Coastal Restoration Trust of New Zealand

Coastal Dune Ecosystem Reference Database

Incident wind approach angle and topographic steering through excavated foredune notches Technical Report

Nguyen, D.; Hilton, M.; Wakes, S.; Simons-Smith, T.
Journal / Source
Artificial foredune ‘notching’ aims to increase sand transport between the beach and backdune environments to enhance biodiversity and increase foredune resilience. Recent research has examined the morphodynamics of dunes in the lee of notches, however the conditions that favour sand transport through the notches, and the associated processes of topographic steering, flow acceleration and deceleration, have not been systematically examined. It is important to examine the relationship between these processes because on most temperate coasts regional winds cross shorelines at oblique angles. This study combines field-based experiments and Computational Fluid Dynamics (CFD) modelling to examine the relationship between the relative incident wind approach angle, topographic steering and flow acceleration through an excavated foredune notch located at St Kilda, New Zealand. Secondary wind speed and direction is recorded in the notch by 12 ultrasonic anemometers and compared with incident wind conditions measured at 6.5 m above the foredune crest. Field data is used to validate the CFD simulations, which is used to examine the flow dynamics at a larger spatial scale, across the beach, as well as through the notch. Wind speeds inside the notch are strongly correlated with incident wind angles ranging from −6° to 77° relative to notch orientation. Wind flow realignment to notch orientation, compression and acceleration through the notch occurs when the relative incident wind angle is less than 27° for a notch oriented 77° to the shoreline. Wind flow separation and deceleration occurs when the relative angle is greater than 27°. An empirical model relating incident wind speed, direction and notch activation (i.e. when sand transport occurs) is developed. The threshold of incident wind speed for notch activation increases when the relative wind angle increases, and vice versa.