Process-based Two-dimensional Numerical Model Of Longshore Sediment Transport And Morphological Evolution

Sandy coasts with bundant nature resources and beautiful scenery are the habitat of human living.In recent years,the development and utilization of sandy coasts has become more and more frequent.Artificial constructs break the original balance of hydrodynamics on sandy coasts,which accelerates the evolution of morphology.Longshore sediment transport is an important factor affecting long-term evolution of morphology on sandy coasts.Therefore,it is of great significance to establish a numerical model which reasonably describes the characteristics of longshore sediment transport and to investigate the morphological evolution under the influence of human activities.In order to overcome the shortcomings of traditional one-line model,an unstructured-grid,two-dimensional morphological evolution model was developed based on the proposed suspended-load longshore sediment transport formula with cross-shore distribution and the developed real-time coupled model for wave-induced longshore current.The numerical model was further applied to investigate longshore sediment transport and morphological evolution on sandy coasts influenced by human constructs.The main works and conclusions are summarized as follows.(1)A formula for suspended-load longshore sediment transport,with the capability of describing the cross-shore distribution,was developed based on the energy principle.A large number of measured data were collected for calibrating transfer coefficients in the formula.A relationship between the transfer coefficient of breaking waves and the Dean number was proposed.Comparisons between calculated results and field data indicated that the cross-shore distribution of suspended-load longshore sediment transport under different wave conditions could be described reasonably by the presented formula.(2)Bulk longshore sediment transport was achieved by integrating the presented suspended load transport formula along the surf zone.In order to evaluate the presented formula and some other commonly used formulae of bulk longshore sediment transport,high-quality measured data were collected.Comparisons showed that predicted results of the formula proposed in this dissertation,the van Rijn formula and the formula suggested by “Code of Hydrology for Harbour and Channel” agreed well with the measurements.(3)A two-dimensional model for simulating wave-induced longshore current was developed through real-time coupling of the random wave model,SWAN,and the nearshore circulation model FVCOM.Effects of surface roller,wave-induced horizontal mixing and bottom shear stress under combined waves and currents were incorporated in this model.Sensitivity analyses were made to investigate the effect of empirical coefficient ? in the kinetic energy of surface roller and ? in wave-induced horizontal mixing.Recommended values of model coefficients were also proposed based on numerical tests.Field observations and laboratory measurements were collected to examine the performance of this model.Results indicated that the transformation of wave nearshore and the circulation pattern of wave-induced currents were reasonably described by this model.(4)A two-dimensional unstructured-grids model for simulating morphological evolution was developed based on the finite volume method.This numerical model integrated four submodels including a wave model,a nearshore circulation model,a sediment transport model and a morphological evolution model.Influences of local seabed slope on the bed load transport,effects of wave breaking on the suspended load transport,and longshore sediment transport in the swash zone were incorporated in the sediment transport model.In the morphological evolution model,the mass conservation equation of sediment was solved utilizing the finite volume method.In order to test the performance of the numerical model,measured data in physical models concerning coastal morphological evolution influenced by breakwaters were collected.Results showed that morphological change near the breakwater was reasonably described by the numerical model.(5)Longshore sediment transport and shoreline change in Jinghai Bay with and without the breakwater of Huilai power plant were investigated using the morphological evolution model.Before the construction of breakwater,the direction of net longshore sediment transport was from East to West at the top area of Jinghai Bay which was in a state of erosion.After the construction of breakwater,the direction of net longshore sediment transport became from West to East causing significant deposition at the top area of the Bay.Comparisons between satellite remote sensing images and numerical results indicated that the shoreline evolution in Jinghai Bay was reasonably described by the presented model.