The influence of backdune morphology on air flow dynamics through an excavated foredune notch

Abstract

On many metropolitan and developed coasts foredunes are narrow, vegetated, highly stable and confined by hinterland development. Such foredunes are most likely to erode, rather than landward migration, in response to ongoing eustatic sea-level rise. Foredune notching may be undertaken on such coasts to facilitate sand transport through the foredune zone and accomplish degrees of foredune landward migration; however, the efficacy of this method has not been examined in relation to the backdune topography, which in many instances takes the form of a dyke or similar infrastructure.

Computational Fluid Dynamics (CFD) is used to investigate how the space behind a notch, and the slope of the seaward face of the backdune topography, modifies near-surface wind through foredune notches. Incident winds are simulated parallel to the notch long axis and the effects of changing backdune morphology on the secondary winds through the notch are examined. Swale widths between 3 and 53 m and hinterland gradients between 0° and 90° are examined.

Air flow through the notch is strongly influenced by the morphology of backdune infrastructure. Wind speed increases through the notch as the spacing behind the notch increases and the slope of the hinterland topography decreases. An increase in spacing reduces the landward extension of wind recirculation in the lee of the notch. To maximise notch efficiency and sediment accumulation in the lee of the foredune the minimum spacing should be 8 and 30 m when the slope of the backdune infrastructure is 20° and 90°, respectively.