Surveying" (US ACE 1991). A concise but authoritative overview of tidal 

 datums and their various uses is given in Hicks (1 985, 1986). 



Basic concepts of tides and tide measurement 



The tide is defined by NOS as the periodic rise and fall of the water 

 resulting from gravitational interactions between the sun, moon, and earth 

 (Hicks 1989). The term "tide" refers to the astronomically forced changes 

 in water level, which is deterministic or predictable. Astronomical or 

 daily tide should be distinguished from other contributions to changes in 

 water level, including meteorological forcing (seasonal changes in sea 

 level, changes caused by daily wind or to passage of weather fronts, etc.), 

 terrestrial inputs (rain runoff or river discharge), and atmospheric forcing 

 (change in air pressure). The distinguishing of periodicity produced by 

 astronomical forcing and by diurnal and longer-term wind forcing, for 

 example, has not been well addressed in tidal datum definition. Similarly, 

 the distinguishing of variation in daily water (tidal) level and in annual 

 water elevation does not seem to have been well addressed in the United 

 States with regard to definition or determination of a navigation datum. In 

 the following, standard NOS definitions for selected tidal datums are given 

 for reference in this report. 



Low water is the minimum height reached by a falling tide. Low 

 water occurs according to the periodic tidal forces and the acting 

 meteorological, hydrologic, and oceanographic conditions. For tidal 

 datum computation, the minimum height is not considered a low water 

 unless it contains a tidal low water (Hicks 1989). 



Mean high water (mhw) and mean low water (mlw) are, 

 respectively, the averages of all the high- water heights and low- water 

 heights observed over the NTDE. For stations with time series shorter 

 than 19 years, simultaneous comparisons with a control station are made 

 to determine the equivalent datum of the NTDE. Also, strictly speaking, 

 to be counted as a high, each high water must be 0.10 ft or more above, 

 and must occur 2 hr or more later than the adjacent low waters. 

 Analogous considerations hold for definition of low waters. 



The mean range of tide, denoted as "Mn" by NOS, is the difference in 

 height between mhw and mlw. Thus, Mn corresponds to a tidal range 



Chapter 2 Water Level Definition, Measurement, and Properties 1 1 



