Coastalock™ Performance on a Permeable Breakwater Slope

Abstract

Coastal areas face increasing threat from erosion. The use of coastal protection structures is imperative to counter this. Concrete “armouring” is widely used due to durability and cost-effectiveness. Conventional methods of “armouring” result in habitat degradation and loss of biodiversity. ECOncrete developed Coastalock, an armour unit designed to provide coastal protection but also to create marine habitats.

Initial tests conducted on impermeable slopes in deep water conditions revealed, that tightly placed units experienced pressure gradients across the top layer, leading to failure. The aim of this study is to investigate the hydraulic performance of Coastalock, both with and without modification, within permeable breakwater structures. The study examines the influence of toe berms on different surfaces, assessing the armour layer’s susceptibility to sliding.

The research aims to bridge existing knowledge gaps regarding Coastalock behavior under varying wave conditions through literature review and physical model tests conducted in the 2D wave flume at TU Delft.

Structure from motion photogrammetry enabled the creation of 3D models of the armour layer after wave attack, facilitating the tracking of armour layer deformation of selected test series. The research includes measurements of overtopping discharge and reflection coefficient.

The findings shed light on failure mechanisms observed in Coastalock armour layers on permeable core slopes, attributed to built-up pressures exceeding self-weight and interlocking capabilities during wave run-down. 'Breathing' involves upward movement perpendicular to the slope during wave run-down and downward movement during wave run-up. Friction and partial interlocking contribute to the formation of a bulge in the armour layer, growing in size and magnitude, leading to extraction.

Observing increased 'breathing' and extraction thresholds with larger inter-unit void sizes, was confirmed for permeable cores. No stability increase compared to impermeable core was found, attributed to reduced maximum run-down levels. A lowering of overtopping and reflection was found for the permeable core.

The protrusions implementation necessitated a new configuration, termed the 'Protrusion Optimized' configuration, with an orientation change from cavity upwards to downwards at SWL. 10% protrusions reduced the stability number threshold for 'breathing', while 22.5% protrusions prevented filter layer migration and reached up 𝑁𝑠=4.2 without 'breathing' or extraction. A stability increase was found in 𝑠0𝑝=0.02 conditions, attributed to the ‘reservoir effect’. Incorporating protrusions and transitioning to the 'Protrusion Optimized' configuration increased reflection due to increased surface area and reduced permeability.

Changing the orientation location towards the midpoint between SWL and the bottom row or facing all units upwards resulted in increased stability in terms of ‘breathing’ and extraction, as well as a downslope shift of the damage location. Upwards-oriented units led to smoother slopes and higher reflection coefficients, both attributed to the water retaining properties of the cavity.

The presence of toe berms, showed no significant impact on damage progression or the location. Downslope movement well below the threshold indicative of near extraction was observed.

Recommendations for advancing 22.5% protrusions are proposed, advocating a Ns=2.6 for surging waves in deep water conditions. This design offers notable overtopping reduction, orientation flexibility, and reduced concrete usage and project duration.