Nature-based solutions (NBS) are defined as the sustainable management and use of nature for tackling societal challenges such as climate change and flooding. A great challenge in the large-scale implementation of nature-based solutions (NBS) for coastal safety along sandy coastlines lies in dealing with the finite nature of sand resources and with the impacts on the marine ecosystem caused by sand extraction activities. Truly sustainable NBS must not only produce ecosystem services (ES), but must also minimize negative externalities which may be generated in areas that are distant from where the NBS is constructed. This includes (1) increasing the use of alternative sand sources to safeguard high-quality offshore sand resources on the longer-term, and (2) impact mitigation of sand extraction. To date, no integrated method is available that allows to balance these trade-offs between offshore sand extraction and its onshore use for the development of NBS.

The goal of SUSANA is therefore to develop a coupled ES-model that allows to assess and compare the ES at the NBS with the ES at the sand extraction sites, supporting the development of a long-term strategy for the sustainable use of sand in NBS. SUSANA aims for a breakthrough innovation in the implementation of NBS in coastal areas as the developed model will be a first in kind to allow for a systems’ perspective analysis across the NBS impact zone. Specific focus is on the dune-in-front-of-dike principle as a working case. SUSANA investigates how alternative sand sources affect the ecosystem functions that support dune development and that provide the key ES in the coastal zone, i.e. vegetation development, aeolian transport and dune erosion, as well as their reciprocal interactions. A combined empirical-computational research approach is used, including the parametrization of existing dune development and erosion models to include dynamic vegetation - sand interactions, the integration of the key ecological and biophysical characteristics and feedback mechanisms into an integrated dune development model, and a social valuation experiment. From this, the impacts on the ES coastal safety, prevention of undesired sand deposition, and recreational experience are quantified by integrating the key ecological and societal characteristics.

SUSANA quantifies how important ES (water quality regulation, food production and climate regulation) are affected by sand extraction in the near-field, using in situ measurements of the functional processes underpinning these ES. Specific focus will be on collecting data from extraction sites with different extraction regimes and in situ sediment composition, and at reference sites. Extracting large volumes of sand also results in increased sediment dispersal in the far-field, hitherto only studied on a case-by-case, single-activity basis. For the first time, cumulative impact forecasting will be developed accounting for the main seabed-disturbing activities and the impact on ES of sensitive ecological receptors. The results will feed the development of a coupled ES-model of dune-in-front-of-dike and sand extraction sites, allowing to balance trade-offs and synergies between offshore sand extraction and its onshore use for the development of NBS. The sustainability of future scenarios for the implementation of dune-in-front-of-dike are investigated through this model. Together with an analysis of the feasibility of cost-beneficial (re)use cases, this will result in a long-term strategy on the sustainable use of sand in NBS.

The following concrete research outcomes are envisioned:

-                      Boundary conditions of alternative sand for ES of dune-in-front-of-dike

-                      Sand volume needs for dune-in-front-of-dike using alternative sand sources

-                      Parameterized models for coastal safety functions

-                      Measured sediment characteristics for two alternative sand sources

-                      Ecosystem services model for dune-in-front-of-dike

-                      Sand extraction impacts on ecosystem functioning near- and far-field

-                      Cumulative risk map of human-induced increases in sediment deposition

-                      Ecosystem services model of sand extraction sites

-                      Feasibility criteria for cost-beneficial reuse + case-identification

-                      Long-term strategy for sustainable use of sand in NBS

-                      Coupled ecosystem services model dune-in-front-of-dike - sand extraction sites