DELTA SIMULATIONS USING A ONE-LINE MODEL COUPLED WITH OVERWASH

摘要

By adding a simple fixed-point sediment source to a numerical one-contour-line coastal evolution model, we investigate how the distribution of wave approach angles affects the evolution of wave-dominated deltas. These experiments are motivated by recent findings that shoreline evolution is strongly dependant on wave approach angle, and that the ability for alongshore sediment transport to flatten a bump along a sandy coast decreases as deep-water waves approach from more oblique angles. Waves approaching from sufficiently oblique angles (high-angle waves) result in a shoreline instability. Simulations using the one-line numerical model, which includes a simple parameterization of barrier overwash, show that, for the same sediment supply and wave energy, deltas prograde faster and with a more pronounced aspect ratio if the proportion of high-angle waves is increased. An asymmetry in the wave climate increases the tendency towards shoreline instability on the downdrift delta wing; simulations exhibit overwashing spits that extend from the river mouth. This asymmetrical delta evolution is reminiscent of Sf. Gheorghe (St. George) lobe of the Danube Delta, Romania. Other simulations with an initially high then later reduced sediment input rate resemble the basic form of the Ebro Delta, Spain, with lobes recurving towards the mainland extending from both sides of the river mouth. These preliminary investigations demonstrate that reshaping of deltas by waves and subsequent overwash can give rise to surprisingly complex shapes and behaviors