Hard stabilization methods have traditionally been employed to mitigate coastal erosion. Concrete armour is widely used due to its high level of dependence, robustness, ease of production and cost effectiveness (Cooke et al., 2020; Pikey and Cooper, 2012). It is inevitable that coastline ‘armouring’ will continue to rise because of the growing human population and urbanization, desire for and value of coastal property, opposed to predicted climate change (Chapman and Underwood, 2011). The environmental impact of such 'armouring' on coastal systems can be detrimental, resulting in a degradation or destruction of habitats and the loss of ecologically trivial species (Gittman et al., 2015).

The CoastalockTM, a single-layer armour unit, aims to blend coastal protection with marine habitat creation. This armour unit is designed to mimic inter- and sub-tidal habitats, with chemical composition of substrate and micro and macro features that provide niches for various species. The key feature of CoastalockTM is the cavity that is integrated into the design, that caters to diverse marine life needs depending on its orientation (ECOncrete Tech Ltd., 2019). CoastalockTM's hydraulic performance is under research. Preliminary tests conducted in the Hydraulic Engineering Laboratory (HEL) of the Technical University of Delft (TUD) on a 2V:3H impermeable slope in deep water conditions highlighted that with tight placement of the units significant pressure gradients across the top layer led to damage. The introduction of spacings between units for enhanced permeability improved stability significantly (Gutiérrez et al., 2023). A redesign of the unit was proposed incorporating protrusions to enforce the spacings between the blocks (Molenkamp, 2022).

This research focuses on evaluating the influence of a porous core on the hydraulic performance of a CoastalockTM armour layer, specifically assessing its stability, overtopping, and reflection on a 2V:3H breakwater slope in deep water conditions—from the toe to just below the crest. A pivotal aspect of this research is the investigation of the impact of protrusions on the hydraulic performance. Furthermore, the study explores the influence of different toe configurations, aiming to comprehend the vulnerability of the armour layer to sliding. Toe scour falls outside the scope of this study.

Wave transmission at low-crested coastal structures has been studied, based on physical model tests with trapezoidal impermeable, permeable and perforated structures. The differences between wave transmission at impermeable and permeable structures are relatively limited. For a perforated hollow structure with an impermeable vertical screen in the middle, the wave transmission is significantly less than for perforated structures without an impermeable vertical screen; the blocking of the orbital motion by the screen significantly reduces wave transmission. The effectiveness of an impermeable vertical screen to block the orbital motion and consequently reduce wave transmission assists designers of artificial reefs to design structures that reduce wave transmission. Empirical expressions based on a hyperbolic tangent function have been derived to describe the test results. For permeable structures also available data for emerged structures has been used in the analysis, and the newly introduced expression appears to be accurate for both submerged and emerged permeable structures.

ECOncrete®‘s Coastalock is an ecologically designed armour unit, providing an alternative and/or a complement to traditional armour layers with ecologically enhanced armouring that provides shoreline stabilization, while also creating well-defined local ecosystems that mimic natural rock pools. The 2D physical model tests performed at TU Delft focused on the stability, reflection and overtopping of a slope with regularly placed single layer Coastalock armour. The 2V:3H slope had an impermeable core, no wave breaking on the foreshore and no rock toe. The stability was seen to double, with stability number Ns (Ns= Hs/ΔDn50) increasing from roughly 2 to 4 and above, by increasing the porosity between the blocks from spacing the units from 0percent to 25percent. So less concrete use led to more stability. The mean overtopping discharge could be characterized by a roughness factor of gamma_f = 0.610 (for 25percent spacing). A key goal of the Coastalock development is to demonstrate that with the use of innovative eco-engineered armour unit design it is now possible to add ecological considerations into the design process to promote biodiversity and provide ecosystem services, achieving both structural and ecological goals.