Abstract
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 hv/h results in decreased wave run-up on the dike by up to 16.5% at hv/h = 1.0.
Highlights
The interaction and synthesis of ecology and hydraulics has recently been termed “ecohydraulics”, and this field concerns fluid mechanics and engineering methods helping to foster a more thorough understanding of the complex plant–flow interactions (Carus et al 2016; Maddock et al 2013; Nestler et al 2008; Spencer et al 2016)
Wave energy dissipation is dependent on various vegetation and wave parameters, and on the complex interaction of those parameters that vary in space and time (Koch et al 2009), making it difficult to provide a simple equation to quantify the wave height reduction due to a foreshore geometry and a coastal salt marsh
Augustin et al (2009) observed that wave energy dissipation increases with decreasing water level, Anderson and Smith (2014) and Peruzzo et al (2018) point out an increased wave energy dissipation with increasing vegetation density, whereas Rupprecht et al (2015) reported that wave reduction is more dependent on vegetation biomass and plant stiffness
Summary
The interaction and synthesis of ecology (vegetated foreshores) and hydraulics (wave action) has recently been termed “ecohydraulics”, and this field concerns fluid mechanics and engineering methods helping to foster a more thorough understanding of the complex plant–flow interactions (Carus et al 2016; Maddock et al 2013; Nestler et al 2008; Spencer et al 2016). The values and services provided by salt marshes are, e.g., blue carbon storage, habitat provision, ecosystem functioning, and the reduction of construction and maintenance cost, compared with dikes and sea walls without vegetated foreshores (King and Lester 1995; Barbier et al 2011; Purcell et al 2020). Recent research on wave–vegetation interaction has revealed relevant percentages of wave attenuation due to wave energy dissipation (King and Lester 1995; Koch et al 2009) as well as reduction of currents and bed shear stresses and further, trapping, stabilization and covering of sediment by roots and rhizomes (Bouma et al 2010; Cahoon et al 1996). If a salt marsh erodes, it depends on the bed topography and the wave propagation, and on sediment and vegetation characteristics forming the salt marsh bed (Christie et al 2019)
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