc.. <br /> <br />As can be seen in the discussion aboye, there are few instances where you will <br />be able to measure recession rates in the field. Much experience and <br />judgement are generally required to estimate recession rates for channel <br />erosion. Because of this, a table has been compiled for your use which <br />relates recession rates to narrative descriptions of banks eroding at <br />different rates. <br /> <br />19 <br />