Fluvial deltas have worldwide socio-economic importance as human development and infrastructure centers and provide several ecosystem services, including storm protection and nursery habitats. Their subsurface architecture also holds clues to past climate and sea-level change that can be reconstructed from stratigraphy. A significant challenge in inverting stratigraphy is separating the signals of external forcing, such as variations in sea level, and internal processes, such as the dynamics of the fluvial surface and channel network variations. In a previous work, we analyzed laboratory flume data from the Tulane Delta Basin using an experimental run with oscillating sea level conditions and constant sediment supply. We found that the dynamics of the fluvial surface play an important role in delaying the response of the upper portion of the subaerial topset. To further quantify this phenomenon, we couple this flume experiment with a numerical modeling framework that integrates the topset with a subaqueous offshore region or foreset. The numerical model can explain the topset slope, convexity dynamics, and sediment partitioning between the topset and the foreset under sea level variations. For example, it captures how during sea-level rise (SLR), low sedimentation near the topset's center reduces the subaerial slope and increases convexity, while during sea-level fall (SLF), high sedimentation increases the slope and concavity. Moreover, the model can explain the counterintuitive observation of higher sediment topset bypass to the foreset under SLR than SLF due to the reduction in subaerial slope, partially explained by a higher presence of active channels during SLR than SLF. These results underscore the importance of internal processes such as fluvial surface and channel dynamics, which can result in net erosion during SLR and net deposition during SLF, potentially complicating the reconstruction of paleo sea-level from deltaic deposits.

Estuaries are dynamic coastal features that support industry, food production, and recreation, and provide habitat for numerous animal species. Their typically low surface gradients make estuaries vulnerable to sea level rise, storms, and high river water discharge. This vulnerability combined with the large number of people who often live near estuaries has led to increasing efforts over recent decades to improve our understanding of how to minimize flooding and protect people and property. Despite these efforts, however, we still lack the tools to quantify the relationship between changes in estuarine morphology and flood risks. In particular, the interplay between bathymetric changes and water levels during storm conditions remains poorly quantified. To address this knowledge gap, we present a general enthalpy framework for modeling the evolution of estuaries that couples a low gradient subaerial topset and a subaqueous offshore region or foreset. Sediment transport in both the subaerial and subaqueous domains includes a non-linear term that relates sediment flux, local slope, and a threshold of motion. With this approach, we describe the evolution of the bathymetric profile and sediment partitioning between topset and foreset under a range of sea-level variations scenarios. We find that in some cases upstream sections of the topset can undergo erosion during periods of sea-level rise and deposition during sea-level fall, contradicting traditional stratigraphic models. These counterintuitive bathymetric changes could potentially lead to shifts in the location of maximum water levels along the estuary not accounted for by models of storm inundation.