Dredged-material emplacement
DECODE, or the Determination of the
Ecological Consequences of
Dredged-material Emplacement,
was an umbrella group established in November 2000. It's aim
was to harmonise efforts and increase information transfer
between the many beneficial use-related research projects within
the UK. DECODE focussed on the utilisation of fine-grained dredged
material for environmental enhancement, habitat sustainability and
flood defence. The core of DECODE was a group of experts in many
fields from marine ecological research to environmental
regulation.
Beneficial use of fine-grained maintenance dredged
material
Dredging and disposal of dredged material constitutes one of the
most important issues in coastal zone management. In the UK,
approximately 40 million tonnes of dredged material are annually
disposed of to the marine environment at estuarine and offshore
sites licensed by Defra. Typically, about 80% of the material
arises from maintenance dredging, mostly muddy sand. Under the Food
and Environment Protection Act, as part of the UK licensing
procedure for the disposal of dredged material, the licensing
authority must 'have regard to the practical availability of any
alternative methods of dealing with them [disposed materials]'. In
recent years, this consideration of alternatives has shifted the
emphasis from disposal per se to whether relocation can be managed
in such a way as to derive environmental or other benefits. As a
result, a number of 'beneficial use' options have developed whereby
the material is regarded as a potential resource and used to
recharge or recreate intertidal habitats.
Capital dredged material, which typically is comprised of
relatively coarse material compared to maintenance dredged
material, is more suited to such schemes as its behaviour once
deposited is more predictable. In contrast, the fine material
obtained from maintenance dredging is less likely to remain at the
disposal site and its potential for detrimental indirect ecological
effects is increased. Consequently, at present, the beneficial
placement of maintenance dredged material within the UK is limited
to small-scale trials. There are several reasons for this. Our
current lack of understanding of the biological processes following
deposition: phyto- and zoobenthic recolonisation may have profound
effects on the stability of sediments, and hence, the fate of
deposited material in both the short and long term. In addition,
our lack of knowledge of the rates of invertebrate recovery, and
how they are affected by other factors, limits our ability to
predict the indirect effects of sediment placement on bird and fish
populations. This is particularly important as the majority of
beneficial use schemes are situated on estuarine intertidal
habitats, areas important for sustaining such populations.
Dredged material has been shown to successfully protect eroding
and/or create new saltmarshes which, in time, are capable of
functioning like natural systems. In a similar way, mudflats can be
created or, more usually, biologically impoverished mudflats can be
enhanced, resulting in much more productive systems than
before.
The concept of beneficial use using dredged material must not,
however, be seen as a solution to all problems arising from
dredgings management by port authorities. It can have its own
logistical, legal, economic and environmental limitations. Each
disposal case should be assessed on its own merits. Nevertheless,
under certain circumstances, the concept of beneficial use of
dredged material can result in solutions that satisfies the needs
of the industry, regulator and to society at large.
Advantages of Beneficial Use Schemes
Flood and Coastal Defence
Relative sea level rise creates increased tidal energies that
are eroding the soft coastlines of various parts of the UK,
particularly of the South East. These soft coastlines comprise
mudflats and saltmarshes that stabilise existing sea defences.
Studies have shown the importance of saltmarshes for wave energy
dissipation: they can act as efficient wave buffers. In South East
England, saltmarshes are eroding at an unprecedented rate. This has
important consequences for seawalls: the wall toe becomes exposed
resulting in underpinning and slippage of the facing blocks, the
front wall becomes destabilised with the removal of the weight of
the marsh, and the amount of wave energy reaching the seawall is
increased. Consequently, the finances required to build and
maintain a wall without a fronting saltmarsh are significantly more
than those with a marsh. For example, a seawall behind 6m of
saltmarsh 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). The use of maintenance dredged material to protect
and/or create saltmarshes in the South East (see examples below) is
currently being viewed as a cost-effective way of maintaining
seawalls.
Sediment Cell Maintenance
Many tidal estuaries are 'in regime', i.e., there is a net
balance between the amount of material being deposited and eroded.
It is a dynamic, self-regulating process. If excess erosion takes
place (e.g., during a period of high-velocity flow), the fact that
the riverbed becomes deeper reduces the speed of the flow, allowing
deposition to occur. If excess accretion occurs, the flow is forced
through a smaller cross-section, speeds up and becomes capable of
re-eroding the accreted material. Such a balance may be disturbed
when an estuary is dredged. Continuous removal may eventually lead
to erosion of intertidal banks and saltmarshes. The 'in estuary'
placement of dredged material during beneficial use schemes, either
by trickle charging or direct intertidal placement, ensures that
perturbations to an estuary's cell maintenance during essential
dredged is minimised.
Habitat conservation/enhancement
Nature conservation bodies in the UK have become increasingly
concerned at the progressive loss of intertidal habitats, including
saltmarshes and mudflats, due to erosion and reclamation.
Saltmarshes are a rare and specialised habitat and are important
for a variety of nature conservation reasons. Overall, there are
only 44,370 ha of saltmarsh in Great Britain, this compares with
approximately 1,300,000 ha of peatland and 350,000 ha of ancient
semi-natural woodland (1989 figures), themselves rare in national
terms. Many of the plants which occur there survive nowhere else
and are specifically adapted to the high and often changing
salinities of the soils and regular tidal immersion. Saltmarshes
are among the most natural ecosystems remaining in Britain. These
habitats support important, and sometimes specifically-adapted,
invertebrates which include a number of rare species, and are home
for a wide variety of breeding birds. Mudflats are productive
systems, inhabited by huge numbers of surface-dwelling, marine
invertebrates and consequently are very important in supporting
large bird and fish populations.
Examples of Fine-Grained Beneficial Use Schemes in the South-East
Saltmarsh creation. Horsey Island, Essex, UK.
Horsey Island lies within the inlet of Hamford Water, and is
part of the Walton Backwaters. The island is strategically
important in that it provides protection against wave action for
the Backwaters, and is thus in a position to mitigate erosional
impacts. 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 saltmarsh growth (Salicornia sp.) 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 saltmarsh
plants.
Saltmarsh creation. Suffolk Yacht Harbour, Levington, Orwell Estuary, Suffolk, UK
Suffolk Yacht Harbour is situated on the east bank of the
Orwell, 6km upriver of Felixstowe. Changes in the river have
resulted in a loss of intertidal mud levels in the Orwell adjacent
to the harbour. The Harbour 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
dredged material is very fluid and 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 saltmarsh plants
to colonise.
Saltmarsh creation, Titchmarsh Marina, Walton on the Naze, Essex, UK
Titchmarsh Marina was constructed during the early 1970s to the
south of the Walton Backwaters, close to Walton on the Naze. The
marina suffers regular deposition as fine sediment settles from
suspension and needs to be removed by frequent dredging. To allow
for intertidal recharge of fluid dredged material, excavated earth
was used to construct a bund on the west side of the marina. In
1998, sediments from the marina berths were dredged using the
marina's grab dredger and the material placed into a barge before
being broken down into a slurry which can be pumped into the
recharge area. During two three or four month periods of 1998 and
1999, approximately 10,000 tonnes of mud (per year) was pumped into
the bounded area. During 2001, more mud was recharged to raise the
tidal height further to allow saltmarsh plant development.
