Westerschelde Research Articles

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.

Western Scheldt Dike, Doel, East-Flanders Province, Belgium.

Western Scheldt Dike, Doel, East-Flanders Province, Belgium.

The impact of wind‐waves and sea level rise on the morphodynamics of a sandy estuarine shoal

SummaryIntertidal shoals are pronounced morphological features found in many estuaries worldwide. Apart from maintaining an ecologically unique intertidal environment, shoals also protect adjacent dyke systems by attenuating waves. The fate of sandy shoals under anticipated sea level rise (SLR) scenarios is underexplored.The current research investigates the long‐term morphodynamic evolution of estuarine sandy shoals under forcing by short fetch, locally generated wind‐waves, tides, and SLR by means of a numerical, process‐based model (Delft3D). The focus lies on a sheltered shoal complex in the Western Scheldt, the Netherlands. Starting from the initial, 1963 bathymetry, we model 50‐year morphodynamic development with schematized wind‐wave forcing. We analyze in detail the impact of locally generated wind‐waves on shoal formation. Finally, we impose regional SLR of 1.10 m and 1.95 m for 100 years.Model results show that, on the spatial scale of intertidal flats, small, locally generated wind‐waves lower and widen the shoals while the adjacent channels deepen. However, on the estuarine system scale, wind‐waves do not lead to fundamentally different channel–shoal patterns and morphodynamic evolution trends. This suggests that channel–shoal formation is mainly due to tide residual sediment transports, with wind‐waves playing a secondary role. SLR leads to a notable intertidal area loss, despite a continuous heightening of the shoals, implying that morphodynamic adaptation lags behind SLR. The inclusion of wind‐waves does not fundamentally change the reaction of the estuarine shoal to SLR. Future research may focus on exploring the impact of including multiple sediment classes.

Population collapse or human resilience in response to the 9.3 and 8.2 ka cooling events: A multi-proxy analysis of Mesolithic occupation in the Scheldt basin (Belgium)

This paper explores the impact of environmental, e.g. sea level rise, and climatic events, e.g. abrupt cooling events, on Mesolithic populations (ca. 11,350 to 6600 cal BP) living in the western Scheldt basin of Belgium and Northern France. The Mesolithic in this study-area has been extensively studied during the last few decades, leading to an extensive database of radiocarbon dates (n = 418), sites (n = 157) and excavated loci (n = 145). A multi-proxy analysis of this database reveals important changes both chronologically and geographically, which are interpreted in terms of population dynamics and changing mobility and land-use. The results suggest a population peak and high residential mobility in the Early Mesolithic, followed by a population shift and increased intra-basin mobility in the Middle Mesolithic, possibly triggered by the rapid inundation of the North Sea basin. The situation during the Late Mesolithic remains less clear but a possible reduction in the mobility seems likely. Currently there is little evidence supporting a causal link between these diachronic changes in human behavior and the 9.3 and 8.2 ka cooling events. Most of the observed changes seem more in response to long-term climatic and environmental changes during the Early and Middle Holocene, hinting at considerable resilience.

Tracking controller for ship manoeuvring in a shallow or confined fairway: Design, comparison and application

The present study introduces a new tracking controller for fast-time simulation in shallow or confined water. The performance of the developed controller is investigated by simulation studies with a container ship in a confined tidal environment. Firstly, a novel tracking controller was designed and applied in a fast-time manoeuvring simulator with a mathematical manoeuvring model identical to that of the full mission bridge simulators of Flanders Hydraulics Research (FHR). Prior to the simulation runs, a statistical AIS-analysis was carried out to determine the reference speeds along the trajectory. Then, the proposed controller was tested via navigation simulations on the Western Scheldt River, flowing through the Netherlands and Belgium, with realistic environment scenarios. Comparisons were conducted between the proposed tracking controller and the present prescience model based tracking controller (PMTC) used in the simulator both in calm water conditions and in tidal conditions covering a full tidal cycle. The results show that both tracking controllers are able to provide satisfactory tracking performance. Nevertheless, compared with PMTC, the proposed tracking controller reveals some important advantages such as a better speed control, a significant reduction of the computational cost, etc. In conclusion, the designed tracking controller can be applied for fast-time simulation in shallow or confined water with high accuracy.

Decabromodiphenylether trends in the European environment: Bird eggs, sewage sludge and surficial sediments

Concern on relatively high levels and the potential bioaccumulation of decabromodiphenylether (BDE209) has led to a European 8-year monitoring program on trends in BDE209 concentrations in birds, sewage sludge and sediments from seven countries. BDE209 was analysed in four environmental matrices: sparrowhawk eggs (UK), glaucous gull eggs (Bear Island, Norway), sewage sludge (UK, Ireland and the Netherlands) and sediment (France, Germany, the Netherlands, UK and Ireland). BDE209 was detected in most of the glaucous gull and sparrow hawk eggs but neither increasing nor decreasing trends in these BDE209 levels were observed. An indication for debromination of BDE209 in sparrowhawk eggs was found. BDE209 concentrations in sediments ranged from very low ng/g (88 ng/g on an organic carbon (OC) basis) concentrations, in the rivers Elbe, Ems, Seine and the Outer Humber, to high μg/g (120 μg/g OC), in the Western Scheldt, Liverpool Bay and River Mersey. Apart from decreasing values in the Western Scheldt sediment no further decreases in BDE209 concentrations were observed over time, neither in sediment nor in sewage sludge showing that the voluntary emissions control program of the bromine industry only had a local effect. In contrast to the sewage sludge samples from the Netherlands (mean 355 ng/g dry weight (dw) or 1026 ng/g OC), the BDE209 concentrations in the UK increased at all sites from 2006 to 2011 (8092 ng/g dw or 22,367 ng/g OC). The BDE209 levels in several UK sediments and sewage sludge were still very high at the end of the program in 2012, most likely caused by frequent use of BDE209 in the textile industry. This may be indicative of the persistence of BDE209 and the limited degradation into lower brominated congeners in sediment, although it cannot be excluded that ongoing BDE209 emissions have played a role as well.

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.

Sediment Disposals in Estuarine Channels Alter the Eco‐Morphology of Intertidal Flats

Dredging of navigation channels in estuaries affects estuarine morphology and ecosystems. In the Western Scheldt, a two‐channel estuary in the Netherlands, the navigation channel is deepened and the sediment is relocated to other parts of the estuary. We analyzed the response of an intertidal flat to sediment disposals in its adjacent channel. Decades of high‐frequency monitoring data from the intertidal flat show a shift from erosion toward accretion and reveal a sequence of cascading eco‐morphological consequences. We document significant morphological changes not only at the disposal sites, but also at the nearby intertidal flats. Disposals influence channel bank migration, driving changes in the evolution of the intertidal flat hydrodynamics, morphology, and grain sizes. The analyzed disposals related to an expansion of the channel bank, an increase in bed level of the intertidal flat, a decrease in flow velocities on this higher elevated flat, and locally a decrease in grain sizes. These changes in turn affect intertidal flat benthic communities (increased in quantity in this case) and the evolution of the adjacent salt marsh (retreated less or even expanded in this case). The shifts in evolution may occur years after dredged disposal begins, especially in zones of the flats farther away from the disposal locations. The consequences of sediment disposals that we identify stress the urgency of managing such interventions with integrated strategies on a system scale.

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.

Trends in the Seaward Extent of Saltmarshes across Europe from Long-Term Satellite Data

Saltmarshes provide crucial functions for flora, fauna, and humankind. Thus far, studies of their dynamics and response to environmental drivers are limited in space and time. Satellite data allow for looking at saltmarshes on a large scale and over a long time period. We developed an unsupervised decision tree classification method to classify satellite images into saltmarsh vegetation, mudflat and open water, integrating additional land cover information. By using consecutive stacks of three years, we considered trends while taking into account water level variations. We used Landsat 5 TM data but found that other satellite data can be used as well. Classification performance for different periods of the Western Scheldt was almost perfect for this site, with overall accuracies above 90% and Kappa coefficients of over 0.85. Sensitivity analysis characterizes the method as being robust. Generated time series for 125 sites across Europe show saltmarsh area changes between 1986 and 2010. The method also worked using a global approach for these sites. We reveal transitions between saltmarsh, mudflat and open water, both at the saltmarsh lower edge and interior, but our method cannot detect changes at the saltmarsh-upland boundary. Resulting trends in saltmarsh dynamics can be coupled to environmental drivers, such as sea level, tidal currents, waves, and sediment availability.

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.

Morphodynamic Feedback Loops Control Stable Fringing Flats

We apply a 2‐D horizontal process‐based model (Delft3D) to study the feedback mechanisms that control the long‐term evolution of a fringing intertidal flat in the Western Scheldt Estuary. The hydrodynamic model is validated using a comparison with measurements on the intertidal flat and the sediment transport module is calibrated against long‐term morphology data. First, the processes that lead to net sediment exchange between channel and flat are studied. Then, long‐term simulations are performed and the dependency of sediment fluxes on the tidal flat bathymetry, and the corresponding morphodynamic feedback mechanisms are explained. In the long run, relatively stable states can be approached, which are shown to be typical for wave‐dominated fringing mudflats. The system behavior can be explained by the typical feedback mechanisms between the intertidal bathymetry and the hydrodynamic forces on the flat. In the subtidal domain, the impact of small (5–10 cm) wind waves increases with a rising elevation due to decreasing water depths. In the intertidal domain, the wave impact increases with increasing cross‐sectional slope due to wave shoaling. These relationships result in negative (stabilizing) morphodynamic feedback loops. The tidal current velocities and tide‐induced bed shear stresses, on the other hand, are largely determined by the typical horizontal geometry. A stabilizing feedback loop fails, so that there is no trend toward an equilibrium state in the absence of wind waves.

Self-organization of a biogeomorphic landscape controlled by plant life-history traits

Feedbacks between geomorphology and plants are increasingly recognized as key drivers shaping a variety of landscapes. Most studies of biogeomorphic interactions have focused on the influence of physical plant characteristics, such as stem and root density, on landscape morphodynamics without considering the role of life-history traits. However, pioneer plants can have very different colonization behaviours. Fast colonizers are characterized by a high number of establishing seedlings that produce homogenous vegetation patterns. In contrast, slow colonizers are characterized by a low number of establishing seedlings that are able to expand laterally, resulting in patchy vegetation patterns. Here we combine biogeomorphic model simulations and field observations in the Western Scheldt Estuary, the Netherlands, to show that colonization behaviour can influence the evolution of wetland landscapes. We find that colonization by fast colonizers favours stabilization of pre-existing channels and consolidation of the landscape configuration. In contrast, colonization by slow colonizers facilitates the formation of new channels and thereby actively facilitates further landscape self-organization. Our findings underline the key role of life-history traits in steering landscape self-organization across different biogeomorphic systems, and potentially the long-term resilience of these landscapes to disturbances. Fast-colonizing plants stabilize wetland landscapes, whereas slow-colonizing plants promote channel formation according to biogeomorphic model simulations and field observations.

Location and probability of shoal margin collapses in a sandy estuary

AbstractChannel bank failure, and collapses of shoal margins and beaches due to flow slides, have been recorded in Dutch estuaries for the past 200 years but have hardly been recognized elsewhere. Current predictions lack forecasting capabilities, because they were validated and calibrated for historic data of cross‐sections in specific systems, allowing local hindcast rather than location and probability forecasting. The objectives of this study were to investigate where on shoal margins collapses typically occur and what shoal margin collapse geometries and volumes are, such that we can predict their occurrence. We identified shoal margin collapses, generally completely submerged, from bathymetry data by analyzing digital elevation models of difference of the Western Scheldt for the period 1959–2015. We used the bathymetry data to determine the conditions for occurrence, specifically to obtain slope height and angle, and applied these variables in a shoal margin collapse predictor. We found 299 collapses along 300 km of shoal margin boundaries over 56 years, meaning that more than five collapses occur on average per year. The average shoal margin collapse body is well approximated by a 1/3 ellipsoid shape, covers on average an area of 34 000 m2 and has an average volume of 100 000 m3. Shoal margin collapses occur mainly at locations where shoals take up a proportionally larger area than average in the cross‐section of the entire estuary, and occur most frequently where lateral shoal margin displacement is low. A receiver operating characteristic curve shows that the forecasting method predicts the shoal margin collapse location well. We conclude that the locations of the shoal margin collapses are well predicted by the variation in conditions of the relative slope height and angle within the Western Scheldt, and likely locations are at laterally relatively stable shoal margins. This provides hypotheses aiding the recognition of these features in sandy estuaries worldwide. Copyright © 2018 John Wiley & Sons, Ltd.