Hydrodynamic Equations for Coastal Boulder Movement: Reflections on a Recent Review

Abstract

Hydrodynamic equations are frequently employed in the study of coastal boulder movement to estimate the height of tsunami or storm waves responsible for initiating dislodgement. A recent review has challenged the use of such equations and, through a test using a data set of boulders that were moved during storms over winter 2013–14 in the Aran Islands (Ireland), concludes that the equations are flawed. To evaluate this claim, this study revisits the foundation of the equations and reflects on the recent review. We conclude that, although the review is timely and welcomed for the questions it prompts, it arguably does not provide grounds to pronounce the equations flawed. This is due to various considerations, including (1) the equations have been misapplied to physical settings other than as originally intended, (2) wave-type parameters for estimating tsunami and storm wave heights from boulder measurements appear reasonable and not flawed when used within the intended physical settings, (3) the Aran Islands may not be an appropriate location to test the equations because of their physical setting, (4) the exclusion of storm surge and wave setup effects in the test is likely to underestimate the calculated height attained by storm waves, and (5) for comparison, recently revised equations are used to recalculate the Aran Islands boulder data set, indicating that all but 33 boulders demonstrably moved by the 2013–14 storms may be explained by the maximum storm wave height proposed by the review and that all these moved boulders might be explained if the effects of storm surge are included. Analytical tools, such as provided by hydrodynamic equations for boulder dislodgement, are considered important in the contribution they make to coastal risk assessment and hazard management.