Geoscience Reference
In-Depth Information
Table 10.2
Volumes of marine sand
and gravel extracted from the western
European shelf for the period 1992-97
(OSPAR Commission 2000).
Country
Total
×
10
3
(m
3
)
Average
×
10
3
(m
3
yr
−
1
)
Belgium
11,000
1833
Denmark
30,500
5083
France
13,200
2200
Germany
17,000
2833
Iceland
No information
No information
Ireland
Zero abstraction
Zero abstraction
The Netherlands
104,200
17,366
Norway
710
118
Portugal
No information
No information
Spain
No information
No information
Sweden
Zero abstraction
Zero abstraction
UK
81,600
13,600
Table 10.3
Summary of UK extraction of terrestrial and marine aggregates (EMU 2004).
Total area
Total area
Percentage
Volume extracted
Maximum
Average
licensed (sand
worked
area worked
annually (England
size of site
size of site
and gravel)
annually
annually
and Wales)
270 km
2
150 km
2
tyr
−1
9.45 km
2
1km
2
Terrestrial
55
70
Μ
<
1300 km
2
150 km
2
tyr
−1
Marine
11.5
23
Μ
shallow linear furrows 1-3 m wide and 0.2-
0.3 m deep (Kenny & Rees 1994, 1996). Other
extraction methods result in saucer-shaped de-
pressions up to 200 m in diameter and 8 -10 m
deep, with slopes of
c
. 5° (Wenban-Smith 2002;
EMU 2004). Together with the direct physical
impacts on the sea bed, typically 5% of dredged
material is returned to the sea bed during the
dredging process, but this may increase to 60%
when screening is used to increase the propor-
tion of gravel in the cargo. There are thus also
impacts from the settling of sediments through
the water column back to the sea bed.
The impacts of aggregate dredging are de-
pendent upon the nature of the substrate being
dredged. In physical terms, impacts include:
1
The formation of dredge tracks or depressions
on the sea bed (Fig. 10.11).
2
Disruption of the surficial sediments, leading
to release of sands and finer sediments from the
matrix and from underlying deposits.
3
Generation of sediment plumes at the sea-bed
by the action of the drag head, and plumes at the
sea surface which then settle to the bed.
4
Changed grain-size characteristics of the bed
sediments (McCauley et al. 1977), which might
be an increase in gravel content through the ex-
posure of coarser sediments (Kenny et al. 1998),
an increase in the proportion of fine sands
(Desprez 2000; van Dalfsen et al. 2000), loss
of silt- and clay-sized grains (Boyd et al. 2003,
2004) and organic material (EMU 2004).
5
Temporary changes in the sediment trans-
port regime at and near the bed. Typically, this
includes the introduction of a local 'slug' of sandy
sediment onto the bed, derived from settling of
sediments rejected by the screening process on
board the dredge and from sands released from
the sea bed. Initially, these sands are generally
deposited within a few hundred metres of the
dredge site. Silts, clays and organic matter are
distributed more widely, and all are then liable
to redistribution by waves and currents.
It has long been recognized that the sediment-
ary environment (composition, texture, struc-
ture and stability) is a major control upon the
benthic biological assemblage (Holme 1961,
1966; Holme & Wilson 1985). Studies of impacts
