Western Scheldt Estuary Research Articles

The shape of fringing tidal flats in engineered estuaries

For the management of estuaries and the preservation of tidal flats it is crucial to understand the tidal flat shape and development. Previous work focused predominantly on the quasi-equilibrium shape of tidal flats along open coasts with a dominant cross-shore flow and wave exposure. This paper evaluates the shape of fringing tidal flats in engineered estuaries, where longshore velocities generally dominate. Using a long-term (20 years) topographic data set of an anthropogenically modified estuary in the Netherlands (the Western Scheldt estuary), we relate key profile shape parameters and changes over time to natural and anthropogenic processes. In an engineered estuary, the tidal flat shape depends on the estuary geometry, hydrodynamic forcings and human interventions. In contrast to open coast tidal flats, the presence of the channel and dominant longshore flow determines the available cross-shore length (accommodation space) of the tidal flat and the shape of the tidal flat. This accommodation space defines the maximum tidal flat height and opportunity for marsh development. We propose the use of the Index of Development, indicating to what extend tidal flats have space to develop. This index is not only influenced by longshore and cross-shore flow, but also (or even more) by hydraulic structures, dike realignments and channel migration. Especially the latter two strongly influence the accommodation space and thereby the maximum tidal flat height and the opportunity for marsh development. For large stretches of the Western Scheldt, the accommodation space is too small, and the majority of the tidal flats do not vertically extent to mean high water. The success of tidal flat and marsh restoration projects depends on the accommodation space.

A vertebra of a small species of Pachycetus from the North Sea and its inner structure and vascularity compared with other basilosaurid vertebrae from the same site.

In the Western Scheldt Estuary near the Belgian-Dutch border, middle to late Eocene strata crop out at the current seafloor. Most vertebrae of large Eocene basilosaurid taxa from this area were previously described in several papers. They represent three morphotypes: elongated vertebrae of a large species of Pachycetus (Morphotype 1b), a not-elongated vertebra of a large 'dorudontid' basilosaurid (Morphotype 2) and 'shortened' vertebrae of a new, unnamed taxon (Morphotype 3). This article deals with a still undescribed, smaller vertebra, NMR-16642, from this site. Our first aim was to date it by dinoflagellate cysts in adhering sediments. Yielding an age of about 38 Ma, it is one of the very few remains of basilosaurids from Europe, of which the age could be assessed with reasonable certainty. The vertebra, Morphotype 1a, is assigned to a small species of Pachycetus. High-quality CT scans are used to differentiate between NMR-16642, Morphotype 1a, and the large species of Pachycetus, Morphotype 1b. Another aim of this paper is to investigate the inner structure and vascularity of the study vertebra and that of the other morphotypes (1b, 2, 3) from this area by using high-quality CT scans. Notwithstanding differences in size, shape and compactness, the vertebral inner structure with a multi-layered cortex of periosteal bone, surrounding two cones of endosteal bone appears to be basically similar in all morphotypes. Apparently, this inner structure reflects the ontogenetic vertebral growth. An attempt to reconstruct the vascularity of the vertebrae reveals a remarkable pattern of interconnected vascular systems. From the dorsal and, if present, ventral foramina, vascular canals are running to a central vascular node. From this node a system of vascular canals goes to the epiphyseal ends, giving rise to separate systems for cortex and cones. It is the first time that the vascularity of vertebrae of archaeocetes is investigated.

Comparison of methods for the radiological impact assessment of aquatic releases to the waters in the low countries

Accurate assessment of the radiological impact of liquid discharges on the marine environment is challenging despite all developments in recent years. The lack of consensus on this type of assessment manifests itself even stronger when transborder issues are expected, such as in the Low Countries. Belgium and the Netherlands operate nuclear power plants with discharges in the shared estuary of the Western Scheldt, therefore if there are safety concerns, information on both sides of the border must be coherent. This work provides a comparison of two computational methods used for assessment of aquatic releases in the Western Scheldt estuary and the adjacent North Sea.The work demonstrates a fair degree of consistency in modelling the uptake and fate of key anthropogenic radionuclides. Nevertheless, there are also considerable differences found in sediment and sea species with concentrations ranging by over two orders of magnitude in some cases. These explainable differences are methodological in nature, occurring in codes that underwent extensive validation during development. Therefore, the outcomes of this work clearly demonstrate the need to produce explicit guidance that is specifically tailored to the (inter)national water system of concern. This should not be limited to releases from nuclear power plants, but also include other nuclear applications. For all these reasons, more intensive collaboration and model harmonisation across borders is essential, signalling the direction for future investigations.

Annual biogeochemical cycling in intertidal sediments of a restored estuary reveals dependence of N, P, C and Si cycles to temperature and water column properties

Estuarine intertidal sediments are important centres for organic matter remineralization and nutrients recycling. Nevertheless, there is limited understanding regarding how these processes occur along the salinity gradient and their seasonality. Here, we report on the seasonal biogeochemical cycles from three types of intertidal sedimentary habitats (freshwater, brackish and marine) located in the Western Scheldt estuary (The Netherlands and Belgium). A full year of solute fluxes, porewater nutrient and sediment pigment concentrations at a monthly resolution revealed clear differences in the biogeochemistry of the three sites, indicating that environmental conditions determined the local nutrient dynamics. Temperature controlled sediment oxygen consumption rates and nutrient fluxes, but also affected pore water nutrient concentrations up to 14 cm deep. Fresh and brackish sediments had a net influx of dissolved inorganic nitrogen (DIN) (−1.62 mmol m−2 d−1 and -2.84 mmol m−2 d−1, respectively), while only the freshwater sediments showed a net influx of phosphate (−0.07 mmol m−2 d−1). We estimated that intertidal sediments remineralized a total of 10,000 t C y−1, with 97% of mineralization occurring in the brackish and marine parts. Overall, sediments removed 11% (1500 t N y−1) and 15% (∼200 t P y−1) of the total nitrogen and phosphorus entering the estuary from riverine input. Moreover, observations revealed the historical improvement of water quality resulting from water treatment policies. This spatiotemporal study of OM remineralization and early diagenesis in estuarine systems highlights the importance of intertidal sediments for estuarine systems. Our observations can be used in models to predict estuarine biogeochemistry or assess climate change scenarios.

Large-scale morphological changes and sediment budget of the Western Scheldt estuary 1955–2020: the impact of large-scale sediment management

Abstract In the Western Scheldt estuary, like in many estuaries, safe navigation, flood protection, and ecological targets require a balanced and sustainable sediment management. A thorough understanding of the morphodynamic functioning of the estuary and its response to changes in hydrodynamics (natural sediment transport) and large-scale interventions is imperative. This paper presents a detailed overview of over 65 years of morphological changes and a comprehensive sediment budget of the Western Scheldt estuary that is based on analysis of a unique series of frequent bathymetric surveys and historical data on human–sediment interactions of dredging, dredge disposal and sand mining. Solving the sediment budget reveals an annual sediment import of 2.2 million m3. The highest sediment imports occurred between 1980–1994 and 2005–2020 (2.9 and 3.7 million m3/year). A 1.3 million m3/year net export prevailed between 1994 and 2005. Natural variations in the hydrodynamics (e.g., tidal asymmetry and amplification) and sediment transports cannot explain the derived temporal variations in sediment import rates. Anthropogenic activities play a dominant role. Capital dredging of the main navigation channel has led to channel deepening and significantly increased dredge and disposal volumes. Disposal on tidal flats and in the secondary channel leads to a long-term storage of sand and, consequently, a local increase in bed level and a sand deficit in the remainder of the system that results in increased sediment imports. Large-scale disposal in the western part of the estuary can (temporarily) reverse the sediment exchange between the estuary and its mouth area, as observed between 1994 and 2005. Apparently, large-scale anthropogenic reallocation of sediment by dredging and/or disposal as part of navigation channel improvement, sand mining or nourishment essentially influences the morphological development of the Western Scheldt estuary. This reveals responsibilities as well as opportunities of sediment management for the Western Scheldt and similar estuaries worldwide.

Breaching and liquefaction in subaqueous retrogressive flow slides

Although retrogressive flow slides in subaqueous sandy slopes can be very large and cause substantial damage, the failure mechanisms of such slides are not very clear yet. This study analyses two well-monitored flow slides in a shoal margin in the Western Scheldt estuary in the Netherlands: a natural flow slide that eroded 300 m into the edge of the shoal and an artificially induced flow slide that was triggered by dredging and eroded only 30 m of the shoal margin. Both slides were simulated with a newly developed numerical model that describes the physics of slow retrogressive breaching and the much faster retrogression of statically liquefied fine to medium (silty) sands. The simulations show that the differences in trigger and size can be explained by assuming that in the larger slide both retrogressive breaching and static liquefaction took place, while in the smaller one only breaching occurred. The main contribution of retrogressive liquefaction to the larger slide was the generation of a temporary high-erosive density flow that proved sufficient to create such a high, near-vertical slope that the breaching process could continue over a long period and distance. It is therefore likely that both breaching and static liquefaction play a role in large natural flow slides.

Satellite-Derived Trends in Inundation Frequency Reveal the Fate of Saltmarshes

Some of the world’s coastal saltmarshes experience loss in area due to environmental changes, such as relative sea level rise and limited sediment supply. We use satellite remote sensing to assess changes in inundation (flooding) frequency in tidal basins and investigate the bio-physical interactions with saltmarshes. We apply a simple automated method to retrieve time series of inundation frequency change and seaward habitat change of saltmarshes and tidal flats from Landsat-5 TM satellite imagery between 1985 and 2011, for a number of contrasting tidal basins (estuaries, deltas) globally. We evaluated the satellite-derived information on inundation frequency with such information obtained from elevation and tide gauge data for the Western Scheldt estuary, showing good agreement. Application of the method on all study sites reveal which tidal basins are stable or net emerging and which tidal basins are net drowning, but also show large spatial variation in the changes in inundation frequency within each basin. Tidal basins experiencing an overall significant increase in inundation frequency (Mississippi Delta and Venice Lagoon) were associated with an overall loss of saltmarsh area. Satellite-derived temporal and spatial information on inundation frequency helps to assess the fate of saltmarshes in light of sea level change, changes in sediment supply and subsidence.

Observed and modelled tidal bar sedimentology reveals preservation bias against mud in estuarine stratigraphy

AbstractMud plays a pivotal role in estuarine ecology and morphology. However, field data on the lateral and vertical depositional record of mud are rare. Furthermore, numerical morphodynamic models often ignore mud due to long computational times and simplifications of mixed depositional processes. This study aims to understand the spatial distribution, formative conditions and preservation of mud deposits in the intertidal zone of bars in high‐energy sand‐dominated estuaries, and to elucidate the effects of mud on morphology, ecology and stratigraphic architecture. To meet these objectives, field data (historic bathymetry, bio‐morphological maps and sediment cores of the shoal of Walsoorden, Western Scheldt estuary, the Netherlands) were combined with complementary hydro‐morphodynamic numerical modelling (Delft3D). Based on the field observations, two types of mud deposits were distinguished: (1) mudflat deposits, which are thick (>10 cm) mud beds at the surface associated with high elevations and low accumulation rates; and (2) mud drapes, which are thin (millimetre to centimetre) buried laminae that form and preserve at a wide range of elevations and energy conditions. Model results show that deposition on mudflats occurs just after high‐tide slack water in areas shielded from high flood velocities, suggesting that mud accumulation is mostly controlled by elevation, flow velocity and flow direction. Mud accumulation increases shoal elevation, sometimes to supratidal levels. This reduces flow over the shoal, which in turn reduces chute channel formation, stabilises bar morphology and decreases local tidal prism. These effects further promote mud deposition and vegetation settling. Although observations show that mud cover at the surface is relatively high (20%–40% of the intertidal area), mud constitutes only a small percentage of the total estuary volume (ca 5%) revealing that only a small fraction is preserved in the stratigraphy. Due to this mismatch between surface and subsurface expression of mud, interpretations of estuarine stratigraphy risk underestimating the influence of mud at the surface on morphodynamics and habitats.

Contrasting behaviour of sand and mud in a long‐term sediment budget of the Western Scheldt estuary

AbstractUnderstanding trends in estuarine sediment fluxes is of great interest to sustainable estuarine management addressing anthropogenic interferences and climate change. The long‐term sediment budget of the Western Scheldt estuary, Netherlands, is investigated by a detailed analysis of a unique and long‐term bathymetric data set and data of a three‐dimensional subsurface model (‘GeoTop’). Different sediment types show contrasting transport behaviour. The Western Scheldt narrowed and deepened, while the estuary exported sand (1.5 to 2.5 million m3 per year) and imported mud (0.5 to 1.5 million m3 per year) over the 1860 to 1955 period. The eroded sand originated from the channels in the seaward part of the estuary and was dispersed in all directions. A significant amount of mud permanently deposited in the side branches, which were also gradually reclaimed. These results suggest that sediment characteristics potentially play a crucial role in deriving long‐term sediment budgets and morphodynamic behaviour of estuarine environments. Future morphodynamic sand–mud model studies may reproduce and further explain the underlying transport processes of the current study.

Facilitating Salt Marsh Restoration: The Importance of Event-Based Bed Level Dynamics and Seasonal Trends in Bed Level Change

Intertidal salt marshes provide a range of valuable ecosystem services which typically increase with marsh width. Understanding the drivers for salt marsh expansion versus retreat is thus key to managers. Previous research highlights the influence of short-term (daily/event) bed level dynamics on germination and establishment and subsequent vegetation presence. However, more recent literature suggests the importance of medium-term seasonal bed level dynamics on viable seed availability and subsequent vegetation presence. This study aims to assess event-based and seasonal bed level dynamics for vegetation presence in natural and semi-natural salt marshes and to provide generic thresholds for vegetation presence. To gain insight into bed level dynamics, data was used from autonomous Optical and Acoustic Surface Elevation Dynamics sensors (O-SED and A-SED) around the edge of natural and semi-natural salt marshes. Sensors were installed at vegetated and unvegetated measurement station Field observations from 22 O-SEDs deployed at 4 well-established natural salt marshes in the Western Scheldt estuary and 4 O-SEDs at a well-established semi-natural salt marsh in the Wadden Sea were reanalyzed. Six novel A-SEDs were deployed at a pioneer semi-natural salt marsh in the Ems-Dollard Estuary. The measurement duration at all salt marshes was at least 1 year. The A-SED sensor was successfully validated against manual measurements. Furthermore, vegetation data and water level data were obtained. No significant difference was observed between natural and semi-natural salt marshes. However, a significant difference between vegetated and unvegetated measurement stations for short-term bed level dynamics was observed. Vegetation was found to be present at locations restricted by short-term bed level variability smaller than or equal to 12 mm, emphasizing the presence of a short-term threshold. Although trends in the non-growing season were significantly different between vegetated and unvegetated stations, seasonal thresholds for vegetation presence were not found. The findings imply that knowledge of bed level-dynamics in well-established natural marshes can be used to predict vegetation presence in constructed semi-natural marshes. The importance of local short-term dynamics for vegetation presence instead of longer-term dynamics highlights possibilities for developing favorable conditions for vegetation presence in marsh restoration projects and the construction of new salt marsh ecosystems.

Alluvial connectivity in multi‐channel networks in rivers and estuaries

Channels in rivers and estuaries are the main paths of fluvial and tidal currents that transport sediment through the system. While network representations of multi‐channel systems and their connectivity are quite useful for characterisation of braiding patterns and dynamics, the recognition of channels and their properties is complicated because of the large bed elevation variations, such as shallow shoals and bed steps that render channels visually disconnected. We present and analyse two mathematically rigorous methods to identify channel networks from a terrain model of the river bed. Both methods construct a dense network of locally steepest‐descent channels from saddle points on the terrain, and select a subset of channels with a certain minimum sediment volume between them. This is closely linked to the main mechanism of channel formation and change by displacement of sediment volume. The two methods differ in how they compute these sediment volumes: either globally through the entire length of the river, or locally. We compare the methods for the measured bathymetry of the Western Scheldt estuary, The Netherlands, over the past decades. The global method is overly sensitive to small changes elsewhere in the network compared to the local method. We conclude that the local method works best conceptually and for stability reasons. The associated concept of alluvial connectivity between channels in a network is thus the inverse of the volume of sediment that must be displaced to merge the channels. Our method opens up possibilities for new analyses as shown in two examples. First, it shows a clear pattern of scale dependence on volume of the total network length and of the number of nodes by a power law relation, showing that the smaller channels are relatively much shorter. Second, channel bifurcations were found to be predominantly mildly asymmetrical, which is unexpected from fluvial bifurcation theory.

Impact of Wind on Tide-Induced Advective Salt Transport in A Well-Mixed Estuary

The tide-induced net advective salt flux in well-mixed estuaries consists of five terms according to the method from Kjerfve. The term resulted from the vertical variation in salinity can be negligible in well-mixed estuaries with four tide-induced salt flux terms, known as F1-F4. To explore the effects of wind on these salt fluxes, the current-salinity analytical model combined with the perturbation analysis is extended by including wind. Analytical expressions for the four salt fluxes are derived separately in the present model. Under the assumption that only the M2 tidal component is accounted for and the salt flux generated by diffusion is not studied, the tide-induced net advective salt flux Qsx is in the seaward direction without the wind effect. By applying the Western Scheldt estuary case, the wind influence on the tidal advection salt flux (TASF) distribution in the F4 term was investigated. The phase difference between zero-order velocity and first-order salinity (Δϕ) at the surface layer of the estuary is larger than 90° and smaller than 90° at the bottom layer, which leads to landward TASF in the surface layer and seaward TASF in the bottom layer. The distribution of Δϕ is not uniform in the horizontal direction with wind included, which differs from the result without wind. In the case of seaward wind with the speed of 18 m/s, the decrease in the zeroth-order velocity phase (ϕu) at the surface layer is larger than that of the first-order salinity phase (ϕs) downstream, which leads to an abnormal seaward TASF in this region. Owing to the surface stress caused by wind, the Stokes compensation flow in the middle and lower reaches increases/decreases with the increase of the landward/seaward wind, while the upstream situation is opposite. Thus, the first-order velocity in the middle and lower reaches increases/decreases with the increase of the landward/seaward wind, while the upstream situation is also opposite. The first-order salinity also increases/decreases with the increase of landward/seaward wind, while the upstream salinity tends to zero. Therefore, the tide-induced net advective salt flux Qsx increases/decreases with the increase of the landward/seaward wind, which is contrary to the usual recognition.

The vulnerability of tidal flats and multi-channel estuaries to dredging and disposal

Shipping fairways in estuaries are continuously dredged to maintain access for large vessels to major ports. However, several estuaries worldwide show adverse side effects to dredging activities, in particular affecting morphology and ecologically valuable habitats. We used physical scale experiments, field assessments of the Western Scheldt estuary (the Netherlands), and morphodynamic model runs to analyse the effects of dredging and future stresses (climate and sediment management) on a multi-channel system and its ecologically valuable intertidal flats. All methods indicate that dredging and disposal strategies are unfavourable to long-term morphology because dredging creates and propagates the imbalance between shallow and deeper parts of the estuary, causing a loss of valuable connecting channels and fixation of the tidal flats and main channel positions, while countering adverse effects by disposal strategy has limited effectiveness. Changing the disposal strategy towards main channel scour disposal can be economically and ecologically beneficial for the preservation of the multi-channel system. Further channel deepening will accelerate the adverse side effects, whereas future sea-level rise may revive the multi-channel system.

What Came First, Mud or Biostabilizers? Elucidating Interacting Effects in a Coupled Model of Mud, Saltmarsh, Microphytobenthos, and Estuarine Morphology

AbstractMud accretion and establishment of biostabilizers, such as microphytobenthos and saltmarsh vegetation, govern the development of estuarine morphology. Mud facilitates saltmarsh survival and microphytobenthos growth, which in turn promotes sedimentation and reduces mud erosion. Consequently, an increasing extent and thickness of mud cover might lead to a stabilization of large‐scale estuarine morphology. To disentangle the interactions between saltmarsh establishment, microphytobenthos colonization, and mud layer formation, we use our novel eco‐morphodynamic model applied to the Western Scheldt estuary. Our model shows that presence of dynamic saltmarsh vegetation and microphytobenthos enhances predictions of mud location in the computations compared to field data. Saltmarsh establishment is partly determined by the antecedent mud content in the bed, resulting in varying emerging vegetation coverage between model experiments of a generic saltmarsh and a saltmarsh species that requires prior mud for establishment. In contrast to microphytobenthos enhancing seasonal mud accretion during their growth period, saltmarshes promote largest accretion when lower biomass and high water levels are present. Interestingly, thick long‐term mud is enhanced despite the biostabilizers seasonal growth. The combination of saltmarsh and microphytobenthos leads to expanding saltmarsh cover and mud area. Generally, mud layer thickness is governed by the ratio of hydroperiod and maximum flow velocity that is mediated by the biostabilizers. On estuary scale, the presence of intertidal vegetation leads to increased mud volumes in the intertidal. Mud layers are enhanced in extent by a mud‐dependent species and in thickness by a generic species. Thus, local biostabilization alters large‐scale morphology controlling long‐term estuarine development.

Identifying Hydro-Geomorphological Conditions for State Shifts from Bare Tidal Flats to Vegetated Tidal Marshes

High-lying vegetated marshes and low-lying bare mudflats have been suggested to be two stable states in intertidal ecosystems. Being able to identify the conditions enabling the shifts between these two stable states is of great importance for ecosystem management in general and the restoration of tidal marsh ecosystems in particular. However, the number of studies investigating the conditions for state shifts from bare mudflats to vegetated marshes remains relatively low. We developed a GIS approach to identify the locations of expected shifts from bare intertidal flats to vegetated marshes along a large estuary (Western Scheldt estuary, SW Netherlands), by analyzing the interactions between spatial patterns of vegetation biomass, elevation, tidal currents, and wind waves. We analyzed false-color aerial images for locating marshes, LIDAR-based digital elevation models, and spatial model simulations of tidal currents and wind waves at the whole estuary scale (~326 km²). Our results demonstrate that: (1) Bimodality in vegetation biomass and intertidal elevation co-occur; (2) the tidal currents and wind waves change abruptly at the transitions between the low-elevation bare state and high-elevation vegetated state. These findings suggest that biogeomorphic feedback between vegetation growth, currents, waves, and sediment dynamics causes the state shifts from bare mudflats to vegetated marshes. Our findings are translated into a GIS approach (logistic regression) to identify the locations of shifts from bare to vegetated states during the studied period based on spatial patterns of elevation, current, and wave orbital velocities. This GIS approach can provide a scientific basis for the management and restoration of tidal marshes.

Dietary bioaccumulation of persistent organic pollutants in the common sole Solea solea in the context of global change. Part 2: Sensitivity of juvenile growth and contamination to toxicokinetic parameters uncertainty and environmental conditions variability in estuaries

The amount of potentially toxic chemicals in a fish depends on various environmental factors, such as temperature and feeding ecology, which can be affected by Global Changes (GC). The main objective of the present work was to study the relative influence of temperature, food quality and food availability on the growth and contamination of juveniles of common sole (Solea solea), a marine flatfish species known to be a relevant indicator of the nursery quality. It focuses on two Persistent Organic Pollutants (CB153 and L-PFOS) of legacy and emerging concern, respectively. To achieve this, we used a toxicokinetic (TK) model in which toxicant flows are mechanistically predicted using a bioenergetic model based on the Dynamic Energy Budget (DEB) theory. This modelling framework was applied to juvenile sole from the Gironde estuary (SW France) and allows accounting for the influence of environmental conditions on fish biological processes involved in toxicant fluxes. To compare their respective influence on model predictions of age, length, and contamination at puberty, we included in a global sensitivity analysis: (1) environmental variability gathered from literature for this particular estuary and (2) TK parameters (i.e. assimilation efficiency AE and elimination rate k˙e) variability and uncertainty gathered from literature about each contaminant but for different fish species and experimental conditions. Then, model predictions were confronted to fish contamination measurements from the Gironde Estuary with different combinations of TK parameter values from literature. Results highlighted a key role of diet composition on fish contamination and growth while water temperature only affected growth. It stressed the need to focus on GC impact on benthic communities and their consequences on juvenile fish diet for future work on GC scenarios. Furthermore, for both chemical, the range of variability of TK parameters from experiments led to underestimated fish contaminations. The best model fits were obtained using TK parameter values from model applications: from Mounier et al. (n.d.) for CB153 (Solea solea, experiment, AE=0.8 and k˙e=0 d−1) and from de Vos et al. (2008) for PFOS (food chain of the Western Scheldt estuary, The Netherlands, AE=0.8 and k˙e=0.8 10−2.d−1).

Morphodynamic Evolution of a Fringing Sandy Shoal: From Tidal Levees to Sea Level Rise

AbstractIntertidal shoals are vital components of estuaries. Tides, waves, and sediment supply shape the profile of estuarine shoals. Ensuring their sustainability requires an understanding of how such systems will react to sea level rise (SLR). In contrast to mudflats, sandy shoals have drawn limited attention in research. Inspired by a channel‐shoal system in the Western Scheldt Estuary (Netherlands), this research investigates governing processes of the long‐term morphodynamic evolution of intertidal estuarine sandy shoals across different timescales. We apply a high‐resolution process‐based numerical model (Delft3D) to generate a channel‐shoal system in equilibrium and expose the equilibrium profile to variations in wave forcing and SLR. Combined tidal action and wave forcing initiate ridge formation at the seaward shoal edge, which slowly propagates landward until a linear equilibrium profile develops within 200 years. Model simulations in which forcing conditions have been varied to reproduce observations show that the bed is most dynamic near the channel‐shoal interface. A decrease/increase in wave forcing causes the formation/erosion of small tidal levees at the shoal edge, which shows good resemblance to observed features. The profile recovers when regular wave forcing applies again. Sandy shoals accrete in response to SLR with a long (decades) bed‐level adaptation lag eventually leading to intertidal area loss. This lag depends on the forcing conditions and is lowest near the channel and gradually increases landward. Adding mud makes the shoal more resilient to SLR. Our study suggests that processes near the channel‐shoal interface are crucial to understanding the long‐term morphodynamic development of sandy shoals.

Salt Marsh Establishment and Eco‐Engineering Effects in Dynamic Estuaries Determined by Species Growth and Mortality

Growth conditions and eco‐engineering effects of vegetation on local conditions in coastal environments have been extensively studied. However, interactions between salt marsh settling, growth, and mortality as a function of hydromorphology and eco‐engineering lack sufficient understanding to forecast morphological development of dynamic systems. We predict salt marsh establishment with an ecomorphodynamic model that accounts for literature‐based seasonal settling and life‐stage‐dependent growth and mortality of a generic salt marsh species. The model was coupled to a calibrated hydromorphodynamic model of an intertidal bar and, on a coarser grid, to the entire Western Scheldt estuary. To quantify the importance of eco‐engineering effects we compared the dynamic model results to a static model approach. The ecomorphodynamic model reproduces spatial pattern, cover, and growth trends over 15 years. The modeled vegetation cover emerges from the combination of a positive and a new negative eco‐engineering effect: vegetation reduces tidal flow strength facilitating plant survival while the developing salt marsh increases the hydroperiod, which limits large‐scale marsh expansion. The reproduced spatial gradient in vegetation density by our model is strongly correlated to their life‐stages, which underlines the importance of age‐dependence when modeling vegetation and for predictions of the stability of the marsh. Upscaling of the model to the entire estuary on a coarser grid gives implications for grid size‐dependent modeling of hydrodynamics and vegetation. In comparison with static model results, the eco‐engineering effects reduce vegetation cover, showing the importance of vegetation dynamics for predictions of salt marsh growth.

The heterogeneity of mudflat erodibility

The prediction of the erosion of mudflats is hampered by inaccurate estimates of the erodibility distribution of the sediment bed. To investigate how erodibility varies in space and what the vertical distribution over the sediment depth is, comprehensive observations of the sediment properties, hydrodynamics and bed-level changes were conducted on an intertidal flat in the Western Scheldt Estuary, the Netherlands. The erosion potential on a mudflat is determined by the critical shear stress for erosion (τe), erosion rate coefficient (M) and local hydrodynamic conditions. A clear difference in hydrodynamic forcing was observed, leading to significant bed level variations at the low water line, where erosion often occurs during very shallow water condition, and a nearly constant bed level at the upper part. The erosion parameters τe and M could be determined over a sediment bed of 12 cm at the low water line. The erosion coefficient M can be considered constant with depth, although there is a large spreading. A clear vertical variation of τe was found: τe increased significantly downward from 0.10 Pa at the sediment surface to 1.13 Pa at 12 cm below the surface. Additionally, there was a strong indication that the presence of diatoms enhanced τe in the upper 2 mm of sediment by five times of the abiotic τe (from 0.09 Pa to 0.46 Pa). These findings lead to the following improvement for predicting morphological changes of tidal mudflats: (1) very shallow conditions should be better simulated, (2) the vertical distribution of τe should be considered. Otherwise, erosion rates can be overestimated, especially during extreme events, because exposure of the deeper well-consolidated layer likely occurs; and (3) an appropriate description of the effect of diatoms should be considered as part of the bottom boundary condition.

Effects of Shoal Margin Collapses on the Morphodynamics of a Sandy Estuary.

Shoal margin collapses of several million cubic meters have occurred in the Western Scheldt estuary, the Netherlands, on average five times a year over the last decades. While these collapses involve significant volumes of material, their effect on the channel‐shoal morphology is unknown. We hypothesize that collapses dynamicize the channel‐shoal interactions, which could impact the ecological functioning, flood safety, and navigation in the estuary. The objective is to investigate how locations, probability, type, and volume of shoal margin collapse affect the channel‐shoal dynamics. We implemented an empirically validated parameterization for shoal margin collapses and tested its effect on simulated estuary morphological development in a Delft3D schematization of the Western Scheldt. Three sets of scenarios were analyzed for near‐field and far‐field effects on flow pattern and channel‐shoal morphology: (1) an observed shoal margin collapse of 2014, (2) initial large collapses on 10 locations, and (3) continuous collapses predicted by our novel probabilistic model over a time span of decades. Results show that a single shoal margin collapse only affects the local dynamics in the longitudinal flow direction and dampen out within a year for typical volumes, whereas larger disturbances that reach the seaward or landward sill at tidal channel junctions grow. The direction of the strongest tidally averaged flow determined the redistribution of the collapsed sediment. We conclude that adding the process of shoal margin collapses increases the channel‐shoal interactions and that in intensively dredged estuaries shoal margins oversteepen, amplifying the number of collapses, but because of dredging the natural morphological response is interrupted.