Fluvial and marine controls on combined subaerial and subaqueous delta progradation: Morphodynamic modeling of compound‐clinoform development

TLDR

Fluviodeltaic systems often exhibit compound‑clinoform geometry, a subaerial/subaqueous delta couplet, with subaqueous development varying with fluvial input and basin hydrodynamics. The study introduces a model of fluviodeltaic progradation driven by terrestrial floods and coastal storms. The model couples fluvial and shallow‑marine sediment dynamics through a shock‑condition surf zone boundary, simulating subaerial aggradation and shoreface progradation alongside subaqueous foreset progradation with minimal topset growth. Under steady sediment supply and sea level, the model produces prograding deltas with compound‑clinoform geometries, showing that fluvial input, wave/current fields, storm frequency, flood discharge, and grain size govern sediment partitioning and that these dynamics explain key natural delta morphologies such as shoreline‑clinoform separation and differential progradation rates.

Abstract

Fluviodeltaic systems commonly display a compound‐clinoform geometry that consists of a subaerial/subaqueous delta couplet. The extent of subaqueous delta development varies significantly and, in modern systems, is a function of fluvial input and basin hydrodynamics. We present a model of fluviodeltaic progradation in which the repeated occurrence of characteristic terrestrial floods and large coastal storms drives fluvial and shallow marine morphodynamics, respectively. We couple fluvial and shallow marine sediment dynamics via the surf zone, which we collapse to a shock condition and treat as a moving boundary. With steady sediment supply and sea level and simple basin geometry, our model naturally develops prograding deltas with compound‐clinoform geometries. The subaerial delta grows via fluvial aggradation and shoreface progradation, whereas the subaqueous delta expands through foreset progradation, with only minor topset aggradation. The interplay of fluvial input with the wave/current field controls the basic partitioning of sediment between subaerial and subaqueous deltas and, by extension, the compound‐clinoform geometry. Increasing the frequency or magnitude of coastal storms, decreasing flood frequency or discharge, and reducing grain size all increase the fraction of sediment delivered to the shallow marine environment and the extent of subaqueous delta progradation relative to subaerial delta development. Our model, which emphasizes the intrinsic coupling of fluvial and shallow marine sediment dynamics and downplays the importance of allogenic fluctuations, can explain many of the first‐order morphologic features of natural delta systems, including significant lateral separation of the shoreline and clinoform rollover and differing rates of subaerial and subaqueous delta progradation.