Geoscience Reference
In-Depth Information
using the time of arrival of material derived from
the Mount St Helen's eruption of May 1980
(Chapter 2), which had been carried to the shelf
by the river. Only 17 months after the eruption,
the ash had been transported to the north-north-
west, at least 125 km along the shelf and 40 km
across it (Ridge & Carson 1987).
Polymodal shelf sediments aid the delineation
of shelf sedimentary facies and infer character-
istics of the sediment environments. Woolfe et al.
(2000) analysed the silt and sand fractions of 300
samples from the central GBR shelf (Fig. 10.9)
to show that the muddy sediments supplied by
the Herbert River (Group 2) either are trapped
in sheltered environments close to the coast or are
deposited below the 10 m depth contour on open
coasts. Sediment accumulation rates measured
using
210
Pb and
137
Cs profiles from core samples
are 0.7-12.3 kg m
2
yr
−1
(mean
c
.4kgm
2
yr
−1
)
(Brunskill et al. 2002). These muds are separated
from the coastline by an erosional (or at least
highly mobile) sandy subtidal zone (Group 1),
and to seawards, much of the shelf is occupied
by a muddy, medium to coarse-grained terrigen-
ous or calcareous sand (Group 3) of very low
accumulation rate (generally
bathymetry and digital side-scan sonar) to pro-
duce three-dimensional digital terrain models
of the sea bed, which show sediment distribu-
tion and bedforms. Repeated surveys can show
net changes in bed elevation, and are now stand-
ard industry practice in assessing sedimentary
impacts of offshore activities. Grabs and corers
remain essential elements of field studies (Aller
et al. 2004), and nowadays include refined corers
that sample the water-sediment interface undis-
turbed. Accumulation rates of sediments can be
made in muds using various radio-tracers (e.g.
Brunskill et al. 2002).
Measurements of currents and waves are
now generally of very high quality, for exam-
ple using acoustic devices such as Acoustic
Doppler Current Profilers (ADCPs) deployed
from buoys, vessels or at the sea bed. Instru-
ment packages deployed at the sea bed can
allow simultaneous measurements of near-bed
sedimentary processes over periods of days to
weeks (e.g. Cacchione et al. 1999). Sediment
traps have limited use in shelf studies, because
of strong horizontal transport and resuspen-
sion events (e.g. Topçu & Brockman 2001;
Thomas & Ridd 2004). At the bed itself, time-
series measurement of sediment accumulation
or erosion is possible at a single point (e.g.
Larcombe et al. 1995; Thomas & Ridd 2004).
Direct measures of bedform migration are pos-
sible through use of video cameras, or sector-
scanning sonar. Acoustic techniques also exist
to assess sediment transport, especially of sand
(Thorne & Hanes 2002). Optical devices now
exist for measuring grain-size distributions
in situ
(Gartner et al. 2001; see also Van Walree et al.
(2005) for progress on acoustic techniques), and
although real-time field measurements of sedi-
ment transport rates are increasingly common,
they generally remain scientific and investigative
techniques rather than being applicable tools.
The algorithms applied to satellite images are
improving (Binding et al. 2003), but are not yet
sufficiently robust to be generally applicable to
sedimentary studies. Most shelf sediment trans-
port occurs near the sea-bed, where information
from satellites is weakest, especially when sedi-
ment transport rates are high.
0.1 kg m
2
yr
−1
).
Clearly, on many shelves, characterizing sedi-
ments by mean grain size is a simplification. Vari-
ous computer programs are used by industry to
predict sediment transport on continental shelves,
some of which use a single grain size. Although
such a simple approach can sometimes fit the
purpose, predictive models of shelf sediment
transport are increasingly being demanded in
assessments of human impacts, whether regard-
ing marine engineering projects or regional stud-
ies of pollution dispersal and accumulation, so
that more complex models need designing and
testing. Porter-Smith et al. (2004) have com-
bined information on shelf sediment texture with
hydrodynamic models to delineate regions of the
Australian continental shelf, based on sediment
transport processes.
<
10.2.3.9 Measuring sedimentary processes
Increasingly, studies on shelves use sea-bed
mapping techniques (high-resolution swath-
