Controls on the growth of wave-influenced river deltas: The roles of fluvial sediment composition and basin depth

Deltas are among Earths most important depositional landforms. They host megacities, serve as ecological hotspots, and control fluxes of water, sediment, and nutrients between terrestrial and marine domains. Nearly all river deltas are subject to some degree of wave-influence, and most river mouths are wave-dominated. These systems exhibit strong spatiotemporal variability across a range of scales and form under hydrodynamic conditions that are highly nonlinear. As a result, the processes involved in the formation and evolution of wave-influenced deltas are poorly understood, limiting our ability to predict how they will respond to future changes. Here we explore the factors governing wave-influenced delta evolution using a coupled flow-wave-transport model (Delft3D-SWAN). We present the first physics-based simulations to correctly reproduce the morphological attributes commonly observed in wave-influenced deltas (smooth cuspate shorelines, simple distributary networks, systems of barriers and lagoons) and capture the emergent processes that govern their evolution. We show that wave-influenced deltas grow through a combination of shoreface accretion, crevasse splays, and distributary channel avulsions. Shoreface accretion occurs when fluvial sediment is deposited in the nearshore faster than it can be removed by wave-driven currents, and manifests as spit / barrier growth and migrating sand waves. Splays and avulsions lead to aggradation on the delta top, but also increase the area over which fluvial sediment is distributed, effectively decreasing the deposition rate at each channel mouth and in turn limiting shoreface accretion. We show that for a given wave climate, the relative importance of these processes determines a deltas morphology and is controlled by the fluvial sediment composition and the receiving basin depth via the distributary channel network. In systems with shallow receiving basins or high sand loads, distributary channels are shallow, wide, and unstable. Network reorganization is frequent, creating deltas with smooth shorelines, many distributaries, and few barrier features. By contrast, a deep receiving basin or an abundance of cohesive fluvial sediment leads to enhanced levee formation, stabilizing distributary channels and reducing network complexity. Simpler networks deliver proportionally more water and sediment to individual distributary mouths, favoring shoreface accretion and leading to deltas with high protrusion angles and an abundance of barrier features. Our results provide insight into the processes involved in wave-influenced delta growth and the factors governing those processes. This information can be used to improve stratigraphic interpretation of delta deposits and helps inform sediment management practices for improving delta resilience in the face of anthropogenic pressures, such as land use and climate change.