70 D37. Advances in Coastal Sedimentology, Hydrodynamics, and Geomorphology

Monday, 10 October 2022
8:00 AM - 12:00 PM
407

MODELING HIGH-ENERGY WAVES FROM THEIR BOULDERS: MODERN FIELD DATA AND FROUDE NUMBER APPROACH

Paper number: 70-13
Program: Technical Sessions
Time: 10:55 AM – 11:10 AM

Authors

Abstract

There is no explicit method to differentiate the wave characteristics from their boulders. Indices such as boulder mass, inland distribution, deposit characteristics, and Froude number have been used to clarify the wave origin (storm or tsunami) of boulder transport. Among them, the Froude number (Fr) approach has widely been cited and used for the determination of flow depth of boulder transport. Despite field validations, many past studies judged the wave origin for their boulders using the criteria of Fr = 1 for storms and Fr = 2 for tsunamis.

The purpose of this study was to validate the Froude number approach using the boulders transported by a Mediterranean tropical-like cyclone, the Medicane Zorbas (28 September 2018), that impacted the Peninsula Maddalena (south-eastern Sicily). UAV photogrammetric survey, active surveillance camera system, and tracker software were used to assess the wave flow during the movements of eight boulders (short axis length > 0.3 m). The mass of the boulders varied from 0.8 to 41.3 tonnes. The flow depth and flow velocity of the wave flow for the incipient motion of the boulders were extracted. The Froude number of the wave flow for the boulders was calculated using the possible combinations of average, maximum, and minimum flow depth and flow velocity.

The Froude number varied from 0.56 to 1.72 with an average of 1.06. The calculated flow depths of the boulder transport with the tsunami hypothesis (Fr = 2) were significantly lower compared to the measured flow depths. Therefore, the criteria of Fr = 1 for discriminating storm boulders from tsunami boulders was validated in this study. However, past studies reported that the Froude number of tsunamis including the 2004 Indian Ocean tsunami varied from 0.7 to 2.0 in shallow water and on the coast. Therefore, caution is needed when only using the Froude number approach to confirm the wave origin because tsunami boulders can be misinterpreted as storm boulders with the Froude number approach.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-377249

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