SAND DEPOSITION BY GRAINFALL BEYOND THE LEE SLOPE OF A LARGE PARABOLIC DUNE

Studies of grainfall have primarily focused on sand deposition onto the lee slopes of large dunes, but sand transport well beyond the lee slope of active dunes has been noted informally. We measured the amount of grainfall-deposited sand in wooded backdunes in the lee of Green Mountain, a 40 m high parabolic sand dune on the southeastern shore of Lake Michigan, starting after deciduous leaf fall in Nov., 2017. Green Mountain is an active dune, migrating about 1 m/y, with an unvegetated interior, permanent plant cover on the exterior limbs, and a summertime dune grass cover on the lee slope of its nose.

Cloth sample bags clamped beneath funnels mounted on 1.4 m high posts served as grainfall traps, ensuring that any sand captured was airborne. We installed 10 traps along the projection of the dune axis (trend 67°) from the toe of the lee slope to 165 m inland, and later added two traps to reach 215 m inland. Four more traps were placed along an arc 20 m from the toe of the dune, at ≈20 m and ≈40 m to each side of the dune axis, to sample off-axis transport. Sediment was collected from the traps during two windstorm events, and collecting bags were changed intermittently throughout the fall and winter to monitor sediment transport over longer intervals.

All traps collected sand in each interval. The total mass of sediment transported as grainfall between Nov. 17, 2017 and Apr. 23, 2018 ranged from 9,306 g/m² at 10 m from the toe to 1,406 g/m² at 165 m from the toe. The mass of grains transported fits an exponential model m = e^{-ax}, where x is the distance from the toe of the dune and a ranges between 0.011 and 0.019. The traps lateral to the dune axis captured slightly more sand than the trap at 20 m on the dune axis, suggesting that sand transport may be fairly evenly dispersed in a broad front downwind from the dune. Two thirds (66.8%) of the total grainfall occurred during the strongest storm, Dec. 4­–6, 2017, with winds from the SW to W >10 m/s for 24 hours and gusts >18 m/s for more than 12 hours. The Dec. 21–Feb. 27 period, which saw 11 events with sustained winds at least briefly >10 m/s, accounted for 27.5% of total grainfall. The Feb. 27–Apr. 23 interval accounted for only 0.4% of total deposition. The results suggest that grainfall transport inland of dunes is episodic and storm dependent but can deposit appreciable quantities of sand into these environments.