Environmental Engineering Reference
In-Depth Information
TIDAL ACTION AND CURRENTS
Although breaking wave systems appear to dominate coastal geomorphology, tidal waves
and tsunamis (see Chapter 11) are also important in a number of respects. Geomorphic
activity is concentrated in the surf zone, whose maximum vertical and lateral extent is a
product of tidal range as well as of wave height (see box, pp. 357-8, Figure 2). They
work together in storm surges which, although relatively infrequent, may have major
Figure 17.2
The development of longshore and rip currents.
WAVE FORM AND ACTION
key processes
The origin and general behaviour of waves in transmitting energy was outlined in
Chapter 11. Important relations exist between
wavelength
(
L
),
wave base
(
L
/2) and water
depth. Energy is converted to work on meeting the coast, and its geomorphic impact
depends on a combination of wave form and coastal material properties. Wave form is
the outcome of offshore features of approaching waves, modified by water depth in the
inshore zone, coastline geometry and wind-induced wave direction.
Reflected waves
rebound from cliffs terminating in deep water and meet incoming waves to form a
standing wave
which does not break. In all other cases, as waves enter water depth
below
L
/2 and bed friction destabilizes the orbital path of water particles, they are
transformed into
breaking waves
(Figure 1). The retarded wave increases steadily in
height to conserve energy and thereby raises the potential energy of the wave, which is
released as it breaks. The
starting
height of the wave offshore is determined by the wind
environment but its
breaking
height increases with the rate of
shoaling
(shallowing or
shelving) of the nearshore zone. The higher the wave, the greater the energy it delivers to
the shore.
