Air flow over foredunes and implications for sand transport

Abstract

More than 4000 hourly wind profiles measured on three topographically different foredunes are analysed and discussed. Wind flow over the foredunes is studied by means of the relative wind speed: the ratio between wind speed at a certain location and the reference wind speed at the same height. Relative wind speeds appear to be independent of general wind speed but dependent on wind direction. For perpendicular onshore winds the flow over the foredune is accelerated due to topographic changes and decelerated due to changes in surface roughness. Accelerations dominate over decelerations on the seaward slope. The pattern of acceleration and deceleration in relation to wind direction is more or less comparable for different foredunes, but the magnitudes differ. An increase in foredune height from 6 to 10m leads to an increase in speed-up near the top of the seaward slope from 110 to 150 per cent during onshore wind, but further increase of foredune height from 10 to 23 m appears to have little effect, due to increased roughness and deflection of flow.Topography also influences the direction of the flow. Between beach and top, the flow deflects in the direction of the normal during onshore winds. During offshore winds the flow is deflected to the parallel. Near the dunefoot, deflection is always in the direction of the parallel, and increases with steeper topography. The maximum deflection near the dunefoot was 90", over a 23 m high dune, observed during offshore winds.Patterns of erosion and sedimentation resulting from winds from different directions can be explained by the observed accelerations and decelerations. Owing to speed-up on the seaward front of the foredune, sand transport capacity of the wind increases, which results in erosion if vegetation is absent. During strong onshore wind, sand is lifted near the dunefoot and moves over the foredune in suspension. During weaker winds, vertical wind velocities do not exceed fall velocities of the sand grains, and most of the sand is deposited near the dunefoot.

Foredune accretion under offshore winds

“…Further field research into airflow patterns in dune systems (e.g. Arens et al, 1995;Frank and Kocurek, 1996;Hesp, 2002;Kocurek et al, 1992;Tsoar, 1983;Walker, 1999;Walker and Nickling, 2002) as well as wind tunnel studies and numerical modelling (Parsons et al, 2004a,b;Schatz and Herrmann, 2006 ;van Boxel et al, 1999;Walker and Nickling, 2003) has yielded a more detailed understanding of lee-side air flow. The three broad categories--attached and undeflected flow, attached and deflected flow, and separated flow--determined by Sweet and Kocurek (1990) and modified by Walker and Nickling (2002) are dependent primarily on dune topography and the approach angle of the wind.…”

Aeolian sediment transport on a human‐altered foredune

“…These actions create unvegetated gaps in the dune crest. As wind direction becomes more nearly parallel to a beach/dune contact, much of the sediment moves alongshore rather than into the dune (Sarre, 1989;Arens et al, 1995). An exception is where gaps in the crest increase local wind speeds and sand transport rates, making portions of the dune landward of them susceptible to inundation by wind-blown sand (Rosen, 1979;Gares and Nordstrom, 1995).…”

Flow deflection over a foredune

“…Bradley (1983) and found an inverse relationship between incident flow direction and speed, such that when the flow was more oblique to the dune crestline, the flow speed decreased over the dune (cf. Arens, 1995;1996;Lynch et al, 2010;Jackson et al, 2011), which implies that less sand can be delivered to the foredune crest and lee-side region. Arens et al (1995) found that, as winds became more oblique, the effective slope (i.e., aspect ratio) diminished and, in response, transport rates up the stoss slope decreased because topographically-forced flow acceleration is not as pronounced as with perpendicular approach angles.…”

“…For flow approaching a steep, 70-to 90-m high scarp at an angle of 45° to normal, it was found that flow separated in front of the scarp base and was deflected alongshore, while higher up the scarp the flow crossed at an oblique angle. Arens et al (1995) argued that flow deflection increased with increasing foredune height. They found…”