Consequences of limited sediment supply for long-term evolution of offshore tidal sand waves, a 3D model perspective

Field data show that offshore tidal sand waves in areas where sediment supply is limited have different characteristics (shape and dimensions) compared with their counterparts in areas with sufficient sediment supply. So far, only the initial formation of tidal sand waves on a sediment-starved shelf has been studied with a 2DV model that ignores variations along the crests. In this study, a 3D non-linear morphodynamic model is used to investigate the effects of sediment availability on the long-term evolution of offshore tidal sand waves. Overall, the simulated sand waves have characteristics that resemble those of observed sand waves. The mature sand waves that develop in the case of limited sediment supply (i.e., thickness of erodible sediment layer is smaller than the height of sand waves) are more three-dimensional, i.e., having isolated and more irregular crestlines compared with those in the case of sufficient supply. With decreasing sediment supply, sand waves have larger spacings between successive crests, smaller heights and they migrate faster. These differences in the characteristics of the sand waves start to occur once the hard bed underneath the erodible sediment layer is exposed

Morphodynamic impact of sea-level rise on the Western Scheldt estuary and its mouth region:insights from an idealized modeling study

Estuaries lie at the interface of land and sea, and are particularly vulnerable to sea-level rise (SLR). Understanding the impact of SLR on the long-term (order decades to centuries) morphodynamic evolution of estuaries is of great importance to successfully manage these areas, such as maintaining shipping routes and preserving ecosystems. An analysis of historical water level data at Vlissingen (Figure 1) between 1900 and present revealed that the mean sea level has been rising at about 2 mm/yr. Moreover, these data show that the amplitude of the dominant tidal constituent (M2) has been rising as well during this period (Figure 2), which most likely is caused by the rising mean sea level (Pickering et al. 2012, Idier et al, 2017). The specific aims of this study are 1) to investigate the impact of SLR (2 mm/yr) on long-term evolution of the Western Scheldt and its mouth region, 2) to systematically explore sensitivity of model results to different rates of SLR (0-10 mm/yr), and 3) to address the combined effect of SLR and changes in tidal characteristics on the evolution of the estuary. To this end, the coupled SWAN-Delft3D numerical model is used, which accounts for both flow and waves. A curvilinear grid is created, which extends from Ghent to 30 km seaward. As a wave climate, a highly simplified wave forcing (constant wave height, wave periods and wave direction) is considered. The methodology employed is that first the model is spun-up until a bathymetry is obtained that is comparable to observations. Subsequently, the latter bathymetry is used to address the objectives An important model result is that stronger tidal currents are crucial to prevent sedimentation in channels caused by SLR. This result and other findings will be discussed during the presentation

Understanding coastal morphodynamic patterns from depth-averaged sediment concentration

This review highlights the important role of the depth-averaged sediment concentration (DASC) to understand the formation of a number of coastal morphodynamic features that have an alongshore rhythmic pattern: beach cusps, surf zone transverse and crescentic bars, and shoreface-connected sand ridges. We present a formulation and methodology, based on the knowledge of the DASC (which equals the sediment load divided by the water depth), that has been successfully used to understand the characteristics of these features. These sand bodies, relevant for coastal engineering and other disciplines, are located in different parts of the coastal zone and are characterized by different spatial and temporal scales, but the same technique can be used to understand them. Since the sand bodies occur in the presence of depth-averaged currents, the sediment transport approximately equals a sediment load times the current. Moreover, it is assumed that waves essentially mobilize the sediment, and the current increases this mobilization and advects the sediment. In such conditions, knowing the spatial distribution of the DASC and the depth-averaged currents induced by the forcing (waves, wind, and pressure gradients) over the patterns allows inferring the convergence/divergence of sediment transport. Deposition (erosion) occurs where the current flows from areas of high to low (low to high) values of DASC. The formulation and methodology are especially useful to understand the positive feedback mechanisms between flow and morphology leading to the formation of those morphological features, but the physical mechanisms for their migration, their finite-amplitude behavior and their decay can also be explored

CYCLIC BEHAVIOR OF A SHOAL-CHANNEL SYSTEM IN THE WESTERN SCHELDT ESTUARY

Many tidal embayments feature shoals that cyclically form and migrate in a specific direction. The period between successive formations of shoals varies among the tidal embayments. This cyclic behavior characterizes many ebb-tidal deltas seaward of tidal inlets. However in tidal estuaries such as the Western Scheldt, cyclic behavior occurs as well. This is shown in Figure 1, which presents the morphological evolution of the seaward area of Western Scheldt near Vlissingen between 1974 and 2012. Initially, a shoal detaches from the northern part of the shoal “Hooge Platen”, which starts to migrate towards north until it merges with the northern shoal (“Spijkerplaat”). Subsequently, a new shoal detaches again and start to move north. The period between the successive shoal detachments is approximately 30 years. Beside shoal migration, the adjacent northern channel (“Schaar van Spijkerplaat”) also migrates in the northern direction (indicated by the circles). The northern migration of the shoal-channel system is also visible in Figure 2, which shows the evolution of the bedlevel along a cross-channel transect (red dashed line in Figure 1) between 1964 and 2014. While previous research has mainly focused on understanding the cyclic behavior of shoals in tidal inlet systems, little is known about the mechanisms underlying the cyclic behavior of shoals and channels in estuaries. The overall aim of this study is to quantify the physical mechanisms that are responsible for the observed migration of the shoal-channel system in the Western Scheld