A ROBUST IN-SITU SETTLING VELOCITY BOX FOR COASTAL SEAS

A new instrument for measuring the in situ settling velocity able to w ithstand the harsh conditions common in coastal waters is introduced. This settling box traps a parcel of fluid and measures the decaying co ncentration with an array of optical backscatter probes; output from t hese four sensors is then combined to produce a single settling veloci ty distribution. Preliminary deployments have been undertaken in the r iver Elbe and have produced encouraging results. Once the box is close d, an initial period of turbulent fluctuations is followed by settling and a smooth decrease in concentration. Settling velocity distributio ns recorded at transitions from flood/ebb tides are distinct from thos e at ebb/flood transitions; median settling velocity was found to incr ease with sediment concentration.