Wave climate and nearshore lakebed response, Illinois Beach State Park, Lake Michigan

Abstract

The Lake Michigan outer nearshore zone (water depths ≈5 to 25 m) off Illinois Beach State Park is subjected to a spectrum of wave conditions, including those generated by major storms. Only under these major storm conditions is there a realistic potential for wave-lakebed interaction (and associated wind-driven currents) to cause a significant net modification to the outer nearshore lakebed, which, in turn, may promulgate change in the inner nearshore (surf) zone. Analysis of bathymetric and sediment grain-size data, used in conjunction with published wave hindcast data, wave propagation modeling, and previous studies in the area, indicates that this potential occurs, most likely, on a scale of years. Although such storms can generate bottom currents well in excess of what is required to mobilize the fine to very fine sands that are present, little compelling evidence was found, on the basis of gross comparisons with previous studies (1946, 1973, 1978), that there have been rapid or pronounced changes in the outer nearshore lakebed. Nonetheless, grain-size data suggest that sands are episodically transported in a net southerly direction. At least over the time period represented by this and previous studies, any modification of the outer nearshore zone lakebed would seem to be modest and gradual. If so, whereas the outer nearshore zone may be a factor in the long-term adjustment of the shoreline, any rapid change in shoreline position, or any conspicuous change in the rate of shoreline adjustment, is more likely controlled by factors other than those linked to the outer nearshore zone.