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.