AEOLIAN TRANSPORT OF SAND ACROSS A FOREST CANOPY IN THE LEE OF A LARGE PARABOLIC DUNE

Green Mountain Beach Dune (GMB) is a 50 m high, 300 m long parabolic dune migrating into a forest along the southeastern shore of Lake Michigan. Grainfall was monitored under the forest canopy beyond the lee slope of the dune in a series of 16 traps consisting of vertically oriented funnels with cloth bags around the narrow ends, set on 1.4 m high posts. The farthest trap was 215 m beyond the base of the dune. Sand was collected and weighed periodically over 3 years. A separate set of traps, consisting of vertically oriented cups set on poles 1 m high, were deployed on the bare lee slope of the dune during two strong wind events. The mass of sand in a trap at position x at the end of period i, M_i(x), can be modeled by the exponential decay equation,

M_i(x) = C_i exp(-A_i x).

For the forest series C_i depends on the amount of sand that reaches the canopy and varies from period to period, and the exponential decay rate, A_i, varies slightly from period to period. A_i for the lee slope series is larger than A_i for the forest series. Mean grain sizes, S_i(x), of sand in the traps on either the lee slope or under the canopy can be modelled by the linear equation,

S_i(x) = B_i - D_i x.

The initial grain size, B_i, is approximately the same for both the lee slope and forest series while the rate of decrease in grain size D_i is larger for the lee slope series than for the forest series. Rotating vortices of suspended sand were observed on the bare lee slope of the dune during strong wind events. Rotating airflow was measured using anemometers set on the slope, and short intense bursts of sand transport were detected with modified miniphones.

Our hypothesis is that suspended sand launched off the parabolic dune reaches a boundary layer near the top of the canopy where turbulence associated with flow over the rough surface keeps some of this sand in suspension for distances greater than 200 m. Thus, under some circumstances the presence of a forest canopy may aid the transport of suspended aeolian sand.