AbstractsEarth & Environmental Science

The erosion problem in Hai Hau coastal district (Vietnam) is the subject of this thesis. The exact processes that drive this ongoing erosion however are not well defined yet, and therefore two main goals are defined. The first goal is to gain more insight into the processes that drive the ongoing erosion in Hai Hau. The second goal is to predict the future developments of the nearshore area with following consequences for the coastal defense system. The most important results are: 1. Large amounts of wave energy coming from north-eastern directions are blocked by the Ba Lat delta. The only waves reaching the northern part of the Hai Hau coast refract around the delta. These refracted waves are lower than the waves in the other parts of Hai Hau. 2. Modeling south-western currents along the Ba Lat delta showed that the flow is restored at a distance of approximately 13 km south of the southern spit of the Ba Lat delta. Due to the asymmetrical shape of the Ba Lat delta this reattachment point is expected to be located more south in reality. 3. Alongshore sediment transport modeling by waves only, resulted in southward directed transport in winter, with increasing transport capacities in southern direction. Summer transport directions are north. In the annual transport a point of diverging transport direction has been observed near Van Ly. 4. Lowering of the seabed in front of the dikes resulted in a reduction of sediment transport capacities for all locations. The transport gradients, and hence the eroded volumes, decrease as well. These results lead to the following conclusions: a) The sheltering effect of the Ba Lat delta on the sediment transport in Hai Hau is a reduction in sediment transport capacities in the northern parts during winter monsoon periods. This transport increases in southern direction. During summer monsoon periods waves are not hindered to reach the shore and transport capacities are more constant. b) conclusions regarding the annual erosion were based on the assumption that the large scale residual currents reattach in the middle of Hai Hau. Currents along the coast are expected to increase in southern direction from 0 m/s in the North to 0.2 m/s in the South. c) The present annual eroding volume is approximated at 546.000 m3/year. 4) Lowering of the seabed in front of dikes over the width of the surfzone due to ongoing erosion, reduces the width over which sediment is transported in longshore direction. This also leads to lowering of alongshore sediment transports rates and eroded volumes. An equilibrium situation is not to be expected in the near future.