The movement of noisy sandwaves in the Strait of Dover

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Abstract
Large sandwaves between the Sandettié and Outer Ruytingen Banks are noisy when the tide is running. The noise appears above the crests of the sandwaves as a “plume” when recorded by a 30 kHz or 100 kHz echo sounder and as a well-defined noise band when observed by 300 kHz sector-scanning sonar. Surveys carried out between December 1970 and July 1974 showed that two large sandwaves moved to the west at an average speed of 0·18 m d”1. Slope angles and seabed profiles were consistent with this movement which was confirmed when the crest of one sandwave was positioned with reference to a fixed bottom marker fitted with a long-life acoustic transponding tag. Noise from bottom features was shown to increase with current speed and at 125 cm s”1 (2'/2 knots) the maximum spectrum level was estimated at 139 dB ref 1 μPa Hz−1 at 1 m for 300 kHz. The results are discussed in relation to the hypothesis that underwater sound sources might be used by fish as indicators of tidal flow or as acoustic beacons by migrants on passage. While there are no field measurements to show that bottom noise is generated within the frequency range to which marine fish might be expected to respond, flume studies suggest that this is quite likely. Echo-sounder and sonar observations show that neither herring nor plaice avoid noisy bottom areas in the sea.

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