necessary to make full use of the available information about sea level variability or tlie 

 elevation of land surfaces referenced to a tidal datum. 



This report examines astronomical tides from an engineering standpoint, defines widely 

 used tidal datums based on tide records, and presents tide statistics. Many features of water 

 level records must be identified in the coastal zone that are not due to astronomical tides. 

 The causes of these other phenomena are not examined in detail, but a few references are 

 given for additional study. Other reports in the Coastal Engineering Manual (CEM) series 

 will discuss the two major nonastronomical causes for sea level variability— storm surges and 

 tsunamis. Tsunamis are discussed in Camfield (1980). 



A description of astronomical tide phenomena and water level records which illustrate 

 astronomical tides and nontidal phenomena that affect tlie water level near the coast are 

 presented in Section II, An introduction to the classical basis for tide prediction, used by 

 the National Ocean Survey (NOS) of the National Oceanic and Atmospheric Administration 

 (NOAA), and tide prediction services of other nations is presented in Section III. The newer 

 concepts of tide analysis, based on the application of an admittance function (Munk and 

 Cartwright, 1966; Zetler, Cartwright, and Munk, 1970; Godin, 1976), are not discussed in 

 this report because these concepts are not yet used in the United States for official tide 

 predictions. 



Tidal datums are described in Section IV, with particular attention given to the 

 distinction between the often confused datums of mean sea level (MSL) and the National 

 Geodetic Vertical Datum (NGVD) of 1929, formerly called the Sea Level Datum of 1929. 

 Sources of information about tidal datums and sample bench-mark indexes and descriptions 

 are also identified. 



The establishment of datums near large inland water bodies at a significant elevation 

 above MSL presents special problems. As a result of the increasing strength of the 

 gravitational vector from the Equator to the poles, the distance between two level surfaces 

 (defined as normal to tlie gravitational vector) decreases from the Equator to the poles. This 

 factor must be accounted for in assigning a single elevation value to the equilibrium surface 

 of a lake with a large extent in the north-south direction. Two systems of elevation 

 adjustments are widely used; both systems and their application to the establisliment of 

 datums in the Great Lakes region are discussed in Section V, 



Variations in sea level relative to the land over periods longer than a year are discussed in 

 Section VI, Although these changes are small within a year, their cumulative effects during 

 the lifetime of many engineering structures can be significant. Long-term changes in mean 

 water level can result from the withdrawal of subsurface water or minerals from the ground, 

 increased loading of the ground by dams, deposition of sediment, or from long-term changes 

 in the shape of the Earth. Even when the proximate cause of past changes in sea level 

 relative to land is known, predictions of future changes cannot be made with great 

 confidence. 



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