the Great Lakes, is subject to greater wind-induced surface fluctuations, 

 that is, wind setup, than any other Lake. Wind setup is discussed in 

 Section 3.86, 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 3-7 

 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 Lake Survey Center, 

 National Oceanic and Atmospheric Administration, U.S. Department of 

 Commerce. 



Table 3-7. Short-Period Fluctuations in Lake Levels at Selected Gage Sites 



*The mean surface elevation (MSE) refers to the water level that represents 

 the average water elevation for the period of record. It corresponds to 

 the mean surface elevation given in Table 3-6. 



3.84 SEICHES 



Seiches are standing waves (Fig. 3-40) of relatively long periods 

 that occur in lakes, canals, 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 tsunamis. Standing waves 

 of large amplitude are likely to be generated if the causative force which 

 sets the water basin in motion is periodic in character, especially if the 

 period of this force is the same as, or is in resonance with, the natural 

 or free oscillating period of the basin. (See Section 2.5, Wave Reflection.) 



3-78 



