Chapter 13 Modelling of sedimentation and self-weight consolidation of cohesive sediments

Abstract

Over the past decade, sedimentation and self-weight consolidation of cohesive sediments in dam reservoirs, especially those close to the hydropower intakes and in harbours, has been recognized as a serious problem. Removal of these sediments by dredging and sediment flushing requires knowledge of the properties of the deposited sediment; hence using both physical and numerical modelling, a study was conducted to parameterize the sedimentation and self-weight consolidation processes. In order to achieve the above-mentioned goals, an extensive experimental program was conducted in a plexiglass settling column test apparatus, 3.8 m high and 0.3 m in diameter. In this study, 30 tests with different initial conditions (different initial heights and initial sediment concentrations) were conducted. Three different natural sediments from the Dez reservoir, the Shahid-Abbaspur reservoir and the Karun River near the port of Khorramshahr were studied. For each test, the water-sediment interface was recorded for several days. In order to measure pore water pressure the piezometric head at different elevations was also recorded during the test. In some of the tests, the effects of sediment flocculation and temperature on the sedimentation and consolidation processes were investigated. A one-dimensional nonlinear finite strain model was also developed in this study. The finite volume method was applied to solve the governing equations, and the experimental data were used to verify the applicability and accuracy of this model. The results of this study show that the sedimentation and consolidation processes are affected not only by the characteristics of the water-sediment mixture, but also by the initial height and density of the mixture. Four separate logarithmic relationships were found between sediment density and time in the sedimentation and consolidation processes. A dimensionless analysis was carried out and the two dimensionless parameters of Π H =( H f / H 0 ) a h ( C 0 / C 0 * ) b h (to normalize height of sediment) and Π t = t W s /H 0 ) a t (C 0 /C 0 * )b t (to normalize time) were developed to transform the water-sediment interface height curves (i.e. surface settlement curves) into normalized curves. Applying these parameters, it was found that the normalized surface settlement curves for various tests (with different initial conditions) superimpose upon one another; thus, the dimensionless parameters allow the development of a single normalized curve for a wide range of tests. In order to calibrate a h , b h , a t and b t in the dimensionless parameters, two reference experiments were required. Application of the numerical model to the testing condition shows that the model is capable of predicting void ratio ( e ), sediment concentration ( C ) and water-sediment interface height ( H f ) at any desired time and depth during both the sedimentation and consolidation processes. In order to calibrate the coefficients of consolidation for a particular real case, the numerical model can use the results of a single reference sedimentation and consolidation experiment for that case. A sensitivity analysis was performed to assess the influence of consolidation parameters (permeability and compressibility) and finite strain coefficients ( f (e) and g (e) ) on the accuracy of the numerical model. The results show that the accuracy of the model results depends to a great extent upon the permeability coefficient, especially at a high rate of permeability. Also, the sensitivity analysis shows that the accuracy of the model results is more sensitive to the value of f (e) than to that of g (e) .