Incident wind direction and topographic steering through foredune notches

Authors: Duc Nguyen*, School of Geography, University of Otago, New Zealand, Mike Hilton, School of Geography, University of Otago, New Zealand, Sarah Wakes, Department of Mathematics and Statistics, University of Otago, New Zealand, Tom Simons-Smith, Dunedin City Council
Topics: Coastal and Marine, Geomorphology
Keywords: foredunes, constructed notches, aeolian processes, topographic steering
Session Type: Virtual Paper
Day: 4/9/2021
Start / End Time: 1:30 PM / 2:45 PM
Room: Virtual 45
Presentation File: Download

Constructing foredune notches has been undertaken in some countries to modify the foredune morphology and dynamics. Notches are intended to increase the flux of wind and sand from the beach to backdune environment. The nature of aeolian processes of notched foredunes has been barely reported. Our research examines the changes of wind flow dynamic after constructing foredune notches, specifically the sensitivity of notch orientation to incident wind conditions.
The study site is notch C (orientation = 190 degrees where 90 degrees = shore-parallel) at St. Kilda Beach, Dunedin, New Zealand. Flow is characterised by field experiments and Computational Fluid Dynamics (CFD). 12 ultrasonic anemometers are placed along the notch long and cross axes at 0.2m, 0.46m, 1.05m and 2.51m heights. The simulated CFD flow is validated with field observation.
The results show that constructing notches provide significantly flow steering over the foredune. When incident wind direction changes from 206 to 265 degrees, flow is steered toward notch orientation. Flow is accelerated across the surface of the depositional lobe downwind of the notch. Wind speeds measured at notch centre and the lobe decrease. Wind direction at the lobe shifts from notch orientation to alongshore. When incident wind direction is 265–281 degrees, flow steering occurs but wind decelerates up to the lobe. When incident wind direction is greater than 281 degrees, flow at notch centre is complex. Topographic steering is observed at up to 1.05m height. These findings will have significance for coastal managers implementing foredune notching as a dune management strategy.

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