Geoscience Reference
In-Depth Information
particularly unstable and prone to storm surges. Rocky
coasts are more enduring but failure is often dramatic
when it occurs. Evidence of the geomorphic and human
consequences of historic sea-level change demonstrates
the changeable character of the coastline and the problems
posed by impending sea-level rise. Increasing apprecia-
tion of coastline complexity and sensitivity is leading to
more varied, holistic and pragmatic strategies in its
management. Cnut's message is being heard!
KEY POINTS
1
Moving water and air shape the coastline and modify the effects of other sub-aerial processes of weathering
and erosion. Water energy is derived from wind-driven breaking waves and, to a lesser extent, tidal waves
and currents. It is applied through hydraulic pressure, water jets and sheets. Wind energy deflates sand
from the backshore to create and subsequently alter littoral barriers.
2
Breaking waves establish shore-normal and longshore currents of water and energy. The precise wave
form depends on water depth, the shelving angle of the inshore zone and the direction of approach
determined by wind and coastline configuration. Tidal waves and currents also move large bodies of water
around the coast. Tidal frequency, range and coastline shape determine their velocity.
3
Coastal marine geomorphic processes are concentrated in the foreshore or intertidal zone and inshore surf
zone, influenced by breaking wave style and height. Hydraulic shock, attrition and corrasion erode rock
coasts and comminute their debris, aided by water-layer weathering in the intertidal zone. Sediment
movement depends on the availability and direction of water currents at or above the entraining velocity
for individual particles. Storm waves inevitably cause the greatest amounts of erosion and sediment
movement.
4
Soft-sediment coastal landsystems develop within partially enclosed estuaries or lagoons and as coast-
parallel barriers. Aeolian processes may ornament the barriers with dunes. Deltas form a third landsystem
at the seaward extension of fluvial sediments, shaped by the wave and current environment. However,
75 per cent of all coasts are rocky, with their own characteristic rock platform, cliff and cliff remnant
landforms. Biogeomorphic processes create important coastal structures in the form of salt marsh,
mangrove swamp, reefs, bioherms and vegetated dunes.
5
Coastal patterns are discernible at the global scale. Morphotectonic distinctions between leading and
trailing-edge coasts are reflected in the respective dominance of rock/erosion-dominated coasts and river-
fed/depositional coasts. Tidal patterns vary according to diurnal/semidiurnal frequency and tidal range,
whilst high to moderate wave energy patterns depend on location within or outside storm belts. Complex
isostatic-eustatic adjustment throughout the Holocene has created patterns of emergent and submergent
coasts. Global sea levels are now rising slowly as a consequence of global warming through ocean-
atmosphere-ice sheet coupling.
FURTHER READING
Davis, R. A. and Fitzgerald, D. M. (2004) Beaches and Coasts, Malden, MA, and Oxford: Blackwell. A copiously illustrated
modern textbook, black-and-white photographs and two-tone figures, with a good introduction to global and tectonic
coastlines, followed by comprehensive cover of coastal geomorphology.
French, P. W. (2001) Coastal Defences: processes, problems and solutions, London and New York: Routledge. An excellent
volume, complementing geomorphological texts with the coastal management side of dynamic contemporary
coastlines.
