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44 . such as an annual flood, because it is the frequent flood discharges that dominant the long-term degradation or aggradation process. A design example for determining equilibrium slope is given in Section 5.2.2.2. In addition to the prediction of a long-term equilibrium channel slope, degradation or aggradation from a rare flood event, such as a 100-year flood, should also be evaluated because the maximum short-term degradation or aggradation usually will occur during a rare storm. The maximum degradation or aggra- dation may not be evident after the storm event. Prediction of ultimate degradation or aggradation of a storm can generally be made using mathe- matical models. For projects where a detailed quantitative analysis can he justified, a series of programs have been developed by Simons, Li & Associates, Inc. to determine the river response to a flood event. The sediment routing model utilizes the hydraulic conditions determined by the U.S. Army Corps of Engineers' HEC-2 program, but recognizes that the channel bed and banks are movable and adjustable, responding to scour and deposition. Application of the model requires knowledge of the characteristics so the model. can be calibrated and/or adjusted for each individual application. . . . . . . 3.6 General Scour General scour usually occurs when flow area is contracted by embankments, channelization, and accumulation of debris. Scour at contractions occurs because the flow area becomes smaller than the normal stream and average ve- locity and bed shear stress increase. Hence, there is an increase in stream power at the contraction and more bed material is transported through the contracted section than is transported into the section. As bed level is lowered, velocity decreases, shear stress decreases, and equilibrium is restored when the transport rate of sediment through the contracted section is equal to the incoming rate. . . . .