Abstract
Foredune growth results predominantly from sand that is blown from the
beach and backshore. Predictions of multi-year potential sand supply
that are based on time series of wind speed and direction measured at a
regional (offshore or coastal) meteorological station, however, often
grossly overpredict measured deposition volumes on the foredune. This is
commonly ascribed to supply limiting factors, such as beach surface
moisture or shell deposits, or to fetch limitations. Here we show that
differences between regional and local (i.e., on the beach) wind
characteristics can also contribute substantially to this
overprediction. Using wind data collected during a five-week field
experiment on a Dutch beach backed by a 20-m high, steep (1:2) foredune
we found that the wind speed on the beach is lower and that the wind
direction on the backshore is more alongshore than expected from the
regional wind data. Both the difference in speed and direction were a
function of the regional wind direction, with the largest speed
reduction (to about 70% of the regional value) for shore-normal winds
and the largest alongshore deflection (about 15 degrees) for
shore-oblique winds. When these functional dependencies are applied to a
10-year series of regional wind data, we found that the potential annual
onshore sand transport at our site, predicted with the aeolian sand
transport equation of Hsu (1971), reduces from 86 to 24 m3/m. The latter
is now comparable, although still somewhat higher than the measured
annual deposition volume of 10 to 15 m3/m. Further analysis of the
computations shows that most of this reduction is due to the difference
between regional and local wind speed. In future work we will explore
how much of the remaining overprediction is due to surface moisture and
fetch limitations.