Mangrove forests reduce wave attack along tropical and sub-tropical coastlines, decreasing the wave loads acting on coastal protection structures. Mangrove belts seaward of embankments can therefore lower their required height and decrease their slope protection thickness. Wave reduction by mangroves depends on tree frontal surface area and stability against storms, but both aspects are often oversimplified or neglected in coastal protection designs. Here we present a framework to evaluate how mangrove belts influence embankment designs, including mangrove growth over time and failure by overturning and trunk breakage. This methodology is applied to Sonneratia apetala mangroves seaward of embankments in Bangladesh, considering forest widths between 10 and 1000 m (cross-shore). For water depths of 5 m, wave reduction by mangrove forests narrower than 1 km mostly affects the slope protection and the bank erodibility, whereas the required embankment height is less influenced by mangroves. Sonneratia apetala trees experience a relative maximum in wave attenuation capacity at 10 years age, due to their large submerged canopy area. Once trees are more than 20 years old, their canopy is emergent, and most wave attenuation is caused by trunk and roots. Canopy emergence exposes mangroves to wind loads, which are much larger than wave loads, and can cause tree failure during cyclones. These results stress the importance of including tree surface area and stability models when predicting coastal protection by mangroves.

Low-energy, non-tidal lake beaches are known to be subject to longshore morphodynamics, but little is known about how they are driven by wind and wave-driven currents. Lake Markermeer is a shallow (∼4 m deep), wind-dominated lake, of approximately 700 km². A gradient in wind-induced water level set-up at the leeward shore induces a flow from the shallower to the deeper parts of the lake, thereby generating a large-scale, horizontal circulation. Flow measurements and results from a numerical Delft3D model of the lake show that these circulations impact the nearshore currents greatly, even more than wave-driven longshore currents for most wind conditions. From nearshore measurements at the first study site in lake Markermeer, we found a clear relation between longshore sediment transport capacity and the measured longshore volume flux. The model numerical can predict flow direction and magnitude for any wind condition. Using wind statistics, the net transport capacity for a short period or a long term mean can be predicted. The relation is confirmed for a second study site, which shows a distinct net transport capacity that could not be explained from wave-driven longshore flow alone. Concluding, large-scale lake circulations are of great significance for the morphological development of low-energy, non-tidal beaches in shallow, wind-driven water bodies. Knowledge of these circulations and their dependence on wind characteristics is a crucial factor to better understand and predict sediment losses of lake beaches.

To address the important research question of whether implicit (bottom friction) or explicit (stem drag) dissipation models are most appropriate for the prediction of wave attenuation due to aquatic vegetation, the Simulating Waves Nearshore (SWAN) spectral wave model has been extended with an explicit frequency-dependent dissipation model for submerged and emergent vegetation. The new explicit model is compared to existing explicit and implicit dissipation models in SWAN, and the distinguishing features of each of the dissipation models are quantified. The present work verifies the implementation of the new and existing dissipation models, outlines their distinguishing features, and compares model predictions against experimental data. The emphasis is on the transformation of the spectral wave periods Tm0;1 and Tm 1;0 over a canopy. Model evaluation based on academic and laboratory cases allows for recommendations regarding applicability of the three dissipation models, where the new method has the broadest applicability, since it bridges the gap in applicability between the other two dissipation models. The implementation of Jacobsen, McFall, and van der A (2019; A frequency distributed dissipation model for canopies; Coastal Engineering, 150, 135-146) is publicly available in SWAN version 41.31B.

The risk of coastal storm flooding is deteriorating under global warming, especially for the heavily urbanized deltaic cities, like Shanghai. The Nature-Based Flood Defense (NBFD), as an eco-friendly design alternative for hard infrastructure against coastal flooding, is gaining attention. Nevertheless, the vulnerability of saltmarsh due to the biological instability, resulting in the uncertainties on coastal protection, is considered the bottleneck challenge that hinders the broad application of the NBFD concept. We argue that except for direct wave attenuations by the above-ground vegetation during storms, the gradual sediment trapping and consolidating during the non-storm period is a more crucial function of coastal saltmarsh, which mitigates storm waves by forming a broader and higher intertidal morphology. This benefit is an important value of saltmarsh-based coastal protection but is largely neglected in many NBFD studies. Taking Chongming Dongtan Shoal (CDS) as a case study, we demonstrated that over 2/3th wave attenuation during storms is contributed by the saltmarsh morphology, and less than 1/3th is from the saltmarsh vegetation. The relative contribution of the saltmarsh morphology on wave mitigation is even enhanced under the increasing storm grades from 100 yrs. to 5000 yrs. return levels. To promote this idea for broader application, the cost-benefit analysis of three artificial NBFD solutions (e.g., submerged breakwater, timber piles, and sand nourishment) are compared. We identified an optimal measure of the submerged breakwater for CDS, which minimizes the ecological impact and maximizes the cost-benefit. Moreover, the wave-free zone behind the breakwater increases the chance of vegetation establishment, helps suspended sediment trapping, hence fostering a beneficent cycle for saltmarsh restoration. In summary, ignoring the contribution of saltmarsh morphology on wave attenuation largely underestimated the benefits of vegetation-based coastal protection, which should be greatly emphasized to provide a solid basis for developing NBFD.

The morphodynamic behaviour of low-energy beaches is poorly understood, compared to that of exposed coasts. This study analyses the morphological development of sandy, low-energy beaches and the steering hydrodynamic processes. Four densely-monitored study sites in the non-tidal lake Markermeer in the Netherlands offered a unique opportunity to examine the relation between their hydraulic boundary conditions and morphodynamics. Regular bathymetric surveys were executed at all locations. Furthermore, the wave climate was monitored at one of these four sites. All four sites exhibit a commonly found low-energy beach morphology, with a narrow beach face and a low-gradient, subaqueous platform. This platform reaches an equilibrium depth quickly and then stays relatively stable. The stable elevation of the platform is located near Hallermeier's depth of closure. A sediment budget analysis over time demonstrates that the beach faces at all study sites have eroded during more energetic periods, and sediment accumulated offshore. During the monitoring periods of 2 to 4 years, the elevation of the platforms reached an equilibrium, but other morphological dimensions are still developing. The new insights gained from this study enable the prediction of platform elevations along sandy beaches in low-energy, non-tidal environments, and have contributed to our insight in the underlying processes driving the morphological evolution.

Tidal flats provide valuable ecosystem services such as flood protection and carbon sequestration. Erosion and accretion processes govern the ecogeomorphic evolution of intertidal ecosystems (marshes and bare flats) and, hence, substantially affect their valuable ecosystem services. To understand the intertidal ecosystem development, high-frequency bed-level change data are thus needed. However, such datasets are scarce due to the lack of suitable methods that do not involve excessive labour and/or costly instruments. By applying newly developed surface elevation dynamics (SED) sensors, we obtained unique high-resolution daily bed-level change datasets in the period 2013-2017 from 10 marsh-mudflat sites situated in the Netherlands, Belgium, and the United Kingdom in contrasting physical and biological settings. At each site, multiple sensors were deployed for 9-20 months to ensure sufficient spatial and temporal coverage of highly variable bed-level change processes. The bed-level change data are provided with synchronized hydrodynamic data, i.e. water level, wave height, tidal current velocity, medium sediment grain size (D₅₀), and chlorophyll a level at four sites. This dataset has revealed diverse spatial morphodynamics patterns over daily to seasonal scales, which are valuable to theoretical and model development. On the daily scale, this dataset is particularly instructive, as it includes a number of storm events, the response to which can be detected in the bed-level change observations. Such data are rare but useful to study tidal flat response to highly energetic conditions. The dataset is available from 4TU.ResearchData (https://doi.org/10.4121/12693254.v4; Hu et al., 2020), which is expected to expand with additional SED sensor data from ongoing and planned surveys.

Foreshores consisting of both bare tidal flats and vegetated salt marshes are found worldwide and they are well studied for their wave attenuating capacity. However, most studies only focus on the small scale: just some isolated locations in space and only up to several years in time. In order to stimulate the implementation of foreshores serving as reliable coastal defense on a large scale, we need to quantify the decadal wave attenuating capacity of the foreshore on the scale of an estuary. To study this, a unique bathymetrical dataset is analyzed, covering the geometry of the Westerschelde estuary (The Netherlands) over a time-span of 65 years. From this dataset, six study sites were extracted (both sheltered sites and exposed sites to the prevailing wind direction) and divided into transects. This resulted in 36 transects covering the entire foreshore (composed of the bare tidal flat and the vegetated salt marsh). The wave attenuation of all transects under daily conditions (with and without vegetation) and design conditions (i.e. events statistically occurring once every 10,000 years) was modelled. Overall, the spatial variability of the geometry of a single foreshore was observed to be much larger than the temporal variability. Temporal changes in salt marsh width did not follow the variability of the entire foreshore. Both under daily and design conditions, vegetation contributes to decreasing wave energy and decreases the variability of incoming wave energy, thereby decreasing the wave load on the dike. The southern foreshores, sheltered from the prevailing wind direction, were more efficient in wave attenuation than the exposed northern foreshores. A linear relation between marsh width and wave attenuation over a period of 65 years was observed at all marshes. The present study provides insights needed to calculate the length of a salt marsh to obtain a desired minimum wave attenuating capacity.

Global change amplifies coastal flood risks and motivates a paradigm shift towards nature-based coastal defence, where engineered structures are supplemented with coastal wetlands such as saltmarshes. Although experiments and models indicate that such natural defences can attenuate storm waves, there is still limited field evidence on how much they add safety to engineered structures during severe storms. Using well-documented historic data from the 1717 and 1953 flood disasters in Northwest Europe, we show that saltmarshes can reduce both the chance and impact of the breaching of engineered defences. Historic lessons also reveal a key but unrecognized natural flood defence mechanism: saltmarshes lower flood magnitude by confining breach size when engineered defences have failed, which is shown to be highly effective even with long-term sea level rise. These findings provide new insights into the mechanisms and benefits of nature-based mitigation of flood hazards, and should stimulate the development of novel safety designs that smartly harness different natural coastal defence functions.

Vegetated foreshores adjacent to engineered structures (so-called hybrid flood defenses), are considered to have high potential in reducing flood risk, even in the face of sea level rise and increasing storminess. However, foreshores such as salt marshes and mangrove forests are generally characterized by relatively strong temporal and spatial variations in geometry and vegetation characteristics (e.g., stem height and density), which causes uncertainties with regards to their protective value under extreme storm conditions. Currently, no method is available to assess the failure probability of a hybrid flood defense, taking into account the aforementioned uncertainties. This paper presents a method to determine the failure probability of a hybrid flood defense, integrating models and stochastic parameters that describe dike failure and wave propagation over a vegetated foreshore. Two dike failure mechanisms are considered: failure due to (i) wave overtopping and (ii) wave impact on revetments. Results show that vegetated foreshores cause a reduction in failure probability for both mechanisms. This effect is more pronounced for wave impact on revetments than for wave overtopping, since revetment failure occurs at relatively low water levels. The relevance of different uncertainties depends on the protection level and associated dike height and strength. For relatively low dikes (i.e., low protection levels), vegetation remains stable in design conditions, and plays an important role in reducing wave loads. In case of higher protection levels, hence for more robust dikes, vegetation is less important than foreshore geometry, because of expected stem breakage of the vegetation under these more extreme conditions. The integrated analysis of uncertainties in hydraulic loads, dike geometry and foreshore characteristics in this paper enables the comparison between nature-based flood defenses and traditionally engineered solutions, and allows coastal engineers to design hybrid flood defenses worldwide.

The authors regret that the correct affiliation of co-author Zhenchang Zhu should be ‘Department of Estuarine and Delta Systems, Royal Netherlands Institute for Sea Research and Utrecht University, 4400AC, Yerseke, The Netherlands’. The authors would like to apologise for any inconvenience caused.

One of the services provided by coastal ecosystems is wave attenuation by vegetation, and subsequent reduction of wave loads on flood defense structures. Therefore, stability of vegetation under wave forcing is an important factor to consider. This paper presents a model which determines the wave load that plant stems can withstand before they break or fold. This occurs when wave-induced bending stresses exceed the flexural strength of stems. Flexural strength was determined by means of three-point-bending tests, which were carried out for two common salt marsh species: Spartina anglica (common cord-grass) and Scirpus maritimus (sea club-rush), at different stages in the seasonal cycle. Plant stability is expressed in terms of a critical orbital velocity, which combines factors that contribute to stability: high flexural strength, large stem diameter, low vegetation height, high flexibility and a low drag coefficient. In order to include stem breakage in the computation of wave attenuation by vegetation, the stem breakage model was implemented in a wave energy balance. A model parameter was calibrated so that the predicted stem breakage corresponded with the wave-induced loss of biomass that occurred in the field. The stability of Spartina is significantly higher than that of Scirpus, because of its higher strength, shorter stems, and greater flexibility. The model is validated by applying wave flume tests of Elymus athericus (sea couch), which produced reasonable results with regards to the threshold of folding and overall stem breakage percentage, despite the high flexibility of this species. Application of the stem breakage model will lead to a more realistic assessment of the role of vegetation for coastal protection.

This paper describes a fully probabilistic safety assessment of the Dutch North Sea coast, in which stochastic properties of both hydraulic loads and strength of the flood defences have been taken into account. The study has led to an overview of failure probabilities along the coast with high spatial resolution. Both dikes and dunes have been considered. Failure probabilities at individual locations have been combined to flooding probabilities per dike ring area. The vast majority of the Dutch coastal defences is quite secure in terms of flooding. This study demonstrates that generally, the Dutch dunes provide a higher degree of safety than the sea dikes. When incorporating the consequences of flooding to the analysis, the calculated flooding probabilities can be used to determine flood risks. The probabilistic method, presented in this paper, enables accurate balancing between avoided flood risks and investments to reinforce the flood defences.

Vegetated foreshores such as salt marshes, mangrove forests and reed fields can reduce wave loads on coastal dikes due to depth-induced wave breaking and wave attenuation by vegetation. Here we present field measurements of wave propagation over salt marshes during severe storm conditions, a modelling approach to describe the effect of vegetated foreshores on wave loads on the dike, and a probabilistic model to quantify the effect of vegetated foreshores on failure probabilities of the dike due to wave overtopping.