The Impact of Ammophila arenaria Foredune Development on Downwind Aerodynamics and Parabolic Dune Development

HART, AT.; HILTON, M.J.; WAKES, S.J., and DICKINSON, K.J.M., 2012. The impact of Ammophila arenaria foredune development on downwind aerodynamics and parabolic dune development. Journal of Coastal Research, 28(1), 112-122. West Palm Beach (Florida), ISSN 0749-0208. Ammophila arenaria (marram grass) is highly invasive in temperate, southern hemisphere dune systems. Ammophila arenaria is known to form relatively large, uniform foredunes. However, the impact of A. arenaria invasion on adjacent transgressive dune systems is relatively poorly understood. This study (1) documents foredune and parabolic dune development and A. arenaria invasion at Mason Bay, New Zealand and (2) investigates the impact of A. arenaria foredune development on aerodynamic flow patterns and resultant parabolic dune sedimentary dynamics. Over a period of 40 years, A. arenaria invasion in Mason Bay transformed foredune morphology from an irregular and hummocky morphology to a continuous, densely vegetated foredune complex up to 11 m high. An incipient parabolic dune, initiated from a large blowout, was present prior to A. arenaria invasion. Parabolic dune development occurred through downwind migration of the depositional lobe at an average of 24 m y(-1) between 1958 and 1978. Subsequent parabolic development occurred through decreased depositional-lobe migration (0.79 m y(-1) between 1989 and 2002) and deflation surface enlargement as A. arenaria increased in extent and density across the dune. Significant vertical accretion and increased stability of the foredune occurred during this period. Computational fluid dynamics was used to model flow over foredune topographies associated with the native sand binder Desmoschoenus spiralis and the exotic A. arenaria. Ammophila arenaria foredune development has significantly altered the aerodynamic and sedimentary dynamics of the parabolic dune. During onshore SW flows, higher sheltering was modelled in the lee of the A. arenaria foredune with velocities reduced to between 60% and 70% of ambient across the deflation surface. Flow recovery downwind of the A. arenaria high foredune was 81%, compared to 94% for the D. spiralis low foredune with flow influenced up to 40-45 times the A. arenaria foredune height. Reduced flow velocities, increased foredune stability, and stabilisation of the parabolic dune have resulted in a transition from a highly mobile to a highly stable dune.