TIDES AND TIDAL DATUMS IN THE UNITED STATES 



by 

 D.L. Harris 



I. INTRODUCTION 



The coastal engineer must often measure the deptFi of the water or the height of the land 

 near the shore. A reference point which can be used as the zero in making these 

 measurements is essential. If the sea surface were level, and its position relative to the land 

 were constant in time, the choice of a reference point would be simple— depths would be 

 measured below and heights above the waterline. 



The sea surface, however, is not level and its elevation is not constant in time. The sea 

 surface rises and falls once or twice daily in response to the varying gravitational attraction 

 by the Sun and Moon. Irregular changes due to the action of the wind and atmospheric 

 pressure are generally continuous. Violent changes occasionally occur in response to seismic 

 disturbances; very slow changes occur in response to subsidence or emergence of the land, 

 changes in the volume of the ocean or the shape of the Earth, and other unidentified causes. 



Thus, if the zero of the vertical scale is based on the elevation of the sea surface, it is 

 necessary to use some statistical value such as the mean water level. Moreover, the mean 

 must be referred to a fixed interval of time. The expression "tidal datum plane" has been 

 widely used as a reference for the vertical scale based on tide records. This practice appears 

 to have encouraged many engineers to use reference datums as planes in many situations 

 where this was not justified. Therefore, this expression is now looked on with disfavor 

 (Mitchell, 1969, p. 21). The preferred term is tidal datum or tidal datums. Several tidal 

 datums are in common use. Others have been used in the past or proposed for use in the 

 future. Each datum has been introduced for a specific purpose and has often been used for 

 other purposes without a fuU understanding of how well it might satisfy this secondary use. 

 A Umited use of several of these datums wiU be necessary before an adequate base has been 

 established for a rigorous definition. 



The variability of sea level must be considered in a variety of problems. In international 

 law, national sovereignty is determined from the mean low water (MLW) Une. In many 

 States, the seaward Umit of private property is determined by the mean high water (MHW) 

 Une. The elevation of the extremely low water levels is often critical for navigation. The 

 elevation of extremely high water levels is critical for the design of coastal powerplants and 

 flood protection procedures. 



The theory of sea level changes is not so highly developed that satisfactory predictions 

 can be obtained without observations. The density of observations is not great enough in 

 either space or time to permit satisfactory interpretation without the use of theory. Thus, 

 an awareness of many of the physical processes which produce changes in sea level is 



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