Promising Building with Nature (BwN) solutions for flood protection, such as vegetated foreshores, inherently have a dynamic nature. Therefore there is a relatively large degree of uncertainty with respect to their contribution to flood protection. This hampers innovation and the implementation of vegetated foreshores in flood risk management in the Netherlands and abroad.
We aim to develop new methods to assess how, and how much vegetated foreshores can contribute to flood risk reduction. The project will lead to a better understanding of (uncertainties in) the functioning and stability of these ecosystems and the development of novel governance arrangements. This requires integration of knowledge from ecology, biogeomorphology, hydraulic engineering, and governance.
By field observations on several sites and flume measurements we will analyse fundamental ecological and physical processes for various types of wetland vegetations. The knowledge obtained will be applied in one implementation case study for a location in the Netherlands where dike reinforcement is needed in the future. This case study integrates fundamental knowledge from all the disciplines. It is used to design governance and implementation arrangements, and to demonstrate how vegetated foreshores can contribute to flood risk reduction.
The project will provide the knowledge, methods and tools (e.g. a maptable) required for the design and implementation of vegetated foreshores as a safe, ecologically desirable, and cost-effective alternative in flood management. Strong cooperation with end-users from the private sector, government and non-governmental organizations is embedded in the project to enhance the implementation of our findings in practice.
Application of wetlands in front of dikes (=vegetated foreshores), as a cost-effective method of flood defence, is hampered by lack of insight in (1) their long-term dynamics and (2) the compatibility of safety goals with the conservation aim to maximize diversity.
With respect to long-term dynamics we identified three critical thresholds that need to be quantitatively understood to enable reliable modelling. These are (1A) thresholds related to wetland decay/shrinking, (1B) thresholds for establishment or expansion, and (1C) density thresholds for positive biogeomorphic feedbacks. We will quantify these thresholds for 4 different wetland types (Spartina, Scirpus, Phragmites and Salix) with contrasting plant growth strategies (i.e. woody versus non-woody and predominantly rhizomal versus non-rhizomal expanding).
Regarding compliance of safety and conservation goals, we note that wave attenuation is mainly determined by standing biomass, whereas maximum diversity is generally achieved at maximum habitat heterogeneity. This raises the question to which extent both services are compatible. We will (2A) study in the field how different management strategies (like seasonal mowing/pruning or summer grazing) affect wave attenuation and biodiversity and (2B) model the dependence of ecosystem services on age and characteristics of the vegetation under different management strategies.
We will provide fundamental insight in how plant-growth strategies affect both thresholds driving wetland dynamics and trade-offs in their ecosystem services by comparing four contrasting wetland types. These fundamental insights will enable application by providing the knowledge basis for long-term biogeomorphical modelling (sub-project 2), safety analyses (sub-project 3) and management evaluations (sub-project 4).