Patterns and controls of topographic change within the deflation basins of a trough and bowl coastal blowout

In North-West Europe and throughout the world, dune systems are increasingly stabilised by vegetation, due to both human intervention and changes in climate. Blowouts are erosional hollows produced by wind erosion in vegetated or semi-vegetated dune systems. Where active blowouts are present in vegetated dunes, they provide a source of dynamism and sediment in an otherwise fixed environment. The transition from a fixed vegetated dune to an active blowout is poorly understood, however, and anthropogenic attempts to reactivate areas of bare sand in dunes are often unsuccessful. In this study, we measured topographic change at a monthly resolution in the deflation basins of one bowl and one trough blowout over a 23-month period in Ainsdale Sand Dunes National Nature Reserve, North-West England. Our results show that monthly surface change in the blowouts did not correlate strongly with the transport capacity of winds measured at a meteorological station 10 km south of the site. Precipitation was found to have a moderate negative correlation with all indices of surface change, that is, the more it rained the less the surface changed. Inter-annual (23 months) patterns of topographic change in the bowl and trough blowouts were distinctly different. In the deflation basin of the bowl blowout erosion predominantly took place on the erosional walls facing into the prevailing winds, while minimal change occurred on the erosional walls facing away from the prevailing wind direction. This produced a moderate negative correlation between surface change and slope, that is, the steeper the slope, the more erosion occurred. In the trough blowout, erosion took place in the centre of the deflation basin and minimal change was measured on the easternmost erosional wall, which faced the prevailing winds. Patterns of monthly topographic change were highly variable and demonstrated that changes in the direction of above-threshold winds can cause blowout walls and floors to ‘flip’ from erosional to depositional surfaces. These findings highlight the variability and complexity of surface change in blowouts and demonstrate that patterns vary because of landform morphology and climatic conditions.