July 11th Lake Michigan Seiche

Shortly after the derecho event that quickly passed through the Northern Illinois area on Monday, July 11th, the Illinois shore of Lake Michigan experienced the rare phenomenon known as a seiche. A seiche is an oscillation of water in an enclosed or partly enclosed body of water caused by the conservation of momentum of the pile-up of water against a shore. Specific to the seiche that occurred on Monday, the extremely gusty winds from the morning derecho event caused a pile up of water on the Michigan and Indiana shore of Lake Michigan, and the water rebounded back to the Illinois shore causing an increase in wave heights measured in Lake County and Calumet Harbor.

The increase in wave heights at the locations measured cannot be attributed to the observed offshore winds for several reasons. After the passage of the derecho, the wind speeds expectedly dropped; however, the wave heights continued to rise, which is the opposite of what is typically expected. A decrease in wind speed is usually correlated with a decrease in wave heights. Aside from that, in any marine environment, a westerly wind blowing offshore would not be conducive to wave height increases on the western shore. It can be determined that the wave height increases that Lake and Cook county experienced is due to a seiche event.

The graphs below are the plotted wind speed and wave heights for four different locations along the Illinois shore during the 24 hour period from 12:00am to 11:59pm on July 11th. Both the passage of the derecho and the arrival of the seiche are readily evident in all four locations.

                The three above beaches are all located along the Lake Michigan coast in Lake county Illinois. The readings are from buoys located in the water along the shore. The graphs are displaying the wave height and the associated wind speed on a temporal scale with a single hour resolution for July 11th, 2011. On all three graphs, the passage of the derecho is easily indicated by a spike in the wind speed between 8:30-9:00am CDT. The indication of the seiche caused by the derecho starts to appear at the earliest 9:30 at Waukegan beach with the wave height level starting to increase. The wave heights at the other two beaches peak between 11:00am and 12:00pm and slowly decline through the afternoon and evening. Despite showing the first indication of an increase in wave heights, the Waukegan beach did not peak until shortly after 2:00pm, and the heights there also slowly declined through the rest of the day.

                The Calumet Harbor station had a much higher temporal resolution than the Lake County beach buoys did, allowing a better visual discrimination of the oscillating nature of the seiche. The wave height readings were recorded every six minutes, allowing for a better big picture of the situation. The oscillations began to increase in amplitude between 9:00-10:00am , and they peaked just before 1pm with a difference of over 2 feet in wave heights. Although the amplitude of the wave height differences remained higher than normal for the rest of the day, it steadily decreased from the peak through the afternoon into the evening and overnight hours.

 



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