TRIDISMA—3-D Sediment Transport Measurements by Acoustics

The EC-funded TRIDISMA project was established to develop a prototype instrument to measure the profiles of suspended sand flux close to the seabed using non-intrusive acoustic methods. The instrument, consisting of 6 vertical, closely-spaced transducers, operating at frequencies between 0.5 and 5MHz, used coherent Doppler to measure vertical velocities, cross-correlation for the 2 horizontal velocities and backscattered intensity for the concentration and size. Tests of the instrument in an oscillating water tunnel with steady flows and oscillatory currents indicated that the coherent Doppler was able to measure current profiles (steady and oscillatory) accurately (to within 1–2% compared to reference instruments, but that the cross-correlation methods were suitable for only steady currents. Backscattered intensity produced both size and concentration estimates which compared well to time-averaged pump-samples taken from the tank. It was conclude that this combination of sensors could provide useful flux-profile measurements in steady flows but not in wave-dominated environments.