A simple method for calculating in situ floc settling velocities based on effective density functions

We present a simple method to obtain settling velocity estimates of fine-grained suspended sediment using the following: (1) in situ floc volume fraction measurements, (2) calculations of in situ floc mass, (3) measurements of in situ floc size, (4) laboratory measurements of primary particle size and (5) laboratory measurements of % organic matter obtained through loss on ignition. This method is based on the mass distribution over the particle size spectra. Floc properties are incorporated by using information concerning the primary particles and the ratio between the volume concentration and the mass concentration of the in situ measurements. In this manner, the changes in effective densities caused by flocculation and biological mediation are approximated. This method relies solely on measurements conducted with a particle-sizing instrument and on an analysis of water samples. The method is applied to measurements obtained with a LISST, and data from three different estuaries in different climatic zones are used. The settling velocity estimates found using this method are compared to settling tube measurements and to mass settling method estimates, as well as to calculations using fractal dimension methods from the literature. Settling velocities calculated using the method presented in this study lie within the ranges of the three other methods, demonstrating that this simple method produces results within the ranges of settling tube velocities and those predicted by the more comprehensive, multiparameter fractal dimension models. The presented method allows for easy intercomparisons between studies by utilising instruments such as the LISST, and it is believed that this method would also produce reliable settling velocities using other instruments that yield a volume distribution as well as a proxy for total volume and mass concentration, e.g., some camera systems.