Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 266

Ap

Fig. 6.38 The Kelvin rotating tidal wave travels anticlockwise in the northern hemisphere, decreasing in amplitude inward toward the ampho-
dromic point, Ap , of zero displacement.

Force balance and Rossby
radius of deformation (L).

Horizontal pressure
gradient force

Coriolis
force

L

Fig. 6.39 Topography and bottom flow associated with the edge of an anticlockwise-rotating Kelvin tidal wave. The rotary component is
neglected for clarity.

this distance the amplitude of any Kelvin wave has reduced
to 1/ e , 0.37 of its initial value.
We may usefully summarize the vector variation of tidal
currents by means of tidal current ellipses whose ellipticity is
a direct function of tidal current type and vector asymmetry

(Fig. 6.40). For example, the inequality between ebb and
flow on the northwest European continental shelf is largely
determined by a harmonic of the main lunar tide. Since sed-
iment transport is a cubic function of current velocity it can
be appreciated that quite small residual tidal currents can