Imbricated Coastal Boulder Deposits are Formed by Storm Waves, and Can Preserve a Long-Term Storminess Record

Abstract

Coastal boulder deposits (CBD) are archives of extreme wave events. They are emplaced well above high tide, and may include megagravel clasts weighing tens or even hundreds of tonnes. But do they represent storms or tsunami? Many are interpreted as tsunami deposits based simply on clast size and inferences about transport, despite the fact that there are no direct observations documenting formation of these inbricated boulder clusters and ridges. In this study, we use force-balanced, dynamically scaled wave-tank experiments to model storm wave interactions with boulders, and show that storm waves can produce all the features of imbricated CBD. This means that CBD, even when containing megagravel, cannot be used as de facto tsunami indicators. On the contrary, CBD should be evaluated for inclusion in long-term storminess analysis.