Floods in alluvial rivers are characterized by channels incision in the alluvial bed during the rising stage and their filling up in the falling stage; this phenomenon is well known and called scour and fill. In this work scour and fill during floods will be described as bar migration in the alluvial bed; when bar trough reaches the riverbank protection, the undermining of longitudinal protection structures may occur if the river bed level is lower than the bank foundation structure. Scour or trough depths below the average riverbed level can be predicted with formulas calibrated using flume data or derived theoretically. In this paper four different formulas for alternate bars will be discussed and their applicability to field data is tested. A simple criterion for riverbank foundation depth estimate is proposed and its applicability to braided rivers is derived by using a correction factor.
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