Understanding overtopping flow thickness and velocity over coastal defences (part 2): How Much Overtopping Water Is Too Much for a Person?

Picture this: you're out for a walk on a breakwater or a coastal promenade. The wind’s picking up, the sea’s a bit rough, and suddenly—splash!—a wave rolls over the top. Not exactly dangerous, just part of the charm of being near the sea… right?

But now imagine it’s not just a splash. The water keeps coming, faster and thicker. You try to stay upright, but the ground gets slippery, the flow gets stronger, and you wonder—at what point would it actually knock you over?

That’s the question a group of coastal engineers led by Van der Meer and Bruce set out to answer. Specifically, they asked: how much wave overtopping can a person tolerate while remaining safely standing?

This matters more than ever. As sea levels rise and storms become more intense, it's not always feasible—or affordable—to design coastal defences that block every last drop. Some overtopping may be acceptable, especially on wide-crested structures like promenades or rock-armoured breakwaters. But we need to be sure that allowing a little water doesn’t turn a pleasant seaside walk into a safety hazard.

Now, here’s the brilliant bit: the researchers didn’t just run simulations. They used real people, real waves, and real overtopping flows.

Back in 1992, the team conducted full-scale tests in the Delta Flume in the Netherlands. Two volunteers were strapped into safety harnesses and stood on a grass-covered dike while controlled waves overtopped it. At a flow of around 10 litres per second per metre, they stood their ground easily for over an hour. But when the discharge increased to about 25 l/s·m, they were swept off their feet—briefly, but conclusively.

Fast forward to 2020. A more modern version of the experiment was carried out using the Wave Overtopping Simulator on a real dike with a 1:4 slope. This time, the measurements were far more detailed. High-speed cameras, paddle-wheel sensors and even drones were used to track how fast and how thick the water flowed during each simulated wave event. A volunteer was positioned either on the horizontal crest or partway down the slope, standing in the path of the incoming water.

So, what did they find?

It turns out it’s not simply the amount of water that matters—it’s the combination of velocity and thickness. When the water rushes in quickly and with enough depth, your boots lose grip. You don't fall over like a tree being pushed. You slide, like someone hitting black ice. The key failure mechanism is slipping, not tipping.

The team then analysed the data and proposed a practical guideline. It’s not overly complicated, and it gives coastal designers a clear threshold for how much overtopping flow can be considered safe for pedestrians.

📐 Practical Design Criteria

Condition	Flow velocity (v)	Flow thickness (h)
Safe on horizontal surface (crest)	≤ 4 m/s	≤ 0.20 m
Safe on slope (1:4 gradient)	≤ 4 m/s	≤ 0.12 m

They also suggested a simple empirical rule of thumb to help assess safety under varying conditions:

$$ h = 0.34 - 0.036 \cdot v $$

So, for example, if overtopping water arrives at 4 m/s, the thickness should be no more than 20 cm. If it’s coming in at 7 m/s, it should be kept under 10 cm. Conditions above this curve are likely to cause instability for the average person.

A few other insights came out of the work. First, there’s no need to adjust the flow thickness for air content—although the water often contains 30–50% air, the effects on density and volume cancel each other out when it comes to drag. Second, it’s not the average flow that matters most, but the peaks. Even if the overtopping lasts only one second, that can be enough to cause a fall. That’s why the design should focus on peak flow events, not just mean discharge.

And most importantly, this study confirms that pedestrian safety during wave overtopping can be quantified with real data—not just assumptions. By using full-scale tests with actual people and carefully measured flow characteristics, the team has provided a solid foundation for setting safety thresholds in coastal engineering projects.

Next time you’re designing a sea wall or a breakwater promenade and someone says, “It’s fine if a little water gets over the top,” you’ll know exactly how to test whether that’s really safe—or if someone’s going to end up sliding across the walkway in wet trainers.

If you're working on a coastal project and want to factor in safe overtopping limits for people, feel free to reach out. These findings could be a game-changer for balancing cost, climate resilience, and public safety.

See ya! César

Reference

Van der Meer, J.W., Steendam, G.J., Bruce, T., & Klein Breteler, M. (2022). Admissible post-wave overtopping flow for persons on a horizontal surface. Journal of Coastal and Hydraulic Structures, 2, 1–22.
https://doi.org/10.48438/jchs.2022.0015