Geoscience Reference
In-Depth Information
8.3.3 Controls on sediment accumulation and
transport
wave crests are reorientated is known as re-
fraction and it is important in distributing wave
energy along a shoreline. A related process of
diffraction, by which wave energy is transferred
laterally along the crest, also occurs as wave crests
bend. Because swell waves have longer wave-
lengths they interact with the sea-bed further off-
shore and hence arrive more fully refracted than
short wavelength waves. Full refraction tends to
arrange wave crests parallel to the shoreline and
to equally distribute energy alongshore. Short
period, locally generated waves exhibit greater
energy differences alongshore due to incomplete
refraction.
Waves undergo further modifications as they
approach the shore and a series of zones are
defined according to wave behaviour near the
coast (Fig. 8.8). As waves interact with the sea-bed
and slow, they become higher. The zone in which
this takes place is known as the shoaling zone.
Wave steepening may lead to instability as the
wave becomes too high relative to its length. It
then 'breaks' as gravity collapses the waveform.
A range of breaking criteria has been determined
empirically (Komar 1998), and several breaker
types are recognized in a continuum including
surging, collapsing, plunging and spilling breakers
related to the mode of energy release.
Landward of the breaker zone, wave energy
continues to propagate shoreward in what is
known as the surf zone (Fig. 8.8). This is a zone
of often intense turbulence as waves reform,
break again and generate secondary wave and
current motions. Much wave energy is dissipated
in the surf zone through breaking, turbulence
and sediment transport. The surf zone is vari-
able in width. Wide surf zones are associated
with large waves, which dissipate their energy
through a series of spilling breakers. Lower
energy beaches have narrow surf zones in which
wave energy is dissipated through breaking close
to the shore. Reorganization of wave energy
can lead to generation of secondary wave forms
(edge waves) that have longer wave periods
than incident waves. These have crests that are
oblique or perpendicular to the shoreline. They
can be stationary or may propagate along the
shore and give rise to widely spaced inequalities
Sediment transport in the nearshore zone is a
highly complex phenomenon that occurs through
a variety of interlinked processes operating at
spatially and temporally variable intensity. A
synopsis is presented here; for a more thorough
review of sediment processes in the nearshore
the reader is referred to Komar (1998) or
Woodroffe (2002). The dominant force in sedi-
ment transport and geomorphological change
in the coastal zone is wave action. Waves are
formed by winds blowing over a water surface.
They are typically described in terms of wave
length, height and period. Once formed, waves
radiate out from the generation area and the
wave form propagates across the water surface.
Typically, a wide range of wave sizes (defined by
wavelength and period) are produced by winds
at the source area. The waves formed directly in
the area of generation tend to be of various sizes
and the water is very 'choppy'. Larger waves
are produced by winds of greater speed and
duration, and greater fetch distances. As waves
travel further from the generation area they
become sorted and amalgamated. Longer waves
travel faster and arrive at distant coasts first as
a regularly spaced set of large waves (termed
swell). Oceans have large fetches and hence large
waves are produced. Coasts facing open oceans
are thus typically dominated by such conditions.
In semi-enclosed seas, most waves are generated
by more proximal winds blowing across restricted
fetches and hence the waves are smaller and
more variable in their dimensions.
As waves move from deep to shallow water,
friction with the sea-floor retards the wave,
which maintains its energy by steepening. Waves
with longer wavelengths 'feel' the sea-floor
first and are slowed comparatively far from
the shore. The point at which significant wave-
sediment interaction begins is known as wave
base (see Chapter 1). It occurs at variable depth
depending on the length of the waves. Waves
approaching a shoreline obliquely thus change
the orientation of their crests in response to
bottom bathymetry. This process by which the
