GLOBAL SIGNATURES OF EXTREME WAVE EVENTS: INVESTIGATING THE MASS-TOPOGRAPHY RELATIONSHIPS OF COASTAL BOULDER DEPOSITS

Coastal boulder deposits (CBD) are emplaced above high water (AHW) during extreme wave events, and occur worldwide on high-energy rocky coasts. In many cases it is unclear whether storms or tsunami were responsible for boulder dislodgement and transport. CBD are sometimes interpreted as tsunami-related based mainly on the large size of included boulders, the argument being that storm waves may be incapable of entraining such masses, but recent observations show that storms can move megagravel >600t near sea level, and even at distances >100m inland and 15m AHW, boulders >10t have been created and moved by storm waves.

Can data patterns from verified storm deposits help us understand depositional mechanics in locations where CBD emplacement mechanisms are unclear? To address this question, we compiled a database of CBD measurements., using published studies in which authors reported not only boulder masses but also the elevation and distance inland at which the boulders lie. By amalgamating data from many locations, we achieve a broad spread relating boulder masses to the widest possible range of elevations, distances inland, and coastal steepnesses.

Our reference set includes recent observations from western Ireland and the Philippines, which show that storm waves shifted megagravel in the 200-600t range at near-sea-level locations, that boulders in the 10-50t range were created and transported at distances up to 200 m inland, and that clasts of order 1t can be moved at elevations more than 40 m AHW. These amalgamated data define a parameter space for storm-wave deposition of boulders.

Many CBD interpreted as tsunamigenic fall within this space, indicating that their masses and topographic locations may in fact be consistent with storm-wave transport, even in cases where application of wave-transport equations suggested that was unlikely. These results reinforce the growing consensus that large coastal boulders are not necessarily a priori tsunami signatures, and that storm waves interacting with coasts are more powerful than hydrodynamic equations predict. Defining a mass-topography parameter space for storm-wave clast transport provides a tool that can be applied in any location as a first-order test of whether a storm origin for CBD should be considered as a realistic interpretation.