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Waves

How we deal with backwash

The Coastal Ocean Basin (COB), a consortium of the Universities of Ghent and Leuven, and Flanders Hydraulic Research Institute, have created the scientific ground for the AllWaves reef setup. 

In 2020 we started off with experiments in COB’s wave flume facility. This concrete wave flume is 30m long, 1m wide and 1.2m high, such that a 2D slice of the passive wave absorber system can be investigated. Based on desk research that has been done prior to the tests, a passive wave absorber system was selected. This was subject to further investigation in terms of reflection coefficients and backwash velocities. The experimental set-up, test matrix and analysis results serve as design guidance for the passive wave absorber system in our test surf pool in Knokke-Heist.

It is specifically designed to avoid backwash and wave reflection. This results in soft slopes where incoming waves break and disappear seamlessly. After an AllWaves wave breaks, the water flows back into the pool without disturbing the next perfect, rolling set of waves. Thanks to that, there is no need to interrupt the surfing session to calm down the water. Waves can roll nonstop.

“Generating perfect wave conditions in recreational surf pools depends on the wave generation system and wave transformation phenomena. Backwash results in steep cross waves and confused wave conditions. Our experiments have led to a state-of-the-art solution delivering a generic reef, where more than 90% of the remaining energy is dissipated.”, say Prof. Peter Troch and Dr. Maximilian Streicher, Coastal & Ocean Basin, Ghent University, Belgium.

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Analysing man-made waves in scale models

Belgian surf pioneer Frank Vanleenhove, the founding father of the surfclub “Surfers Paradise” and the sister company “Lakeside Paradise”, evaluated different wave profiles in our 1/10 and 1/20 scale models. He looked at wave heights, shapes, styles and speeds. 

Frank brings in years of surf experience. Through his early-stage involvement, we integrated surfers’ expectations towards an artificial wave pool and imbedded that in our design. As waves of 2 m are in the model 20 cm, the evaluation is a mainly visual check of the different wave profiles. We looked at 48 different profiles considering all level of users with different preferences.

“Most people focus only on the shape of the lip, the shoulder, the pocket and tube of a wave when observing a wave. To be able to know whether a wave carries enough energy to surf on, you need also to focus on the waves’ backside. We want to see a rather flat shoulder with a vast amount of water that pushes the face in a perfect way (see picture). Secondly, we want to see what happens in the area just in front of the face. When water is sucked into the wave, we experience the wave as a qualified wave”, Frank explains.

When energy is not transferred correctly between the wavemaker and the water, trailing waves are created. These are extra waves in between the actual surf waves, which usually represent wasted energy, reduced efficiency and unpleasant surf sensation. “In none of the settings, we observed trailing waves.“, Frank adds. “I can’t wait to go and catch the waves in the full-scale centre. Countdown has started”.

In cooperation with:

With financial support of: