Under a standing wave two types of bed response can occur depending on the relative size of the sediment: L-type, which occurs for relatively fine sediment, and ''N-type,'' which occurs for relatively coarse sediment. The specific concern of the present note is with the N-type response, which involves scour in the area of bed between the node and antinode of the standing wave field and accretion under the node. The conditions necessary for the N-type response to occur are examined and a new method for predicting the equilibrium N-type scour profile is described. The method is based on a general characterization of N-type profile geometry and on physical reasoning applied to available experimental data. Predictions of field-scale scour using the new method are generally much greater than scour predictions based on existing interpretation of wave flume experimental results.
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