Abstract
Blowouts are characteristic features of many natural coastal foredunes.
These dynamic bowl- or trough-shaped depressions act as conduits for
aeolian transport of beach sand into the more landward dunes. Along many
inhabited coasts foredunes and their blowouts have been planted with
vegetation to retain the sand in the foredune, facilitate blowout
closure and hence function as sea defense. The resulting vegetated and
uniform foredune has, subsequently, contributed to a widespread
reduction in the biodiversity of the backdunes. Present-day dune
management therefore increasingly involves artificially creating
blowouts to maintain and improve backdune biodiversity. The design
criteria are high, aiming to postpone or prevent blowout closure as long
as possible. Such dune restoration projects often follow a
learning-by-doing approach, as information on the underlying aeolian
processes, including airflow patterns that steer blowout development, is
scarce. Here, we focus on airflow patterns measured in a man-made trough
blowout in Dutch National Park Zuid-Kennemerland excavated in winter
2012. The blowout is approximately 100 m long and up to 11 m deep, and
has a trapezoidal plan view that narrows from 100 to 20 m in the
landward direction. It is approximately aligned with the dominant
southwesterly wind direction and hence obliquely with the roughly N-S
coastline. Four ultrasonic 3D anemometers, sampling at 10 Hz, were
installed in winter/spring 2017 from the mouth of the blowout, across
its basin, on to the depositional lobe and have been operational since.
The wind recordings at a nearby weather station operated by the Royal
Netherlands Meteorological Institute serve as the offshore reference.
Wind speed-up through the blowout varied with offshore wind approach
angle, and was generally strongest (140%) when the wind was aligned
with the blowout axis up to approximately 30° to the south of this axis.
Intriguingly, winds approaching with the same angle from the north did
not accelerate. We suspect that this asymmetry in speed-up is invoked by
the asymmetric blowout shape, with a substantially steeper northern than
southern sidewall. Wind deceleration on the lobe was also a function of
offshore wind approach angle, with the largest deceleration (40%) for
winds approaching from the north of the blowout axis. Winds with
approach angles up to 70° were all steered into the blowout, to become
approximately aligned with the blowout axis at the landward blowout end.
On the lobe, however, the wind closely followed the offshore wind
direction. Future work will focus on modelling air flow patterns with
computational fluid dynamics, and exploring the relationship between the
airflow patterns, blowout morphology and sand transport pathways using
additional field observations.