Post‐disturbance evolution of a prograded foredune barrier during a sustained dynamic restoration project—the role of wind speed, wind direction and vegetation

Abstract

AbstractDisturbed (de‐vegetated) coastal dunes can follow divergent evolutionary pathways, evolving to one of a range of possible morphologies in response to spatial and temporal variations in windiness (i.e. the speed and direction of competent winds) or vegetation cover (i.e. the magnitude and rate of vegetation loss). Here we examine the influence of these factors on the evolution of two adjacent barriers during a sustained dune restoration programme, at Doughboy Bay, Aotearoa/New Zealand. We document a shift from a stable prograded foredune barrier to a mobile dune state over a 15‐year period. The spatial and temporal patterns of sedimentation across the barriers reflect alongshore variations in incident wind direction. Changes in vegetation resulted from an alongshore gradient in wind speed, with little decline in vegetation cover recorded towards the more sheltered sections of the dunes and almost total loss of vegetation towards the centre of the bay. Subsequent changes in vegetation cover continued to reflect these gradients in wind speed and direction, with an increase in vegetation cover associated with relatively sheltered sections of coast and a continued decline or no change in vegetation cover across the more exposed parts of the barriers. Finally, localized sediment accumulation across both barriers is strongly associated with the presence of dune‐building species. This case demonstrates the influence of wind speed, wind direction and vegetation on post‐disturbance foredune morphologies and barrier development. We propose a three‐stage conceptual model of barrier evolution consistent with existing biogeomorphic disturbance models: (1) an initial ‘reaction’ phase, characterized by high vegetation cover and relatively little topographic change; (2) an ‘activation’ phase, characterized by low vegetation cover and rapid rates of topographic change; and (3) a ‘stabilization’ phase, characterized by increasing vegetation cover and topographic stability. Dune restoration interventions of this type would clearly benefit from an analysis of incident wind conditions prior to commencement and careful consideration of where and how to reintroduce native sand‐binding species.