InSiPID: A new low-cost instrument for in situ particle size measurements in estuarine and coastal waters

Abstract

In estuaries dominated by cohesive sediments, tidal variation in the size distribution of suspended particulates has important implications for automated suspended sediment monitoring, since instrument calibration must typically take account of both sediment concentration and particle size. Particle size variation at tidal time-scales also directly affects the parameterisation of numerical sediment transport models through its effect on the settling velocity. In situ measurements of particle size are preferred, since the behaviour of complex particle aggregates (flocs) is strongly influenced by turbulence intensity. A variety of in situ particle size measurements systems have been developed, mostly based upon either direct photographic or video imaging, or the analysis of optical or acoustic scattering. However, existing systems are expensive and/or unsuited to small-boat deployment. This paper describes a new low-cost particle sizing system that can easily be deployed within shallow estuarine and coastal waters. The In situ Particle Imaging Device (InSiPID) combines twin CCD video cameras with fully automated digital image acquisition and processing algorithms for the extraction of size and shape statistics. The system has a useful imaging range of approximately 4 to 3000 μm. It has been tested in concentrations in the range 5 to 400 mg l − 1 , and provides near real-time analysis of image ensembles using a compact portable computer. The image processing algorithms are specifically designed for the analysis of flocculated particles, and perform well against manual analysis of laboratory standards. A trial deployment within a small UK estuary reveals a clear bimodality in particle size, with the proportion of suspended material contained within these distinct micro- and macro-floc modes varying both within and between spring and neap tides. InSiPID is considerably cheaper than alternative systems. It is also more flexible in that additional functionality can be implemented in software without any major alteration to the underlying hardware.