Determination of suspended sediment concentrations and particle size distributions using pressure measurements

We present an automated, in-situ technique for measuring suspended-sediment concentrations and particle-size distributions. The technique uses pressure transducers to continuously monitor the fluid density, which is shown to be directly related to the mass of sediments suspended in the water column. We estimate the depth-integrated mass of 5 g of suspended sediments with an accuracy of 3%, corresponding to a suspended sediment concentration of 542 +/- 14 mg L-1. Particle-size distributions of the sediments were estimated from the rate of change of the suspended-sediment concentrations coupled with Stokes' Law. We evaluated the accuracy using three size classes of precision glass microspheres. Good agreement between observed and calculated fall times was only found for the smallest size fraction (106 to 150 mu m). Larger particles (180 to 250 mu m and 300 to 425 mu m) fell slower than predicted by Stokes' Law, presumably due to the onset of turbulent non around the particles. Also, equal masses of two different sizes of microspheres did not fall independently of each other. The larger microspheres were slowed by thr presence of the smaller particles while the smaller microspheres were accelerated, presumably due to entrainment of smaller particles in the wake of the larger particles. Application to laboratory characterization of soil sedimentation processes, as well as field applications for automated suspended sediment monitoring, prov ides the ability for real-time determination of depth-integrated sediment characteristics.