Table 3-4. Fluctuations in water levels, Great Lakes System (1900 through 



1977) 



' Elevations are in meters (feet) above mean water level at Father Point, Quebec. International Great Lakes Datuu ( IGLD) (1955). 



^ To convert to 1935 Datum, add datum factor to IGLD (NOS 1935 = IGLD + datum factor). 



Low water datun is the zero plane on NOS Charts to which charts are referred. Thus the zero, (low water) datum on a NOS Lake Superior 

 chart Is 182 meters (600 feet) above mean water level at Father Point, Quebec. 



In addition to seasonal and annual fluctuations, the Great Lakes are 

 subject to occasional seiches of irregular amount and duration. These 

 sometimes result from a resonant coupling which arises when the propagation 

 speed of an atmospheric disturbance is nearly equal to the speed of free waves 

 on a lake (Ewing, Press, and Donn, 1954; Harris, 1957; Platzman, 1958, 

 1965) . The lakes are also affected by wind stresses which raise the water 

 level at one end and lower it at the other. These mechanisms may produce 

 changes in water elevation ranging from a few centimeters to more than 2 

 meters. Lake Erie, shallowest of the Great Lakes, is subject to greater wind- 

 induced surface fluctuations, i.e. wind setup, than any other Lake. Wind 

 setup is discussed in Chapter 3, Section VIII, 6 (Storm Surge and Wind Setup). 



In general, the maximum amount of these irregular changes in lake level 

 must be determined for each location under consideration. Table 3-5 shows 

 short-period observed maximum and minimum water level elevations at selected 

 gage sites. More detailed data on seasonal lake levels and wind setup may be 

 obtained for specific locations from the National Ocean Service, National 

 Oceanic and Atmospheric Administration. 



4. Seiches . 



Seiches are standing waves (Fig. 3-48) of relatively long periods that 

 occur in lakes, canals, and bays and along open seacoasts. Lake seiches are 

 usually the result of a sudden change, or a series of intermittent-periodic 

 changes, in atmospheric pressure or wind velocity. Standing waves in canals 

 can be initiated by suddenly adding or subtracting large quantities of 

 water. Seiches in bays can be generated by local changes in atmospheric 

 pressure and wind and by oscillations transmitted through the mouth of the bay 

 from the open sea. Open-sea seiches can be caused by changes in atmospheric 

 pressure and wind or by tsunamis. Standing waves of large amplitude are 

 likely to be generated if the causative force which sets the water basin in 



3-96 



