Small-scale nature-based solutions for protection of sandy coasts

Nature-based solutions (NBS) are promising methods to enhance biodiversity and adapt to climate change in coastal areas. However, upscaling NBS to replace conventional methods requires knowledge about their performance from multiple perspectives, e.g., biodiversity, coastal safety, and economy. In recent years, great efforts have been put into researching NBS pilots of sandy solutions. Some of the most prominent examples are found in the Netherlands, e.g., the Sandmotor, the Hondsbossche Dunes, and the Prince Hendrik Sand dike. These are examples of large-scale interventions with nourished sand volumes of hundreds of thousands to millions of cubic meters. In contrast, this study focuses on small-scale NBS pilots of sandy solutions with nourishment volumes of hundreds to thousands of cubic meters. Two NBS pilots in Sweden are described and analysed, and the advantages and disadvantages of small-scale NBS are discussed in relation to larger-scale interventions.

The first pilot was installed in 2018 in the Furusund navigational fairway in the Stockholm Archipelago. A few hundred cubic meters of sand was nourished to a beach subject to erosion due to ship-generated waves. The nourishment protects an eroding bluff and prevents the loss of forest areas with high nature values. Compared to hard solutions, e.g., a rock revetment, the small-scale beach nourishment supplies sand to a small sandy beach down-drift used for recreational purposes. Since the implementation, a significant part of the nourishment has already been eroded, and the expected lifetime of this intervention is in the order of a few years.

The second pilot was installed at Fortuna beach, located in the narrow sound between Sweden and Denmark. The area has a low-energy wave climate, and the nourishment was designed to protect a beach in front of a residential area from storm erosion recurring with decadal frequency. The beach and dune area were nourished with approximately 3000 m³ of sediment. The area has limited offshore sand resources that can be extracted without adverse environmental impact. Therefore, the beneficial use of sediment dredged from local marinas and a mixture of sand and seaweed from nearby beach clean-ups was used to carry out the project. Within a year after the nourishment, a storm with a recurrence period of approximately 5-20 years hit the coast, but the dune volume still exceeded the volume before the measures.

Experiences from the small-scale sandy solutions are that the limited extent of the interventions facilitates financing and permitting processes, which can be a bottleneck in upscaling NBS. The limited volume of nourishments makes it easier for beneficial use of dredged material, which in many cases is viewed as a waste rather than a resource. Both the volume and timing can be adapted to nearby dredging operations, thus reducing the cost of maintenance of small marinas with a high cultural value. The short lifetime and low safety level of small-scale NBS can be a disadvantage but allow for more flexibility, and no-regret solutions compatible with adaptive pathway approaches to climate change adaptation.