Adaptation of focused beam reflectance measurement to in-situ particle sizing in estuaries and coastal waters

Abstract

Recent advances in focused beam reflectance technology have resulted in new instruments with the potential for particle size determination in natural waters. A commercially manufactured instrument based upon this principle has been adapted to allow the size distribution of suspended particulate matter in estuaries and coastal waters to be determined in-situ. After calibration, the standard instrument has been shown to be capable of providing accurate, undisturbed particle size distribution data over a wide range of particle concentrations (10 mg l −1–50 g l −1), irrespective of current speed and, to a large extent, particle composition and density. Data obtained with the in-situ instrument in the Humber Estuary have revealed large temporal and spatial variations in the size distribution of suspended particles over a tidal cycle. Particle sizes were largest near the bed during periods of slack water where median diameters exceeded 500 μm. This was attributed to the sedimentation of large aggregates. Rapid reductions in median particle size coincided with periods of strong tidal current acceleration after low water slack current. At maximum current velocity on the flood tide, mobilisation of sediment from the bed gave rise to larger particles in the water column. Multiple regression analysis indicated that a combination of current velocity and particle concentration provides the best predictor of particle size.