Lee Hau Yee*, Anbarasu Elavenil
School of Science and Technology
*Email:
Abstract
Coastal dams are crucial in preventing wave overtopping and mitigating coastal flooding. The three commonly used coastal dam structures are vertical seawalls, curved seawalls, and riprap dams. Along with these, we designed three more dam models (triangular, zigzag, and inverted triangle) for this investigation, possessing characteristics similar to the aforementioned commonly used dams. Using physical models, past papers have looked into how the overtopping of dam-break structures affects the inundation height behind the structures. Through this research, we used such physical models to understand how different dam designs affect seawater overtopping by either creating in-phase waves or redirecting wave impact and diffusing wave energy to create antiphase waves. This research is highly relevant in our current times, as climate change has exacerbated extreme weather conditions such as ice cap melting and rising sea levels, which lead to overflowing seawater over coastal dams, causing flooding and soil erosion. We used compressed foam models to simulate wave impacts in a wave tank across 6 different wave frequencies to analyse which dam designs are most suitable in preventing the overtopping of seawater at coastal areas through the smallest value of the maximum vertical displacements of water, concluding that the inverted triangle dam is the most effective in reducing overtopping due to its narrow bottom lacking outward protrusion and redirection of wave energy downwards, followed by the zigzag dam, vertical seawall, riprap dam and curved seawall. The triangular dam, which was used as the control, was the least effective model due to the upward reflection of wave energy.
Introduction
Coastal erosion is one of the many issues brought on by the onslaught of climate change. Climate change has caused an increase in extreme weather events, making storms more violent and aggressive. Coastal erosion is not just a distant concern for scientists or governments—it’s an issue reshaping our shorelines, affecting all of us. In Hemsby, England, coastal erosion has progressed to such an extent that residents had to be evacuated as houses were about to collapse into the sea. According to, erosion causes about USD 500 million annually in coastal property losses, damages and loss of land. Coastal dams are a crucial piece of the puzzle in combating coastal erosion.
Some common coastal dams include vertical seawalls, curved seawalls and riprap dams. Vertical seawalls are commonly employed in coastal defence due to their ability to withstand high-energy wave impacts. Curved seawalls are designed to redirect wave energy away from the structure and reduce overtopping. Riprap dams, consisting of loose stones or boulders, serve as a flexible coastal defence mechanism, as well as providing a more natural coastal appearance. However, the effectiveness of the curved seawall and riprap dams is highly dependent on factors like size and local wave climate, and may not be as effective as anticipated under certain circumstances.
A possible cause of overtopping is the constructive interference of coastal waves. When coastal waves hit the dam, the reflected wave may be in phase with the coastal waves, causing the waves to be amplified and overtop the dams. Thus, it is critical to understand how waves are reflected.
Experimental design
To further investigate specific design considerations that affect overtopping, we will also be experimenting with three other models: triangular, inverted triangle and zigzag-shaped dams as shown in Fig. 1. By carving the six dam models out of compressed foam, we measured the maximum vertical displacement of water, by taking a video of the side profile and measuring the greatest height the water reached, for six different wave frequencies in the experimental setup as shown in Fig. 2. The glass panel moves back and forth at 6 different speeds, generating different frequencies of waves, which have a different amount of energy. Hence, we can analyse the effectiveness of each dam type and predict the amount of overtopping.
We hypothesized that curved seawall dams are the most effective in reducing overtopping, followed by inverted triangle dams, zigzag dams, riprap dams, vertical seawall dams and lastly, triangular dams.
Results and discussion
The trend lines for the 6 different dam types indicate that the inverted dam is the most effective in reducing overtopping, followed by the zigzag dam, vertical seawall, riprap dam and curved seawall. The triangular dam, which was used as the control, was the least effective model.
From the data collected, the relationship between the maximum vertical displacement of water and the frequency of the wave is linear for vertical and inverted triangle dams. This may be due to these dams not reflecting much wave impact, absorbing wave impact instead, reducing the creation of in-phase waves, indicating the effectiveness of these dam types. Next, we compared the curved and inverted triangle dams. We found that the curved seawall has a significantly higher maximum vertical displacement of water than the inverted triangle dam. This could be due to the bottom of the curved seawall reflecting wave energy upwards, suggesting that the curved shape of the curved seawall is not ideal in reducing wave impact. The riprap dam model caused more maximum vertical displacement of water than the zigzag dam. The ineffectiveness of the riprap dam may indicate a limit to the amount of energy the rocks can dissipate. The remaining wave energy would have been reflected upwards by the general shape of the dam, increasing the maximum vertical displacement of water. Meanwhile, each triangular piece in the zigzag dam pushes the water downwards. This may suggest that reflecting water downwards is more effective than dispersing wave energy.
Conclusion
In conclusion, inverted triangle, zigzag and vertical seawall dams effectively reduce overtopping, with inverted triangle dams being the most effective. This suggests an overarching parapet is effective in blocking water from overtopping. Furthermore, as the triangular dam was designed to mimic coastal regions, and is the least effective in reducing overtopping, it highlights the importance of coastal dams in protecting coastal areas from flooding and erosion. Unexpectedly, the commonly used curved seawall and riprap dams may have certain limitations, especially with high-frequency waves. To reduce overtopping in real life, a parapet can be attached to existing dams to redirect the water downwards, as shown in Fig. 4. We hope that our findings on the relationship between the shape of coastal dams and the overtopping will help contribute to the development of better dam designs that can withstand the more volatile coastal conditions climate change brings about and reduce flooding and coastal erosion.
An area of further study can be to learn more about how the different designs of riprap and semi-permeable dams affect energy absorption, while also preserving the natural ecosystem. Lastly, we can also delve deeper into how high and low frequencies affect seawater overtopping.
References
Briganti, R., Musumeci, R. E., Van der Meer, J., Romano, A., Stancanelli, L. M., Kudella, M., … & Schimmels, S. (2022). Wave overtopping at near-vertical seawalls: Influence of foreshore evolution during storms. Ocean Engineering, 261, 112024. https://doi.org/10.1016/j.oceaneng.2022.112024
Chikomba, T. (2023, July). Coastal erosion causing homes in England to slide into the sea. GB News. Retrieved January 8, 2025, from https://www.gbnews.com/news/coastal-erosion-homes-sea-norfolk
Comfort, J. A., & Single, M. B. (1997). Literature review on the effects of seawalls on beaches. Department of Conservation, Wellington, New Zealand.
Kerpen, N. B., & Schlurmann, T. (2016, May). Stepped revetments – revisited. In Proceedings of the 6th International Conference on the Application of Physical Modelling in Coastal and Port Engineering and Science (Coastlab16) (pp. 1–6). Ottawa, Canada.
U.S. Climate Resilience Toolkit. (n.d.). Coastal erosion. U.S. Climate Resilience Toolkit. Retrieved January 8, 2025, from https://toolkit.climate.gov/topics/coastal-flood-risk/coastal-erosion
