3.81 ASTRONOMICAL TIDES 



Tide is a periodic rising and falling of sea level caused by the 

 gravitational attraction of the moon, sun, and other astronomical bodies 

 acting on the rotating earth. Tides follow the moon more closely than they 

 do the sun. There are usually two high and and two low waters in a tidal 

 or lunar day. As the lunar day is about 50 minutes longer than the solar 

 day, tides occur about 50 minutes later each day. Typical tide curves for 

 various locations along the Atlantic, Gulf, and Pacific coasts of the 

 United States are shown in Figures 3-36 and 3-37. Along the Atlantic 

 coast, the two tides each day are of nearly the same height. On the Gulf 

 coast, the tides are low but in some instances have a pronounced diurnal 

 inequality. Pacific coast tides compare in height with those on the 

 Atlantic coast but in most cases have a decided diurnal inequality. 

 (See Appendix A, Figure A-10.) 



The dynamic theory of tides was formulated by Laplace (1775) and 

 special solutions have been obtained by Doodson and Warburg (1941) among 

 others. The use of simplified theories for the analysis and prediction 

 of tides has been described by Schureman (1941), Defant (1961) and Ippen 

 (1966) . The computer program for tide prediction, currently being used 

 for official tide prediction in the United States is described by Pore 

 and Cummings (1967). 



Data concerning tidal ranges along the seacoasts of the United States 

 are given to the nearest foot in Table 3-5. Spring ranges are shown for 

 areas having approximately equal daily tides; diurnal ranges are shown 

 for areas having either a diurnal tide or a pronounced diurnal inequality. 

 Detailed data concerning tidal ranges are published annually in Tide Tables, 

 U.S. Department of Commerce, National Ocean Survey. 



3.82 TSUNAMIS 



Long period gravity waves generated by such disturbances as earth- 

 quakes, landslides, volcano eruptions and explosions near the sea surface 

 are called tsunamis. The Japanese word tsunami has been adopted to replace 

 the expression tidal wave to avoid confusion with the astronomical tides. 



Most tsunamis are caused by earthquakes that extend at least partly 

 under the sea, although not all submarine earthquakes produce tsunamis. 

 Severe tsunamis are rare events. 



Tsunamis may be compared to the wave generated by dropping a rock in 

 a pond. Waves (ripples) move outward from the source region in every 

 direction. In general, the tsunami wave amplitudes decrease but the 

 number of individual waves increases with distance from the source region. 

 Tsunami waves may be reflected, refracted, or diffracted by islands, sea- 

 mounts, submarine ridges or shores. The longest waves travel across the 

 deepest part of the sea as shallow-water waves, and may obtain speeds of 

 several hundred knots. The travel time required for the first tsunami 

 disturbance to arrive at any location can be determined within a few 

 percent of the actual travel time by the use of suitable tsunami travel- 

 time charts. 



3-71 



