Geoscience Reference
In-Depth Information
environmental ranges for their occurrence (e.g.
Potts & Jacobs 2003; Chapter 9), but these ranges
cover a variety of corals, some of which have
variable ways of obtaining energy and some of
which are adapted for variable conditions.
10.2.3.1 Tide-influenced shelves
Tidal forces affect the whole globe, but their influ-
ence on the world's oceans is heavily modified
by the bathymetry of the continental shelves and
by the shape of the coastlines. Tides may occur
daily (diurnal) or twice-daily (semi-diurnal): for
most places around the world, the semi-diurnal
tide is dominant. The dynamics of many contin-
ental shelves are dominated by tides, especially
where tidal ranges are high (Chapter 1), but even
in areas of low tidal range, tidal currents can be
strong locally, for example in the Torres Strait,
Australia (Harris 1989, 1991) and the Hayasui
Strait, Japan (Mogi 1979). On most shelves,
the flood and ebb current directions are gener-
ally opposed, but as they change in speed they
also change direction, so that, overall, the tide
describes an open ellipse, with a net direction and
magnitude (Stride 1982). Should the threshold
for bed sediment transport be exceeded by one
or both of the tidal currents, net bed sediment
transport results. Over long periods, the result
is a pattern of regional bed-sediment-transport
pathways, which begin at bed sediment 'parting
zones', where they tend to be associated with lag
surfaces, and end at bed sediment 'convergences',
where there is an accumulation of sandy sedi-
ment. On the UK shelf, individual transport
10.2.3 Sediment accumulation processes and
disturbance events
The wide distribution of modern continental
shelves means that, as a group, they experience
a range of environmental processes (Fig. 10.3)
and thus contain a diverse group of depositional
environments. The energy required to trans-
port sediments can be derived from a number
of sources. In most cases, the dominant energy
supply is either tidal or weather-related, al-
though there is generally a mixture of both. It is
also important to consider the relative impact of
daily (generally low-energy) versus episodic high-
energy phenomena (e.g. hurricanes, tsunami).
Finally, there are very well-developed models
of across-shelf transport (e.g. Fig. 10.5), but it is
now acknowledged that, for most shelves, the
along-shelf component of sediment transport is
greater than that across-shelf, especially where
the coastline (or shallow bathymetry) is relatively
straight (e.g. Great Barrier Reef shelf, Otago
shelf, Texas-Louisiana shelf, this chapter).
Wind-driven along-shelf flows
Estuary
Estuarine
plume
10
Int
e
rnal waves
20
Physical and
biological mixing
of sediment column
30
40
Steep gradients in
cross-shelf sediment flux
Inner shelf Mid-shelf
Fig. 10.5
Processes of cross-shelf transport. (Adapted from Nittrouer & Wright 1994.)
