Morphodynamics of ebb-tidal deltas

Abstract

Ebb-tidal deltas are bodies of sand that are located seaward of tidal inlets. The latter connect the open sea with a back-barrier basin and separate barrier islands. The morphology (e.g., sand volume, geometry, shoal formation) of ebb-tidal deltas evolves continuously, both due to natural processes as well as due to human interventions. Both aspects were investigated in this thesis. A human intervention that affected the morphology of ebb-tidal deltas is the closure of the Zuiderzee by which the size of the back-barrier basins of Texel Inlet and Vlie Inlet was reduced. By combined use of analytical and numerical models it was found that the length of a back-barrier basis affects the spatial symmetry (center of mass) of ebb-tidal deltas through its control on the phase of the cross-shore tidal velocity in an inlet. It was also found that the back-barrier basin length affects the sand volume of an ebb-tidal delta by controlling the characteristics of the principal tide, residual currents, and overtides. An overtide-component with a magnitude such as present seaward of Texel Inlet and Vlie Inlet has a large impact on the local net sand transport. The phase difference between the principal tide and the overtide such as observed seaward of Texel Inlet and Vlie Inlet causes net landward (seaward) sand transport for short (long) back-barrier basins. This suggests that the observed sand import into the Wadden Sea after the closure of the Zuiderzee is caused by the changed result of the imposed overtides on the net sand transport in the inlets. The natural evolution of many ebb-tidal deltas involves the formation of coherent shoals that migrate towards the coast. Such shoals were found on numerous ebb-tidal deltas of the Wadden Sea. The average period between successive attachments of these shoals varies among inlets and ranges between 4 and 130 years. For inlets that are composed of a single channel, there is a positive relationship between the period between successive shoals and the tidal prism. Moreover, there is a negative relationship between the migration speed of shoals and the tidal prism in these systems. For tidal inlets with multiple channels no clear relationships were found. A negative relationship between the migration speed of shoals and the tidal prism was also found in a numerical model study. Shoals formed on the simulated ebb-tidal deltas when the bathymetry was locally out of balance with the wave conditions. They grow and migrate due to the combined effect of wave-driven residual currents and high sediment concentrations over the shallow structure. The depth-averaged sediment concentration is largest at the top of the shoal due to increased near-bed wave orbital velocities, which increase with decreasing water depth. Landward-directed residual currents over the shoal originate from spatial variations in radiation stresses due to the dissipation of wave energy over the shallow bathymetry. Sediment transport driven by asymmetric wave orbital velocities was found to contribute to maintaining the shoal as a coherent structure.