The first five have periods that range from a few minutes to a few 

 days; the last two have periods that range from semi-annually to many 

 years. Although important in long-term changes in water elevations, 

 climatological and secular variations are not discussed here. 



Forces caused by the graviational attraction between the moon, the 

 sun, and the rotating earth result in periodic level changes in large 

 bodies of water. The vertical rise and fall resulting from these forces 

 is called the tide or astronomical tide; the horizontal movements of 

 water are called tidal currents. The responses of water level changes to 

 the tidal forces are modified in coastal regions because of variations in 

 depths and lateral boundaries; tides vary substantially from place to 

 place. Astronomical tide generating forces are well understood, and can 

 be predicted many years in advance. The response to these forces can be 

 determined from an analysis of tide gage records. Tide predictions are 

 routinely made for many locations for which analyzed tide observations 

 are available. In the United States, tide predictions are made by the 

 National Ocean Survey, National Oceanographic and Atmospheric Administratioi 



Tsunamis are generated by several mechanisms: submarine earthquakes, 

 submarine landslides, and underwater volcanos. These waves may travel 

 distances of more than 5,000 miles across an ocean with speeds at times 

 exceeding 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 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 

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



Surges are caused by moving atmospheric pressure jumps and by the 

 wind stress accompanying moving storm systems. Storm systems are signifi- 

 cant 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. 



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 surge to occur 

 coincidently at the shore on the open coast. It is difficult to determine 

 how much rise can be attributed to each of these causes. Although astro- 

 nomical 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 interaction between tides and storm surge in 

 shallow water is not well defined. 



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