Modelling the morphodynamics of a tidal shelf sea

Results are presented of numerical simulations of the basin-scale morphodynamics of an idealised, semi-enclosed, energetic tidal shelf sea. The basin dimensions and tidal characteristics resemble those of the Southern Bight of the North Sea. The simulations were carried out using a finite-difference model of bed-level change, designed for use with an existing numerical hydrodynamical model. A ‘bottom–up’ approach was chosen, which allows investigation of basic physical effects. A bed-load sediment-transport formulation was used, assuming uniform grain size. The computations focus on the basin-scale evolution of the seabed (hundreds of kilometres), span several tens of thousands of years, and represent various scenarios of sea-level change. Morphological features with length scales of tidal sandbanks and smaller were not included. The results show that the basin will export sediment and deepen. Also, it will expand by the accumulation of the eroded sediment in the deeper waters to the north. The deepening causes reduction of the flow velocities and the net sediment transport, resulting in decreasing rates of morphodynamic evolution. The resulting large-scale seabed topography partly resembles the current bathymetry of the Southern Bight of the North Sea. The topography is mainly the result of sediment redistribution along the longitudinal axis of the basin, caused by asymmetry in the velocity field of the amphidromic system. The feedback of the developing bed levels to the water motion is dominated by the increase in water depth, and much less by the seabed topography. Externally prescribed changes in sea level change the wavelengths of the tide and the seabed pattern; they also change the speed of the morphodynamical evolution.