Bedform Geometry and Bedload Sediment Flux in Coastal Wave, Current, and Combined Wave-Current Flows

Ripples and megaripples aggregate our coastlines, inlets, and rivers. This dissertation provides evidence for the contribution of small scale bedform migration to the evolution of larger coastal morphology. Using the connection between scales as a justification for continued research into the dynamics of growth and sediment flux associated with the mobility of ripples and megaripples in our coastal areas, this dissertation examines the dynamics between current dominant, wave dominant, and combined wave-current dominant bedforms on the multi-ripple and intra-ripple scales. Finally, this dissertation proposes that the bedload sediment flux is correlated with the total kinetic energy in the flow field as well as the bed shear stress. However, the results provide evidence to suggest that the kinetic energy formulation may provide higher skill than shear stress based models when detailed boundary layer measurements are unavailable. On the multi-ripple scale, bedforms are shown to have an adjustment time scale for growth or decay that is a function of the total kinetic energy in the flow. The relationship between the bedform volumetric growth or decay and the bedload sediment flux is modeled using a time varying sediment continuity analysis. Additionally, findings show that the bedload sediment transport associated with bedform migration is a function of the total kinetic energy in the flow field. The bedload sediment transport is shown to be robustly modeled using a modification of the energetics set of bedload transport equations regardless of whether the flow is current dominant, wave dominant, or combined wave-current flow, such that the structure for a combined roughness and bedload transport model is outlined supporting that the bedload sediment transport can be represented as a function of the total kinetic energy in the combined wave-current flow. The unified model may be applicable to predicting large scale coastal change without dependence on a robust estimate of the bed shear stress. On the intra-ripple scale, dynamics of bedload transport were investigated in both mean flows and oscillatory flows at the mobile sediment layer of the ripple crest. The mobile bed layer was shown to have a decay in applied stress with up to an 80% reduction in shear stress between the top of the mobile layer and the immobile layer, and a sign change of the applied shear stress at the immobile surface in oscillatory flow. At some of the smallest scales of sediment transport, findings have implications for understanding the mechanisms of bedload transport under waves and currents as well as the shear structure within the mobile layer. A momentum integral method formulated to estimate the bed/wall shear stress was validated and extended to investigate the gradients of momentum through the mobile layer as well as through separated flows and flows around complex geometries. The technique will be useful to investigations with detailed measurements or simulations of the boundary layer momentum structure