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<br />Depth of Scour in Alluvial Channels <br /> <br />The depth of scour in a channel bed is a function of the amount of sus- <br />pended sediment, size and placement of bed material, and magnitude of erosion <br />stresses exerted by the stream. In the design of bank protection, estimates of <br />the depth of scour are needed so that the protective layer is placed suffi- <br />ciently low in the streambed to prevent undermining. Design procedures in <br />Hydraulic Engineering CirculHr-ll (Searcy, 1967) recommend that the rip rap <br />layer extend a minimum vertical distance of 5 ft (1.5 m) below the streambed, <br />and on a continuous slope with the embankment (fig. 11). The l"ecommendation <br />may not be adequate for reasons described in this report. <br /> <br />For typical channels that are not affected by degradation or aggradation <br />(caused by mining, landslide", or changes in flow regime), scour and fill of <br />the channel boundary is a coo.tinuing and natural phenomenon. Changes in chan- <br />nel shape and size are related, in part, to discharge and suspended-sediment <br />load, as shown by Leopold and Maddock (1953). Their studies indicate that <br />scour and fill of a riverbed during a flood is directly related to changes in <br />suspended-sediment concentration. <br /> <br />Scour and fHi of the channel boundary results in fluctuations about a <br />mean elevation or position as part of a continuing process in natural streams. <br />Therefore, there is some limit in the magnitude of the channel changes unless <br />the stream is aggrading or degrading. Studies of streambed scour at 21 sites <br />were made to determine typical depths of scour for various sizes of bed mate- <br />rial and flow conditions. Depths of scour (table 15) were measured at sites on <br />streams with sand, gravel, .md cobble beds. Sites were selected that were <br />unaffected by bridge piers or other cultural features, were generally in <br />straight reaches, and were without features such as bedrock or large boulders <br />that may cause localized seoUl:. Measurements of the channel bed elevation were <br />made systematically (either annually or monthly) by soundings or depth fathom- <br />eter at the same location frOln a boat, cableway structure, or by wading. <br /> <br />28 <br />