River deltas are a compelling target for numerical simulation because they contain seemingly organized patterns and shapes at a variety of scales. For instance, most river-dominated deltas, regardless of size, have triangular to semi-circular planform shapes, channel networks, and channel bifurcations. The common presence of these features among most deltas in the world (Caldwell et al., 2019; Nienhuis et al., 2020) suggests there are consistent underlying physical processes controlling delta form and behavior. In this review, we discuss how numerical modeling, and more specifically a type of modeling focused on the morphodynamic feedback, has helped explore some of these key physical processes over the last 15 years.

In response to a growing number of natural and anthropogenic threats, the long-term sustainability of coastal river deltas and wetlands has come into question worldwide. Tools such as remote sensing and numerical modeling have been implemented in an effort to monitor and predict the hydro-geomorphological evolution of our coasts. Hydrological connectivity is known to play an important role in deltaic evolution by delivering flow, sediment, and nutrients to the interior of deltaic islands/wetlands. However, estimating connectivity typically requires detailed field work or numerical modeling, which is difficult to implement over broad spatial and temporal scales. In the present work, we investigate the potential of using remote sensing to estimate hydrological connectivity in the Wax Lake Delta (WLD) and Atchafalaya Delta region of the Louisiana coast. During a three-hour window, five difference maps of water level in the WLD and surrounding wetlands were collected using UAVSAR L-band radar in repeat-pass interferometric mode. We then modeled the WLD subsection of the domain using a 2D shallow-water hydrodynamic model configured to run on the same discharge, tide, and wind conditions as recorded at nearby monitoring stations during the observational window, with vegetation parameterized as a source of additional drag in the deltaic islands. Modeling allowed us to determine the relative influence of tides, vegetation, and wind on WLD water levels, which could then be extrapolated to infer the behavior throughout the rest of the domain. Over the observational window, UAVSAR measured a cumulative loss of over 22 megatons of water from non-channelized wetlands as tides fell. We find that the model tends to under-predict the observed water level draw-down, as well as the degree of hydrological activity in proximal islands that we observe in the UAVSAR data. Models that neglect the influence of wind underestimate the volume of water leaving the islands by up to two-thirds, suggesting the importance of wind on deltaic hydrodynamics during the observational window. With the information gained from the numerical modeling, as well as the computation of information theory statistics, we extend the WLD results to analyze and quantify the water level behavior in the surrounding wetlands and Atchafalaya delta.

Deltas are important landforms on Mars because they indicate past fluvial activity, contain a sedimentary record amenable to study, and have the potential to store past signs of life. The sedimentary records that are visible today on satellite and rover images are used to deduce the fluvial and climate history of Mars. To do this we use our understanding of deltas on Earth, as deltas are well-studied landforms. However, when making interpretations it is very important to understand the differences in delta morphodynamics and stratigraphy between Earth and Mars. Even though the processes on Mars are similar, the water discharge, sediment flux, and grain size sorting can be significantly different due to for example gravity, sediment density, presence of ice, and lack of ecology. In this research, we focus on the effect of gravity and its net effect on delta morphology and stratigraphy. From preliminary experiments, we expect a significant effect of gravity on grain size sorting, because suspended sediment (fine grains) is affected more by gravity than bedload transport (bigger grains). In addition, we expect bigger grains to travel in suspension on Mars under the same boundary conditions. We will study the net effect of these water and sediment fluxes on morphology and stratigraphy by numerical modelling in Delft3D. This software is typically used for river and coastal systems on Earth, but we adapted the software by identifying all explicit and implicit dependencies on gravity, so it can also be used for deltas on Mars. We developed a 2DH hydro-morphodynamic model of a simplified delta to test different scenarios. The model consists of a straight channel flowing into a sloping basin. In the comparison between our Earth and Mars scenarios, gravity is the only parameter varied. We are currently experimenting with different upstream boundary conditions that we keep equal between the Earth and Mars scenario. We are testing equal discharge, water level, and sediment input. Equal discharge reveals the effect of gravity on sediment flux, equal water level the effect on water and net sediment flux, and equal sediment supply focusses on the sediment sorting that is mostly overshadowed in the other scenarios by the sediment flux difference.