Field observations of the threshold of sediment motion by wave action

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Abstract
Two field experiments were carried out to determine critical conditions at the threshold of sand motion beneath irregular sea waves. The experiments were performed outside the breaker zone at Blackpool Sands, Start Bay, Devon, in waters of depth 4–10 m. From synchronous video and near‐bed velocity records, critical conditions at the threshold of motion have been established in terms of the measured free‐stream velocity amplitude. Despite the irregular nature of the (swell) waves and the mixture of grain sizes on the bed, agreement between the present field results and previous laboratory results is reasonable, provided that proper allowance is made for the presence of sand ripples. In order to define threshold motion conditions on a more rational basis than in terms of the free‐stream velocity amplitude, calculations of the bottom stress have been made on the basis of laboratory‐derived wave drag coefficients. The threshold‐motion conditions thus established from the field data are in good agreement with sediment threshold values from Shields' curve derived from laboratory work, at least for cases in which it has been possible to calculate the skin‐friction contribution to the total bed shear stress.

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