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An efficient, self-orientating (swinging), sand trap

Mike Hilton 1, William Nickling2, Sarah Wakes1, Douglas Sherman4, Teresa Konlechner1, Mark Jermy3, Patrick Geoghegan3
1University of Otago, Dunedin, New Zealand, 2University of Guelph, Guelph, Canada, 3University of Canterbury, Christchurch, New Zealand, 4University of Alabama, Tuscaloosa, USA

Accurate sand transport measurements are difficult because of the large spatial variability in aeolian transport rates, both horizontally and vertically, even on level surfaces. Large numbers of sand traps may be required, depending on the topography. Such traps should be light and easily deployed and recovered. The current paper describes a high efficiency, cheap (US$2 materials), sand trap. It is constructed from a 0.21m length of 0.04m diameter PVC tubing. The entrance is moulded and the rear of the trap cut to support a 63micron square-mesh bag. A set of up to 10 traps can be mounted (skewered) on a 1.0m long fibreglass rod so that the traps can rotate 360o.

The ability of the traps to self-orientate (swing) into the wind, and the stability of the traps and the mesh bags, was tested in the University of Canterbury’s closed-circuit wind tunnel in flows up to 40ms-1; both empty, and containing 30-60g of weights. Empty traps set at 45o to the flow oriented at around 8ms-1 and 10ms-1 when fully weighted. Traps orientate in the field at lower speeds because of the inherent variability of natural flows. Computational Fluid Dynamics was used to simulate the wind flow through different configurations of traps. It was found that there was only a small decrease in mass flow rate for multiple traps on a single pole or in rows. Adding an extra row of traps 0.25m behind the first row resulted in only a 10% loss of mass flow rate for the traps in the second row.

The efficiency of the traps was evaluated in a 15m recirculating wind tunnel in the Wind Erosion Laboratory, Department of Geography, University of Guelph, by comparing the overall efficiency of the trap design compared to an isokinetic sampler of a similar design. The flux and associated mass caught in the unmodified trap is somewhat less than that caught in the iso-kinetic trap as a result of the blocking caused by the mesh bag. The differences however, are very small (-2.16% to -5.37%) with an overall average for the runs of -3.95% (SD = 1.9%).

The swinging traps were compared in the field with the ‘box traps’ of Sherman et al. (2014). The swinging traps do not trap sand within 1cm of the bed so they underestimate the total sand flux, but the predictive models for the flux profiles are not statistically different at the 95% level.

Sherman, D.J., C. Swann & J.D. Barron (2014) A high-efficiency, low-cost, aeolian sand trap. Aeolian Research, 13, 31-34.