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.

Sea turtles, with six of the seven species listed as endangered by the IUCN, face significant threats. Their eggs exhibit sensitivity to fluctuations in temperature, salinity, and moisture during their 6-8-week incubation period. Inundation of these eggs can substantially decrease their viability and influence sex ratios. This thesis centers on mitigating the wave runup-driven flooding of sea turtle nesting beaches. Mitigating nest flooding can involve relocating at-risk nests to higher elevations. This thesis aims to assist in responsible nesting relocation by providing flood risk information as a function of beach elevation. The study site, Ras Baridi, is a significant nesting site for green turtles (\textit{Chelonia mydas}) in the Red Sea. The research focuses on the development of a methodology for assessing flood risk in environments with limited data.

The risk of sea turtle nests to flooding at Ras Baridi was assessed by employing two different metamodels, BEWARE 2.0 and HyCReWW, that were recently developed to assess wave runup in coral reef environments at low computational costs. These models employ different strategies to model wave runup. BEWARE 2.0 uses real bathymetry, whereas HyCReWW schematizes the input bathymetry. 40-year hindcast datasets of waves and water levels from 1978-2018 were used in combination with 10 m horizontal resolution bathymetry data from the Allen Coral Atlas as model inputs. HyCReWW exhibited maximum runup at an 80 m reef width (0.95 m for 1 year return period (RP), 1.95 m for 40 year RP) BEWARE 2.0’s highest runup was at an 80 m reef width (1.35 m for 1 year RP, 2.7 m for 40 year RP). Median inundation durations from BEWARE 2.0 and HyCReWW were 4 and 7 hours, respectively. Beach elevations associated with 5-year return periods were identified as suitable minimum nesting elevations to mitigate inundation risk.

BEWARE 2.0 and HyCReWW results for Ras Baridi lacked direct real-world validation, prompting a methodological validation approach. Six validated XBeach 1D NH models in fringing reef environments globally were utilized for comparison against BEWARE 2.0 and HyCReWW. The comparison revealed increased median scatter indices and root-mean-square (RMS) errors for both BEWARE 2.0 (0.07 and 0.1 m increases) and HyCReWW (0.1 and 0.15 m increases) with low-resolution bathymetry (10 m horizontal). BEWARE 2.0 had lower median RMS errors and scatter indices for high-resolution (2 m horizontal) and low-resolution bathymetry than HyCReWW, as well as smaller spreads. Consequently, utilizing BEWARE 2.0 results is recommended for Ras Baridi and similar data-scarce coastal environments over HyCReWW.

In summary, this thesis addresses the pressing issue of sea turtle nest flooding, providing a methodology to assist in responsible nest relocation in low data environments. The 5-year return period runup elevation of the BEWARE 2.0 results (1.25-1.75 m along the beach) has been identified as a suitable minimum nesting elevation in Ras Baridi, and the likelihood of flooding as a function of beach elevation has been provided. These results can aid coastal managers in making informed decisions for the protection of these endangered species.