This project highlights the value of natural coastal ecosystems in lowering the wave loads on dikes and providing erosion-resistant elevated soil fronting the dikes. More specifically, we provide insight on i) the importance of elevation and width of both tidal-flats and marshes for wave run-up onto the dikes, ii) the importance of tidal flat elevation (changes) on the long-term marsh development, iii) the use of ‘green’ management measures to stabilise tidal flats and thereby facilitate marsh expansion, iv) the topsoil erosion resistance of marshes compared to bare mudflats under fast water flow, as would occur during a dike breach and v) the erosion resistance of marsh cliffs about grazing management and sediment type, as this drives the marsh width in front of a dike.
Figure 1-Top: Dike protected by a salt marsh in the Dutch Wadden Sea. Middle-left: measurement of beach wrack on the dikes after the storm. Middle-right: salt marsh edge. Bottom-left: biodegradable artificial reefs deployed in the tidal flats of Griend. Bottom-right: cattle grazing in a salt marsh. Photos by: Beatriz Marin-Diaz.
Motivation and practical challenge
From the coastal areas near my home in Barcelona to the Dutch coast that I now study, I acknowledge that hard engineering measures such as dikes are at many places needed for flood protection (Figure 1 middle-left). However, the required engineering measures may become less intrusive if we know how to combine them with natural foreshore ecosystems such as salt marshes, resulting in hybrid solutions (Figure 1 top). The salt- and flooding tolerant plants growing in these marshes may contribute to coastal protection by stabilising the soil and reducing waves, in addition to providing other important ecosystem services like water quality improvement, carbon sequestration and habitat for biodiversity (Figure 1 middle-right). This is highly promising, for example, by preserving and restoring natural ecosystems like salt marshes and mudflats, about 100 km of dikes along the Wadden Sea may, in theory, convert their safety status from insufficient to safe. To date, uncertainties about the actual effectiveness still hamper the practical implementation of these ecosystem-based measures. Hence, as an environmental biologist, I am highly motivated to generate knowledge enabling the implementation of ecosystem-based coastal defences and thereby protecting and restoring declining coastal ecosystems.
In this project, I address key knowledge gaps about the functioning of natural coastal ecosystems such as salt marshes and mudflats for flood defence and how their management may benefit both flood protection and ecological value.
- The role of tidal flats and salt marshes on wave attenuation and reduction of wave run-up on sea dikes was quantified by three-year-long measurements along the Wadden Sea coast (Figure 1 middle-left and map).
- By analysing a 20-year time series of aerial images and elevation maps, I identified which factors drive salt marshes development at several locations in the Wadden Sea.
- The potential of utilising artificial reefs for salt marshes expansion by accreting the fronting mudflats was experimentally explored by creating with many colleagues, a unique artificial reef experiment (see location on the map). I focused on the wave attenuation and elevational effects (Figure 1 bottom-left).
- The topsoil erosion mechanisms of salt marshes and mudflats, important for minimising the depth of dike breaches. With a fast-flow flume, I studied the resistance of salt marshes and tidal flats with different soil and vegetation properties on fast flow erosion, which could occur during a dike breach.
- The effect of grazing management on lateral erosion resistance, important for understanding its effect on marsh width. With wave flumes, I tested the lateral erodibility of marsh soils collected in areas with different grazing management, marsh age and marsh elevation (Figure 1 bottom-right).
Relevant for whom and where?
All authorities involved in the design of ecosystem-based coastal defence, like water-boards, nature managers, and NGOs.
Progress and practical application
Figure 2 summarises the main findings of coastal ecosystem dynamics about coastal protection, along with an illustration highlighting the importance of ecosystem connectivity.
Figure 2. Illustration summarising the findings and the importance of the ecosystem connectivity. For example, shellfish reefs may reduce waves and trap sediment, which may be beneficial for the salt marsh expansion. In return, salt marshes protect the dike by providing erosion resistant soils in front of the dikes and reduced wave run-up during storms. Illustration by: Beatriz Marin-Diaz.
Locations studied near the north and south Dutch coast and the artificial experiment in the north Island of Griend.
Referring to the Figure 2 summary of findings (for details, see the related outputs):
- Reduced run-up: Salt marshes (even with short vegetation) always reduced the wave run-up compared to bare mudflats.
- Marsh expansion: Marsh expansion offshore is related to an increase in elevation of the adjacent tidal flats. Marsh expansion offshore is related to an increase in elevation of the adjacent tidal flats. Engineering measures may only achieve marsh formation at locations with low sediment deposition.
- Biodegradable artificial reefs potential: The experiment shows the potential to change tidal flat morphology. However, the reef’s dimensions should be larger, and the selection of the material should match the hydrodynamic exposure.
- Resistance to fast water flow top erosion: The salt marsh vegetation traps fine sediment and organic matter, creating a highly resistant cohesive top layer. Only pioneer vegetation in sandy places was not erosion-resistant, comparable to bare mudflats. However, marshes for flood safety should not be built with sand as these can be sensitive to lateral erosion.
- Grazing for reduced erodibility: Both small (e.g. hares, geese) and large herbivores (i.e. cattle) grazing reduce saltmarsh lateral erodibility on fine-grained soils. However, intensive cattle grazing may however compact the soil, which may negatively impact the marsh resilience to sea-level rise in areas with low sediment supply.
Recommendations for practice
- We recommend preserving existing marshes because they provide stable soil and wave run-up reduction on the dikes independently of the season, vegetation state or grazing management. To prevent marsh erosion and create new marshes, focus on managing the elevation of the fronting tidal flats and stimulate their net sediment accretion, for example, by improving the ecosystem connectivity.
- For future marsh restorations, using fine-grained sediments or silt are recommended rather than using coarse sand, which makes marshes sensitive to erosion or may take very long to become erosion resistant.
- High-intensity grazing should be avoided as it is negative for biodiversity and may lower the soil elevation. However, we recommend preserving small herbivores such as hares and geese in the low salt marshes as they may limit erosion and enhance biodiversity without any potentially negative side effects.
Last modified: 27/02/2022
NIOZ Royal Netherlands Institute for Sea Research
Initiating and upscaling mussel reef establishment with life cycle informed restoration: Successes and future challenges
Life cycle informed restoration enhances mussel bed formation. However, mussel biomass depends on sediment burial. Larger-scale processes resulted in partial burial of the structures. Therefore, optimisation of the structures is required to overcome technical failure at exposed sites.
Bevat: Publication open access journal
On the use of large-scale biodegradable artificial reefs for intertidal foreshore stabilisation
A large-scale experiment was conducted on the tidal flats of the Dutch Wadden Sea, by installing biodegradable artificial reefs along 630 m. Waves, sediment dynamics and sediment properties around the structures were monitored over three years.
Bevat: Publication open access journal
How grazing management can maximise erosion resistance of salt marshes
We aimed to determine how salt marsh management (i.e. grazing by large vs. small grazers vs. artificial mowing), marsh elevation and marsh age affect soil stability (i.e. soil collapse) and intrinsic lateral erodibility of salt marshes (i.e. particle-by-particle detachment).
Bevat: Publication open access journal
- Marin‐Diaz, B., Bouma, T. J., & Infantes, E. (2019). Role of eelgrass on bed‐load transport and sediment resuspension under oscillatory flow. Limnology and Oceanography. https://doi.org/10.1002/lno.11312.
- Marin‐Diaz, B., Govers, L. L., van der Wal, D., Olff, H., & Bouma, T. J. (accepted with minor revisions). The importance of marshes providing soil stabilization to resist fast-flow erosion in case of a dike breach. Ecological Applications.
- Marin‐Diaz, B., Govers, L. L., van der Wal, D., Olff, H., & Bouma, T. J. (in prep.). Lateral and top erosion in sandy systems: case study of a managed fetch-limited barrier island.
- Marin‐Diaz, B., Govers, L. L., van der Wal, D., Olff, H., & Bouma, T. J. (in prep.). Using salt marshes for coastal protection: effective but hard to get where most needed.
- Nauta J., Temmink, R.J.M., Fivash, G.S., Marin-Diaz, B., Cruijsen, P.P.M.J.M., Reijers V.C., Didderen, K., Penning, E., Olff, H., Heusinkveld, J.H.T., Lamers L.P.M., Lengkeek, W., Christianen, M.J.A., Bouma, T. J. & van der Heide, T., Govers, L. L. (in prep.). Temporal changes in food web assembly on artificial reef structures.
Reflection: Foreshores - useful for manageable flood safety or just beautiful nature?
In the 19th and early 20th centuries, salt marshes were mainly created for agriculture in the North of the Netherlands. More recently, we have reconsidered the value of these vegetated foreshores for flood safety.