Coastal boulder deposit research three decades on: advances and outstanding questions

Coastal boulder deposits (CBD) are supratidal accumulations that can include clasts many metres in diameter. Emplaced by extreme waves (whether storms or tsunami) on high-energy rocky coasts, they take many forms, including isolated blocks, small clusters, and well organized coast-parallel ridges of imbricated clasts. CBD occur in many topographic settings, from low-elevation shore platforms to cliffs as tall as 50 m. CBD provide unique sedimentary archives that preserve a record of past high-energy wave events; but they remain poorly understood.

The study of CBD is very young. They were not on the research radar until work on Australian occurrences in the early 1990s (primarily by Jonathan Nott and Edward Bryant) asked whether these unusual deposits with their anomalously large clasts were due to extreme storm waves or tsunami. Their papers kickstarted wider interest, but progress was sluggish until the twin advances of accessible high-resolution satellite imagery and drone-based photogrammetry made it possible to perform longitudinal analyses of these slow-evolving features. The number of studies grew dramatically, from a few per decade in the 1990s to many per year in the past 15 years. But the overall number of studies is still low: about 10-30 new studies annually in recent years, totaling about 230 published papers since 2010. For comparison, there were almost 1500 papers on sandy beaches/coastal dune systems published last year alone.

The most significant breakthrough of the past three decades came from before-and-after studies of changes wrought by coastal storms, which revealed unanticipated power and bore-like uprush behaviour of coastal storm waves, and documented transport of boulders previously thought unmovable except by tsunami. This new knowledge forced a re-examination of the physics and hydrodynamics of wind-driven waves. Numerical modelling and wave tank experiments provided increased understanding of complex non-linear processes that can drive wave amplification in steep near-shore environments, helping explain how storm waves can impart forces necessary for transport of enormous boulders. Given that many CBD were originally interpreted to be of tsunami origin (based on clast size and inferred lack of storm wave power), these insights render many coastal boulder deposits open to re-analysis, particularly in areas prone to both tsunami and strong coastal storms.

But the most fundamental questions remain unanswered. For example, although several attempts have been made to provide an analytic relationship between boulder size and the characteristics of emplacing waves (e.g. wave height, bore velocity), the precision and predictive value of existing equations is limited at best. Therefore, increased experimental and theoretical analysis of near-shore hydrodynamics and breaking wave behaviour is a critical need. Furthermore, it remains impossible to definitively conclude whether coastal boulder deposits have been deposited by storm waves or tsunami in many cases, unless there are historical records or before-and-after observations, and more field exploration and measurement are required.

The bottom line is that after three decades of CBD research, we are still far from answering fundamental questions about these fascinating and enigmatic deposits. Many coastal boulder deposits remain undocumented, and there is much work to be done!