Tsunamis are long-period waves generated by several mechanisms: submarine 

 earthquakes, submarine landslides, and underwater volcanos. These waves may 

 travel distances of more than 8000 kilometers (5000 miles) across an ocean, 

 with speeds at times exceeding 800 kilometers per hour (500 miles per hour). 

 In open oceans, the heights of these waves are generally unknown but small; 

 heights in coastal regions have been greater than 30 meters (100 feet). 



Seiches are long-period standing waves that continue after the forces 

 that start them have ceased to act. They occur commonly in enclosed or 

 partially enclosed basins. 



Wave setup is defined as the superelevation of the water surface due to 

 the onshore mass transport of the water by wave action alone. Isolated obser- 

 vations have shown that wave setup does occur in the surf zone. 



Storm surges are caused by moving atmospheric pressure jumps and by the 

 wind stress accompanying moving storm systems. Storm systems are significant 

 because of their frequency and potential for causing abnormal water levels at 

 coastlines. In many coastal regions, maximum storm surges are produced by 

 severe tropical cyclones called hurricanes (see Chapter 3, Section VII, for 

 description and prediction of hurricane waves) . 



Prediction of water level changes is complex because many types of water 

 level fluctuations can occur simultaneously. It is not unusual for surface 

 wave setup, high astronomical tides, and storm surges to occur coincidentally 

 at the shore on the open coast. It is difficult to determine how much rise 

 can be attributed to each of these causes. Although astronomical tides can be 

 predicted rather well where levels have been recorded for a year or more, 

 there are many locations where this information is not available. 

 Furthermore, the interactions between tides and storm surge in shallow water 

 is not well defined. 



1 . Astronomical Tides . 



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

 tational 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 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 3-44 and 3-45. 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 App. A, Fig. 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). 



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