The global coastline - Fundamentals of the Physical Environment - page 401

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 17.3 ). 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 breakingheight 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.

Waves break when the critical ratio of water depth to wave height lies between 0·6 and 1·2, around a mean value
of 0·78. In other words, average waves break in water depths a little less than their own height, and so low waves
run farther into shallower water than high waves before breaking. The shoaling angle is also important. Waves break
close inshore where it is steep and farther out on flat shores, measured by the breaker coefficient , B.

B = H/LS 2

where H = wave height, L = wavelength and S = bed slope. The breaking style influences the way in which wave
kinetic energy is used, and four styles are recognized ( Figure 17.4 ). The breaker coefficient falls from spilling to surging
styles as bed slope angles increase. Most waves do not approach the coastline orthogonally, with wave crests parallel
to the shore, but are driven obliquely onshore or meet an indented coastline. Waves are retarded around headlands
but drive on less impeded into bays. Such refracted waves alter the pattern of energy flux at the coast, with energy
convergence around headlands and divergence in bays (see Plate 17.1 ).

varying the amplitude of the surf zone. Tidal processes
dominate coasts, with macrotidal ranges over 4 m, and
breaking waves truly dominate only in the microtidal
environment.
Tidal ebb and flow stimulate tidal currents with
substantial fluxes of water, energy and sediment around
the coastline ( Figure 17.6 ). Water density differences based
on salinity, and the extent of water body mixing or
separation, vary the patterns of circulating currents.
Current velocities vary with the size of tidal passes and
the frequency of tidal inundation. Tidal wave velocity in
open waters is less than 0·05 m s -1 but can reach 0·3-3
m s -1 through confined passes. Semi-diurnal tides move
approximately twice as much water in one day as diurnal
tides at about twice the diurnal velocity. The duration of
intertidal exposure varies by the same token and, with it,
the opportunity for and nature of drying out, weathering
and biological activity above the low-water mark. The
extent of the intertidal zone depends on coastline configu-
ration and tidal range. Foreshore area exposed and

(a)

Seawall/ cliff

>45°

(b)

Surf Breaking wave

L
2

Wave base

Figure 17.3 (a) Reflected and standing waves, and (b) the
effect of shoaling on the orbital path of water particles, leading
to breaking waves.