A multi-scale hybrid long-term morphodynamic model for wave-dominated coasts

Process-based modules based on conservative equations to solve the transport of waves, currents and sub-aqueous sediment, and behavior-oriented modules based on empirical descriptions of cliff erosion, bed-load and terrestrial aeolian sand transport are coupled with up-scaling measures and stability-maintaining approaches in a parallel code to simulate decadal-to millennial-scale morphological evolution of wave-dominated coasts. An application to the southern Baltic Sea for a hindcast of Holocene morphogenesis of the Darss–Zingst peninsula demonstrates the robustness of the model. The model is then used to investigate the morphogenesis and evolution of spits in an idealized fetch-limited sandy coastal environment. Evolution of the spit system can be categorized into three periods according to the simulation results. In the early period all initial coastline perturbations are amplified due to the high-angle wave effects. Size competition between adjacent spits starts afterwards. Interaction between adjacent spits dominates the middle period. Smaller spits tend to retrogress due to unbalanced sediment budget and this sediment loss feeds further growth of their larger neighbors. The retrograded spits are either smoothed or evolve into barrier-lagoons, and relatively stable large-scale spits originating from larger spits are developed in the late period. Low-frequency storms impinging from a regular high angle onto the coastline facilitate the development of spits in the early period, but become a hindering factor afterwards. Terrestrial aeolian transport not only plays a key role in stabilizing the spits, but is also revealed as an important factor for long-term coastline erosion.