SEDVEL: An underwater balance for measuring in situ settling velocities and suspended cohesive sediment concentrations

An automated instrument (SEDVEL—Sedimentation Velocity) was developed to directly measure the mass-concentration and the mass-distribution of settling velocities of suspended particulate matter (SPM) in situ. This instrument consists of an underwater balance, which directly measures the variation in time of the immersed weight of particulate matter (PM) as it settles on a plate located at the bottom of a settling tube, under quiescent conditions. SEDVEL operates underwater and samples in situ for deployment periods of a few days. SEDVEL produced consistent and reproducible results when tested both in the laboratory and in the field under SPM dry-concentrations of 5 to 200 mg l− 1. Errors in the estimates of maximum dry-concentrations from SEDVEL measurements were less than 30% in 69% of the cycles analysed in the laboratory. The corresponding figure for in situ measurements was 50%. These errors are likely related to uncertainties in the calculation of the dry-density of flocs and in the definition of the zero position (ZP) of the SEDVEL balance. The slow settling particles/flocs (Ws < 1 mm s− 1) represented 32–98% of the total mass of SPM at two sites of deployment in Cleveland Bay (Australia). Distinct settling behaviours were observed between the two sites and among different tidal stages associated with differences in the floc population; the aggregation of which probably varied as a function of the bottom grain size, shear stress, resuspension, advection and organic content of SPM.