declination of the moon and sun with the earth, and the relative position of 

 these three astronomical bodies, influence the pattern and magnitude of tides. 

 The timing and magnitude of high tide and low tide at any given location can 

 be closely predicted by changes in these and other astronomical constituents. 



Three major types of tides can be distinguished, based upon the pattern and 

 frequency of occurrence of high and low tides during a tidal day. Tidal 

 bulges form on both sides of the earth in response to a balance of forces 

 associated with gravity, centripetal acceleration, and gravitational attraction 

 (Komar 1976). The earth rotating on its axis typically produces semidiurnal 

 tides, with two tidal maxima and minima every 24 hr and 50 min. This 

 period exceeds the length of a solar day because of the advance of the moon's 

 orbit. Diurnal tides display, on the average, one maximum and one minimum 

 each day. Diurnal tides occur where the typical cycle is complicated by other 

 astronomical factors. A combination of these characteristics, where two high 

 and low tides having large diurnal inequality occur daily, are known as mixed 

 tides. Mixed tides are commonly classified as being predominantly semi- 

 diurnal or diurnal (Defant 1958). Important tide-generating constituents, ex- 

 pressed as a ratio of the major components influencing diurnal tides to those 

 influencing semidiurnal tides, are used to measure these variations. 



In general, tidal range is highest when the moon is full or new, and when 

 the moon, sun, and earth are in alignment, or syzygy. These alignments 

 produce the spring tides, in which the high tide is higher and the low tide is 

 lower than average. Tidal range is lowest during the first and third quarters, 

 when the moon and sun align perpendicularly with the earth, a condition 

 known as quadrature. These less-pronounced high and low tides are col- 

 lectively known as neap tides. The sequence from spring to neap to spring 

 takes about 28 days, or one lunar month. Both spring and neap tides are 

 affected by drag forces, which cause tidal range to lag syzygy and quadrature 

 by up to 1.5 days. 



Longer period cycles are generated by varying lunar and solar distances, 

 caused by the ellipticity of both the moon's orbit around the earth, the earth's 

 orbit around the sun, and the sun's declination. The moon's orbit around the 

 earth is by far the more important of these. From perigee, when the moon is 

 closest to the earth, to apogee, when it is farthest, represents a change in 

 distance of about 13 percent. The earth is closer to the sun in early January 

 by about 3 percent than its farthest position in early July, causing tidal range 

 to be slightly greater in the Northern Hemisphere in the winter and fall than in 

 the spring and summer. 



Tidal range or magnitude is strongly affected by the depths and configura- 

 tion of the land masses encountered. Davies (1964) introduced coastal 

 classification divisions based on the spring-tide range, namely microtidal 

 (< 2 m), mesotidal (2-4 m), and macrotidal (> 4 m). Tidal magnitude is 

 influenced by the depths and configuration of the land masses encountered in 

 crossing the continental shelves, which cause local resonances and reflections 

 at land boundaries. 



Chapter 2 Relevant Processes and Factors 



15 



