# Protecting the Coast: A Simulation of Artificial Reefs

David Carvalho Author
Fábio Cruz Reviewer

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Perspectives on the Sea
In Perspectives on the Sea, we look into how simulation can help us face societal challenges, particularly those related with the ocean.

We have detailed here how coral reefs are more than a thriving ecosystem.
They provide natural protection to the coastline against harsh sea conditions.

Artificial reefs can recreate this protection profile. However, estimating how well these structures fare requires intense modeling and computation.

But there can be other ways to get the job done.

In this post, we showcase how this assessment can be made not only with the aid of simulation but also with real data from hydraulic experiments and Machine Learning models.

# Protecting the Coast: A Simulation of Artificial Reefs

We saw that natural coral reefs are incredibly efficient structures in protecting the coastline. So much so that about 200 million people worldwide rely on the protection they provide.

Regrettably, they are facing a truly existential threat.

Inventive solutions to recreate their features are becoming widespread.
For instance, custom-designed shapes can now be 3D-printed — providing crevices, refuges and shelter spots to altogether new ecosystems:

Video 1: 3D-printed reef for ecological protection. A whole new ecosystem was born after 3 years. Credits Seaboost Ecological Engineering

But what if reef components were used to protect the coastline?
For that, a more analytical approach is needed.

## Simulating reef components: a laborious computational task

The effect of arbitrary structures on complex environments can lead to setups which tend to be intensive to simulate because they rely on the geometry of the specific piece as well as on the materials used.

Fig. 1: Simulation of the flow field around two different stacking reefs structures, respectively in the inset of (a) and (b). This computation suggests that stacking reefs longitudinally to increase the influence area of the slow-flow zone is not particularly functional. Credits: [1]

To make matters more challenging, the behavior of the structure can vary wildly depending on the incoming fluid conditions, such as its viscosity or velocity [2].

With both reef and flow parameters to consider, the number of simulations needed to be run increases prohibitively.

Another challenge in simulating arises mostly because the currents formed inside and around the structure are oftentimes turbulent [3], requiring refined computationally-intensive frameworks to solve notoriously hard equations of fluid dynamics [3, 4].

Take the simple example of a stack of hollow tubes, shown in Fig. 2.

Using Fluent, a Computational Fluid Dynamics (CFD) simulator, and real data from tank experiments, the flow around the reef can be estimated and contrasted [3].

The velocity components of the fluid show turbulent features and can be seen occurring in the region shielded by the 3-tube reef as the water flows from left to right.

Fig. 2: (top) 3D depiction of the reef for various tube arrangements. (bottom) By fixing one coordinate and setting incoming flow conditions, the velocity vector of the fluid can be seen in the two remaining coordinates through (left) Particle Image Velocimetry experiments and (right) the CFD simulation package Fluent. Credits: [3]

This example illustrates an important point when modelling any intervention on a coastal scenario: for a simulation to be consequential, the properties of each component must be properly estimated.

Needless to say, this is yet another challenge coming our way.

### Dragging along

Wave overtopping is easier to mitigate if incoming waves move slower.
The kinetic energy they carry can be transferred from the water to the rough surface of the reef component.

As fluid moves along it a drag force $$F_D$$ in the direction of the fluid at each point of the object boundary is generated:

$F_D \propto \frac{1}{2} C_D \rho |\mathbf{u}|^2$

where $$\rho$$ is the fluid density and $$\mathbf{u}$$ its velocity at that point. The object response is quantified through the drag coefficient $$C_D$$.

This coefficient is challenging to quantify since it depends not only on the object geometry but also on the fluid properties [2].

Fig. 3: Estimation of the drag coefficient $$C_D$$ for various reef geometries for a fixed fluid regime [5] as done in [6].

One way out is to abstract the geometrical properties of each structure (such as their porosity and permeability) into classes and then estimate the drag coefficient of each class.

In the context of providing coastal protection for the South Korean coastline, this is done in [6], where a total of 24 reef components are proposed and their response benchmarked.

Fig. 4: 3D geometries of 24 different reef components considered for a South Korean Coastal Protection study. Credits: [6].

The upshot coming from this optimization task can be very substantial.

For instance, the geometrical variations from component AR17 to component AR20 (shown in Fig. 4) can lead to drag coefficients 6 times larger [6].

But the space of all shapes and materials seems huge.

We must start somewhere.

## An actual artificial reef component in action

Instead of aiming at generalizations, we can focus on a particular reef component geometry — ideally one easy to manufacture — and fine-tune it with the aid of a handful of parametric features in order to ensure optimal wave attenuation.

Take the rooftop tile-like piece studied for this task in [7, 8].
This shape was chosen so it can both dampen the incoming waves and collect sand.

Fig. 5: Depiction of the reef component from (a) the side and (b) from the top. Credits: [8]

The response of the reef is then studied in a controlled tank environment where hydraulic experiments are conducted [8]. This allows to ascertain which reef properties are optimal in attenuating incoming waves.

How? From Fig. 6, it is a simple idea.

From the right, many incoming wave conditions can be generated by means of a paddle, of varying wave height $$H_0$$ and wavelength $$L_0$$. These can be lumped into a single parameter, the wave steepness $$H_0/L_0$$.

As the wave propagates and moves over the reef, it will dampen.

At a point right after the reef, measurements of the transmitted wave height $$H_t$$ lead to a transmission coefficient $$C_t = H_t/H_0$$, which quantifies the reef protective power.

Fig. 6: Sketch of how the transmission coefficient $$C_t$$ is measured. The reef design parameters, which are meant to be optimized, are also depicted. Credits: [8]

For this geometry, only two design parameters are needed:

• Crown width $$x/B$$, where $$B$$ is the crest width i.e. the reef length across the channel and $$x$$ the cross-shore size where sediments are deposited.
• (Local) submergence ratio $$l_s/h$$, a measure of how tall the structure is ($$l_s$$) with respect to the channel baseline water level $$h$$.

The transmission coefficients $$C_t$$ are assessed from a multitude of incoming wave conditions by taking a series of 9 measurements along the neighborhood of the reef (depicted and numbered in Fig. 7)

Fig. 7: Hydraulic experiment setup. Waves are generated on the right and propagated toward the reef flat. Credits: [8].

Costal protection entails more than wave attenuation. Reefs can help mitigate erosion and this process can also be studied in this type of experiments.

The reef slope (shown in the setup of Fig. 7) is made with gravel and sand grains to scale. Precise measures of their displacement allow for morphological changes to be tracked in time.

Fig. 8: (top) Preparation setup outlining the composition of the reef slope; (bottom) changes to ramp morphology for different time intervals. Credits: [7]

### Machine-learning the wave attenuation

The relationship between so many parameters is highly nontrivial. Neural Networks are particularly well suited for this sort of task.

By supervising the model with high-quality hydraulic data, the transmission coefficients are learned for each wave and reef parameters.

To achieve this, a dataset with 192 configurations, comprised of 7 physically-relevant quantities measured along the channel around the reef are used.

After successfully training, the model infers how the coefficient change with respect to those 3 characteristic ratios of the reef, allowing optimal design parameters combinations to be found.

Fig. 9: Relationship between the transmission coefficient $$H/H_0$$ and the reef design parameters, with respect to: (top) wave steepness; (middle) submergence ratio; (bottom) crown width. Credits: [8]

## Data & Simulation: A Symbiosis for the Future

Simulating the response to many wave conditions of artificial reef components of arbitrary shapes and materials is a computationally-intensive endeavor.

But successful and alternative optimization steps can be taken incrementally.

With the aid of hydraulic experiments, high-quality data can be used to train Neural Networks, allowing the behavior on appropriate reef design features to be understood and optimal combinations fine-tuned.

The symbiosis between simulation and experiments is extremely important and will guide our understanding into more complex domains.

Reefs are truly multi-functional structures.

In the next post, we look into how artificial reefs can be used close to beaches… to enhance their surfing conditions

## References

[1] - Liu, Tsung-Lung, and Dong-Taur Su. “Numerical analysis of the influence of reef arrangements on artificial reef flow fields.” Ocean Engineering 74 (2013): 81-89

[2] - Reynolds Number A standard way to characterize flow regimes is with the aid of the Reynolds number $$Re$$.

[3] - Jiao, Li, et al. “Numerical simulation and PIV experimental study of the effect of flow fields around tube artificial reefs.” Ocean Engineering 134 (2017): 96-104

[4] - Reynolds-averaged Navier-Stokes Equations (RANS) In this context, the RANS are used to capture finer, small-scale flow features.

[5] - For the estimates of the coefficients in Fig. 3, a Reynolds number $$Re \approx 10^2$$ was used.

[6] - Woo, Jinho, et al. “Characterizing Korean general artificial reefs by drag coefficients.” Ocean Engineering 82 (2014): 105-114

[7] - Kim, Taeyoon, et al. “Wave attenuation prediction of artificial coral reef using machine-learning integrated with hydraulic experiment.” Ocean Engineering 248 (2022): 110324

[8] - Kim, Taeyoon, et al. “Improved coastal erosion prevention using a hybrid method with an artificial coral reef: Large-scale 3D hydraulic experiment.” Water 12.10 (2020): 2801

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