HUMID AND COOL CLIMATE RATHER THAN DROUGHTS LEAD TO COASTAL BLOWOUTS DEVELOPMENT IN THE GREAT LAKES REGION

Coastal blowouts along the southeastern shores of Lake Michigan develop during the periods of increased regional precipitation and cooler climate, which cause rise in the lake level and shoreline erosion by waves and storms. During low levels of Lake Michigan backshore is wide and incipient foredunes develop and grow, trapping the sand, thus limiting sand accretion on established foredunes and parabolic dunes further inland. From December 1998 to August 2014 Lake Michigan level was lower than average, which allowed continuous development and growth of incipient foredunes. 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. Lake Michigan level began rising in late 2013, surpassing the long term average in August 2014, and in July 2017 reached 177.06 m, or 1.49 m higher level than in January 2013. The backshore was shortened by 35-40 m and many incipient foredunes were partially or completely eroded, with 1-3 m high cliffs in places. In the early stages of the lake transgression more than 95% of sand from eroded foredunes was transferred into a foreshore, while winds moved the remaining 5% of sand on top of the retreating foredunes. In the late stages of transgression blowouts will develop if storms breach foredune ridges and create blowout throats for an easy transfer of sand from the backshore to the dunes. Most of sand from eroded foredunes and relatively narrow backshore will then be transferred inland and promote growth of blowouts and parabolic dunes. A geomorphic analysis of about 20 km long unpopulated shoreline indicated that all recent blowouts were initiated through a breach in the foredune or through a breach in the arm of a parabolic dune on shores that lacked foredunes. The average spacing of 0.4 km between the blowouts allowed most of sand derived from eroded foredunes to be transferred inland rather than in the foreshore. The blowout healing commences during low lake levels, when incipient foredune formation impedes blowout throats and stops further transfer of sand inland.