This research focuses on the implementation of the Building with Nature (BwN) approach in the design of artificial coral reefs. The goal is to explore environmental preferences and design tools for coral development, considering both coral habitat potential and coastal protection services. The research also aims to optimize artificial reef design for a case study in Addu City, Maldives.

A conceptual model is developed based on an extensive literature review, incorporating critical engineering and ecological variables relevant to coral habitat potential and coastal protection services. This conceptual model serves as a practical design tool, fulfilling three primary purposes: identifying variables that require further examination to explore coral habitat potential, identifying variables that require analysis to explore coastal protection services, and identifying design variables that influence physical-chemical and biological variables for potential integral solutions.

The research also utilizes OpenFOAM, a numerical modeling tool commonly used in coastal engineering, to design artificial reefs. OpenFOAM accurately models flow and turbulence regimes, crucial for the ecological and biological functioning of coral reefs. An OpenFOAM numerical model is set up, calibrated, and validated for the case study in Addu City. Three design alternatives with different slopes are evaluated using a multi-criteria analysis (MCA), considering criteria such as coastal protection, coral habitat potential, and costs. The numerical model is applied to assess flow regimes and wave transmission for different design variables.

Based on the MCA results and the evaluation of criteria, an artificial reef with a mild slope of 1:3 is selected as the optimal integral solution for the case study. This design performs well in terms of coastal protection, coral habitat potential, and costs. The research demonstrates that the conceptual model and OpenFOAM numerical model are valuable design tools for assessing and optimizing artificial reef design, adhering to the principles of the BwN approach.

In conclusion, this research contributes to expanding knowledge and developing tools for implementing the BwN approach in artificial reef design. By considering both coral habitat potential and coastal protection services, the design of artificial reefs can be optimized to provide multiple benefits, including environmental and socio-economic values. The conceptual model and numerical modeling tool offer practical assistance in the design process, promoting the integration of nature and engineering for sustainable coastal solutions.

The Delta21-project is a proposed alternative solution to the increasingly higher risk of flooding in the southwestern delta of the Netherlands. Located at the mouth of the Haringvliet, it has three main ambitions: flood protection, storage of renewable energy and the recovery and preservation of nature. Flood protection is achieved by the installation of high capacity pumps, that pump out water from the rivers towards the sea in the case of a high river level, in combination with a closed storm surge barrier during a severe storm at the North Sea. Storage of renewable energy is accomplished by an artificial lake in which turbines generate renewable energy and the aforementioned pump capacity can empty the water back out to sea. The project does however pose various challenges regarding the local ecology, morphology and coastal maintenance. In this thesis, the impact on the latter is studied. The objective is to study the consequences of this Delta21-project on the alongshore sediment transport and resulting erosion volumes. Using a sediment transport proxy, the offshore wave climate nearMaasvlakte 2 is reduced to 100 representative wave conditions. This reduced wave climate is then transformed to the nearshore by the wave-model SWAN, after which the coastline modelling software UNIBEST-CL+ is used to calculate the alongshore sediment transports and erosion for the coastal profile above NAP -8.0m. In Model 0, the models are firstly validated to check whether the models are an accurate depiction of the present conditions in the area of Maasvlakte 2. This validation is done by comparing the measured erosion and performed coastal nourishments for Maasvlakte 2 to that of the modelled erosion. This confirms the model’s validity and applicability for modelling the effect of the Delta21-project in Model 1. In addition, the implementation of tidal data from an external Delft3D-model is modelled. However, due to the inability of UNIBEST to model the tidal effect above NAP -8.0m accurately based on the available data and model, no conclusive remarks can be given on the effect of this. In Model 1, the alongshore transports and erosion due to implementation of the Delta21 project are then modelled. The extent and characteristics are summarised as being similar to those found at Maasvlakte 2. This study furthermore shows that the grain size can have a significant impact, with alongshore sediment transports and erosion a factor of 2.5 higher for a grain size of 160 mm instead of 370 mm. A large temporal and spatial uncertainty and variability in wave energy and consequent sediment transport and erosion for Delta21 can be expected, as was also observed for Maasvlakte 2. This leads to a total bandwidth of the estimated erosion for the current design of the Delta21 coastline in between 0.4 and 1.2 millionm3/year, for a grain size of 370 mm. This study shows that the current layout for the Delta21-project will behave similarly toMaasvlakte 2 in regards to alongshore sediment transport and erosion above NAP -8.0m.

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.