GSA Connects 2021 in Portland, Oregon

Paper No. 81-8
Presentation Time: 9:50 AM


BODENBENDER, Brian, PhD1, YURK, Brian2, HANSEN, Edward C.1, HARLOW, Blake2, SCHAETZL, Randall3 and DEVRIES-ZIMMERMAN, Suzanne1, (1)Department of Geological & Environmental Sciences, Hope College, P.O. Box 9000, Holland, MI 49422-9000, (2)Department of Mathematics and Statistics, Hope College, 27 Graves Place, Holland, MI 49423, (3)Geography, Environment and Spatial Sciences, Michigan State University, 673 Auditorium Rd, East Lansing, MI 48864

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, Mi(x), can be modeled by the exponential decay equation,

Mi(x) = Ci exp(-Ai x).

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

Si(x) = Bi - Di x.

The initial grain size, Bi, is approximately the same for both the lee slope and forest series while the rate of decrease in grain size Di 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.