The use of a LISST-100 laser particle sizer for in-situ estimates of floc size, density and settling velocity

Abstract

A LISST-100 laser particle sizer was used to measure in-situ particle size spectra (PSS), mean grain size (DM) and volume concentration (VC) of flocculated suspended sediments in Danish coastal waters. DM and VC were found to vary by up to 23% in tens of seconds. Consequently, an averaging period of 1 min for the LISST measurements is proposed. Based on the average values of DM and VC, and when the concentration of total suspended matter (TSM) is known, it is shown how to compute the in-situ mean effective density (Δρ) and mean settling velocity (WS) of suspended flocculated sediments using the LISST. Δρ versus DM, and WS versus DM relationships were found to be in good agreement with published data obtained by use of video systems. This lends credibility to the method used, and also suggests that laser diffraction instruments are capable of measuring DM and VC of flocs quite accurately. This is supported by a selected literature survey. The fractal dimension (Df) of the particles was calculated for different sites and seasons, and variability in the data interpreted in relation to the energy conditions prevailing in the study areas prior to fieldwork.