currents for some purposes. These are particularly well documented 

 for Lake Michigan. These disturbances are called long waves because 

 their wavelengths extend for several miles. The waves can produce 

 severe damage in marinas and have at times been responsible for loss 

 of life. They have been studied by Donn and Ewing (1956), Ewing, Press' 

 and Donn (1954), Harris (1957), Platzman (1958, 1965), Hughes (1965), 

 and Irish (1965) . Their effect on sediment transport has not been 

 documented. 



7. Lake Tides . 



Water motion generated in the Great Lakes by tides and by subsurface 

 flow are infinitesimal when compared to the causes discussed previously. 



8. Effects of Water Stratification . 



If pronounced horizontal gradients in water density exist in a basin, 

 currents may be generated as denser water seeks to flow under lighter 

 water. Currents of this type are significant in the ocean and are some- 

 times significant in estuaries where freshwater flows over saltwater. 

 Such currents are rarely significant in the Great Lakes. 



From late spring to late autumn the water in the Great Lakes is 

 stratified with warm water near the surface and cold water at greater 

 depths. Internal waves can form on the discontinuity between warm and 

 cold water. The presence of internal waves greatly complicates the 

 description of water motion in a lake. 



III. CHARACTERISTICS OF FLOW 



Turbulence Near the Lake Bottom . 



The water motion near the bottom of a lake is the net result of all 

 currents and waves present. At the bottom (or a short distance below the 

 bottom if made up of sedimentary material) , the water velocity perpendicular 

 and along the bottom must be zero. The shearing forces generated between 

 adjacent fluid layers in this region lead to the generation of small scale 

 irregular motions knoxvn as tuvhulenae . Collins (1963) discusses the incep- 

 tion of turbulence under gravity waves, and concludes that for waves with 

 a period of 6 seconds in water about 20 feet deep, the flow will be turbu- 

 lent for wave heights of about 1 foot. Lower waves may produce turbulence 

 in lesser depths. The turbulence intensity is directly proportional to the 

 square of the total speed of the water particles a short distance above 

 the lakebed. Turbulent motions die off rapidly when velocity shear 

 decreases. It is not uncommon to observe pulses of sedimentary material 

 picked up into the fluid column as a wave crest passes and to find much 

 of the material returned to the seabed before the next wave crest passes. 



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