Measuring saltation and creep with high spatial and temporal resolution

Measurement of aeolian sand transport rates with high temporal and spatial resolution is crucial for further progress in developing proper sand transport equations, in testing and developing numerical models of sand movement by wind, and in the modelling of sand dunes, ripples and other aeolian forms. Observational research on the behaviour of sand grains in natural sediments under natural conditions is presented herein. This study uses the established measuring principle of a Saltiphone (Spaan and Van den Abeele, 1991) and a webcam commonly used in personal computers. The webcam frame transfer is triggered every 0.1s by a sonic anemometer. Consecutive frames are compared and analysed in real-time by a computer program. The webcam signal also provides the number of moving grains also the grain size of the moving grains and this for every grain that has moved in a time step of 0.1s. Problems to determine the transport near the thresholds and the difficulties in the determination of thresholds from measured mean transport rates are discussed, and proposals for dealing with the problems are made. An iterative technique to determine the thresholds of transport (ITT) from high resolution transport measurements is presented. In this way, constitutive equations for sand transport in terms of wind speed can be tested. If viable, they can be employed to infer estimates for the thresholds by minimising the root-mean-square error between measured and calculated transport data. Alternatively, the fluid and impact thresholds for aeolian sand transport are determined from the field measurements on a beach by analysing the onset and breakdown of saltation in gust and lull intervals (GLM) of rising and falling wind speeds, respectivel