EM 1110-2-1100 (Part II)
30 Apr 02
Figure II-5-6.
Spring and neap tides (Shalowitz 1964)
(b) Diurnal inequality can be explained as follows. The tidal bulge is centered along a line from the
center of the moon or sun to the center of the earth. The tidal bulge at a given sublunar or subsolar (location
on the earth nearest the moon or sun) location has an equivalent bulge on the opposite side of the earth, i.e.,
on a line drawn from the sublunar or subsolar point through the center of the earth on the opposite side of the
equator. If the sublunar or subsolar point appears at a given north latitude, the peak of the corresponding tidal
bulge on the opposite side of the equator will appear at a corresponding south latitude. Thus, a point on the
same north latitude but 180 deg in longitude from the sublunar or subsolar point will show a reduced
amplitude.
(c) A schematic example of the daily inequality is presented by Dronkers (1964) for the simple case of
an earth-moon system. Referring to Figure II-5-7, the moon is located in the direction M and earth is rotating
about the polar axis P. The deformed water surface resulting in response to the tide-producing forces is
shown in the figure. Four locations (I - IV) are indicated to demonstrate the effect of location on the diurnal
inequality. The fluctuating tide can be seen as the deviation in the deformed surface from a line at constant
latitude on the undeformed spherical surface corresponding to each location. Location I corresponds to an
observer on the equator. In this case, it can be seen that the tidal deformations from static conditions are
equal; therefore, there is no diurnal inequality, each high tide is equal. However, at locations II and III, the
inequality is evident with the second tide being substantially lower than the first. In the extreme case,
location IV exhibits a diurnal tide only due to its location with respect to the deformed water surface.
(d) The combinations of astronomical forcing that define spring and neap cycles and diurnal inequalities
is further modified by local bathymetry and shoreline boundary influences. All of these factors combine to
produce tidal envelopes that vary from location to location. The result is site-specific tidal signatures, which
can be classified as semidiurnal, diurnal, or mixed. Examples of these classes of tides are shown in
Figures II-5-8 and II-5-9. Tides along the Atlantic coast are generally semidiurnal with a small diurnal
inequality. Typical east coast envelopes for Boston, MA; New York, NY; Hampton Roads, (Hampton), VA;
II-5-10
Water Levels and Long Waves
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