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This manuscript reviews the progresses made in the understanding of the dynamic interactions between coastal storms and salt marshes, including the dissipation of extreme water levels and wind waves across marsh surfaces, the geomorphic impact of storms on salt marshes, the preservation of hurricanes signals and deposits into the sedimentary records, and the importance of storms for the long term survival of salt marshes to sea level rise. A review of weaknesses, and strengths of coastal defences incorporating the use of salt marshes including natural, and hybrid infrastructures in comparison to standard built solutions is then presented. Salt marshes are effective in dissipating wave energy, and storm surges, especially when the marsh is highly elevated, and continuous. This buffering action reduces for storms lasting more than one day. Storm surge attenuation rates range from 1.7 to 25 cm/km depending on marsh and storms characteristics. In terms of vegetation properties, the more flexible stems tend to flatten during powerful storms, and to dissipate less energy but they are also more resilient to structural damage, and their flattening helps to protect the marsh surface from erosion, while stiff plants tend to break, and could increase the turbulence level and the scour. From a morphological point of view, salt marshes are generally able to withstand violent storms without collapsing, and violent storms are responsible for only a small portion of the long term marsh erosion. Our considerations highlight the necessity to focus on the indirect long term impact that large storms exerts on the whole marsh complex rather than on sole after-storm periods. The morphological consequences of storms, even if not dramatic, might in fact influence the response of the system to normal weather conditions during following inter-storm periods. For instance, storms can cause tidal flats deepening which in turn promotes wave energy propagation, and exerts a long term detrimental effect for marsh boundaries even during calm weather. On the other hand, when a violent storm causes substantial erosion but sediments are redistributed across nearby areas, the long term impact might not be as severe as if sediments were permanently lost from the system, and the salt marsh could easily recover to the initial state. (C) 2017 Elsevier B.V. All rights reserved.

Many overlapping mechanisms have been proposed to control horizontal seaward expansion of marshes and rates of elevation change that are associated with it. Key questions to resolve are: i) whether simple geomorphological conditions such as elevation are a reliable predictor of marsh expansion rates; ii) whether there are seasonal vegetation-induced effects on elevation change (both, increase and decrease of elevation); and iii) how steep the spatial gradient of elevation change is from the bare tidal flat into the vegetated marsh? These questions have been addressed with a two-scale study approach performed on two contrastingly wave-exposed marshes in the Scheldt Estuary (SW Netherlands and N Belgium) where Scirpus maritimus is the dominant pioneer species. On the one hand, we investigated the relations between large-scale, geomorphological parameters (elevation, slope) and clonal marsh expansion rates at both sites. On the other hand, we performed a small-scale monthly field monitoring over two years at the same two marshes where we investigated the relations between spatio-temporal variations in vertical elevation change and spatio-temporal variations in vegetation properties along cross-shore transects. We found that at the sheltered site, clonal expansion rates are almost twice as high as at the exposed site. Furthermore, expansion rates at the sheltered site related well to elevation. At the exposed site, this relation was less strong as wave exposure might cause a dominant disturbance. Moreover, we found clear seasonal elevation change patterns that closely followed the seasonal vegetation cycle, with prevailing increase in elevation in summer when above-ground biomass was maximal and decrease in elevation in winter when plant shoots had largely decayed. Especially at the exposed site, the presence of vegetation has a positive effect on increase in elevation within the marsh. Finally, our results show that clonal marsh expansion succeeded at elevations for which previous studies at the same locations showed that individual shoots could not establish, emphasising the importance of clonal integration for both survival and lateral expansion in disturbance-driven ecosystems.

The importance of ecosystem engineering and biogeomorphic processes in shaping many aquatic and semiaquatic landscapes is increasingly acknowledged. Ecosystem engineering and biogeomorphic landscape formation involves two critical processes: (1) species establishment, and (2) scale-dependent feedbacks, meaning that organisms improve their living conditions on a local scale but at the same time worsen them at larger scales. However, the influence of organism traits in combination with physical factors (e.g. hydrodynamics, sediments) on early establishment and successive development due to scale-dependent feedbacks is still unclear. As a model system, this was tested for salt marsh pioneer plants by conducting flume experiments: i) on the influence of species-specific traits (such as stiffness) of two contrasting dominant pioneer species (Spartina alterniflora and Schpus mariqueter) to withstand current-induced stress during establishment; and ii) to study the impact of species-specific traits (stiffness) and physical forcing (water level, current stress) on the large-scale negative feedback at established tussocks (induced scour at tussock edges) of the two model species. The results indicate that, not only do species-specific plant traits, such as stiffness, exert a major control on species establishment thresholds, but also potentially physiologically triggered plant properties, such as adapted root morphology due to sediment properties. Moreover, the results show a clear relation between species-specific plant traits, abiotics (i.e. sediment, currents) and the magnitude of the large-scale negative scale-dependent feedback. These findings suggest that the ecosystem engineering ability, resulting from physical plant properties can be disadvantageous for plant survival through promoted dislodgement (stem stiffness increases the amount of drag experienced at the root system), underlying the importance of scale-dependent feedbacks on landscape development. (C) 2015 Elsevier B.V. All rights reserved.

There is a growing recognition of the important role of scale-dependent feedback for biogeomorphological landscape formation, where organisms locally improve survival and growth but at the same time negatively affect organisms at larger distance. However, little is known on how scale-dependent bio-geomorphic feedback is influenced by organism traits in combination with abiotic forcing. This was studied by measuring in a flume, the flow patterns around patches of three contrasting marsh species (Spartina anglica, Puccinellia maritima and Salicornia procumbens), using the flow acceleration around vegetation patches and deceleration within vegetation patches as quantitative proxy for the negative and positive feedback to the vegetation performance. The importance of external forcing was assessed by comparing three realistic current velocities: 0.1, 0.2 and 0.3 m s− 1. Our results showed that the dense clonal growth of stiff Spartina anglica shoots caused strongest flow deviations, irrespective of the applied current velocity. In contrast, the more sparsely growing, shorter stiff shoots of Salicornia procumbens induced much less flow deviation, allowing more water to pass through and over the vegetation canopy. The dense but highly flexible shoots of Puccinellia maritima caused strong flow deviations at low velocities, which diminished at higher velocities due to bending of the vegetation. Overall, these hydrodynamic results demonstrate that plant species traits interact with environmental conditions in creating scale-dependent feedbacks explaining why the effects of vegetation on landscape formation in saltmarshes are species specific.

The morphogenesis of tidal channel networks that dissect intertidal flats and marshes has been studied especially by morphodynamic modeling, while relatively few empirical data exist on high-resolution field observations. Here we measured the spontaneous formation and evolution of a tidal channel network in a newly constructed tidal marsh (Scheldt estuary, Belgium) over a period of 4 years, by high-accuracy topographic surveying with a temporal resolution of 1 year at high spatial resolution considering all channels deeper than 0.1 m. As a reference, topographic measurements with a similar high resolution were performed in a nearby, mature natural tidal marsh network. Based on the field surveying and additional GIS processing, we derived several geometric and hydraulic parameters (channel width, depth, eroded volumes, cross-sectional area, length profiles, drainage density, mainstream length, tidal discharge and watershed area), and compared the evolution of geometric relationships in the constructed marsh with the natural tidal marsh. In this way we have evaluated how fast an equilibrium state was attained. We found that after 2 to 3 years of tidal working the cross-sectional areas of former ditches in the constructed marsh were in equilibrium with the corresponding tidal discharge. Furthermore we observed that the mainstream lengths and the drainage densities for the smaller watershed areas were comparable with the natural tidal marsh, demonstrating the rapid headward growth of newly forming channels and tributary channel formation near the channel heads. Newly formed channels preferentially developed in the low elevation zones of the constructed marsh and channel extension was not significantly influenced by the presence or absence of vegetation. However, the overall channel drainage density and channel cross-sectional areas of the newly formed channels were still lower compared to the natural tidal marsh after 4 years. This indicates that further channel network extension and continued channel deepening can be expected in the coming years.

Intertidal flats and marshes provide important ecosystem services to coastal and estuarine societies, and have therefore been subject to extensive research. Most of these studies, however, have one thing in common: intertidal landscape response has been studied from a rather short-term perspective, mostly less than 100 years. Furthermore, the impact of embankment practices on the development of the remaining intertidal area and its stability has hardly been studied at all. In this paper, a longer term perspective (sixteenth to twenty-first centuries) is used in order to (1) reconstruct intertidal landscape evolution, using both historical and soil maps and (2) assess the effect of stepwise embankment on the intertidal area, both in terms of the surface proportions of the different components of an intertidal area and the vertical and lateral sediment elevation and sedimentation rates. The reconstruction and embankment effect assessment will be applied to the Land of Saeftinghe, an intertidal area in the Western Scheldt estuary that was partially re-embanked in a stepwise manner into an embankment landscape called the Waasland polder area (Belgium/The Netherlands). Important outcomes of this study are: (1) a combination of historical–geographical methods proves to be a valuable tool for past intertidal landscape reconstruction; (2) dike breaching and partial (stepwise) re-embankment are important driving mechanisms for the evolution of the remaining intertidal landscape towards new equilibrium conditions, though the new landscape equilibria can only be reached with a certain time lag; (3) subsequent embankments with a short time interval (15 to 21 years) prevent tidal marsh surface areas from reaching an equilibrium state; and (4) between 60 and 96 years are needed for tidal marsh elevation to reach an equilibrium.

Vegetation is increasingly recognized as an important control on flow and landform patterns in many landscape types. Field studies on the landscape-scale effect of vegetation in fluvial and tidal floodplains are relatively scarce while insights are especially based on flume and numerical models. Large-scale flow patterns and landforms were measured on a vegetated tidal marsh and unvegetated tidal flat, in particular dynamic changes in two-dimensional water surface slopes, flow velocities and directions on an extensive network of locations. It was found that during flooding and drainage of the vegetated tidal marsh, the flow was concentrated in and routed through bare tidal channels, whereas on the unvegetated tidal flat more homogeneous sheet flow occurred. On the marsh the peak flow velocities were lower on the vegetated platform (< 0.1 m/s) compared to their adjacent tidal channels (0.3− 1 m/s), while on the bare tidal flat the channel and platform flow velocities were not significantly different from each other (0.1 − 0.4 m/s). These differences in flow patterns have important implications for the landform differences between vegetated and bare tidal floodplains: (1) for tidal channels with channel widths smaller than 10 m, width-to-depth ratios (= β) were smaller for the tidal marsh channels (β ~ 3) than for the tidal flat channels (β ~ 9) (due to flow concentration towards the channels in the marsh); and (2) the vegetated marsh platform exhibits a clear levee-basin topography (due to vegetation-induced platform flooding from the channels), while there is no levee-basin topography on the bare flat (due to sheet flow). This study not only emphasizes previous findings on the direct effect of vegetation, acting as frictional constraints on flow in tidal marshes, it also suggests that the indirect effect of vegetation, namely its induced micro-topography, influences the flooding and drainage behavior of marsh systems potentially altering feedbacks caused by direct vegetation effects.

Along coasts and estuaries, formerly embanked land is increasingly restored into tidal marshes in order to re-establish valuable ecosystem services, such as buffering against flooding. Along the Scheldt estuary (Belgium), tidal marshes are restored on embanked land by allowing a controlled reduced tide (CRT) into a constructed basin, through a culvert in the embankment. In this way tidal water levels are significantly lowered (ca. 3 m) so that a CRT marsh can develop on formerly embanked land with a ca. 3 m lower elevation than the natural tidal marshes. In this study we compared the long-term change in elevation (delta-E) within a CRT marsh and adjacent natural tidal marsh. Over a period of 4 years, the observed spatio-temporal variations in delta-E rate were related to variations in inundation depth, and this relationship was not significantly different for the CRT marsh and natural tidal marsh. A model was developed to simulate the delta-E over the next century. (1) Under a scenario without mean high water level (MHWL) rise in the estuary, the model shows that the marsh elevation-delta-E feedback that is typical for a natural tidal marsh (i.e. rising marsh elevation results in decreasing inundation depth and therefore a decreasing increase in elevation) is absent in the basin of the CRT marsh. This is because tidal exchange of water volumes between the estuary and CRT marsh are independent from the CRT marsh elevation but dependent on the culvert dimensions. Thus the volume of water entering the CRT remains constant regardless of the marsh elevation. Consequently the CRT MHWL follows the increase in CRT surface elevation, resulting after 75 years in a 2–2.5 times larger elevation gain in the CRT marsh, and a faster reduction of spatial elevation differences. (2) Under a scenario of constant MHWL rise (historical rate of 1.5 cm a- 1), the equilibrium elevation (relative to MHWL) is 0.13 m lower in the CRT marsh and is reached almost 2 times faster. (3) Under a scenario of accelerated MHWL rise (acceleration of 0.02 cm a- 1), the CRT marsh is much less able to keep up with the MHWL rise; after 75 years the CRT elevation is already 0.21 m lower than for the natural marsh. In conclusion, this study demonstrates that although short-term (4 years) delta-E rates are similar in a restored CRT marsh and natural tidal marsh, these ecosystems may evolve differently in response to sea-level rise in the longer term (10–100 years).

Although natural levees and lower basins are typical geomorphic features along tidal marsh creeks, long-term sedimentation and elevation changes in tidal marshes were traditionally studied using 0-dimensional point models without considering spatial variations. In this study, the long-term evolution of the surface elevation of tidal marsh levees and adjacent basins was studied by applying a 0-dimensional, time-stepping model (MARSED) using spatially differentiated model parameter values for levees and basins. Firstly, the model was calibrated using field data on short-term (