Aeolian ramp morphodynamics during storm events

Aeolian ramp, i.e. the area between upper beach and the seaward foredune toe, serves as a transit zone between the source area (beach) and the foredune. Its morphology greatly impacts sand supply to the foredune and is therefore one of the key factors responsible for the foredune growth dynamics. The aim of the study was to analyse the change of aeolian ramp topography during extreme events manifested by strong onshore winds. The research was made on the Łeba Barrier, south Baltic Sea coast, Poland in winter 2022. Two experimental sites, different in foredune size and coastline orientation, were chosen. Storm periods were determined on the basis of 10-minute averages of wind speed and direction obtained from a 10-m-high reference mast located near the experimental sites on the established foredune and hourly data from 2 mareographs located approx. 15 km west and east of these sites.

To analyse changes in near surface wind speed at different directions of wind attack a set of cap and sonic anemometers was arranged along shore-normal transect. The measurement sites within aeolian ramp were placed every 2 m. Based on 1-minute averages and the data from the reference mast, the speed-up factor was calculated for 10-degree wind direction sectors which were determined based on wind directions from the reference mast. 3D TLS measurements allowed for the development of the high-resolution Digital Elevation Model of each of the experimental sites. Several short term changes in aeolian ramp topography during storm events were measured by means of GPS-RTK supported by measurements of 200 erosion pins.

Comparison of sediment budget with wind speed and direction for 6 storm events showed 3 types of aeolian ramp morphodynamics. During strong alongshore winds, due to higher wind speed in the upper ramp than in its lower part, the greatest deflation occurred in the upper ramp. During oblique onshore storm winds, air flow deceleration in the upper ramp contributed to the slight deposition of sand. In both cases the sediment budget in the middle and lower ramp was small. During onshore storm wind events, much stronger deceleration of airflow was recorded up the ramp and change in its topography was insignificant. We found that during almost 2-month survey during which several extreme wind events occurred, the aeolian ramp was eroded in both experimental sites. The greater erosion (up to 60 cm) was recorded within the ramp adjacent to the lower foredune than to the higher one (up to 30 cm) where deceleration of near-surface airflow on aeolian ramp was much more pronounced. The lack of deposition during onshore wind events, despite great deceleration of the airflow, was caused by a significant reduction in the sand source due to beach flooding by storm waves and unsaturation of air stream approaching the ramp. Aeolian ramp erosion affected sand transfer in the entire beach-ramp-foredune system and resulted in the limitation of sand supply to foredune and thus their degradation during storm events.

This study was supported from Polish National Science Centre (grant no. 2018/31/B/ST10/03051).