Page 105 - Fundamentals of Geomorphology

88 INTRODUCING LANDFORMS AND LANDSCAPES

that weaken the rock before abrading it with teeth and may carry previously eroded beach material or fluvial
other hard parts. Grazing animals include gastropods, sediments from the offshore zone to the littoral zone.
chitons, and echinoids (p. 57). Very severe storm waves, storm surges, and tsunamis may
carry sediments from beyond the offshore zone. During
Wave erosion the Holocene, sediment deposited on exposed continen-
tal shelves and then submerged by rising sea levels has
The pounding of the coast by waves is an enormously been carried landwards. In some places, this supply of
powerful process of erosion. The effects of waves vary sediment appears to have dried up and some Holocene
with the resistance of the rocks being attacked and with depositional landforms eroding.
the wave energy. Where cliffs plunge straight into deep Tides and wave action tend to move sediments
water, waves do not break before they strike and cause towards and away from shorelines. However, owing to
little erosion. Where waves break on a coastline, water is the effects of longshore currents, the primary sediment
displaced up the shore, and erosion and transport occur. movement is along the coast, parallel to the shore-
Plunging breakers produce the greatest pressures on line. This movement, called longshore drift, depends
rocks – up to 600 kPa or more – because air may become upon the wave energy and the angle that the waves
trapped and compressed between the leading wave front approach the coast. Longshore drift is maximal when
and the shore. Air compression and the sudden impact waves strike the coast at around 30 degrees. It occurs
of a large mass of water dislodge fractured rock and other below the breaker zone where waves are steep, or by
loose particles, a process called quarrying. Well-jointed beach drift where waves are shallow. Beach drift occurs
rocks and unconsolidated or loosely consolidated rocks as waves approaching a beach obliquely run up the
are the most susceptible to wave erosion. Breaking waves shore in the direction of wave propagation, but their
also pick up debris and throw it against the shore, causing backwash moves down the steepest slope, normally per-
abrasion of shoreline materials. Some seashore organisms pendicular to the shoreline, under the influence of grav-
erode rocks by boring into them – some molluscs, boring ity (Figure 3.18). Consequently, particles being moved
sponges, and sea urchins do this (p. 57). by swash and backwash follow a parabolic path that
slowly moves them along the shore. Wherever beach
Aggradational processes
Sediment transport and deposition
Coastal sediments come from land inland of the shore Beach drift
or littoral zone, the offshore zone and beyond, and Land
the coastal landforms themselves. In high-energy envi- Sand particles
ronments, cliff erosion may provide copious sediment,
but in low-energy environments, which are common Swash
Swash
in the tropics, such erosion is minimal. For this rea- Backwash
Backwash
son, few tropical coasts form in bedrock and tropical
cliffs recede slowly, although fossil beaches and dunes are
eroded by waves. Sediment from the land arrives through
mass movement, especially where cliffs are undercut.
W
Gelifluction is common in periglacial environments. Waves
aves
Nevertheless, the chief sediment source is fluvial erosion.
Longshore current
Globally, rivers contribute a hundred times more sedi- Longshore current
ment to coasts than marine erosion, with a proportionally
greater contribution in the tropics and lower contribu- Figure 3.18 Beach drift.
tion at higher latitudes. Onshore transport of sediments Source: After Butzer (1976, 226)