Environmental Engineering Reference
In-Depth Information
Fig. 1.21
Spring and neap tides. a Spring tide, b neap tide
nature, i.e., two high tides and two low tides
(each of 6 h 25 min) on each lunar day.
It may so happen that, while the sun, the moon
and the earth are in conjunction, the moon is at
perigee, i.e., shortest distance in its orbit from the
earth. The tidal forces are mightiest under such
circumstances, and the tides are higher than the
usual. If in addition, the sun is on the equator,
and the perigee spring tides are the strongest.
This happens in the equinoxes in March and
September.
The tidal currents, produced by the ebb and
In semi-diurnal tide, two high waters and
two low waters occur in each day with relatively
small differences in the respective highs and
lows. Examples of such tides are the tides on the
Atlantic coast of USA. In diurnal tides, only a
single high and a single low water occur each
tidal day. Tides of diurnal type occur along the
northern shore of the Gulf of Mexico, in the Java
Sea and the Gulf of Tonkin. Mixed tides are
characterized by large diurnal inequalities in
heights and/or times of successive high and/or
low waters. In general, this type of tide inter-
mediates between predominantly semi-diurnal
and a predominantly diurnal. Such tides are
prevalent along the Paci
fl
cance in open sea,
but in the near-shore zones (depending on the
size, con
ow of tides, have little signi
guration and topography of the marine
basins), they control the coastal morphology to a
great extent. The coasts can be categorized into
macrotidal (<2 m), mesotidal (2
c coast of the USA and
in many other parts of the world. At Los Angles,
it is typical that the inequalities in the high and
low waters are about the same. At Seattle, the
greater inequalities are typically in the low water,
while in Honolulu, it is the high waters that have
the greater inequalities. Table
1.7
exhibits the
tidal range at different corners of the globe.
In general, tides have two basic components.
4 m) and mi-
crotidal (>4 m) depending on the tidal range.
Tides are normally semi-diurnal in nature.
However, diurnal and mixed tides are also
observed in some places in the ocean and bays.
Hence, tides are classi
-
ed as one of three types,
semi-diurnal, diurnal or mixed according to the
characteristics of the tidal pattern.
Magnitude.
Time.
