Foredunes erosion during high Lake Michigan levels

After three brief episodes of higher than normal levels - in early 1970s, the late 1980s, and in the late 1990s - Lake Michigan level became lower than normal from 1998 until 2014. Lower lake level made backshore wider and exposed sand, which allowed continuous development and growth of incipient foredunes along the southern shores of Lake Michigan. Incipient foredunes were 13 to 50 m wide, 2.5-4.5 m tall, and had about 30 to 50 m wide backshore to the water line. In August 2014 Lake Michigan level rose above the normal and has been rising ever since, likely soon achieving record high level. The rise in lake level dramatically altered coastal sand distribution. The backshore was shortened by 35-40 m and many incipient foredunes were partially or completely eroded, with 2-4 m high cliffs in places. In the past five years most of sand from eroded foredunes was transferred into a foreshore, while winds moved small amount of sand on top of the retreating foredunes. Nearshore sand bars changed in morphology and size, becoming much wider but flatter than sand bars forming during lower lake levels. In many places there were only 10-15 m of foredunes left between the water and parabolic dunes further inland. If storms breach the remaining thin foredune ridges blowout throats will form and allow an easy transfer of sand from the backshore to the dunes. Sand from eroded foredunes and relatively narrow backshore will then be transferred inland and promote growth of blowouts and parabolic dunes. Most foredune erosion takes place during summer storms and seiche events. In the past several years there was 1.1 to 1.3 m difference between highest hourly lake levels recorded in summer and lowest hourly lake levels recorded in winter. Therefore, during summer backshore is about 15-20 m shorter, and wave erosion of foredunes is stronger on stormy days when high waves are common. Linear regression analysis indicates that about 55% of decrease in foredune and backshore width can be associated to the rise in the lake level. The other two factors that determine erosion rates of foredunes are storms and migration of sand bars along the shoreline. During storms lake level can rise 50-70 cm within minutes with waves splashing foredunes' cliffs. During a seiche event on July 20, 2017 there were five large pulses of water sloshing with a maximum amplitude of 96.1 cm.