Vegetation on foreshores in close vicinity to sea dikes may prove beneficial as regulating ecosystem service in the context of coastal defense, dike safety, and flood protection by reducing loads on these defense structures. Predominantly, a decrease in wave heights and bottom shear stresses is hypothesized, which calls for an inclusion in design procedures of coastal defense structures. In contrast to heterogeneous and variable salt marsh vegetation, this study uses surrogate vegetation models for systematic hydraulic experiments in a wave flume, without modeling specific plant species a priori. Froude-scale experiments are performed in order to investigate the effect of salt marsh vegetation on the wave transformation processes on the foreshore and wave run-up at sea dikes. The effect of plant and wave properties on wave transmission, energy dissipation, and wave run-up at a 1:6 sloped smooth dike are presented and discussed, focusing on the wave-vegetation-structure interaction. Vegetated foreshores can contribute to wave attenuation, where an increasing relative vegetation height h v /h results in decreased wave run-up on the dike by up to 16.5% at h v /h = 1.0.

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<p>Biodiversity and nature conservation play an increasingly important role with growing societal awareness, which is reflected in current European legislative frameworks such as the Marine Strategy Framework Directive or the Water Framework Directive, calling for integrative solutions and restoration of good environmental status. Salt marshes provide ecosystem services which can help mitigate climate change and sea level rise threats and simultaneously address coastal squeeze problems. The periodical submergence due to tidal changes creates a special ecosystem with different zones delineated by a landward increasing marsh elevation, which are inhabited by different plant and animal communities. In addition to their ecological value, salt marshes provide coastal protection, as they dissipate wave energy and stabilize otherwise exposed coastal soil lining sea dikes.  </p><p>The "Gute Küste Niedersachsen" research project investigates which environmental properties account for livable and safe coastal conditions along temperate climate coastlines, focusing on the symbiosis of human settlements, nature conservation and sustainable coastal protection. Specifically, the identification of vegetation-mediated ecosystem services within salt marshes at the North Sea coast of Lower Saxony, Germany is addressed here. The overarching goal of the transdisciplinary project is to gain knowledge of natural or nature-based systems and their processes within real-world laboratories at the coast to incorporate proven ecosystem services into standardized coastal protection design guidelines and promote integrated coastal zone management.</p><p>Methods include field observations and experiments, hydraulic laboratory experiments and numerical simulations over the course of 5 years. During the first years, a systematic observation of vegetation regarding distribution patterns, growth, density, and bio-mechanical (e.g. flexural rigidity, area moment of inertia) as well as root properties (e.g. root length density, tensile strength) and their respective seasonality is conducted. Through comprehensive monitoring covering large areas of halophytic meadows, a physical model of heterogeneous salt marshes will be developed. Simultaneous measurements of environmental parameters covering waves, currents and soil properties yield a comprehensive data set for analysis, numerical and analytical modeling purposes. </p><p>Hydraulic experiments modeling the wave-vegetation-soil interaction will be devised based on field data, developing dynamically and geometrically scaled vegetation surrogates. Besides vegetation properties aboveground, a focus will be on previously sparsely considered root system effects that is hypothesized to govern erosional processes in salt marshes.</p>

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<p>The joint-research project "Gute Küste Niedersachsen" is a multidisciplinary approach across spatial and temporal scales investigating ecosystem services for coastal protection. Current national coastal protection concepts predominantly target flood protection and rarely consider additional benefits to coastal ecosystems or vice versa. How maritime landscapes, such as salt marshes, coastal white dunes or a diversification of dike vegetation, can be integrated into approaches of coastal protection without compromising protection levels is the driving question in "Gute Küste Niedersachsen" and heeds recent European Framework directives calls for the restoration of a good ecological status. An in-depth understanding of dynamics within coastal ecosystems, covering eco-hydrodynamics and eco-geomorphodynamics is developed in real world laboratories at the German North Sea coast, as part of the project.<br>Systematic field observations in collaboration between biologists, geo-ecologists and coastal engineers are conducted to identify seasonal changes of vegetation regarding zonation, height, root length density and bio-mechanical parameters like bending stiffness or tensile strength. The differences of bio-mechanical vegetation traits from specific plant species, e.g. the European beach grass <em>Ammophila arenaria</em>, will indicate differences in bio-stabilization states.<br>Complementary field data of topography and soil parameters, e.g. shear and pull-out resistance, among other parameters, are acquired, employing specifically developed instrumentation like the DiCoastar for automatic and digital measurements of shear resistance over rotation angle. Additionally, values such as water and biomass content obtained from soil samples help to elucidate erosion stability of coastal ecosystems.<br>Field campaigns are focused on two real world laboratories, the tidal barrier island of Spiekeroog, Germany, and a coastal mainland section. Spiekeroog offers a variety of dune systems exposed to divergent environmental conditions such as established and recently developing natural dunes at the north-eastern coast, dunes that are used for coastal protection at the north-western coast, dunes in combination with a sea wall that are already supported by sand nourishment at the western coast or established dunes along the south-western tip of the island. Furthermore, the island holds a unique setting with an engineered dune, which was created to integrate a dike system into the landscape. This offers a one-of-a-kind opportunity to investigate differences between six different dune system types within close proximity regarding their vegetation bound bio-mechanical properties and linked soil-bound erosion resistance.<br>In addition, Spiekeroog offers an abandoned dike line, for which a sectional re-planting is rolled out with alternative seed combinations for ecologically upgrading grass dikes and boost plant diversity while coastal protection is maintained. A direct comparison against a sea dike is made at the second real world laboratory situated at the adjacent mainland coast. This setting facilitates the comparison between different biological revetment types and their respective performance in coastal protection regarding wave-soil-vegetation interactions.<br>In a subsequent step, the extensive data set will be used to develop surrogate plant models and mimic nature in hydraulic laboratories and numerical simulations to project system performance under climate change scenarios. Finally, technical guidance as well as policy recommendations will be derived for enhancing ecosystem services of artificial structures for coastal protection.</p>

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Salt marshes are a valuable ecosystem with coastal protection potential, for example by absorbing hydrodynamic energy, increasing sedimentation and stabilizing the soil. This study investigated biomechanical properties of Spartina anglica to improve future models of wave-vegetation interaction. To fully understand the correlations between hydro- and biomechanics, the biomechanical vegetation properties from December 2021 to July 2022 are investigated with specimens collected from the field monthly. 551 specimens were used to determine the vegetation properties during storm surge season with high hydrodynamic forces. Additional geometrical properties were determined for 1265 specimens. Three-point bending tests measured the stiffness S (N/mm) and maximum forces Fmax (N). Different phenological states were observed over time and separated for analysis. These states provide a novel classification of growth periods for evaluating the coastal protection potential of Spartina anglica. Especially during storm season, most specimen were identified as broken shoots with a mean stiffness of 1.92N/mm (using 304 samples) compared to the bottom part of flowering shoots in December and January with a mean stiffness of 2.98N/mm (using 61 samples). The classification of plant properties recognizing phenological differences, based on plant state and seasonality, can be used to explain and reduce variability of biomechanical properties obtained during field campaigns. Additionally, this study shows that March to April is recommended for future investigations focusing on shoot properties during storm surge season, which is the important season for coastal engineers considering vegetation state.