176
5. Tides
';)0·
Im _ _ _ _
~~ '
~ .. 2
"
~
\----,
,
,
,
,
Fig. 5.14: The amphidromic system in the North Sea (adapted from Bearman,
1997); dashed lines denote co-range lines, solid lines denote co-tidal lines
is created with water moving anticlockwise. In Fig. 5.12c, a set of co-tidal lines
is shown schematically for one full tide cycle.
A confined amphidromic system, as shown in Fig. 5.12, produces what is
known as Kelvin waves. Water level along particular co-tidal lines decreases
exponentially with increasing distance from the bank, i.e.:
/'
-ms
.., l"'Ve
,
(5.29)
where s is the distance from the bank and attenuation coefficient m = f / Viii,
In a real situation on the rotating Earth, the amphidromic systems are much
more complicated. Recently, sophisticated numerical models have been developed to predict tides around the Earth. They are based on the Laplace
tidal equations, augmented by the energy dissipation due to bottom friction,
especially over the shelf areas and in the marginal seas. The accuracy of