Over the last decade or so there has been a shift in
thinking about how to handle the disposal of dredged material
derived from port maintenance and similar activities.
The emphasis has turned from disposal to the beneficial use of
such material for environmental gain. For example, beneficial use
has been shown to successfully protect and/or create salt marshes
and mud flats, which in turn may provide useful flood and coastal
defence. These inter-tidal habitats are also important ecosystems
in their own right, and relatively rare in Britain.
However, beneficial use must not be seen as the solution to all
problems. For a start some material may be better suited for such a
use than others.
"Capital" dredged material is typically coarse or bulky material
which, once deposited at a marine disposal site, tends to remain in
place. In contrast, "fine" material (e.g. muddy sand) obtained from
maintenance dredging is less likely to remain at a disposal site.
Sediment movement can be potentially detrimental and have greater
indirect ecological effects.
Within the UK there have been a number of small-scale
trials for beneficial use. This is because of our lack of
understanding of the biological processes following sediment
deposal. The re-colonisation of sediments by marine plants and
animals may have profound effects on the stability and fate of
deposited material, in both the short and long term.
We also don't know enough about how beneficial use affects
invertebrates, which limits our ability to predict any indirect
effects on birds and fish. This is particularly important as most
beneficial-use schemes are situated on estuarine inter-tidal
habitats, areas important for sustaining such populations.
Beneficial use and disposal options have their own
logistical, legal, economic and environmental limitations. Each
case should be assessed on its own merits. Nevertheless, under
certain circumstances, the concept of the beneficial use of dredged
material can result in solutions that satisfy the needs of
industry, the regulator and society at large.
Flood and coastal defence
In southeast England, salt marshes are eroding at an
unprecedented rate. This has important consequences for our flood
and coastal defences as mud flats and salt marshes help to
dissipate wave energy that often leads to erosion.
A sea wall's toe can become exposed, resulting in
underpinning and slippage, and the wall becoming destabilised. The
funds needed to build and maintain a wall without a fronting salt
marsh are significantly more than those with a marsh. For example,
a seawall behind 6m of salt marsh needs to be 6m high; but if the
marsh were lost, the wall would need to be 12m high, and cost an
extra £3,500 per metre (1994 EA figures).
So creating salt marshes is a cost-effective way of
maintaining sea walls.
There is a dynamic, self-regulating process found in many
tidal estuaries: a net balance between the amount of material being
deposited and eroded. Such a balance may be disturbed when an
estuary is dredged. Continuous removal may eventually lead to
erosion of inter-tidal banks and salt marshes.
The "in estuary" placement of dredged material during
beneficial use schemes ensures that potential damage during
essential dredging is minimised.
Nature conservation bodies in the UK have become
increasingly concerned at the progressive loss of inter-tidal
habitats, including salt marshes and mud flats, due to erosion
and reclamation. Overall, there are only 44,370 ha of salt marsh in
Great Britain. This compares with approximately 1,300,000 ha of
peat land and 350,000 ha of ancient, semi-natural woodland (1996
figures), themselves rare in national terms.
Salt marshes are a rare and
specialised habitat and are among the most natural ecosystems
remaining in Britain. Many of their plants survive nowhere else and
are specifically adapted to the high and often changing salinities
of the soils and regular tidal immersion. These habitats support
specifically adapted invertebrates, which include a number of rare
species and are home for a wide variety of breeding
Mud flats are productive
systems, inhabited by huge numbers of surface-dwelling marine
invertebrates. Consequently, mud flats are very important for
supporting large bird and fish populations.
Site re-alignment: Wallasea Island, Essex
The Wallasea Island re-alignment project was undertaken by Defra
in 2006 to create a new wetland on the north shore of this island,
situated in the Crouch Estuary in mid-Essex.
As part of the scheme 550,000m3 of maintenance
material dredged from the approach channels to the Port of
Felixstowe was supplied by Harwich Haven Authority with the aim of
creating an area of salt marsh within the re-alignment site.
The dredged material was pumped ashore and deposited in a
45-metre-wide strip at the rear of the site prior to the breaching
of the sea wall. It had to be contained by a new sea wall and a
clay bund to its seaward side to allow for consolidation. The
topography was raised to a level just below the Mean High Water
Springs level using this sediment-recharge approach.
Experience at Wallasea Island suggests that imported sediment
can colonise quickly: the signs of colonisation by salt-marsh
plants were evident at this site within the same year.
material being pumped ashore at Wallasea Island (© ABP
Salt marsh creation: Orwell Estuary, Suffolk
Changes in the River Orwell have resulted in a loss
of inter-tidal mud levels adjacent to the Suffolk yacht harbour,
which is on the east bank of the river. The harbour authority has
always disposed excavated material from the site locally, and has
been keen to place the regularly available maintenance material on
the adjacent foreshore.
The fluid dredged material is pumped through pipes
positioned within the marina to fixed pipes at the disposal area.
The placement is within wattle hurdles or faggots (bundles of twigs
stapled to the foreshore). Sufficient material has remained to
raise the tidal height on the foreshore to allow salt marsh plants
Salt marsh creation: Horsey Island, Essex
Horsey Island is part of the Walton Backwaters. The island
is strategically important because it provides protection against
wave action for the Backwaters, thus mitigating erosion.
In 1998, the Environment Agency recharged
20,000m3 of mud from Harwich Haven Authority's port
development, between a shingle berm and the sea wall. After nine
months, considerable salt marsh growth had occurred over parts of
the recharge area.
The area was again recharged during January 2001, the aim
to raise the tidal height of the mud surface to facilitate the
establishment of higher salt marsh plant species.