The in situ passive acoustic measurement of shingle movement under waves and currents: instrument (TOSCA) development and preliminary results

An instrumented platform (TOSCA) is described for monitoring coarse-grained sediment transport in high (wave and tidal) energy environments. The system consists of a number of sensors which measure: instantaneous current velocities at three levels; sea surface elevation; and self-generated noise (SGN) caused by intercollision of particles in transit. SGN is related to transport rates, on the basis of laboratory calibration; this, in turn, allows a correlation with simultaneous near-bed shear stress to be established. During the field measurements, PC-based real-time data transmission is realised through the use of a (buoyed) radio telemetry system. TOSCA has been deployed in eastern Christchurch Bay, southern England. Burst data were collected every 2 h, over 10 min, with a sampling frequency of 5 Hz. The data sets obtained permit detailed investigations into near-bed flow structure and shingle movement, on time-scales comparable to those of the turbulence.