Delta shoreline structure has long been hypothesized to encode information on the relative influence of fluvial, wave, and tidal processes on delta formation and evolution. We introduce here a novel multiscale characterization of shorelines by defining three process-informed morphological metrics. We show that this characterization yields self-emerging classes of morphologically similar deltas, i.e., delta morphotypes, and also predicts the dominant forcing of each morphotype. Then we show that the dominant forcings inferred from shoreline structure generally align with those estimated via relative sediment fluxes, while positing that misalignments arise from spatiotemporal heterogeneity in deltaic sediment fluxes not captured in their estimates. The proposed framework for shoreline characterization advances our quantitative understanding of how shoreline features reflect delta forcings, and may aid in deciphering paleoclimate from images of ancient deposits and projecting delta morphologic response to changes in sediment fluxes.

Waves, rivers, and tides play a leading role in shaping delta morphology. Recent studies have enabled predictions of their relative influence for deltas globally, but methods and associated uncertainties have remained poorly described. Here we aim to address that gap and assess the quality of delta morphology predictions compared to observations for 31 deltas globally. We expand on seminal works that quantified the Galloway ternary diagram from the balance between river, wave, and tidal sediment fluxes. Our data includes uncertainties for delta shoreline protrusion angles set by wave influence (14.1°±12° predicted vs. 20.8°±16.1° observed), channel widening, set by tidal influence (53.5±170.8 predicted vs. 6.5±11.5 observed), and number of distributary channels, set by river influence (55.9±127.5 predicted vs. 21.4±43.0 observed). Within the ternary diagram for delta morphology, we find an average error of 8% (±11%, 1 standard deviation), linked to uncertainties in wave and tide sediment fluxes. Relative uncertainties are greatest for mixed-process deltas (e.g., Sinu, error of 49%) and tend to decrease for end-member morphologies where either one of wave, tide, or river sediment fluxes dominates (e.g., Fly, error of 0.2%). Large sources of prediction uncertainties are (1) delta morphology data, e.g., delta slopes that modulate tidal fluxes, (2) data on river sediment flux distribution between individual delta river mouths, and (3) theoretical basis behind fluvial and tidal dominance. Future work could help address these three sources and improve predictions of delta morphology.

Wave-influenced deltas are the most abundant delta type and are also potentially the most at-risk to human-caused changes, owing to the effects of wave-driven sediment transport processes and the short timescales on which they operate. Despite this, the processes controlling wave-influenced growth are poorly understood, and the role of fine-grained cohesive sediment (mud) is typically neglected. Here we simulate idealized river deltas in Delft3D across a range of conditions to interrogate how relative wave-influence and fluvial sediment composition impact delta evolution on decadal-millennial timescales. Our simulations capture the barrier-spit formation and accretion process characteristic of prograding wave-influenced deltas, such as those of the Red (Vietnam), Sinu (Colombia), and Coco (Nicaragua) rivers. Barrier-spit accretion exhibits multi-decadal cyclicity driven by subaqueous accumulation of fluvial sediment near river mouths. Using a range of metrics, we quantify how waves and mud influence delta morphology and dynamics. Results show that waves stabilize and simplify channel networks, smooth shorelines, increase shoreline reworking rates, reduce mud retention in the delta plain, and rework mouth bar sediments to form barrier-spits. Higher fluvial mud concentrations produce simpler and more stable distributary networks, rougher shorelines, and limit back-barrier lagoon preservation without altering shoreline reworking rates. Our findings reveal distinct controls on shoreline change between river-dominated and wave-influenced deltas and demonstrate that mud plays a critical role in delta evolution, even under strong wave influence. These insights could enhance paleoenvironmental reconstructions and inform predictions of delta responses to climate and land-use changes.

A document by Laura Portos-Amill. Click on the document to view its contents.

Waves and water level setup during storms can create overwashing flows across barrier islands. Overwashing flows can cause erosion, barrier breaching, and inlet formation, but their sediments can also be deposited and form washover fans. These widely different outcomes remain difficult to predict. Here we suggest that a breach develops when the sediment volume transported by overwashing flows exceeds the barrier subaerial volume. We form a simple analytical theory that estimates overwashing flows from storm characteristics, barrier morphology, and dune vegetation, and which can be used to assess washover deposition and breaching likelihood. Our theory suggests that barrier width and storm surge height are two important controls on barrier breaching. We test our theory with the hydrodynamic and morphodynamic model Delft3D as well as with field observations of 21 washover fans and 6 breaches that formed during hurricane Sandy. There is reasonable correspondence for natural but not for developed barrier coasts, where traditional sediment transport equations do not readily apply. Our analytical formulations for breach formation and overwash deposition can be used to improve long-term barrier island models.