Climate change puts under pressure existing and future coastal interventions. Growing threats like sea-level rise and intensity of storms require solutions to be adaptable and resilient. Nature-based solutions have shown to tackle these challenges while providing social, environmental, and economic benefits. The role of vegetation in coastal protection is increasingly recognized. Aquatic vegetation reduces erosion, storm surge, and incoming wave height.

Large-scale modeling of waves with spectral wave models such as SWAN is indispensable for the design of coastal structures and the assessment of flood risk. Wave dissipation due to vegetation can be modeled in SWAN as increased bottom friction (implicit modeling) or as an additional dissipation function (explicit modeling). The second assumes that vegetation can be represented as rigid cylinders or plates (canopies) with different properties. While some studies concluded that implicit modeling reproduces the spectral evolution of field measurements more closely, others concluded the opposite.

Within the BE-SAFE project, field campaigns measured the spectral energy distribution over salt marshes in the Dutch Wadden Sea during several winter storms. The vegetated foreshore in front of the coastal dike got submerged over 2 m of water during high tide and storm surge. The measurements deployed wave gauges over the study transect, which was defined between the pioneer zone marsh edge and the near-dike location (300 m behind the salt marsh). Calibrating the implicit and explicit models in SWAN brought the modeled total wave energy decay closer to the measurement. Nevertheless, the spectral shape, which describes the energy distribution over frequencies, still showed significant and not yet understood differences near the dike.

A methodology was executed to investigate the mechanisms that could reduce the spectral mismatch between the SWAN wave model and measurements over vegetation. First, the literature highlighted possible mechanisms that could be incorporated for this purpose. Next, a new frequency-distributed explicit dissipation model of Jacobsen et al. (2019) was implemented in SWAN and compared to implicit and explicit models using lab and field measurements.

The results showed that the newly implemented model accurately captures the physics and the change of spectral shapes for all experimentally tested wave conditions and submergences. In contrast, the existing implicit and explicit dissipation models in SWAN reproduce the spectral evolution only under certain circumstances. In the validation and comparison to the field measurements with a much larger water depth than the vegetation height, the model of Jacobsen et al. (2019) correctly captured the vegetation's physical representation and the dissipation on the wind-sea frequencies. Nevertheless, the amount of energy on low frequencies was largely underpredicted by all frequency-distributed models. Therefore, the model of Jacobsen et al. (2019) was modified to include flexibility in a frequency-dependent reduction factor that reproduced the energy decay of the measurements in all frequency regions. Other mechanisms that could be responsible for the mismatch before and over the marsh are the redistribution of energy by non-linear triad interactions, generation of infra-gravity waves, and near-shore currents caused by horizontal variations on the vegetation properties.

The present research provides the range of conditions in which the tested explicit and implicit energy dissipation functions in SWAN are able to simulate the spectral evolution over rigid canopies and flexible salt-marsh vegetation. A new version of SWAN includes a new frequency-distributed explicit model that performed more accurately than existing models for rigid canopies. The physical insights from the research contributed to developing additional versions of SWAN, which performed closely to the energy distribution of the measurements over deeply submerged and flexible salt marsh vegetation species.

References:
Jacobsen, McFall, Van der A (2019). A frequency distributed dissipation model for canopies. Coastal Engineering, 150, 135-146.

For the design of coastal structures, the hydraulic loads that act on the coast should be known. These are often based on extrapolation of measurements. However, if the physical relation is unknown, large errors could be made. Therefore, a numerical model can be set up to take the most important processes into account.Yet, uncertainties are present in these numerical models as well. Taking for example the spectral wave model SWAN, it is hypothesised that the wave bottom refraction is not always properly modelled. Previous studies concluded that SWAN overestimates this refraction process near tidal channels, leading to an underestimation of waves entering the channel.This study uses the model SWAN in which different sensitivities are tested to assess the effect on refraction. The focus will be on the spatial and directional resolution as well as the wave propagation scheme that SWAN uses to discretise the propagation terms, which are the default SORDUP scheme and the optional BSBT scheme. From previous studies it is known that also physical processes could impact the bottom refraction, however these are only addressed shortly in this study. The aim of this study is to analyse the current performance of SWAN with respect to refraction and to assess how this performance can be improved.First, an analysis of a simple schematic channel case is made in SWAN, where waves propagate from shallow to deeper water. Physical source terms and wind wave-growth were deactivated in this part. It is found that a coarser spatial resolution can lead to a weaker wave refraction. This is caused by the way SWAN determines the turning rate, which is underestimated by SWAN compared to the theory of Snellius. On the other hand, an overestimation of wave refraction was observed for cases were waves should enter the channel. This is partly caused by the increased diffusion. These findings were taken into practice in a study of the Eastern Wadden Sea, specifically along the Ems channel, where waves propagated from deeper to shallower water. A coarser spatial resolution and the BSBT scheme showed less wave penetration from offshore into the Ems channel. The storm of January 2017 was simulated in SWAN after which a comparison was made to measurements. It was found that SWAN underestimated the wave energy for frequencies<0.15Hz. However, this did not improve by refining the resolution in both spatial and directional domain or by applying a different propagation scheme. SWAN thus overestimates the wave refraction due to bottom gradients if waves propagate to shallower water. In case waves travel to deeper water, refraction can be underestimated. If all physical source terms are deactivated, the effects of resolution on refraction are clearly visible. Including the source terms diminishes this effect. Along the channel edges, effects of increased spatial resolution are notable giving a significant wave height of 12-14cm (6-9%) closer to the measured condition. However, the effects at the coast near Uithuizerwad are negligible.

The main objective and secondary objective, which corresponds to the in-depth research are as follows:

(1) To develop guidelines for engineers to include salt marshes in hydraulic infrastructure projects, subject to the Building with Nature philosophy.
(2) To investigate the effect of grazing at salt marshes on the coastal protection property.

This research concerns the development of guidelines for engineers to include salt marshes in coastal protection projects, subject to the Building with Nature philosophy. The results include important design aspects for engineers and an analysis of trade-offs between biodiversity and the coastal protection property of salt marshes.

This research is based on a literature study about the natural and artificial development of salt marsh. Thereafter, the effect of grazing on wave attenuation is modelled by the numerical model SWAN (Simulating WAves Nearshore). The effect of vegetation and the bathymetry is analysed. The vegetation data results from a grazing experiment at the salt marsh at Noord Friesland Buitendijks, in the north of the Netherlands. The results from the literature study and the in-depth research are translated to important design aspects within the engineering design guidelines for salt marsh development.

The main conclusion of the in-depth research is that the effect of vegetation on wave attenuation is small compared to the effect of wave breaking and bottom friction. Moreover, the variations in wave attenuation between the different grazing strategies is also small. However, grazing could be used as an ecosystem management strategy, as from literature it follows that grazing positively affects biodiversity. Although the effect of biotic factors on wave energy dissipation is small, the presence of a salt marsh indirectly affects wave attenuation. The salt marsh vegetation enhances sediment accumulation, which leads to a higher bottom profile elevation compared to the bare foreshore. This should be substantiated by future research.

The main objective of this thesis is to assess the effectiveness of Nature-based Solutions in reducing flood risk in the Galveston Bay by means of wave attenuation. The methodology to answer these questions consists of both literature review and numerical modelling. The wave height reducing capabilities of marsh vegetation, seagrass meadows and oyster reefs are described and assessed quantitatively. Their limitations and optimizations strategies are investigated and applied to the Galveston Bay, Texas.

Begroeide vooroevers zijn er voor wie geen zin heeft in vervelende dijkverhoging, een kale kade zat is of wie een aquatisch ecosysteem wil versterken met buitendijkse terrestrische natuur – voor wie eigenlijk niet? Met zo’n aantrekkelijk concept is het verleidelijk om je rijk te rekenen, maar dat gebeurt niet in het NWO project BE-SAFE. Hoe reduceren planten golfenergie nou precies? Een voorproefje.@en