Nearshore ice formation and sediment transport in southern Lake Michigan
The southern Lake Michigan nearshore zone is ice covered for two to four months each year. Daily observations in January 1991 document the effect of this seasonal ice on sediment transport along the Illinois shoreline of the lake. Three distinct types of ice are found in the nearshore zone. Each of these ice types has a different effect on nearshore sediment but all contribute to ice rafting sediment alongshore and cross shore.The Nearshore Ice Complex (NIC) is a large, 'solid' mass of ice that builds lakeward from the shoreline. At Gillson Beach in southwest Lake Michigan, the NIC reached a maximum width of 180 m and formed ridges up to 4 m above lake level. The volume of this NIC was 420 m$\sp3$ of ice per meter of beach width. This ice entrained 2.3 m$\sp3$ of sand per meter of beach. The NIC is a dynamic feature that grows and decays in response to incident waves and the amount of slush ice in the nearshore zone. Destruction of the NIC by waves leads to ice rafting of sand.Anchor ice is ice that is attached to the lakebed; it forms on sand, pebble and cobble bottoms in the nearshore zone. Anchor ice formation is common; it formed on 14 of 32 nights at one site. Anchor ice forms on cold, clear nights with offshore winds and is released from the bed and floats to the surface with entrained sediment when the water warms during the day. The formation and release of anchor ice transports sand in calm conditions. I estimate that anchor ice rafted 0.85 m$\sp3$ of sand from each m of beach at one site during the winter of 1991. There is a continuum of anchor ice morphologies that is a function of the incident wave energy at the time of anchor ice formation.Slush ice is mm-sized, unconsolidated, mobile ice. Slush ice consists primarily of floating accumulations of frazil ice crystals, although anchor ice and NIC ice are also incorporated into this mobile ice type. Sand is incorporated into slush ice directly from the bed or water column and by transfer of sediment-laden anchor ice and NIC ice to the slush ice zone. Combined ice sampling, video records and drifter returns show that there is a net transport of slush ice and sand to the south and offshore along the southwestern Lake Michigan coast. This study shows that annual ice formation in southern Lake Michigan is removing sand from the sediment-starved nearshore zone.
- Oceanography