Modeling the morphodynamics of shoreface-connected sand ridges

Abstract

The focus of this thesis is on the morphodynamics of shoreface-connected sand ridges, which are large-scale bedforms observed on the inner shelf of coastal seas where storms occur frequently. The main aim was to explore which physical processes control the formation, long-term evolution and main characteristics of these ridges. Existing idealized morphodynamic models were used and extended with potentially important physical processes. The main idea behind these models is that bedforms can evolve due to interactions between waves (which stir sediment from the bottom), storm-driven currents (which transport sediment) and the sandy seabed. The model setting resembles the storm-dominated Long Island inner shelf. The first extension with respect to previous work, as presented in chapter 2 and used in all chapters, is that wave variables are calculated with a shoaling-refraction module. Results (chapter 2) reveal that a necessary condition for growth of ridges is that the transverse bottom slope of the inner shelf exceeds a critical value. Exceeding the critical bottom slope, results in the evolution of ridges with a preferred longshore spacing, growth time and migration speed of about 7 km, 1800 yr and 1 m/yr, respectively. Variations in the latter for a change in offshore wave characteristics are the consequence of a different wave transformation across the inner shelf. In chapter 3 the shoaling-refraction wave module was added to a nonlinear version of the morphodynamic model, meant to study the long-term evolution of shoreface-connected ridges. Simulations show that, in time, the shape of the ridges becomes asymmetrical (steeper seaward flanks), their height saturates and their migration speed decreases. Analysis of the energy balance of ridges reveals that bed slope-induced sediment transports are crucial for the saturation process. Adding subharmonic modes (i.e., eigenmodes with wavelengths larger than that of the initially fastest growing mode), shows a 20% increase in the spacing of ridges in time. The evolution of the individual modal amplitudes is sensitive to the number of subharmonics included and thus is only predictable for a finite amount of time. Interestingly, ridges form patches in the transient stage, which is also observed. In chapter 4 refraction and shoaling processes due to the presence of bedforms (called wave-bedform feedbacks) are also accounted for. Including wave-bedform feedbacks results in a new mechanism for ridge growth, which also works for a flat bed. The spacing and offshore extent of ridges decrease, which is in accordance with observations. The influence of wave-bedform feedbacks on the initial formation of ridges for a bimodal sediment mixture was investigated in chapter 5. Results indicate that ridge growth and migration decrease in case of a mixture. In case that the entrainment of sediment is dependent on bottom roughness the coarsest sediment is found in the troughs, whereas in all other cases fine sediment occurs in the troughs. Some shoreface-connected ridges exhibit grain size patterns, which are in agreement with those obtained by the model, however not the Long Island ones. The modeled maximum variation in the mean grain size over the topography has improved with respect to previous work.