Nearshore circulation and sediment transport

Abstract

The project of "Sand for Hawaiian Beaches" is briefly described in the introduction. Mathematical models are developed to simulate 3-D coastal current structure and to predict the fate of suspended sediments due to offshore sand dredging and dumping operations, thus providing an effective tool for environmental impact assessments. A 2-DH hydrodynamics model provides necessary input and boundary conditions to the 3-D flow model. The effects of waves are incorporated in these models through the wave radiation stresses and thus the models possess the capability of predicting the wave induced longshore and rip currents. A wave transformation model is applied to calculate the radiation stresses and the energy dissipation due to wave breaking and bottom friction. Wind effects are also included through the 2-DH model. The 3-D flow model gives the current field for the 3-D suspended sediment transport model, which deals with fine sand transport due to dredging and dumping operations. The sediment transport model predicts the suspended sediment concentrations and the accumulation of sand on the bottom. The finite volume method as described by Patankar is employed for the 2-D water circulation models as well as for the 3-D suspended sediment transport model. For the 3-D circulation model, a time split technique is applied in order to handle different physical mechanisms separately and a vertical coordinate transformation is employed to obtain convenient finite difference schemes. Effects of the dredging pit on the stability of the bottom profile and of the beach are studied using Ballard's sediment transport model. The filling and the erosion related to the dredging pit are estimated with a 2-DV hydrodynamics model, a k-e turbulence model, and the SUTRENCH sediment transport model. The models are applied to the coastal region off Waikiki and the computational results show that the dredging operation at the Halekulani sand channel has no noticeable adverse effects on the bottom profiles and on the beach. A 24-hour continuous dredging operation will result in a sand deposition of 0.1mm on the nearest live coral area under normal South Swell wave conditions and the assumed critical deposition and erosion shear stresses.