A robust in situ settling velocity box for coastal seas

A new instrument for measuring the in situ settling velocity able to withstand the harsh conditions common in coastal waters is introduced. This settling box traps a parcel of fluid and measures the decaying concentration with an array of optical backscatter probes; output from these four sensors is then combined to produce a single settling velocity distribution. Preliminary deployments have been undertaken in the river Elbe and have produced encouraging results. Once the box is closed, an initial period of turbulent fluctuations is followed by settling and a smooth decrease in concentration. Settling velocity distributions recorded at transitions from flood/ebb tides are distinct from those at ebb/flood transitions; median settling velocity was found to increase with sediment concentration.