Coastal structures such as breakwaters can mitigate the erosive effects of sea level rise by protecting shorelines from wave impact. The Reef Enhancing Breakwater (REB) is a modular, permeable structure designed to dissipate wave energy and boost marine biodiversity by providing suitable habitats. This research investigates the hydraulic stability of the REB under wave loading using a physical model.

 

Experiments were conducted in the Scheldt flume of Deltares with a 1:20 scale model. Both 2 and 3 level structures with simple and complex forms were tested on gravel underlayers of varying roughness. Irregular waves with significant heights up to 12.5 cm and wave steepness ranging from 2% to 4% were used, with varying water depths. As failure was not reached on the horizontal foreshore (main location), the structure was moved closer to the transition slope (alternative location) where plunging breakers can emerge. Smart ReefBlocks with integrated mobility sensors measured accelerations and angular velocities induced by wave motion.

 

Five main types of movements were observed: sliding, rocking, shaking, tilting, and lifting. Sliding was the most common, occurring mainly in the top layer, with a maximum of 4 sliding events per block per 1000 waves. Three limit states were defined: start of motion, start of damage, and failure. Start of motion is when blocks move for the first time, damage is when a block loses interlocking, and failure is defined by a damage level Nod=0.4Nod=0.4.

 

Expected and characteristic values of the stability number (Hm0/ΔDnHm0/ΔDn) were determined for each limit state. Only one displacement occurred at the main location, while failures were reached near the transition slope due to plunging breakers. The 2 and 3 level structures were stable on bed slopes without plunging waves. For non-plunging waves, the start of movement limit state (Ns,mNs,m) ranged from 0.39 to 0.51. When plunging waves occurred, the start of damage limit state (Ns,dNs,d) was 0.94, and the failure limit state (Ns,fNs,f) ranged from 0.94 to 1.11. The start of motion was not determined for setups at the alternative location.

 

For setups at the main location, sliding was analyzed in relation to wave height, wave steepness, water level, and structure height. Higher water levels allowed for higher waves, leading to more instabilities, but excessive submersion reduced movements. Waves with 2% steepness caused more movements than 4% steepness due to higher energy in longer waves. Other factors like underlayer irregularity and increased drag from epifauna were also considered. Most sliding movements occurred for Hm0/ΔDn>0.8Hm0/ΔDn>0.8. The model blocks, made of PLA with lower friction than concrete, started sliding under smaller forces, making the model conservative for sliding movements.

 

The impact of sliding movements on the REB's structural performance was investigated by estimating stresses in the protrusions from data collected by smart ReefBlocks. A conservative model was used to calculate impact forces and check for protrusion rupture. The resulting tensile stresses exceeded the concrete strength, but the model's representativeness is questionable as it differs from the actual protrusion. Further research is needed for a definitive answer.

Reefs are the most important marine ecosystems that support the majority of marine life in our oceans for both temporary and permanent shelter, refuge and food supply. However, reef ecosystems around the world are disappearing and artificial reef structures are an opportunity to help restore them.

This project contributed to the field of artificial reefs, by designing a surface pattern on both millimeter and centimeter scale, that aims to enhance and support biodiversity on the large concrete “ReefBlocks”, by the company Reefy. That serve as both artificial reefs and as functional infrastructure, such as breakwaters and dams.

In an extensive literature research, information on marine environments was collected and three main types of inhabitants that can be found in reef communities were identified, including only those that interact with hard substrates like the ReefBlock. The three different inhabitant types were distinguished based on their unique role within ecosystems, which they need to grow, stay balanced and provide the necessary ecosystem functions.

To understand the ecological requirements for the pattern design, existing knowledge on increasing biodiversity with artificial reef substrates was researched and summarized in a micro marine spatial plan on the scale of an entire ReefBlock. The addition of crevices and pits of varying sizes on centimeter-scale and grooves on millimeter-scale were concluded as the main crucial design features that need to be reflected by the final pattern design.

A design vision to aim for biomimicry of lobed brain corals was formulated for the aesthetics of the proposed pattern design. As brain coral patterns have a visually organic complexity and flexibility that allows for the natural implementation of the identified crucial design features. In order to achieve the pattern design vision that meets all the collected ecological, stakeholder and production requirements, a design toolset was developed that combines the power of generative art with the power of parametric design.

The generative art tool allows the semi-automatic creation of lobed brain coral patterns in a similar process to how actual brain corals, and many other examples in nature, receive their pattern from, in a so-called reaction-diffusion system.

With the generative art tool, a black and white 2D image is generated, which is then used as the input variable for the parametric tool. Which directly translates the 2D image into a perfect 3D surface pattern. This method ensures that the pattern design always meets the various production requirements.

In order to support the choice for the aesthetically best-rated design by the general public, a survey with 60 respondents was conducted with questions based on the stakeholder requirements.

In order to validate the effectiveness of the pattern design within the timeframe of this project, a six weeks lab test was conducted using concrete prototypes of the pattern design and microalgae. The test showed promising results for meeting the defined ecological requirements.

Lastly, a rubber form liner was produced in order to help Reefy along with future field tests with the proposed pattern design, in order to validate the conclusions and insights gained in this graduation project, and in real marine environments, for a much longer period of time.
The design toolset was initially created to support the iterative design process and helped to quickly try, test and compare a multitude of designs. But in the end, this toolset has proven to be very valuable for future use by Reefy as they plan on extending their product line with smaller-scale wall panels and for potential adjustments to the proposed pattern design based on their field tests.

Living Breakwaters have evolved from traditional breakwaters to create multi-purpose structures that provide environmental and social benefits. These breakwaters promote a healthy habitat for flora and fauna, while achieving the same structural capabilities as conventional structures. Reefy has developed a modular living breakwater that can serve both as an artificial reef and a stable breakwater. This study aims to model the impact of a Reefy breakwater on the transmitted wave height, using a process-based numerical model, namely XBeach.

This study builds upon a previous one conducted by van den Brekel [2021], where scaled experiments were performed in a physical wave flume, to investigate the hydrodynamic and ecological functionalities of the Reefy breakwater. In the wave flume experiments a total of 15 structure configurations together with 35 wave conditions, both regular and irregular, were tested. The first 7 of the configurations were 2D structures with a relatively simple shape and a porosity of 20%, and the rest were complex 3D structures with a porosity above 20%. The wave flume was recreated within the numerical model. The influence of the breakwaters on the wave heights are examined through the comparison between the calculated transmission coefficient of the wave flume and the numerical model. However, XBeach, and more specifically XBeach non hydrostatic+ mode which was used, does not resolve the complex interaction between waves, friction and porosity of the structures. As a result, a simple method had to be found to compensate for the loss of these phenomena, while successfully calculating the transmission coefficient.

The findings of this analysis demonstrate that treating the breakwater as an impermeable change in bathymetry, coupled with a 15% reduction in structure height, produces the desired outcomes. From the 35 wave conditions only the 8 irregular wave conditions were chosen to be of interest due to time constraints. Among these 8 wave conditions that were used as an input in XBeach, only 5 of them were satisfying the criterion that describes the range for which the numerical model should produce accurate results (kd<2). The transmission coefficient was defined as the ratio of the transmitted wave height behind the breakwater, and incident wave height in front of the structure. The wave decomposition method used was a modified Guza split method. In pursuit of finding a valid solution, the breakwaters were modeled firstly as impermeable structures with a decreased height, secondly as impermeable structures with a decreased width, thirdly as vegetation with the help of the vegetation module, and lastly the maximum wave steepness criterion as applied in the model was increased (maxbrsteep up to 1.4), without modifying the structure height.

Among the suggested solutions and after being validated for 13 experiments with both simple and complex structures, decreasing the width did not achieve the desired results while the vegetation module failed to produce consistent outcomes. As far as transmission coefficient is of interest, a 15% decrease in the structure height had the smallest mean absolute percentage error of almost 10% and a root mean square error of 8.2%. On the other hand, choosing to increase the maximum wave steepness (to a value of 1.4) is not a valid option for the complex 3D structures, but illustrates even better behavior than a change in the structure height when simple structures with 20% porosity are concerned. To summarize, this thesis provides alternatives to replicate in a simplified way the impact of porous structures, such as Reefy breakwaters, without the need of an extensive and time expensive numerical model.