Geoscience Reference
In-Depth Information
17
CHAPTER SEVENTEEN
coastline
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 ter-
restrial 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 geomor-
phic responses which further alter the coastline and its
sediment fluxes. Eustatic adjustments have climatic and
tectonic origins. Ocean-ice sheet coupling drives Quatern-
ary
glacio-eustatic
responses. It is primarily climate-
driven, but climate itself responds to tectonic as well as
radiative forcing.
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 ratio of land to ocean
area and average sea level. Coastal geomorphology, thus
perceived, classifies the work of the sea into erosional and
depositional processes and landforms and allows us to
concentrate first on the wave environment.
Wave-generated currents
Breaking waves send pulses of water shorewards until
they run out of momentum, whereupon gravity draws
