Inssev: An instrument to measure the size and settling velocity of flocs in situ

Abstract

An instrument has been developed to observe the settling of individual flocs in turbid water in order to measure size and settling velocity spectra of estuarine cohesive suspended sediments. inssev—IN Situ SEttling Velocity instrument—is bed mounted and comprises a computer controlled decelerator chamber that collects a sample of water from which some of the suspended matter is allowed to enter the top of a settling column. The settling flocs are viewed using a miniature video system. Subsequent analysis of video tapes can provide direct measurements of size and settling velocity of individual flocs down to 20 μm. From this information floc density can be estimated. The main feature of the instrument is its ability to video flocs in situ, irrespective of the concentration in the estuary, with as little disturbance to their hydrodynamic environment as possible. Preliminary field testing in the Tamar Estuary has given useful results in flow velocities up to 0.4 m s−1 and in concentrations up to 200 mg 1−1. The video recordings show large numbers of low density flocs with multiple structures linked by fine organic fibres.