Sediment transport: Physical modelling of flocculation

Abstract

A study is made of the behaviour of cohesive sediment in turbulent flowfields, such as are found in strongly tidal river estuaries. A model is developed which incorporates the fact that cohesive sediments, usually clays, consist of particles which can flocculate because of the electrical charges on them. During the cycle of erosion and deposition that occurs in tidal estuaries, the degree of flocculation changes. An equation is formulated for the evolution of the size distribution of the particle aggregates in suspension, taking into account the effect of turbulence both on the rates of flocculation and breakup. Solutions of this equation are obtained using an extension of the quasi-stationary approximation. The results are used to investigate the interpretation of the output of electro-optical turbidity meters. These are important sources of field measurements, but they require calibration before their data can be used for the testing and construction of sediment transport equations. It is shown that the transformation of turbidity-meter readings into density measurements requires statistical information about the sizes of the suspended aggregates. This can be calculated using the size distributions obtained from the model presented here, after they have been combined with an experimental determination of some empirical constants.