EFFICIENCIES OF SEDIMENT SAMPLERS FOR WIND EROSION MEASUREMENT

We investigate the efficiency of three sediment samplers used for studies of wind erosion: a vertically integrating trap (an active, modified Bagnold trap for measuring vertically integrated streamwise sediment fluxes, designed for use in a portable wind tunnel); and two single-point, passive traps, the Leach trap (a small sampler of simple design, mainly for use in wind tunnels) and the Fryrear trap (a rugged sampler for field use). The vertically integrating trap is calibrated using a 'weighed sediment supply' technique in which a weighed sediment source is blown away completely during a calibration run. The single-point traps are calibrated against an accurate isokinetic sampler. The collection efficiency of all three traps is determined both in bulk and as a function of particle size. The results for overall efficiency E (for aeolian sand-sized particles) are: for the vertically integrating trap, E = 1.02+/-0.05; for the Leach trap, E = 0.85+/-0.05 with a slight tendency to increase with wind speed; and for the Fryrear trap, E = 0.90+/-0.05 (with or without a rain hood). Particle size analyses, carried out on the sediments collected by the traps under test and also the isokinetic sampler (assumed to have E = 1 for all particle sizes), show that the particle size distributions of the trapped sediments do not differ significantly from those of the isokinetic sampler. This uexpected result is a feature of the soils used for the tests, for which clay particles are mainly transported as small aggregates or clay skins upon sand grains.