Cyclic channel-shoal dynamics of ebb-tidal deltas

Abstract

Barrier coasts, such as the Wadden Sea, are separated from the open sea by barrier islands, deep channels and ebb-tidal deltas. Because these deltas are shallow, they enhance the offshore wave breaking and other forms of wave energy dissipation, protecting the down-wave islands from the full force of storm waves. A second key role of ebb-tidal deltas in coastal safety is their ability to (temporarily) provide the neighboring coastal areas and tidal basins with sediment to compensate for local erosion. Tidal currents and energetic waves continuously interact with the sandy bed, and as a result, like most systems in nature, ebb-tidal deltas are not static. Instead, ebb-tidal deltas often behave in a cyclic manner, in which channels rotate and periodically breach the ebb-tidal delta. Meanwhile, shoals form, migrate and attach to the downdrift barrier islands (i.e., cyclic channel-shoal dynamics). This cyclic behavior has four characterizing phases: channel rotation, shoal growth, channel breaching and channel deepening. Not only in the Netherlands, but throughout the world ebb-tidal deltas are eroding, which decreases their natural safety functions. This erosion is probably the result of sea-level rise and of large-scale interventions such as land reclamation and dam/dike construction. As a mitigation route, nourishments on the ebb-tidal deltas are being considered. As anthropogenic influences in the coastal environment are expected to increase, there is an urgent need to understand how both natural and human-induced processes drive different aspects of the cyclic channel-shoal dynamics. The overall aim of this thesis is to provide fundamental insight into the cyclic behavior of ebb-tidal deltas. For this, numerical model simulations were conducted using Delft3D and SWAN. The two topics of this thesis are (1) the causes of natural cyclic behavior and its effects on tides, waves and sediment transport, and (2) the influence of human activities and possible ebb-tidal delta nourishments on ebb-tidal deltas.