A MONTE-CARLO-TYPE SIMULATION FOR EXPLORING THE INITIATION OF MOTION CRITERIA FOR BOULDER TRANSPORT BY STORM WAVES

The ability of storm waves to move very large boulders has been documented in recent years. Here, we apply a Monte-Carlo-type model for boulder dislodgement. To account for uncertainties about both the hydrodynamic characteristics at the time of dislodgement and the pre-transport topographic setting of the boulders, the model treats various parameters (e.g., drag and lift coefficients, roughness, and slope) as random variables. This allows us to test a full range of possible conditions and determine initiation of motion criteria. This Monte-Carlo type model acknowledges the fact that we can observe the result of boulder transport, while the initiation of boulder transport is pivotal to understanding the hydrodynamic forces that cause transport.

The input dataset consists of 1,153 boulders that were transported by storms during the winter of 2013-2014 in western Ireland. The boulders range in mass from <1 tonne to 620 tonnes and were observed to move at a range of elevations (0-26 m above sea level) and up to 220 m inland. We know the dimensions, masses, and post-transport locations of all boulders, and we know the season in which they moved, but we do not know the specific wave events that transported them. Therefore. we simulate dislodgement for every boulder 20,000 times, with drag and lift coefficients and roughness set by the algorithm as random variables, which allows us to identify the flow conditions that result in dislodgement for each individual boulder mass. In total, we simulate a Monte-Carlo set of 20,000 realizations for each of the 1,153 boulders in the dataset.

From the dataset generated, we characterize populations with various characteristics(such as mass, location, and distance to the shoreline), and we compare the simulated flow velocities required for transport with hindcast wave conditions for the storm events during the 2013-2014 season when the boulders underwent transport.

This large field-measured boulder dataset, in combination with the Monte-Carlo-type boulder dislodgement model, provides a unique opportunity to link actual boulder-derived flow conditions required for initiation of motion to the offshore or nearshore characteristics of the causative events.