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. ... Read more

The report tackles Galveston's flooding challenges, which are currently in development with the US Army Corps of Engineers (USACE) ring barrier. However, the current design, predominantly addressing coastal flooding, falls short in dealing with pluvial flooding, relying heavily on pumps. This top-down approach neglects environmental and stakeholder considerations, resulting in a decoupled response to compound flooding, lacking adaptability, and overlooking the impact of chronic flooding on local businesses. The central research question revolves around reshaping the Galveston ring barrier in The Strand area for enhanced functionality against both coastal and pluvial flooding, sustainable management of catastrophic and chronic flooding, and improved public space value. The methodology consist of a literature review, fieldwork in 'The Strand,' stakeholder engagement in Texas, the design of multiple alternatives that deal with the issue at hand and evaluation through a Multi-Criteria Analysis. Two alternative designs, sensitive to identified issues, are presented for the Strand area. The outcome emphasizes an interdisciplinary approach incorporating the knowledge of the different academic backgrounds of the team, with designs adaptable for broader implementation in Galveston.
The design alternatives centres on measures to counteract flooding, specifically cloudburst roads, retention areas, and a promenade. Caution is advised in interpreting results, emphasizing the need for further investigation into hydraulic conditions. Climate change effects are underscored, considering sea level rise, precipitation rates, and increased hurricanes. The project area, focusing on a 1 km stretch, offers local adaptation measures, with potential extension to larger areas to explore system behaviour on a larger scale. The study notes the uncommon implementation of sustainable drainage systems in the United States,
highlighting the importance of addressing common failure causes such as incomplete knowledge and poor communication. While two measures for pluvial flooding are examined, the report suggests a more detailed design should consider additional factors like green roofs and their impact on runoff speed and drainage capacity. ... Read more