3D Finite-Element Numerical Failure Prediction of the E.T. Sea Stack Formation at Hopewell Rocks Provincial Park

Abstract

Sea stacks, natural pillars of rock, are borne from shoreline rock cliffs and eventually collapse by wave action and other erosional processes. While historical instances of sea stack failure have occurred within Hopewell Rocks Provincial Park in New Brunswick, comprehensive assessments of sea stack stability and predictions of future failure are in their early stages. This study aims to forecast the future failure of the E.T. sea stack formation. Photographs taken from an unmanned aerial vehicle of the stack are used to construct a digital 3D geometry model of the formation using Structure-from-Motion photogrammetry. To determine the anticipated failure threshold, 3D finite-element (FEM) numerical models of the stack were developed in RocScience RS3 software. Erosion from wave action during high tides is simulated by progressively reducing the width of the stack within the erosion zone. The erosion rate is estimated, by comparing superimposed historical and current images of the stack, to be 19.19 mm/year perpendicular and 2.26 mm/year parallel to the shoreline. Two erosion types are modelled: symmetrical isotropic and asymmetrical anisotropic. Symmetrical isotropic models imply uniform erosion, while asymmetrical anisotropic models feature two orthogonal erosion rates with directional width reductions from the end of each erosion axis. Stability of the formation in the FEM model is evaluated using maximum shear strain, total displacement, yielded elements, and major principal stress values. The symmetrical isotropic models suggest a probable failure period between 101 and 127 years after 2021. The asymmetrical anisotropic models, however, indicate that failure could occur approximately 40 years earlier. Overall, failure of the stack experiencing asymmetrical anisotropic erosion seems to occur when the supporting pillar is about to erode under the stack’s centroid. With further validation, through additional modelling, the centroid of sea stacks has the potential to be used as a guideline for predicting sea stack failure.