A Matter of Hours: Fast Landward Retreat of a Low-Energy Steep Beachface due to the Combined Action of Gravity and Infragravity Waves During Storms.
Abstract
Understanding erosion processes along coastlines is critical for efficient prevention and adaptation solutions for human activities. This is valid for open coasts, but also sheltered environments where the relative contribution of low-energy, infragravity waves is not always known. Along the St. Lawrence Estuary (Canada), yearly shoreline erosion rates can exceed several meters on fetch-limited sandy beaches backing large tidal flats. Such landward retreat tends to accelerate under coastal sea-ice shrinking and therefore a better knowledge of sheltered beach dynamics is necessary to predict future changes. Using the beach of Pointe-Lebel (PTL) as a test case along the Manicouagan Peninsula, the local beach morphodynamics is assessed during two energetic events, with and without coastal sea ice in the nearshore. A combined analysis is conducted involving in situ observations and numerical morphodynamic modelling with a calibrated XBeach model. Both model results and observations are indicating backshore scarping, vertical erosion and the formation of a vertical microcliff. Despite limited incoming wave energy (H0 < 2.5 m) on the tidal flat, the cliff retreated by several meters in only 3 hours during both events. The water level and storm surge are key conditions initializing sediment transport on the beachface under both unfrozen and frozen states, but are not causing direct impacts. Beach erosion is driven by the combined action of gravity waves, inducing sediment remobilization and avalanching on the cliff by collision, and transport by reflected infragravity waves on the steep microcliff. The contribution of infragravity waves is important: highest rate of erosion is concomitant with highest ratio of infragravity over incident wave energy and high ratio of reflection in the infragravity band.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMEP063..06C
- Keywords:
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- 1625 Geomorphology and weathering;
- GLOBAL CHANGE;
- 3020 Littoral processes;
- MARINE GEOLOGY AND GEOPHYSICS;
- 4315 Monitoring;
- forecasting;
- prediction;
- NATURAL HAZARDS;
- 4217 Coastal processes;
- OCEANOGRAPHY: GENERAL