WAVE TANK INVESTIGATIONS OF LARGE-BOULDER TRANSPORT BY STORM WAVES

Coastal boulder deposits (CBD) accumulate above the high tide line, emplaced by high-energy wave events. They include boulders >600t close to sea level, 100t boulders up to about 12m above high water, and small (≈1-3t) boulders up to 51m above the high water mark. How these boulders are transported, however, remains an open question.

We used a 1:100 Froude-scaled model in a laboratory wave tank to test whether storm waves could transport such boulders. The tank was configured to model a characteristic CBD setting, with ramp-and-flat bathymetry offshore, and a cliff-top platform 10m above sea level. Using a JONSWAP spectrum to simulate realistic storm conditions, we examined the interaction of individual waves with large boulders (441-1075t scaled equivalent) situated on the platform. We ran multiple tests using identical sea states while varying boulder size and configuration. Wave gauges at several locations in the tank recorded water surface elevation, and video footage captured wave parameters and wave-boulder interactions that are difficult to measure at full scale.

We found the majority of boulder displacements were caused by a small subset of the incident waves. The keys to boulder transport were pre-impact wave height greater than the cliff, and development of a high velocity bore. Interestingly, the largest waves were not necessarily the most effective: instead, wave-front steepness just before cliff impact seems to determine whether a strong cross-platform flow will develop. Waves that approached the cliff unbroken, with a front slope angle in the range 15°-25°, generated the most powerful bores. These waves moved boulders with masses in excess of those predicted by existing hydrodynamic equations.

This demonstrates the equations that are often used to analyze coastal boulder deposits do not accurately hind-cast the wave conditions required for clast transport. Initiation of motion likely relates to a combination of incoming wave shape, resulting bore velocity, and boulder shape and size, as well as other variables such as where the boulder lies relative to the platform edge. This study demonstrates that storm waves have significant potential to transport large boulders and identifies characteristics that make determine which kinds of waves will be effective.