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In-Depth Information
pathways are up to 550 km long and 170 km
wide (Belderson et al. 1982). The suite of sedi-
mentary facies and bedforms present along such
paths is well-defined on tidally dominated shelves
and is related both to the peak speed of the tidal
current and to sediment availability.
Volumetrically, there is generally little prac-
tical impact that humans can have on these
sediment transport pathways. A small transport
pathway of 30 km in length, 5 km wide and
0.5 m average thickness would contain around
130
wind blowing over the sea surface drags water
along with it, forming wind-driven currents.
These currents are therefore fastest at, and often
limited to, the sea-surface, but where the wind
is of sufficient strength, duration and fetch, the
whole depth of the shelf water mass may be
influenced. A general rule-of-thumb is that the
speed of the surface wind-driven current is up
to
c
. 3% of the wind speed (as measured 10 m
above the sea surface). Therefore, during storms,
wind-driven currents can be of significant mag-
nitude (see also Case Study 9.2).
It is useful to distinguish storm-driven fast
unidirectional flows combined with waves,
compared with the effects of long-period waves
acting alone. In this context, storms include hur-
ricanes, typhoons and cyclones, which directly
influence the shelf with wind-driven currents,
waves and land runoff. The Caribbean region
experiences an average of seven hurricanes
each year, some of which enter the Gulf of
Mexico. The Texas-Louisiana shelf has an even
slope of around 1:1000, with the 80 m contour
located
c
. 80 km offshore. In September 1961,
Hurricane Carla, category 5, took 3 days to
pass 400 km north-north-west across the shelf,
producing a storm sand bed that extended for
>
10
6
t of sediment (assuming porosity of
30% and density of 2.5), which is, for example,
around five times the total weight (25
×
10
6
t) of
aggregate extracted from UK waters each year.
Although apparently reassuring, this is an over-
simplified view because sediment transport rates
along the pathways are generally poorly known,
and thefefore there are difficulties in assessing a
sustainable rate of removal of material. If the
rate of removal exceeds the rate of resupply, or
the deposits are relict, there is the potential to
create a long-term change in bed habitats.
×
10.2.3.2 Storm-influenced shelves
Other reviews have noted that most (perhaps
80%) of the world's shelves are storm-dominated
(e.g. Johnson & Baldwin 1996; Allen 1997). The
effects of both waves and wind-driven currents
are relevant. Low-pressure weather systems over
the sea generate waves with a range of periods
and heights, which may influence shelf sedimenta-
tion directly. Sedimentation on some shelves,
however, can be dominated by swell waves gen-
erated by storms in adjacent sea areas. In deep
water, waves travel at speeds relative to their
wavelength,
200 km along the shelf. The bed was preserved
mostly within the 30 m contour, was thickest,
at up to 9 cm, within the 20 m contour and was
recognizable down to the 50 m depth contour.
At the surface, the bed decreased in grain size
offshore. The storm sand bed fined upwards,
from a sharp basal surface, through planar lam-
inations to shallowly inclined laminations up
to a gradational upper surface. Close to shore,
the upper surface was sharp and truncated
(Snedden & Nummedal 1991). These sediment-
ary data are consistent with numerical models
of shelf flow, which predicted that the wind
regime would have produced:
1
a shoreward flow to the north-west at the
surface;
2
a fast shelf-parallel flow to the south-west of
>
C
=√
(
g
λ
/2
π
)
i.e. wave speed
=√
[(gravity
×
wavelength)/(2
×π
)]
so that long waves travel fastest away from the
centre of a storm. Long waves also carry the
most energy and transfer that energy deepest
into the water column, so that they are able to
mobilize sediment on relatively deep areas of
continental shelves. As well as generating waves,
1.5 m s
−1
in depths around 20 m;
3
an oblique offshore return flow to the south-
south-west or south near the bed (Forristall
et al. 1977; Keen & Slingerland 1993).
