The aeolian development of coastal foredunes is studied at two sites along the Dutch coast. Amounts of sand transport are measured in cross-sections over the foredunes, and changes in surface height are monitored. Sand transport decreases rapidly landward of the vegetation boundary. Near the dunefoot, changes in surface roughness and topography generate turbulence and upward flow, causing a small part of the sand to be transported in suspension. Patterns of transport are found to be closely related to air flow, which in turn is related to topography and vegetation density. With steeper topography, the amounts of sand transported landward from the dunefoot increase, if vegetation density is low. During oblique onshore winds, most of the sand accumulates at the dune front. During perpendicular onshore winds, a large proportion of the sand is deposited landward of the slope. When deflection of flow occurs, landward transport of sand is interrupted.

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.

Fast response wind and turbulence instruments, including sonic anemometers are used more and more in the research of aeolian sediment transport. These instruments provide data on mean wind, but also on friction velocity, wind speed fluctuations and turbulence statistics, such as the U-W and W-T covariance (C UW and C WT ), which are a measure for the momentum flux and the sensible heat flux. This short paper will examine two problems that may arise when using sonic anemometers, namely the low/high frequency losses and the interpretation of the sonic anemometer measurements over sloping or non-homogeneous terrain.

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Rates of aeolian transport on a beach in a temperate humid climateArens, S.M. Published in: Geomorphology DOI:10.1016/0169-555X(95)00089-N Link to publication Citation for published version (APA):Arens, S. M. (1996). Rates of aeolian transport on a beach in a temperate humid climate. Geomorphology, 17(1-3), 3-18. DOI: 10.1016/0169-555X(95)00089-N General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. AbstractShort term field measurements of aeolian transport, using a saltiphone, indicate that actual rates of transport deviate considerably from the potential rates predicted by transport equations. When examined in detail, however, potential rates are approached during spe,zial conditions: mostly when the wind blows offshore, parallel or slightly oblique to the beach and relative humidity is below 85%. Also, the potential rates of transport may be approached when wind speeds considerably exceed threshold speed, even during showers. The main cause of the large deviation of actual from potential rates is in the absence of transport during very wet conditions (prolonged rainfall) regardless of wind speed, and the variation of threshold velocity with time. The threshold velocity increases during wet conditions, with onshore winds or because of the presence of algae. Therefore, potential transport is defined as the transport optimum. To predict the actual rates of transport accurately, insight into the variation of the threshold velocity is essential. Aeolian transport of sand seems to be related to moderate events and not to extreme events, conforming to the principle of magnitude and frequency.