Laboratory experiments on the entrainment by a Concentrated Benthic Suspension

Laboratory experiments on the entrainment by a Concentrated Benthic Suspension

Bruens, A.W.

Civil Engineering and Geosciences

Hydraulic Engineering

2000-01-01

The deposition of cohesive sediments in estuarin and eoestal environments can lead to a range of managerial problems. To deal with these problems it is essential to understand the behaviour of cohesive sediment and its interaction with the flow field. During slack water, cohesive sediment is deposited and a Concentrated Benthic Suspension (CBS) is formed. The period of deposition is often too short for consolidation to occur. As no or only little strength is built up, a CBS cannot resist the increase in tidal pressure gradients after slack water. As a result a flow is then generated in the CBS and turbulence is generated at the base of this layer. The CBS starts to entrain water from the upper layer, resulting in a decrease in density of the CBS and an increase in its thickness (referred to as case 1). If, on the other hand, cohesive sediment is deposited for longer time periods, a strength can develop and a fluid mud or consolidating bed is formed. This layer can resist the increase in pressure gradient and flow and turbulence is generated in the upper water layer. This upper layer now starts to entrain material from the lower layer, resulting in an increase in density of the upper layer and a decrease in thickness of the lower layer (case 2). In the past, experiments concerning the second case of entrainment have been carried out. This report describes entrainment experiments concerning the first case of entrainment. The study consisted of the construction of an experimental device, the execution of experiments and the analysis of the results. The objective of the study was to improve the understanding of the behaviour of CBS and to generate data to validate computer models of CBS dynamics. A rotating annular flume, an experimentaI device often used for entrainment experiments on case 2, was adapted to carry out entrainment experiments on case 1 by the construction of a rotating base plate. An optimization of the operational conditions of the adapted annular flume was carried out. The results indicated that the experimental device was well suited for entrainment experiments on case 1. A first series of entrainment experiments with a layer of saline water below a layer of fresh water were carried out. The results with saline water were compared to the results with a CBS in order to check whether, for these entrainment conditions, CBS behaved as a viscous fluid. Next, a second series of entrainment experiments with China Clay were carried out. Initial conditions were such that a CBS was formed which possessed hardly any strength. The experiments were set up such that the overall Richardson number, which is a function of the hydrodynamïc parameters, was varied. The following conclusions were drawn from the results of the two experimental series: Because of the generation of turbulence in the lower, dense layer, material from the upper less dense layer was entrained and the density of the lower layer decreased as its thickness increased. The entrainment velocity was constant with time, consistent with entrainment theory. The relation between the non-dimensional entrainment rate E and the overall Richardson number Ri« was identical for the two experimental series, indicating that a freshly deposited CBS behaves as a viscous fluid. For the present experiment a relationship of the form E ex: Ri;l was found, identical to the relation found for experiments on case 2. This report is merely a data report. At present, analyses with numerical models are carried out. The result will be reported in a future report.

mud
mudflow
fine sediment

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TU Delft, Department of Hydraulic Engineering

Report 3-00 - Prepared for the European Commission, DG XII MAST3 - COSINUS Project, Contract No. MAS3-CT97-0082.

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