A video system for in situ measurement of size and settling velocity of suspended particulates

A video camera system is described which is designed to make in situ measurements of particle size and settling velocity under conditions of strong currents and high suspended sediment concentrations. The system consists of a Sony Hi8 video camera attached to a miniaturized sediment trap and an underwater light. With the camera lens set at wide angle, the total field of view is 15 mm in the vertical and the camera can resolve particle sizes ranging from 100 mu m to several millimetres. The system is programmed to record a 7-s time series of images every 6 h over a 4-mo deployment. Results from the initial deployment showed settling flocs that ranged from 100 to greater than 500 mu m in diameter. By comparison of sequential images, these particles were found to be settling at speeds ranging from 0.1 to 2.2 mm . s(-1) and a strong correlation existed between particle size and settling velocity. Effective flee density did not significantly change with floc size. These initial tests suggest that this video system can provide valuable information on flee size and settling velocity from long-term deployments in energetic shelf environments.