Intertidal boulder-based wave hindcasting can underestimate wave size: Evidence from Yorkshire, UK

Abstract

Large boulder-size clasts can represent important archives of high energy erosional wave activity at the coast. From tropical coral reefs to high-latitude eroding cliffs, boulders have been used to hindcast the frequency and magnitude (height) of waves produced by both storms and tsunami. Such reconstructions are based on the balance between the hydrodynamic forces acting on individual boulders and the counteracting resistive forces of friction and gravity. We test published models on over 900 intertidal boulders on the coast at Staithes in North Yorkshire, U.K., using repeat high-resolution topographic survey data. We quantified the predicted versus actual rates of boulder movement in the field over a 7-year period (2007–2014). We assessed the degree to which local geomorphology and biology may affect the resistive forces inhibiting transport. We found that 84% of intertidal boulders predicted to be mobile remained stationary over 7 years, suggesting that in some contexts boulder movement may be significantly less than predicted, regardless of boulder volume, shape or location. In situ cementation of boulders to the substrate by marine organisms appears to play a key role in retarding or preventing boulder transport. The implication is that boulder characteristics may not always provide a reliable estimation of wave height on the coast and reliance solely on hindcasting relationships may result in under prediction of the frequency and magnitude of past storm wave activity. We suggest here that more field studies that consider boulder movement are needed, set within specific site contexts, and that these data are used to improve the applicability and robustness of boulder transport models.