Quasi-3D modelling of surf zone dynamics

A quasi-three-dimensional model (quasi-3D) has been developed through the implementation of an analytical 1DV flow model in existing depth-averaged shallow water equations. The model includes the effects of waves and wind on the vertical distribution of the horizontal velocities. Comparisons with data from both physical and field cases show that the quasi-3D approach is able to combine the effect of vertical structures with the efficiency of depth-averaged simulations. Inter-comparisons with three-dimensional simulations show that the quasi-3D approach can represent similar velocity profiles in the surf zone. Quasi-3D morphodynamic simulations show that the bed dynamics in the surf zone represent the relevant 3D effects in the surf zone much more than the depth-averaged computations. It was shown that the quasi-3D approach is computationally efficient as it only adds about 15-20% to the runtimes of a 2DH simulation which is minor compared to a run time increase of 250-800% when switching to a 3D simulationHydraulic EngineeringCivil Engineering and Geoscience

Erosion Negril Beach

The ongoing erosion of the Negril Beach has become worse the past decade. In most places along the coast line, the beach will be gone in approximately 10 years. This will result in a major decrease of incomes that are made by the local tourist sector. To prevent the erosion this study has been performed to find a feasible and affordable solution. An important part of the study is the literature research since several other parties had investigated different aspects of the erosion problem recently. Before any solutions can be brought up different aspects have to be investigated. These are a clear view of the actual problem, a good knowledge of the environment, profiles of the coastline, current patterns and sea grass locations to avoid environmental damage as much as possible. Data was collected during two field trips to Negril in the end of 2006. After collecting this data an analysis was made using different computer models as LITPACK (sediment transport), SWAN (waves) and RMA (currents). The data analysis shows that the primary concern is the erosion at Long Bay since the erosion in Bloody Bay is not that severe. Possible solutions are generated and their validity has been checked. Five alternatives can be distinguished: zero-alternative, nourishment, series of near-shore breakwaters, off shore reef extension and a combination of all. Using a Multi Criteria Analyses (MCA), that ranks solutions by their desired effects (without including costs), objectively the 'best' solution is found. The solution that scores best at our criteria is beach nourishment along Long Bay. When costs are taken into account, this solution seems to be the most efficient. Costs are estimated US$7,000,000. The final recommendation is to execute beach nourishment only at Long Bay. This solution is relatively affordable and shows the highest score in the MCA. The implementation of this solution results in a minimum beach width of about 10 meters for 20 years, about 30 meters in the year of execution. According to local divers the required sand can be found 2km offshore at the beginning of the outer shell. Beside of that it is recommended to perform a separate investigation concerning the water quality to deal with the algae contamination.Hydraulic EngineeringCivil Engineering and Geoscience

A comprehensive sediment budget for the Mississippi Barrier Islands

In order to conceive any realistic plan for post-Katrina island restoration, it is necessary to understand the physical processes that move sand along the littoral drift zone off the coast of Mississippi. This littoral zone influences the character of the Mississippi barrier islands as they exist in an ever changing cycle. To help in this understanding, a sediment transport model was conducted to establish a current sediment budget for the islands. This study evaluated the existing regional sediment transport magnitudes and directions for the Mississippi and Alabama barrier islands fronting Mississippi Sound including daily conditions and hurricanes. A method was developed to incorporate all relevant hurricanes from 1917 to 2010 in the analysis. For the long term average net longshore transport along the southern shorelines of the barriers (i.e. exposed to the Gulf of Mexico), the contribution of year averaged conditions and hurricanes are of similar order of magnitude, although the mean annual percentage occurrence of hurricanes is no more than about 3%. Along the northern shores (Mississippi Sound side) the transport is considerably smaller and the contributions of cold fronts and hurricanes to the sediment transports are more or less equal. For the year-averaged conditions (excluding hurricanes) a westward directed net transport is found. The net effect of the historic hurricanes is also westward in direction. However, for individual hurricanes (e.g. Camille and Katrina) the net transport along Ship Island can be directed eastward due to the dominance of ebb flows after the eye of the hurricane had passed.Hydraulic EngineeringCivil Engineering and Geoscience