The work of the sea - Fundamentals of the Physical Environment - page 529

Chapter 17
The work of the sea
The work of the sea is focused at the coastline, probably the most active and diverse of
Earth's geomorphological features, found in every tectonic and climatic setting. It
stretches for 0·5 M km around the margins of every continent and island - ten times
farther than intra-plate boundaries - and is familiar to most people. Indeed, 50 per cent of
the population of the industrialized world and perhaps 60 per cent of all people live
within 50 km of the sea. The narrow coastal zone occupies less than 0·05 per cent of
Earth's land area but has powerful attractions for agriculture, industry, transport,
residence and recreation. Consequently, we want it to stay where it is! Most of us were
unwitting geomorphologists in our youth as we built sandcastles, doomed by the tide. We
also recall Cnut (Canute), the Anglo-Danish king of England in the early eleventh
century, and his legendary demonstration that even regal power cannot withstand the
relentless motion of tides and waves. The coastline is sensitive to rapid geological and
biophysical change and we are braced to respond to sea-level rise promoted by global
warming.
The coastal zone locates the interaction between terrestrial and marine environments
and embraces several components. The coastline is the outermost limit of permanent
land, which separates the broader coastal hinterland from shore and marine environments.
The backshore occupies land above modern, average high tides but is storm-swept.
Moving seawards, the foreshore (shore) lies between high and low tide limits. Beyond it
is the inshore zone of breaking or shoaling waves , flanked by an offshore zone of
deeper water which occupies inner margins of the continental shelf (Figure 17.1).
Foreshore and inshore zones together comprise the nearshore wave environment. The
coastal zone is a hybrid of terrestrial and oceanic systems at their common boundary,
driven by a series of exogenic and morphotectonic processes, integrated in some respects
and disconnected in others. Wave and tidal (exogenic) energy is at the heart of coastal
processes but we also know that land : ocean area and sea levels are integrated through
tectonic processes and climatic change. They disturb coastal equilibrium through uplift
and isostatic adjustment, triggering geomorphic responses which further alter the
coastline and its sediment fluxes. Eustatic adjustments have climatic and tectonic origins.
Ocean-ice sheet coupling drives Quaternary glacio - eustatic responses. It is primarily
climate-driven, but climate itself responds to tectonic as well as radiative forcing.
WAVE, CURRENT AND TIDAL ACTION
The coast is sculptured primarily by wave action which erodes the land surface in one
place and recreates it elsewhere. This dynamic equilibrium between tide-wave energy
and Earth materials works to maintain an 'average coastline' determined by the average