Towards a nature-based solution for the Brouwersdam beach in the Netherlands

Abstract

Since the Brouwersdam was built as part of the Delta plan, the Grevelingen outer delta undergoes large-scale morphological changes. These changes led to the formation of a shoal adjacent to the dam, which is referred to as the Brouwersdam beach. The beach has been eroding and shifting towards the northeast ever since. The beach has no safety function, as the hinterland is protected from flooding by the Brouwersdam. However, the beach has a large recreational value, which will decrease if no human interventions are taken. Consequently, many stakeholders demand the preservation of the beach. For the Municipality of Schouwen-Duiveland, the loss of recreational value of the beach is estimated to decrease the yearly financial benefits by 0.8 to 5.4 million euro. Hence, their interest in the preservation of the beach is large and therefore they have commissioned this study. This study aims to design nature-based solutions for the ongoing erosion and coastline shift to which the Brouwersdam beach is subjected. The design approach of this research is based on the Building with Nature (BwN) design steps. A thorough analysis of the physical, ecological and social system is followed by the set-up of a Program of Requirements (PoR) for the design. Insight in the physical system is partly obtained by a modelling study in Delft3D-WAVE and UNIBEST-CL+. Based on the PoR, designs are identified, evaluated and selected iteratively. The design iterations, based on the systems’ insights and the PoR, led to the creation of three Nature-based Solutions. The alternative solutions include the creation of a shellfish habitat by making structures of layered brushwood fascine mattresses filled with shells or other hard substratum. The shellfish reef, in which mainly Japanese oysters are expected to settle, provides many ecosystem services such as enhancing biodiversity. Moreover, the structures can be constructed CO2-neutral. The different configurations of the solutions, ensuring coastal protection, are: A groyne configuration, creating multiple pocket beaches, of 1 straight groyne and 4 L-shaped groynes with an orientation slightly tilted towards the west, i.e., the dominant wave direction (Solution A) ; A straight groyne with a west-east orientation, combined with a sediment beach fill between the current coastline and the groyne (Solution C) ; A fishtail groyne, which is a structure with a slightly curved, shore-normal groyne arm and a shore-parallel breakwater arm (Solution E). These solutions were compared with frequent nourishing (solution F) and to the reference situation, i.e. taking no action. The assessment of the solutions by means of a Multi-Criteria Analysis (MCA), based on the categories functionality, costs, recreational value (water and beach recreation separately), ecological value and aesthetics, led to the conclusion that the designs are equally feasible but score differently on the categories. The results of the MCA thus illustrate that the conclusion on the most feasible solution depends on priority. The study concludes that preserving the Brouwersdam beach in a nature-based way is feasible. The MCA tool can potentially support future decision-making processes following this study by clarifying the trade-offs between the different designed solutions. Further exploring the economic benefits of recreational functions will contribute to validating the weight factors in these categories. The results of this study contribute to the discussion on the preservation of the Brouwersdam beach and to the strive for knowledge on ecosystem-based projects within the field of hydraulic engineering.