Suspended sediment transport over the lee slope and forest canopy downwind of a large Blowout/Parabolic dune

Abstract

Green Mountain Beach Dune is a large trough blowout migrating into a deciduous forest on the southeast shore of Lake Michigan, USA. Video cameras focused on the lee-slope of the dune recorded suspended sand transport in turbulent eddies rotating around both horizontal and vertical axes. A line of cups mounted on posts captured grainfall on the lee slope during a high wind event on November 21, 2017. Sediment mass in these traps decreased exponentially with distance from the brink. The relatively high half-length (distance at which 50 % of the sand has been deposited) of 6.2 m suggests that turbulence aids sand suspension. An array of grainfall traps in the forest beyond the base of the lee slope was sampled during 26 intervals over 30 months. Sediment masses in the forest traps decreased exponentially with distance from the dune with a mean half-distance of 46.7 m when leaves were off the trees and 62.6 m when leaves were on. We hypothesize that turbulence above the forest aids in suspending sand which also bounces off leaves and branches along the canopy. Sand in the grainfall traps fines with distance from the dune brink more rapidly in the lee slope than in the forest traps. Transport of sand beyond the lee slope plays a rather small part in the overall dune budget. However the relatively long distances of transport suggest that sand deposited within bogs or lakes can be a proxy for aeolian activity in an upwind dune.