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An evaluation of relative stability can also be made by evaluating <br />• the incipient motion parameters included in the Shields relation <br />(Gessler, 1971). The definition of incipient motion is based on the <br />critical or threshold condition where the hydrodynamic forces acting <br />on the grain of sediment particles have reached a value that, if <br />increased even slightly, will move the grain. Under critical <br />conditions, or at incipient motion, the hydrodynamic forces acting on <br />the grain are just balanced by the resisting forces of the particle. <br />The beginning of motion of bed material is known to be a function <br />of the dimensionless number <br />T <br />F - c <br />* (YS- <br />• where rc is the critical boundary stress, Ys and y are the specific <br /> weight of the sediment and water respectively and Ds is the <br />characteristic diameter of the sediment particle. The parameter F. is <br />often referred to as the Shields parameter. In the region where the <br />shear velocity Reynolds number is 70-500, the boundary is completely <br />rough and F* ranges from 0.047 to 0.06. This procedure can be <br />utilized to determine an allowable or critical tractive force for a <br />given channel soil material. Comparison of the allowable and actual <br />tractive force gives an indication of incipient motion and thus, <br />channel stability. Unfortunately, this procedure was developed on the <br />basis of data collected for sand to gravel material and is not <br />applicable to cohesive soil material. <br />• <br />37 <br />