Formation and long-term evolution of shoreface-connected sand ridges: Modeling the effects of sand extraction and sea level rise

This thesis focuses on shoreface-connected sand ridges, which are large-scale bedforms observed on many storm-dominated inner shelves. A new nonlinear finite-difference model (MORFO56) is used to study effects of sand extraction and sea level rise on the dynamics of these ridges. MORFO56 uses depth-averaged shallow water equations for currents and includes sediment transport and bed updating. The model setting resembles the Long Island inner shelf. In Chapter 2, a series of short-term runs with MORFO56 are conducted to compare initial growth and migration rates of sfcr versus the longshore wavenumber with those computed with earlier models, which are based on stability analysis. It turns out that MORFO56 yields similar initial growth and migration curves to those from earlier models. Furthermore, MORFO56 is able to simulate finite amplitude sfcr for more realistic bottom slopes than earlier models. If subharmonic modes (modes with wavelengths larger than that of the initially fastest growing mode) are included, ridges merge such that one ridge eventually remains in the system. In Chapter 3, the response of sfcr to extraction of sand is studied. After extraction, ridges partially restore on decadal time scales. However, the original sand volume of the ridge is not recovered. Most sand that accomplishes the pit infill originates from upstream areas, as well as from the areas surrounding the pit. Depending on the pit location, additional sand sources contribute: if the pit is located close to the downstream trough, the pit gains sand by reduction of sand transport from the ridge to this trough. If the pit is located close to the adjacent outer shelf, the recovery of the ridge is stronger due to an import of sand from that area. In Chapter 4, the response of sfcr to sea level rise is investigated. A rising sea level increases the height of sfcr and it decreases their migration until they eventually drown. In contrast, in absence of sea level rise, the model simulates sfcr with constant heights and migration rates. In case of including subharmonic modes, sea level rise reduces the merging of ridges, such that multiple ridges occur in the end state, thereby yielding better agreement with observations. In Chapter 5, the dynamics of sand ridges is investigated in a setting that is characterized by coastal retreat and shelf steepening due to sea level rise. New ridges appear in the shallow area of the inner shelf, which remain active in time. Old ridges that were already formed in the antecedent area become less active with the rising sea level. If migration of offshore parts of the ridges vanishes, these parts change orientation to become more shore-parallel compared with the active onshore parts. In case of small landward inner shelf depths and a decreasing rate of sea level rise, the active onshore parts migrate too fast, thereby causing the drowned offshore parts to detach and to become a field of shoreface-detached ridges. The characteristics of the modeled shore-oblique shoreface-connected and more parallel shoreface-detached ridges are in agreement with those of observed sand ridges.