The global coastline - Fundamentals of the Physical Environment - page 400

COASTAL ZONE

Material in transit

BACKSHORE FORESHORE INSHORE OFFSHORE

MORPHOLOGY

Cliff or barrier
Berm edge

HWM

Coastal
plain

Beach face

MSL

LWM

Trough

Longshore bar

SWASH
ZONE

SURF ZONE BREAKER ZONE

WAVE
ENVIRONMENT NEARSHORE ZONE

Figure 17.1 The coastal zone and its component morphology, tide and wave environments, including high-water mark (HWM),
mean sea level (MSL) and low-water mark (LWM).

water back into the sea. This surf zone , between the
breaking waves and the point of maximum run-up , is one
of turbulent water exchange and maximum geomorphic
activity associated with wave-generated currents. Spilling
waves dissipate their energy forwards in the surf zone,
whereas surging waves are reflected back into following
waves. Forward and return pulses - swash and backwash
- move at right-angles to the shore in orthogonal waves,
generating shore-normal (right-angle) currents. Some of
the swash percolates beach sand and gravel, reducing the
immediate backwash volume but leading to later seepage
on the receding tide. The swash from refracted waves
moves diagonally onshore but backwash returns normally
down the maximum slope, resulting in a net longshore
current . Swash and backwash interfere with each other to
some extent and hold water up ahead of the breaking line.
Scarcely visible edge waves are established at right angles
to breaking waves through their incessant mass shoreward
transfer of water, escaping laterally rather than directly as
backwash. Water level is increased or set up when edge
wave and incoming wave crests coincide, and lowered or
set down when they are out of phase. Beach cusps form
where set-up and set-down cause incoming waves to break
farther from (deeper) and nearer (shallower) the shore
respectively, creating a sinuous breaking line. All three
processes establish lateral currents in the surf zone
which eventually drain seaward as powerful rip currents ,
completing a cellular pattern of water movement ( Figure
17.2 ). Lateral and longshore currents are responsible for
longshore drift of sediment. Rip currents cut and
accelerate through rip channels in soft sediments before
dissipating at nearshore rip heads .

Riphead

Onshore waves

Breaker zone

Longshore currents

Beach

Figure 17.2 The development of longshore and rip currents.

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 ( Figure 17.4 ). They
work together in storm surges which, although relatively
infrequent, may have major geomorphic impacts ( Figure
17.5 ). The annual average extent of surges over mean sea
level is about +0·6 m in Britain and up to +1·5 m on
tropical cyclone coasts. Hurricane Hugo created a storm
surge of +11 m on the South Carolina coast of the
United States in 1989. Distinctions between micro- , meso-
and macro tidal ranges (see below and Chapter 11)
demonstrate the persistent impact of tidal waves in