in which t = time of oscillation (seconds) 



X = length of the axis of oscillation (feet) 

 d = mean depth along axis of oscilMion (feet) 

 g = acceleration of gravity (feet/second^) 



$9, Lake Levels . - The Great Lakes have only insignificant tidal 

 variations but are subject to seasonal and annual changes in water level 

 and to changes in water level caused by wind set-up, barometric pressure 

 variations, and by seiches. The average or normal elevations of the lakes 

 surfaces vary irregularly from year to year. During the course of each 

 year the surfaces are subject to consistent seasonal rises and falls, 

 reaching their lowest stages during the winter months and attaining their 

 maximum stages during the summer months. A hydrograph of monthly lake 

 levels from the year i860 to the present is shown in Figure 3U. Table 6 

 summarizes certain lake level data, 



60o In addition to the seasonal and annual fluctuations, 

 the lakes are suDject to occasional seiches of irregular amount and 

 duration. Sometimes these result from variations in barometric pressure, 

 which may produce changes in water surface elevation ranging from a few 

 inches to several feet. At other times the lakes are affected by wind 

 set-up which raise the level at one end and lowers it at the other end 

 of a lake , 



61, In general, the maximum amounts of these irregular changes in 

 lake level must be determined for each location under consideration. 

 Some idea as to the extent of fluctuations which may be expected is given 

 by the following. On Lake Superior a barometric storm in June 1939 caused 

 an oscillation at Marquette with a maxim-om range in surface elevation of 

 7.ij. feet. The storm of 28 November 1905 raised the water level at Duluth 

 Harbor 2,3 feet above normal water stage. The largest flucutations of 

 any of the lakes occur on Lake Erie because of its shallow depth, (see 

 Figure A-n, Appendix A), The largest fluctations on this lake occur at 

 Sandusky, Toledo, and the mouth of Detroit River at the western end of 



the lake, and at Buffalo Harbor at the eastern end of the lake. At Buffalo 

 Harbor the extreme range is 13«7 feet, the highest level on record being 

 9,5 feet above low-water datum on 1 April 1929 and the lowest being h<>2. 

 feet below that datum on 30 January 1939 o The greatest range for any one 

 year was 11,6 feet in 1927, with ? high stage of 9i.3 feet and a low stage 

 of minus 2.3 feet. The least range for any one year was 6,5 feet above 

 and loU feet below datum. The foregoing are based on records for the 

 past 50 years, 



LITTORAL DRIFT 



62. General Characteristics . - Littoral drift may be defined as the 

 material that moves generally parallel to the shore imder the influence 

 of waves and currents. It is sometimes considered as the movement of the 

 material as well as the material itself. Evaluation of littoral drift 

 characteristics is the fundamental basis for functional design of shore 

 protection measures. 



