H = 0.707 (18.5) Jlogg -— ^ = 29.2 m , say 29 m 



H^ = 0.707 (18.5) Jlogg ~ = 27.9 m , say 28 m 

 *************************************** 



VIII. WATER LEVEL FLUCTUATIONS 



The focus now changes from wave prediction to water level fluctuations in 

 oceans and other bodies of water which have periods substantially longer than 

 those associated with surface waves. Several known physical processes combine 

 to cause these longer term variations of the water level. 



The expression water level is used to indicate the mean elevation of the 

 water when averaged over a period of time long enough (about 1 minute) to 

 eliminate high-frequency oscillations caused by surface gravity waves. In the 

 discussion of gravity waves the water level was also referred to as the still- 

 water level ( SWL) to indicate the elevation of the water if no gravity waves 

 were present. In the field, water levels are determined by measuring water 

 surface elevations in a stilling well. Inflow and outflow of the well is 

 restricted so that the rapid responses produced by gravity waves are filtered 

 out, thus reflecting only the mean water elevation. Measurements without a 

 stilling well can be made and the results numerically filtered to obtain the 

 Stillwater level. 



Water level fluctuations classified by the characteristics and types of 

 motion which take place are identified as: 



(a) Astronomical tides 



(b) Tsunamis 



(c) Seiches 



(d) Wave setup 



(e) Storm surges 



(f) Climatological variations 



(g) Secular variations 



The first five fluctuation categories have periods that range from a few 

 minutes to a few days; the last two have periods that range from semiannual to 

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

 climatological and secular variations are not discussed here. 



Forces caused by the gravitational 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 astronomiaal 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. Astonomical 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 Service (NOS), National Oceanic and 

 Atmospheric Administration (NOAA) . 



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