Tracking coastal sediments can provide useful information about coastal dynamics, thereby helping coastal management. However, the highly dynamic conditions of the coasts makes analyzing the trajectories of a huge number of particles challenging. To solve this limitation, the framework of coastal sediment connectivity is designed. In this framework, recent advances in graph theory are used to quantify coastal systems as complex networks. In this context, sediment sinks/sources and pathways represent the graphical nodes and links, respectively. In this work, we take the first step to evaluate the ability of this newly-developed framework in quantifying the basic processes on a sandy beach. Firstly, we used Delft3D to obtain the velocity field and bed-level changes. Then, the Eulerian results were fed into SedTRAILS to simulate the sediment pathways. We show that the current version of the model can correctly calculate the basic metrics of the sediment-connectivity network (e.g., network link strength which is a proxy for sediment fluxes). More specifically, we show that this framework is capable of exploring the initiation of the rip channel formation.

Coastal sediment budgets are a foundational source of information for coastal management decision-making. To quantify these budgets, coastal systems are often divided into “cells” based on jurisdictional boundaries or topography. However, such divisions do not account for the pathways that water and sediment particles actually take. In this study we quantify cell boundaries that emerge from numerical simulations of sand and water pathways in a barrier island-lagoon system in the Netherlands (the Western Wadden Sea). By quantifying Lagrangian particle pathways as a network, we can derive internally well-connected but externally disconnected modules. Here we show that large (O(10 km)) coherent modules develop from flow patterns at tidal timescales (12.5 h), and are persistent through varying tide and weather conditions. Conversely, modules derived from 100 µm sand pathways are less coherent and highly spatially fragmented. The difference in patterns likely relates to the longer timescales associated with sediment transport. These emergent patterns could be used to better inform coastal and estuarine management by providing physics-based sediment cell boundaries.

Estuaries worldwide are experiencing increasing threats from climate change, particularly from the compounding effects of sea level rise (SLR) and varying magnitude of river inflows. Understanding the tidal response of estuaries to these effects can guide future management and help assess ecological concerns. However, there is limited existing understanding on how estuarine tidal dynamics may respond to the compounding effects of SLR and altered riverine inflows in different estuaries. To partially address this knowledge gap, this study used data analysis and scrutinised idealised hydrodynamic models of different estuary shapes and boundary conditions to (i) identify broad effects of SLR on estuarine tidal dynamics under various river inflow conditions, (ii) determine how longitudinal cross-sections are impacted by these effects, and (iii) highlight some implications for environmental risk management. Results indicated that short- to moderate-length, high convergent estuaries experience the greatest and short- to moderate-length prismatic and low convergent estuaries experience the least variations in their overall tidal dynamics (i.e., tidal range, current velocity, and asymmetry). These variations were most evident in estuaries with large riverine inflows and macrotidal conditions. Compounding effects of SLR and altered riverine inflows induced spatially heterogenous changes to tidal range, current velocity, and asymmetry, with transects nearest to the estuary mouth/head and at a three-quarter estuary length (measured from estuary mouth) identified as the most and the least vulnerable zones, respectively. These findings provide an initial broad assessment of some effects of climate change in estuaries and may help to prioritise future investigations.

The ability to model bar migration in the nearshore zone is critically important for predicting coastal morphology. We present a cost-effective wave-averaged 3D model (CROCO) with parameterization of wave-related bedload transport (adapted SANTOSS), and evaluate/calibrate its performance in comparison with data measured in a two-dimensional wave channel. In this model, suspended-load transport (both in and above the wave bottom boundary layer) is resolved by the flow model. We assume that if the tuning parameters are not robust to varying forcing conditions, the parameterization can be considered a failure. The present work is therefore a first step towards a numerical model capable of predicting the onshore and offshore migration of a sandbar under storm and post-storm conditions using the same set of parameters. One way to achieve this goal is to treat the effects of waves and currents in separate formulations to avoid conflicts and redundancy. We show that the implementation and adaptation of SANTOSS in CROCO can achieve this goal, provided that the hydrodynamics are accurately reproduced and the separation between the wave-related bedload and current-related suspended load formulations is respected.

In this paper, we investigate the effects of the bathymetry of the floodplain distributaries using a 2-dimensional hydrodynamic model of the Wax Lake Outlet (WLO), delta and floodplain (Louisiana, USA), using Delft3D. Modelling the tidal-fluvial interaction of this region is challenging because of its complex network of low-lying floodplain distributaries and vegetation coverage. This investigation addresses this interaction by generating the evolution of the flood-map that is used in the calibration and validation phase of the Surface Water and Ocean Topography (SWOT) mission. Accordingly, the model is set up for one week of calm conditions from October 14th to 20th in 2016. Boundary conditions applied at the edges of the 36 × 23 km ² domain include the offshore water level with tidal oscillations, the upstream volumetric river discharge rate and wind time-series (applied spatially uniformly). The topo-bathymetry of the domain is a product of the Pre-Delta-X mission; however, it has considerable uncertainties to represent the small distributaries, especially in the forested section of the region. This paper tackles this uncertainty by parametrically enhancing and calibrating the primary Digital Terrain Model (DTM). The performance of the model is assessed using the water-level time series measured by the stations spread throughout the floodplain, delta and main river. The results show that the morphology of the distributaries play a major role in the hydrodynamic of Wax Lake region under calm conditions. The hydrodynamic of the WLO and floodplain is considerably affected by the discharge capacity of the distributaries (changed by editing their width and depth) due to change of the balance between roughness and topographic convergence. In WLO, higher discharge capacity leads to higher overall roughness friction force, thereby dampening the amplitude of the water-level signal and vice versa. On the other hand, increasing the bottom roughness of the WLO increases the water level in the floodplain, with more considerable effect in the low tides. The calibrated model well matches the measurements along the Wax Lake Outlet. However, the main discrepancies are in the stations further away from the WLO. The tests conducted in this research suggest that the excessive roughness due to uneven bed along the distributaries and vegetation are, respectively, the potentially-responsible parameters for the remaining discrepancies in low- and high-tide periods.

The problem of sandbar migration on the storm timescale is revisited with a 3D wave-resolving hydro-sedimentary model. The latter presents an intermediate approach between expensive wave-resolving two-phase flow models and highly parametrized wave-averaged models. Innovative features include the use of weakly compressible assumptions in the hydrodynamics and morphological acceleration of bed changes to speed up numerical simulations. The model accurately simulates the successive offshore and onshore bar migration observed in a large-scale flume experiment in response to wave forcing representing storm and post-storm (recovery) conditions. The diagnosis of sand transport and the analysis of an ensemble-averaged asymmetric wave cycle reveal the migration mechanisms in each phase. In all cases, sediment resuspension is impacted by breaker-induced turbulence, while sediment transport and bed evolution are primarily the result of the undertow distribution – the breaker-induced seaward undercurrent – across the sandbar. There is also a significant contribution from asymmetric wave-related onshore fluxes, due to greater mobilization and currents during the wave crest period.