Climatology of Sediment Transport on Indiana Shoals, Lake Michigan

Abstract

Seven time-lapse films of the lake bottom covering 2,445 hours of observation were made over a period of 3 years during the months of July through November on Indiana Shoals in southwestern Lake Michigan (water depth 10 m). These films show 25 distinct sediment transport events, consisting of periods of bedform migration or sediment resuspension and occupying approximately 25% of the total record. Simultaneous observations of the surface winds showed that most of the sediment transport occurred during periods of northerly winds, implying that surface waves were the predominant mechanism for sediment remobilization at this location. A simple, empirical, sediment transport forecast model was used to determine threshold criteria for the initiation of bedload and resuspension of the local sediments (fine-medium sand). The model correctly predicted 85% of the total record when forced with estimates of the near-bottom wave orbital velocity calculated from the wind measurements and a parametric dynamical wave model. The critical wave orbital velocity for resuspension was found to be 17.8 cm/s. Estimates of the probability of sediment transport occurring on Indiana Shoals, determined as a function of wind speed and direction, were combined with climatological observations of wind conditions to estimate monthly probabilities of sediment transport. These probabilities are in agreement with previous forecasts based on historical observations of the Great Lakes wave climate.