Observations of aeolian activity on the deflation floor of a foredune trough blowout

Blowouts on coastal foredunes provide efficient pathways for wind-driven sand transport inland. In this way blowouts affect beach-dune sand budgets, facilitate landward dune migration under sea-level rise and stimulate the dunes’ biodiversity by maintaining a heterogeneous landscape.  While our understanding of the dynamics of blowouts on time scales of months to decades is advancing due to the increasing availability of remote sensing data sets, we have limited empirical data and hence understanding of the short-term (hours) aeolian activity in blowouts and how this activity builds up to longer term blowout dynamics.

This contribution documents the first results of a study designed to cover multiple scales of aeolian activity (hours to months) in the foredune trough-blowout system of the Dutch National Park Zuid-Kennemerland. The data used here consist of (i) photographs of one of the deflation basins taken by a time-lapse trap camera every two hours during the day, (ii) wind speeds and directions (at 1-m height) measured by 4 continuously operating ultrasonic anemometers positioned from the seaward side of the same basin to the adjoining depositional lobe, and (iii) wind data (at 10-m height) from a nearby offshore meteorological station. The 120-m long instrumented basin has an approximately linearly upward sloping floor with steep lateral walls and is about 100 m wide at its mouth, reducing to about 20 m at the start of the depositional lobe. Good-quality images were manually classified into five categories with increasing aeolian activity from no transport (class 0), isolated streamers on a small portion of the deflation floor (class 2) to intense streamer activity everywhere (class 4). The first results, based on images collected in fall 2022, indicate that streamer activity (class 2 or higher) is restricted to moments when the wind speed at the blowout mouth exceeds about 5 to 6 m/s and the offshore wind approach direction is within 40 to 50 degrees of the blowout axis. For these directions the wind is steered into the blowout and, depending on the approach angle, accelerates over the deflation floor by up to 50%. Aeolian activity on the deflation floor is absent or very small (class 0 or 1) for larger approach angles (even at offshore wind speeds of 10 – 15 m/s) because the wind is no longer steered into the blowout. These first results thus indicate the importance of the wind speed on the deflation floor and the offshore wind approach angle (relative to the blowout axis) for short-term aeolian activity in the blowout.