CHARACTERISTICS OF AEOLIAN GRAIN TRANSPORT OVER A FLUVIOGLACIAL LACUSTRINE BRAID DELTA, LAKE TEKAPO, NEW-ZEALAND

This paper presents results from one of the few scientific studies to examine the physical characteristics of aeolian sediment transport in an alpine area, where topographically reinforced foehn winds initiate dust storm events. The major objective of this study is to improve kno wledge of aeolian processes in mid-latitude alpine regions experiencin g extreme wind speeds. Of particular interest is the role of surface c haracteristics in contributing to the unusually deep saltation layer w hich is seen to form over fluvio-glacial deposits in the Southern Alps of New Zealand. Sediment was collected at several heights.(0.5, 1, 2 and 4m) and locations over a large alpine braided river delta, and sta ndard laboratory techniques used to examine grain size characteristics . An image processing technique was also used to evaluate grain roundn ess. Grains filtered from the airstream at 0.5m and Im above such surf aces were found to display a mean grain size of approximately 300 to 4 35 mu m, resembling grain size characteristics of saltation clouds pre viously observed in high latitude, cold climate locations, in contrast to desert and prairie environments. Samples collected at 2 and 4m abo ve the surface were found to consist of 60 to 65 per cent sand-sized m aterial, with some grains exceeding 1-1.5mm in diameter. Grain shape a nalysis conducted on silt-and clay-sized grains filtered from the airs tream above mixed sand and gravel surfaces showed such grains to displ ay an increase in grain roundness with height. This characteristic is thought to reflect the airstream's shape-sorting ability and has impor tant implications with respect to the often observed increase in,grain roundness in aeolian deposits with increasing distance from source ar eas. Namely, if more rounded grains are preferentially carried higher into the airstream and therefore into regions of higher wind speed, th ey should theoretically be transported further from the entrainment zo ne before being deposited. The high wind speeds observed, often exceed ing 30ms(-1), are seen to transport significantly larger sediment than reported in the literature for desert and prairie environments. In ad dition, the mixture of grain sizes, and especially the pebble-and cobb le-sized clasts that dominate the fluvio-glacial deposits associated w ith the braided rivers in this mountain region, also appear to increas e significantly the trajectory height of saltating sand grains. As a r esult of these two factors, the depth of the saltation cloud often exc eeds Im. Observations made in this study therefore highlight the need for field and laboratory aeolian process studies to be extended to exa mine grain transport over coarse-grained beds during much higher wind velocities than typically reported in the literature. Such studies wou ld provide a valuable insight into aeolian processes in high latitude/ altitude environments, such as loess genesis. (C) 1997 by John Wiley & Sons, Ltd.