MAKING TSUNAMI IN A WAVE TANK: EXPERIMENTS ON BOULDER TRANSPORT

Coastal Boulder Deposits (CBD) occur worldwide. They include impressively large clasts emplaced above high tide and sometimes at long distances inland. In many instances it can be difficult to determine whether CBD were emplaced by storm waves or tsunami. In the past it was thought that very large clasts were a signature of tsunami events, but recent work has shown that storm waves are capable of creating and moving megagravel, so it is necessary to look more closely at CBD sedimentology to determine the dynamics of tsunami versus storm deposition. Boulder ridges, i.e. CBD where clasts are arranged in long linear coast-parallel accumulations, are a case in point. Boulder ridges are highly organised, They have a steep ocean-facing side made up of imbricated clasts, piled up to a few metres high. The boulders are closely packed on top of one another, several clasts deep. The landward side is more gently sloping, usually tailing off into a scattered boulder field. Some workers argue that boulder ridges can be created only by storm waves, whereas others propose that they are formed by tsunami.

To address this issue, we carried out experiments at Tohoku University to test whether a tsunami-like bore can build organised clast ridges. Within a wave tank, we built a cliffed topography with a flat subaerial platform, similar to the setting of CBD in the eastern Atlantic region. We distributed clasts at different locations in the tank. We used a dam-break set up to create the bores, and ran a series of experiments with differing water depths and topographic configurations. We used Froude scaling parameters, with clasts and topography scaled at 1:100. Experiments were recorded using high-speed video. We found that the final disposition of clasts was not a simple function of bore velocity or depth, and that topographic variations strongly affect the bore-clast interaction. In particular, inland topographic barriers cause strong backwash that can affect clast deposition. Although clasts showed a tendency towards clustering under some conditions, there were never more than a few contiguous clasts, and at most one or two stacked clasts. Generally the deposits were dominated by isolated clasts spread widely over the surface. Where clusters occurred, they were randomly interspersed. No linear deposits or ridges were formed.