Seiche Motion at Little Sand Bay

CEE 618 - Exploration and Investigation of Coastal Processes in the Great Lakes

By: John Reimer

Seiches have very large wavelengths and can be undetected by observers in boats on the water. The effect is caused by disturbances over the water surface most often by meteorological effects. The sources can include wind-induced shear stress and atmospheric pressure disturbances. Upon vertical motion, an impulse is produced that travels the length of the basin at a velocity that is dependent on the water depth. The impulse is then reflected back to the other end of the basin. The repeated motion creates standing waves with one or more nodes experiencing no vertical motion. The frequency of oscillation is determined by the size of the basin, depth, and water temperature. 

 ____________________________________________________________________________________________________________________________________________________________                                                         

Calculation of Lake Superior Seiche

Calculation of a lake seiche is often represented using the Marian formula:

It is assumed that the lake is closed basin with a fundamental mode equal to 1.

NE-SW Seiche

Tn=5.06 Hours

NW-SE Seiche

Tn=34.5 Minutes

 ____________________________________________________________________________________________________________________________________________________________

Installation of Water Level Logger

A water level logger was installed outside of Little Sand Bay to measure absolute pressure. An additional logger was installed on the land surface to measure barometric pressure. The two data sets were combined and used to calculate water depth.

 ____________________________________________________________________________________________________________________________________________________________

Data Analysis

The sampling frequency of the water level logger is set at a 30 second interval. The data is filtered using a 10 minute moving average so that the data can be interpreted for the lake seiche.

Applying a 10 minute moving average the data is shown below. The integrity of the seiching period is intact; however, the amplitude is much less due to the 10-minute average.

 ____________________________________________________________________________________________________________________________________________________________

Spectrum Analysis

Spectrum analysis provides a tool capable of generating information on the complicated mixture of waves. Spectrum analysis is based on the mathematics of Fourier. It describes the complete distributions of wave energies and periods. Spectrum analysis was used below to assess the seiche period at Little Sand Bay.

____________________________________________________________________________________________________________________________________________________________

NW-SE Seiche

The results below depict 12-hour intervals of the NW-SE Seiche.

____________________________________________________________________________________________________________________________________________________________

NE-SW Seiche

The results below depict the entire time period and the NE-SW Seiche.

____________________________________________________________________________________________________________________________________________________________

NE-SW Seiche as compared to Wind Speed and Wind Direction

The results below show the lake seiche as compared to wind speed and direction. The wind speeds during the study period were minimal and period of data collection was approximately 3 days. It is difficult to assign relationships with the three day period. A longer study period and deployment is necessary to develop strong relationships between the wind direction and lake seiche.

____________________________________________________________________________________________________________________________________________________________

Future Recommendations

I would recommend deploying two other water level loggers to assess the seiche motion in both directions. Using additional sensors would be used to determine the seiche period and amplitude created. Additionally, the deployment time should be monthly or carried out during the duration of the ice off period to fully understand the seiche motion.