Coral reefs play a crucial role in coastal protection and ecosystem sustainability. However, they face increasing threats from sea-level rise, ocean warming, and acidification. While coral reef restoration is often proposed as a means to reduce coastal hazards, there remains insufficient field evidence or large-scale laboratory data to validate this claim. A key aspect in assessing the effectiveness of reef restoration is understanding the flow structure within artificial reefs, as wave energy dissipation is closely tied to these dynamics. Not accounting for in-canopy flow in numerical models can lead to inaccuracies in predicting wave dissipation and water level changes.

This thesis investigates wave transformation and dissipation through large-scale wave flume experiments conducted at Deltares' Delta Flume. The experiments utilized a 1:3 scale model of a Maldivian fringing reef, tested under five wave conditions and two water levels. The experimental setup included a 45-meter-wide reef flat, where a 10-meter section was equipped with either 91 or 153 3D-printed artificial reef elements (0.25m high, 0.40m wide). Data were collected using pressure sensors and flow meters to measure wave transformation, while two ADV arrays analyzed in-canopy flow. These CREST experiments, carried out in collaboration with Plymouth University, Deltares, Boskalis, and Coastruction, contribute to the ARISE project, which explores atoll island adaptation to rising sea levels.

The research addresses four primary objectives: (a) evaluating the impact of artificial reefs on wave transformation, frequency dependence, and water level response; (b) identifying in-canopy flow characteristics; (c) linking these flow characteristics to wave dissipation; and (d) assessing the accuracy of an existing in-canopy flow model.

Findings indicate that as water levels rise, the relative height of the artificial reef decreases, leading to a decline in wave height reduction capacity. The reduction ranges from 8.5% to 15.2% at low water levels and from 4.9% to 5.9% at high water levels. Despite reducing incoming wave heights, artificial reefs can, under certain conditions, increase rather than decrease extreme water levels onshore due to the artificial reef-induced drag force amplifying wave setup.

Analysis of streamwise velocity variance revealed significant spatial differences. Velocity variance decreased more in the ADV array located behind the reef elements compared to the ADV array positioned between them, where velocity variance increased. Flow attenuation was found to be more pronounced at lower water levels, with longer and higher waves attenuated more effectively than shorter and lower waves. The modeled canopy wave dissipation rate, was found to be 43-87% lower than observed dissipation, though still within the same order of magnitude. The discrepancy is partly due to the model not accounting for breaker dissipation and non-linear energy transfers. Flow convergence corrections significantly increased flow attenuation and reduced canopy dissipation rates, while ADV selection had minimal impact.

According to canopy flow regime classification, the tested conditions fell between inertia-dominated and general flow. Reducing element spacing could shift the flow further into the general flow regime, enhancing frequency dependence in flow attenuation and wave dissipation. Given its limited wave height reduction capacity, artificial reefs like these would be most effective when integrated into hybrid coastal protection strategies. Such combinations could enhance both coastal resilience and ecological benefits. Ultimately, the CREST experiments provide a valuable dataset for model calibration and validation, contributing to a deeper understanding of reef hydrodynamics and in-canopy flow dynamics.

Low-lying coral islands are highly vulnerable to sea-level rise and climate change, with coral reefs, essential for coastal protection, facing significant decline due to climate change and anthropogenic stressors. Artificial reefs present a potential solution by restoring ecology and mitigating coastal flooding, but their effectiveness and impact on wave dynamics remain unclear. This study investigates the effects of an artificial reef on spectral wave transformation over fringing reefs, combining numerical modelling with SWASH and large-scale experiments conducted in the Deltares Delta Flume. The experiments involved a scaled reconstruction of a fringing reef island with a geometric scaling factor of 3. Numerical modelling was used to optimise the laboratory experiments by adjusting reef placement and element density for maximum wave height reduction.

The results of this thesis show a limited impact of an optimised artificial reef on the reef flat, with effectiveness expected to decrease further as sea levels rise. However, the smaller scale of the reef elements prioritises ecological benefits, and when combined with larger structures, these reefs can still offer valuable benefits for biodiversity and wave mitigation. The experiments provide a valuable new dataset on the hydrodynamics of complex canopies, and can be used to calibrate several numerical models and methods of schematising artificial reefs.