In this study, the excavation process of a low pressure vertical impinging jet in cohesive soil has been investigated. Mass flow excavation is a hydraulic, low pressure, subsea excavation method whereby a large volume flow is applied to the seabed through which the seabed is eroded, and the soil is transported. The erosion process of a cohesive soil by impinging jets depends on many variables (e.g., jet flow velocity, standoff distance, grain size, undrained shear strength). The erosion processes, especially for dynamic pressures of lower than two times the undrained shear strength, are not fully understood. It is still unknown what the influence of different soil and jet parameters are. As a result, it is still unknown what the expected scour rate will be during a mass flow excavation process of cohesive soils. The goal of the research is to quantify and be able to predict the production of a mass flow jet on cohesive soils. For this purpose, the relevant parameters of cohesive soil erosion are investigated, and fluid velocity profiles of the jet are related to clay bed failure mechanisms. Special attention is put on the effect of the consolidation coefficient on the erosion process. Based on literature and experimental research, different soil failure mechanisms have been investigated. The main result of this study is a relation between the main jet and soil parameters, and the erosion velocity development.

At Isle of Palms there is a lot of erosion due to shoal bypassing events. Five TU Delft Hydraulic Engineering students went to the island under supervision of Coastal Science and Engineering. The main scope of the report is to create a thorough understanding of the processes that form, transform and propagate the shoals that cause catastrophic erosion and finding ways to deal with this erosion. The movement of the shoals, but also the entire ebb tidal delta has been analysed for the last 10 years in this report. It is tried to form conclusions from this dataset although some conclusions are still highly uncertain. It is seen that there is not only a episodic erosional trend with a period of around a decade, but also one with a longer period. For both of these types of erosion designs are proposed to mitigate the negative effects of these erosions. It is found that the current practice of 10 yearly nourishments is the most effective way to cope with the erosion. However some notes have to be made that further research can improve the current nourishment strategies.