During the period when sedimentary material is pulled upward by tur- 

 bulent motion it may also be transported in the direction o£ any persis- 

 tent or transient currents. Individual sand particles may be picked up 

 and deposited several times each minute as they are transported either 

 along a beach or in a seaward or landward direction. 



When currents flow past breakwaters, bulkheads, or piling, shear 

 exists between the flow and the immobile solid boundary. Turbulence 

 generated in this way can cause scour near pilings and breakwaters. The 

 turbulence however, never persists very far from the boundary which 

 brought it into existence. 



The construction of a breakwater or groin that interferes with the 

 flow of water parallel to a beach decreases the velocity on both sides 

 of the obstruction and slightly increases velocities near the end of the 

 obstruction. Decreasing the water velocities decreases bottom shear and 

 turbulence. Sedimentary material carried in the neighborhood of the 

 obstruction may come to rest, and unless sufficient turbulence exists, 

 will remain at rest. Observed accretion along updrift sides of groins 

 is a result of this process. 



IV. WATER LEVELS AND PERIODICITY 



Records of lalce level elevations on the Great Lakes go back to 1836. 

 Tlie Lake Survey Center operates many water level gages throughout the 

 Great Lakes. Some are maintained year round, otliers only during the ice- 

 free part of the year. For the purpose of determining monthly and annual 

 averages, one gage has been selected at the master gage site for each of 

 the Great Lakes: Superior, Michigan, Huron, Erie, and Ontario. The 

 latest monthly averages along with historic records, and a 6-month fore- 

 cast, are published regularly by the National Oceanic and Atmos- 

 pheric Administration, Lake Survey Center, Detroit, Michigan. Over the 

 last 113 years, various gages have been employed and several different 

 reference datums adopted and discarded. Surveys were run in 1877, 1903, 

 and 1955 to determine elevations above sea level. The latest determin- 

 ation established the International Great Lakes Datum (IGLD, 1955) based 

 on a uniform, vertical control network tied to the first-order leveling 

 of the United States and Canada. Elevations, actually dynamic heights, 

 are referred to sea level as measured at Father Point, Qiaebec over the 

 11-year period before 1957 fCoordinating Committee, Great Lakes Basic 

 Hydraulic and Hydrologic Data , 1961) . Levels recorded with respect to 

 earlier datums (.e.g. , U. S. Lake Survey, 1903 and 1935) were recalculated 

 to bring them into adjustment with IGLD. 



Long-terra changes in lake levels result from both tilting of the basin 

 due to differential uplift or downwarping of the earth's crust, and more 

 importantly in modem times, from actual changes in the water volume of 

 the basin. Isostatic rebound has been a major cause of crustal movement 

 in the Great Lakes area, i.e., the crust has slowly recovered from the 

 load of Pleistocene glaciers. The effect in this area is a relative tilt 



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