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Environmental Management and Monitoring at the Øresund Fixed Link

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The relatively clean and clear water in the Øresund makes it a difficult environment in which to carry out large dredging operations. For this reason, the Øresunds-konsortiet initiated an environmental monitoring and environmental management programme called " feedback monitoring " to navigate this huge and complicated construction work safely through a set of strict environmental regulations laid down by the Danish and Swedish authorities. Environmental monitoring traditionally uses methods that need a long period of observation before one can judge with statistical certainty whether a development is a lasting change or an occasionally occurring natural variation. In connection with the construction of the Øresund Link, an environmental monitoring programme has been established which allows a much quicker evaluation of impacts, in order to make adjustments in the construction activities as observed effects follow or vary from predictions. This so-called feedback monitoring includes selected variables that over short periods of time show quanti-fiable changes as a result of impacts from the construction work. The use of computer models makes it possible at an early stage to assess whether a feedback action should be taken or not, given the results of the monitoring and the future work plans.
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Abstract
The relatively clean and clear water in the Øresund
makes it a difficult environment in which to carry out
large dredging operations. For this reason, the Øresunds-
konsortiet initiated an environmental monitoring and
environmental management programme called “feed-
back monitoring” to navigate this huge and complica-
ted construction work safely through a set of strict
environmental regulations laid down by the Danish and
Swedish authorities.
Environmental monitoring traditionally uses methods
that need a long period of observation before one can
judge with statistical certainty whether a development
is a lasting change or an occasionally occurring natural
variation. In connection with the construction of the
Øresund Link, an environmental monitoring pro-
gramme has been established which allows a much
quicker evaluation of impacts, in order to make adjust-
ments in the construction activities as observed effects
follow or vary from predictions.
This so-called feedback monitoring includes selected
variables that over short periods of time show quanti-
fiable changes as a result of impacts from the construc-
tion work. The use of computer models makes it possi-
ble at an early stage to assess whether a feedback
action should be taken or not, given the results of the
monitoring and the future work plans.
Introduction
A fixed link between Denmark and Sweden is currently
under construction. The link consists of a combination
of a double-track railroad and a four-lane highway.
The link goes through a submerged tunnel under
Drogden, crossing an artificial island south of Saltholm,
leading to a high bridge crossing the Flinterenden to
Linhamn on the Swedish coast (see Figures 1 and 2).
An environmental monitoring and environmental
management programme called “feedback monitoring”
has been initiated by Øresundskonsortiet to navigate
the huge and complicated construction work safely
through a set of strict environmental regulations laid
down by the Danish and Swedish authorities.
The feedback monitoring programme is designed and
carried out by the Feedback Monitoring Centre, which
has been set-up and run by a co-operation between VKI
in partnership with Toxicon A/B and Danish Hydraulic
Institute (DHI) in joint venture with LIC Engineering
(LIC).
THE FEEDBACK MONITORING CENTRE
The scope of work for the Feedback Monitoring Centre
is to implement, maintain, run and manage Øresunds-
konsortiet’s feedback monitoring programme.
The backbone of the environmental control and
management during the construction phase is the
environmental information system, EAGLE. The Feed-
Terra et Aqua – Number 74 – March 1999
10
Anders Jensen and Jens Erik Lyngby
Environmental Management
and Monitoring at
the Øresund Fixed Link
Figure 1. Map of Denmark and location of Øresund.
Environment
Dredging and handling of large quantities of soil in
the marine environment unavoidably lead to the spill of
a larger or smaller fraction of the finest particles.
The spilled sediment is carried away by currents or
waves and can, even though it consists of unpolluted
material, have an undesirable negative impact on the
ecosystems in the sea. High concentrations of sedi-
ment in the water column can reduce or even com-
pletely switch off the life supporting light coming down
to the seabed, where light-dependent plants such as
eelgrass form large sheltered areas for e.g. schools of
back Monitoring Centre is one of the major suppliers of
environmental data and model results to EAGLE and at
the same time acts as Øresundskonsortiet’s feedback
advisor and manager. Feedback management and
advice is based on the EAGLE system.
Furthermore, the Feedback Monitoring Centre is
Øresundskonsortiet’s environmental monitoring and
supervision unit, which means that a certain level of
readiness is implemented by the Feedback Monitoring
Centre to monitor unforeseen events, e.g. oil spills, or
to react to and evaluate other environmental effects,
which third parties postulate are caused by the con-
struction activities. The Feedback Monitoring Centre is
run by experienced experts in biology, sedimentology
and numerical modelling to secure that the Feedback
Monitoring Centre can deal with all possible environ-
mental problems which may arise during the construc-
tion phase. Furthermore, the Feedback Monitoring
Centre may draw from the comprehensive base of
knowledge shared by the partners involved.
Large construction activities in the marine environment
will unavoidably cause some temporary and permanent
changes or even damages to the environment.
The permanent effects of the fixed link between
Denmark and Sweden, including changes in water
flow, morphology, landscape, and such, have been
minimised through a careful, costly and environmen-
tally optimised design of the fixed link. The permanent
effects, which will be small compared to the temporary
effects, have been described in “The Øresund Link:
Supplementary Assessment of the Impacts on the
Marine Environment of the Øresund Link, Øresunds-
konsortiet 1995”.
In the case of the Øresund Link, the temporary environ-
mental impacts caused by the construction activities
are closely related to the spill of sediment (soil particles)
from the marine dredging and landfill operations.
Environmental Management and Monitoring at Øresund
11
Anders Jensen is a Cand. Scient. in
Physical Geography from the University
of Copenhagen, Denmark (1985).
Since 1986 he has worked with
dredging-related monitoring and
environmental management. He is
presently head of the department for
environmental management at the
Danish Hycraulic Institute.
Jens Erik Lyngby is a Cand. Scient.
from the Univeristy of Aarhus,
Denmark (1981). He is presently Chief
Consultant at VKI, Institute for the
Water Environment, and Project
Manager for the Feedback Monitoring
Centre.
Anders Jensen
Jens Erik Lyngby
Figure 2. View over the fixed link across the Øresund.
Figure 3. Distribution of eelgrass (Zostera Marina).
fish. High sedimentation rates can lead to the burial and
extinction of mussels living in large colonies around
Saltholm.
The Danish and Swedish authorities have laid down an
overall maximum limit of 5% of the sediment spill from
the construction activities. In order to meet the other
environmental restrictions laid down by the authorities,
the Owner has imposed maximum daily and weekly
spill rates. These limitations have been assessed by
numerical modelling of the effect on e.g. eelgrass
caused by various dredging scenarios.
All results from the feedback monitoring programme
are presented to Øresundskonsortiet and the authori-
ties through the advanced environmental information
system EAGLE. EAGLE is based on a geographical
information system and contains all environmental data
related to the fixed link across Øresund. The data is
supplied not only by the work of Feedback Monitoring
Centre, but also by the authorities’ general monitoring
programme and the Contractor’s spill monitoring pro-
gramme.
THE ØRESUND
The Øresund is the easternmost of the three straits
connecting the Baltic Sea with the North Sea.
The Øresund region is densely populated and Øresund
itself and its shores are popular recreation areas.
Through the last 25 years the local communities sur-
rounding the Øresund have invested large sums in
waste water treatment plants and other environmental
improvements, so that the water quality in the Øresund
today has reached a very high standard. A visibility in
the water of more than 10 m is not uncommon, and
eelgrass can grow at water depths greater than 6 m.
The relatively clean and clear water in the Øresund is
a difficult environment in which to carry out large
dredging operations. Even minor sediment plumes
are clearly visible and the relatively strong currents
transport the spill from dredging operations over long
distances.
Hydrography
The currents in the Øresund are like the currents in the
two other Danish straits mainly governed by the
meteorological conditions over the Baltic Sea and the
North Sea. In periods where westerly winds prevail,
inflow to the Baltic Sea takes place causing south-going
currents in the Øresund. In periods with easterly winds
the current in the Øresund is north-going allowing the
Baltic Sea water to flow out. Current velocities over
two nautical miles per hour are not rare in the Øresund,
whereas wave action is limited owing to the lack of
large free stretches.
Terra et Aqua – Number 74 – March 1999
12
Figure 4. Distribution of common mussels (Mytilus edulis) in
the Øresund.
2 mg of dry matter per litre sea water (mg/l), but locally,
in front of wave exposed shores, the sediment concen-
tration can reach 20 mg/l. During calm winter periods
the concentration of particles in the water is close to
zero and during such periods the seabed is visible
down to 20 metres below the sea surface.
Eelgrass meadows
Beds of the marine flowering plant eelgrass (Zostera
marina) cover extensive parts of Danish coastal areas.
The vegetation surrounding the link is characterised by
a few, but dominating, species and here the most
important one is also eelgrass. Eelgrass is found on
sandy seabeds in shallow waters, in general between
1 and 6 m (see Figure 3). The dense meadows of the
shoots with the long, band-shaped leaves act as impor-
tant feeding and breeding grounds for many animal
species, also important commercial fish species. Eel-
grass is an important food source for the mute swan.
Filamentous algae (in Danish FedtMøg) grow in the
eelgrass meadows, which also act as a trap for freely
drifting mats of living, dead or decaying algae, in the
same way as current and wave action are reduced
owing to the plant cover. Around Saltholm, eelgrass
beds cover an area of approximately 60 km2.
Along the coast of Amager eelgrass covers an area of
about 28 km2and along the Swedish coast the area
where eelgrass meadows are found covers at least
48 km2.
Mussel beds
Large parts of the seabed surrounding the Øresund
Link are dominated by hard bottom and places exist
where even the limestone rock is exposed. Such areas
form brilliant places for the common mussel (Mytilus
edulis). The mussel attaches itself to hard substrates,
the rocky seabed, stones, shells of dead mussels or
even living mussels, with a bundle of thin threads
produced by the mussel itself. Mussels may form thick
layers of dead and live mussels. Whenever the
mussels cover more than 40% of the seabed it is
characterised as a mussel bed. Dense populations of
mussels are present in the vicinity of the alignment and
on mixed seabed types in deeper water north-west of
Saltholm. The distribution of mussels in the Øresund is
shown in Figure 4.
The area of mussel beds in the Drogden Channel and in
the vicinity of the alignment is approximately 46 km2.
Thirty per cent of the mussel beds are located at
depths of less than 6 metres. The weight of the living
mussels including their shells is estimated to be around
92 thousand metric tonnes. Twenty per cent of the
mussel beds are found in shallow waters, i.e. at depths
less than 6 m. The mussels in the Øresund are con-
sumed by eider ducks in the spring and by tufted ducks
and golden-eyes during the winter. They are not impor-
tant as a food supply for fish.
The brackish water of the Baltic Sea is much lighter
than the salty water of the North Sea coming through
Kattegat. This difference in density gives rise to the
formation of two or even three water layers in the
Øresund. From time to time the salt water underlying
the brackish surface water will flow from the north into
the Baltic Sea carrying fresh supplies of oxygen-rich
water to the deeper parts of the Baltic Sea. Such
inflows are important for the environment in the Baltic
Sea and great care has been taken to achieve a so-
called zero solution for the Great Belt Link and the
Øresund Link.
In order to compensate for the unavoidable reduction
of the currents in the two straits caused by bridge piers
and other construction elements, compensation dred-
ging is included in the two projects. By increasing the
water depth in the link area by dredging it is possible,
through careful planning and design, to maintain the
natural exchange of water between the Baltic Sea and
the North Sea after the construction of the fixed links.
Geology and sediments
The seabed in the Øresund is formed by the currents
and, in the shallow areas, also by the waves. In the
area close to the link the seabed typically consists of
coarse material such as sand and gravel with occasion-
al stones and boulders. This type of seabed has devel-
oped where the currents have eroded the finer parti-
cles from the underlying glacial till leaving the coarser
material behind as a protective carpet. Beneath the
glacial till, which in general is less than 10 m thick,
limestone is found. Skeletons of microscopic animals
living in a sea more than 60 million years ago form the
limestone.
In the deeper parts of the Øresund north and south of
the link area the seabed has been built up through
thousands of years of deposition of fine particles. 10 to
20 m of mud have been deposited in these areas since
the end of the ice age. This process is still active and
today between 30,000 and 100,000 tonnes of mud are
deposited each year corresponding to an average layer
of around 2 mm. Most of the sediment accumulating
here comes from the Kattegat and the North Sea,
whereas very little comes from the Baltic Sea.
The yearly gross transport of fine-grained sediment
through the Øresund has been estimated to be around
200,000 tonnes/year. Fine-grained sediment comes
from rivers and local shore erosion as well as sedimen-
tation of algae living in the free waters of the Øresund.
Especially after the spring and autumn blooms the
sedimentation of dead algae can often be seen as
white layers in the transition zone between the two
water bodies.
As discussed before, the natural content of particles
(sediment) in the Øresund is very low, generally below
Environmental Management and Monitoring at Øresund
13
DREDGING AND RECLAMATION WORK
During construction of the fixed link approximately
7 million m3of material has to be dredged from the
seabed. The dredging is necessary both for construc-
tion purposes and to compensate for the reduction in
water flow.
The dredged material is utilised for construction of the
artificial island south of Saltholm (Pebberholm) and for
the peninsula located east of Kastrup Airport (see
Figure 2). These landfill operations take place behind
closed bunds made of gravel dikes with a protective
outer layer of stones.
The major dredging operations will be carried out using
different types of dredging equipment. Most of the
dredging of construction channels, work harbours and
other elements will be done using the mechanical
dipper dredger Chicago (see Figure 5) and dredging of
the tunnel trench will be carried out by a hydraulic
cutter suction dredger Castor (Figure 6). A number of
smaller backhoes will be used for the dredging of pier
fundaments and such.
This mechanical dredger works with a bucket which
unloads on barges moored alongside the dredger.
The filled barges are transported to the landfill areas by
tugs, and here the material is unloaded by dozers and
mechanical dredgers.
Seabed material which has broken loose is sucked and
pumped through a pipeline to the landfill area. The
water surplus is drained off through sedimentation
basins where the fine material has time to settle before
the water is returned to the Øresund. Both types of
equipment lose some of the dredged material to the
surroundings. This material is defined as spill if it is
transported away from the work zones or the landfill
areas by currents and waves.
Spill limitations
The spill of fine sediment from dredging and land
reclamation operations causes the major impact on the
marine environment during the construction of the link.
Most of the monitoring variables included in the feed-
back and general monitoring programmes are strongly
dependent on the amount of spill and the dispersal
pattern of the spilled sediment.
An overall spill limit of 5% of the total masses of mate-
rial handled inside the work area has been established
by the Swedish and Danish authorities. Moreover, the
spill of sediment must not hinder fulfilment of the
environmental criteria laid down for the Øresund by the
Danish and Swedish authorities. In order to comply
with those criteria weekly and daily restrictions on
maximum spillage rates have been calculated for the
various parts of the work area. A weekly maximum spill
rate has been established based on numerical model-
ling of the impact on eelgrass. The maximum weekly
spill rate is imposed during the growth season for the
eelgrass, which is from 1/3 to 1/11.
A daily maximum spill rate has been established based
Terra et Aqua – Number 74 – March 1999
14
Figure 5. The dipper dredger Chicago.
Figure 6. The cutter suction dredger Castor.
Figure 7. The spill monitoring vessel Coastal Flyer.
Figure 8. Right, The Feedback Monitoring Centre’s research
vessel Maritina.
actual spillage from the various sources is calculated.
The system including the mobile station is manned
continuously during periods with intensive dredging
activities or when new dredging operations or dredging
equipment are started up.
The daily spill rate reported to Øresundskonsortiet is
used to model and evaluate the results of the other
monitoring variables. The spill measuring programme is
operated by the contractors under close supervision by
Øresundskonsortiet and the authorities.
FEEDBACK MONITORING PROGRAMME
Environmental monitoring traditionally uses methods
that need a long period of observation before one can
judge with statistical certainty whether a development
is a lasting change or an occasionally occurring natural
variation. In connection with the construction of the
Øresund Link an environmental monitoring programme
has been established which allows a much quicker
evaluation of impacts, in order that adjustments can be
made in construction activities as observed effects
follow or vary from predictions. This has been named
feedback monitoring.
Øresundskonsortiet has implemented the Feedback
Monitoring Centre to conduct their Feedback Moni-
toring Programme for the Øresund Link Project in order
to be able to have detailed information on potential
impacts arising from the construction works and to be
able to react quickly in case threshold values are
exceeded. The Feedback Monitoring Centre operates
several vessels and one samples biological samples of
either eelgrass or mussels every week to follow the
development of the ecological conditions in the
Øresund (Figure 8).
Feedback monitoring is used by Øresundskonsortiet to
monitor and model selected variables, which over a
short time show quantifiable changes owing to chan-
on numerical modelling of the sediment dispersal with
respect to fish migration, water quality, the burial and
settling of mussels and the feeding of eiders and
swans. The maximum daily spill rate is imposed
throughout the whole year.
The maximum weekly and daily spill rates may be
increased or decreased during the construction period
depending on the observed impact on the biological
variables. This will, however, not have any influence on
the overall 5% spill limit, which is in force in any case.
Spill monitoring programme
Sediment spill from the dredging and land reclamation
operations occurs at a number of different locations
simultaneously, and therefore a highly mobile ship-
based monitoring system is required. Clouds of
suspended sediment or plumes develop both at the
site of dredging operations and at outlets from deposi-
tion areas (landfill areas) or sedimentation basins.
The monitoring programme is designed to provide real
time and depth/space integrated information about the
actual spill rates from all spill sources. The sediment
concentration and the flux of spilled sediment are
known to vary considerably over space and time
depending on the dredging and deposition activities,
the character of the dredged material and the current
and wave conditions. It is therefore important that the
distribution of the spilled sediment is measured and/or
calculated as space and time integrated values.
Monitoring of the dredging operations is carried out
from a mobile survey vessel, which follows the various
dredging activities, while monitoring of the outlets from
the deposition areas and sedimentation basins is based
on a combination of fixed automatic measuring stations
and the mobile station. Figure 7 shows one of the spill
monitoring vessels operating in the Øresund.
Data from the fixed monitoring stations and the mobile
station are sent on-line to a central computer where the
Environmental Management and Monitoring at Øresund
15
ges in the environmental conditions. The planning and
environmental approval of dredging activities is seen as
the primary tool to ensure compliance with environ-
mental requirements. The combination of measured
conditions of the environmental variables, with predic-
tions based on numerical modelling of the response of
the variables to the actual work plans, is the key factor
for making decisions concerning feedback action at the
construction site.
In the monitoring programme the use of numerical
models is integrated in the feedback system.
The models are applied partly to hindcast the condition
of the variables based on measured hydrographics and
spill emissions and partly to forecast future impact
based on the actual work plans and representative
hydrographic scenarios. The model results are fre-
quently compared with measured conditions to
evaluate the performance of the models and to assess
factors other than the construction works, which might
influence the variables.
Strategies
The environmental impact assessment has shown that
the largest impacts will result from the dispersal of
spilled sediment. Consequently, great effort has been
put into organising the environmental monitoring to
secure that the criteria on the emissions will be met.
Two major tools have been introduced to ensure that
the spill is kept below the limits fulfilling the objectives/
criteria for all variables:
1. The Contractor is made responsible through his
contract for keeping the spill below specified limits
varying in time and space, taking into consideration
the environmentally sensitive periods and areas.
2. A feedback monitoring programme is implemented
covering sediment spill, the dispersal hereof and
biological key variables representing the important
influenced ecosystems.
In theory the Contractors’ fulfilment of the criteria of
maximum spillage alone should reduce the impacts to
acceptable levels. However, because of the unpredicta-
bility of the hydrographical and meteorological regime
and natural variations in the ecosystem, the actual state
of the environment during the construction works must
be monitored to ensure the fulfilment of the general
environmental criteria.
Feedback principles
Feedback monitoring includes selected variables that
over short periods of time show quantifiable changes
owing to impacts from the construction work.
These variables are measured continuously or with a
high intensity and constitute the main instrument for
fast regulations of the construction works.
To ensure fulfilment of the environmental objectives
and criteria, procedures have been established that
specify the actions to be taken on the construction
work, where criteria are in danger of being exceeded.
The feedback monitoring serves as an integral part of
the environmental management system and will im-
pose changes/restrictions in the marine works which
may lead to increased costs. The feedback procedures
include:
assessment and approval of equipment, work plans
and so on, prior to the initiation of operations;
application of threshold criteria and feedback loops
with an agreed code of action; and
–a clear definition of responsibilities of the involved
parties.
The planning and environmental approval of dredging
activities are seen as the primary way to ensure com-
pliance with environmental requirements. This means
that the key factors for making decisions on actions
changing the construction works are the measure-
ments of the environmental variables, combined with
forecasts of the future conditions.
In the monitoring programme the use of computer
models is integrated in the feedback system.
The models are applied in the planning of the dredging
and reclamation operations. They are used to forecast
the sediment dispersion and sedimentation, and the
impact on water quality and the eelgrass beds. The use
of the models makes it possible at an early stage to
Terra et Aqua – Number 74 – March 1999
16
Figure 9. Principals of feedback monitoring.
New work plan from the Contractor
including a spill scenario
Environmental impact assessment of the
work plan based on numerical
modelling of the spill scenario
Are the operational criteria fulfilled
The dredging work starts
Monitoring of selected variables
Are the operational criteria exceeded?
Are the authorities criteria exceeded?
Intensified monitoring
Numerical modelling
YES
YES
NO
NO
NO
Given these demands, the following variables were
selected for monitoring of the feedback organism
eelgrass: shoot density, leaf and root biomass, and
carbohydrates dissolved in the rhizomes. The selected
variables in the mussel monitoring are distribution and
biomass.
FEEDBACK MONITORING AND ACTIONS
The active monitoring procedures of the feedback
programme are described below in the sections cover-
ing sediment and turbidity, eelgrass and mussel moni-
toring. Prior to all activities, the effects of the expected
spills from the planned dredging and reclamation activi-
ties are assessed. A spill scenario is produced on the
basis of the work plans. This scenario is modelled and
shading, sediment transport, sedimentation and effects
on the eelgrass community are forecasted. The model
calculations are based on expected weather situations,
light, currents, waves, and so on. A full year of compre-
hensive hydrographic measurements in the Øresund
(1992-1993) is utilised as the hydrodynamic scenario for
the forecast simulations that are used for environ-
mental impact assessments. If it is anticipated that a
work plan does not comply with the operational criteria,
a new work plan has to be elaborated before the work
can be initiated.
In reality the light, wind, currents, waves, and such do
not end up as anticipated in the forecast modelling.
The summer may be extraordinarily windy or the like.
assess whether a feedback action should be taken or
not, given the results of the monitoring and the future
work plans. The principles of the overall feedback
procedure are illustrated in Figure 9.
The general environmental criteria set by the authorities
in connection with the construction of the Øresund
Link have been transformed into operational and
measurable criteria for the feedback monitoring. The
operational criteria relate to organisms or communities
that are representative for the ecosystem.
In the Øresund, eelgrass meadows and mussel banks
dominate the plant and animal communities, as can be
seen on the map, and these have therefore been
chosen as the most suitable organisms for feedback
monitoring in the Øresund.
In feedback monitoring it is essential that the variable
chosen for measurement meets certain basic
demands:
It must have an unambiguous, easily measurable
relationship to the feedback organism, which repre-
sents the ecosystem concerned.
The measuring result has to be available in a short
time (no more than a few days).
Background material must be available for the deter-
mination of statistically reliable limit values and
criteria for judging an exceeding of the limits.
The impact of various conditions on the variable
should be calculable in advance, that is, some kind
of model of the relationship between cause and
effect has to be available.
Environmental Management and Monitoring at Øresund
17
Figure 10. Diver at work in the eelgrass community.
Figure 11. A photo from a mussel bed south of Saltholm.
Consequently, the effects may be somewhat different
from the ones forecasted. In order to make the next
forecasts more reliable, modelling of the past events is
accomplished. That is, from the beginning of the con-
struction of the link to the actual time. This modelling
uses measured boundary data, measured temperature
and light conditions and other conditions. It is called
hindcast modelling and is carried out several times a
year.
Turbidity and sedimentation
When dredging and reclamation work start, turbidity
and sedimentation surveys are carried out several days
a week. Turbidity is a light measurement that allows for
the calculation of the concentration of sediment parti-
cles in the water.
The surveys serve two main purposes. The monitoring
provides data for the tuning (calibration and verification)
of the sediment model, so that it reflects the real condi-
tions in the Øresund as correctly as possible.
Also data on turbidity and sedimentation rates are
needed to control directly that the work is carried out in
accordance with some of the operational criteria.
The amount of spilled sediment that is transported
around in the Øresund waters may be measured in
different ways: Either as turbidity measured as light
attenuation or as optical back scatter. In some cases
sediment plumes may be mapped using acoustic
methods (Acoustic Doppler Sediment Profiler). In
addition, the sediment fluxes are estimated on the
basis of results obtained from sediment traps deployed
at several stations in the Øresund.
The transport of sediment along the seabed and the
sedimentation are further evaluated on the basis of a
combination of submarine video recordings, dual fre-
quency echosounders and sidescan sonar recordings
of the seabed. Also a number of bottom markers have
been put out on the seabed. Samples of the seabed
are regularly taken with core and grab samplers and
they are analysed for dry matter content and grain size.
The turbidity and sediment monitoring takes place in
the areas most affected by the spilled sediment. The
grey-white plumes of mostly small lime particles can
be observed on the sea surface from the research
vessel. Occasionally, satellite pictures and aerial photo-
graphs are included to support the planning of the
surveys. The submarine video recording surveys are
being planned according to the results of the hindcast
modelling of sedimentation. They do not follow a
regular pattern, but will cover areas where deposition
of spill can be expected.
Turbidity monitoring may trigger feedback actions in
three cases:
If visibility in the water at the bathing beaches along
the Swedish and Danish coasts in the season is less
than one metre in the bathing season. This relates
of course to a criterion concerning the quality of
bathing water.
If sediment plumes with concentrations above
10 mg/l occur simultaneously in the Drogden
Channel and the Flinterenden for a longer period.
This is introduced to comply with a set of criteria
that aims at ensuring unhindered fish migration.
If visibility in the water around Saltholm falls below
one metre for a period longer than two days.
The criterion is intended to allow the mute swan to
graze on eelgrass relatively undisturbed.
If the sedimentation exceeds 15 kg/m2/month
feedback action has to be taken to ensure compli-
ance with criteria concerning mussel beds.
Whenever feedback is initiated the project manager
Terra et Aqua – Number 74 – March 1999
18
The results of a monitoring cruise are compared with
results obtained before the construction of the link
(baseline). If no important change is observed, moni-
toring continues as planned. If an important change in
the variables chosen in the hindcast and forecast
modelling occurs then an assessment must be made
as to whether the criteria will be violated. If the project
management resolves that the change is owing to
activities in connection with the Øresund Link construc-
tion, feedback action is taken in the same way as
previously described.
The mussel programme
The mussel programme also runs every second week,
but in contrast to the eelgrass programme, it runs all
year round. A device called a photo-sampler is operated
from the research vessel. It photographs a fixed area of
the seabed and by use of image analyses the number
and size of the mussels on the picture can be meas-
ured (Figure 11).
Monthly, additional samples of the mussel beds are
taken by scuba divers so that the weight of the live
mussels can be assessed. Once a year the distribution
of mussel beds is mapped using the sound analysis
software system called RoxAnn.
The stations at which sampling takes place by photo-
reports to the Project Director, the Environment and
Authorities of Øresundskonsortiet and action is taken
to change the harmful activities. This can for example
be in the form of a dredger being moved to another
area in the Øresund.
The eelgrass programme
Every second week during the eelgrass-growing
season (March to November) samples of eelgrass are
taken in the area around the Øresund Link. The density
of the eelgrass meadows, the weight of the live parts
of the plants and the storage of energy in the under-
ground parts are recorded. Samples are taken by scuba
divers at ten locations. The locations are chosen accord-
ing to the ongoing dredging and reclamation work and
on the basis of the hindcast modelling (Figure 10).
Some other items are also monitored to supplement
the eelgrass programme. At two sites in the Øresund
light sensors and data loggers are placed permanently
on poles which allows for the calculation of light reduc-
tion. They are visited weekly. Light reduction is also
estimated indirectly from measurements of the growth
of discs of the sea lettuce (Ulva), which are exposed in
Plexiglas cages on buoys in the Øresund. The area
distribution of eelgrass is assessed using aerial photo-
graphs, which are taken every autumn.
Environmental Management and Monitoring at Øresund
19
Figure 12. Example of model calculated sediment spreading as presented in EAGLE.
sampling or by diver are chosen on the basis of the
sites for ongoing work, baseline studies of mussel bed
distribution, and hindcast modelling of sedimentation.
The results of the feedback monitoring cruise are
compared with the results of the baseline. If no impor-
tant change is observed monitoring continues as plan-
ned. If a change occurs to the variables chosen in the
mussel bed monitoring then the project manager
evaluates the reasons for the change. As mentioned
under the eelgrass programme, additional sampling
may be necessary and also hindcast and forecast
sediment modelling to assess whether the criteria will
be violated. If the project management determines that
the change is owing to the construction of the Øresund
Link and that it will eventually result in a violation of a
general environmental criterion, feedback action is
taken in the same way as previously described.
DATA MANAGEMENT
In connection with the environmental monitoring that
runs during the construction of the Øresund Link
Øresundskonsortiet has established an advanced
environmental information system called EAGLE.
All data produced in the feedback monitoring pro-
gramme are stored in a Feedback Monitoring Centre
database. This database contains data collected at the
monitoring surveys as well as data from the baseline
studies, satellite pictures, coastal morphology photo-
graphs and other items.
Other databases in connection with the Øresund Link
contain relevant data. A hydrographic database which
includes immense amounts of data from the hydro-
graphical monitoring programme is run for Øresunds-
konsortiet by Danish Hydraulic Institute (DHI) and
Swedish Meteorological and Hydrographic Institute
(SMHI). Furthermore, Øresundskonsortiet maintains a
database of the earth works, position, amounts, proper-
ties and spills.
Those three data bases are linked together and supply
data, which are transferred to the EAGLE database in
the form of relevant information. This is, for example,
maps based on a geographical information system,
graphs, pictures and reports, also incident reports
describing the possible conclusions and actions in
connection with the feedback procedures. An example
from EAGLE showing model-calculated sedimentation
of spilled sediment is presented on Figure 12.
EAGLE is a highly evolved tool for decision making.
Øresundskonsortiet’s management has direct access
to the latest environmental information through
EAGLE. Furthermore, the Danish and Swedish author-
ities are directly connected to the EAGLE database so
that their information may always be at the same level
as that of Øresundskonsortiet.
Conclusions
Environmental monitoring traditionally uses methods
that need a long period of observation before one can
judge with statistical certainty whether a development
is a lasting change or an occasionally occurring natural
variation. At the construction of the Øresund Link, an
environmental monitoring programme was established
which allows a much quicker evaluation of impacts, in
order to make adjustments in the construction activities
as observed effects follow or vary from predictions.
This so-called feedback monitoring includes selected
variables that over short periods of time show quanti-
fiable changes as a result of impacts from the construc-
tion work.
These variables are measured and modelled contin-
uously or with a high intensity and constitute the main
instrument for management of the construction activi-
ties. In the monitoring programme the use of computer
models is integrated in the feedback system.
The models are applied in the planning of the dredging
and reclamation operations. They are used to forecast
the sediment dispersion and sedimentation, and the
impact on water quality and the eelgrass beds. The use
of the models makes it possible at an early stage to
assess whether a feedback action should be taken or
not, given the results of the monitoring and the future
work plans.
The experience gained so far, with more than 90% of
the dredging work completed, is that none of the
criteria has been violated and the dredging work has
been successfully carried out within the overall time
and budget plans.
References
Tilstånd till uppförande av den svenska delen av Öresunds-
förbindelsen, Vattendomstolens kendelse af 13. juli 1995.
Trafikministeriet & Milj¢- og Energiministeriet
Målsœtninger og Kriterier samt milj¢myndighedernes krav til det
samlede kontrol- og overvågningsprogram for Øresunds-
forbindelsens Kyst-til-kyst anlœg. Januar 1995. (Objectives and
Criteria and the Environmental Authorities’ Requirements for
the Overall Control and Monitoring Programme for the
Øresund Fixed Link Coast-to-Coast Facility, January 1995).
Øresundskonsortiet
Operational Environmental Criteria for the Construction of the
Øresund Link. January 1996.
Terra et Aqua – Number 74 – March 1999
20
... In total, 162 km 2 are covered by seagrass meadows in Øresund [49], whereas blue mussel reefs (M. edulis) cover 46 km 2 [50]. Map of the sampling area and sites. ...
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Seagrass meadows and mussel reefs provide favorable habitats for many fish species, but few studies have compared the associated fish assemblages directly and examined the influence of environmental variables. Knowledge of fish assemblages associated with disparate habitats is needed for the conservation of coastal fisheries and marine spatial planning. Catch per unit effort data derived from fyke nets showed similar species richness and diversity in seagrass meadows and mussel reefs, suggesting that both habitats support elevated marine biodiversity of mobile fauna. However, it was shown that fish assemblage structure differed between those habitats, and also fish abundance in seagrass meadows was significantly higher than in mussel reefs by comparing the data with a multivariate extension of Generalized Linear Models (GLM). Furthermore, employing underwater video recordings to compare fish abundances in high and low water current speed mussel reefs with a Generalized Linear Mixed Model with negative binomial distribution, data revealed similar fish abundances (in terms of the MaxN metric) despite the variation in current speed, probably because the mussel formations provide sufficient shelter, even from high water currents. The commercially important species Atlantic cod (G. morhua), however, was significantly more abundant in the low water current mussel reef. Therefore, restoration efforts targeting G. morhua could benefit from restoring low current mussel reefs. Our study provides input for the conservation of coastal recreational and commercial fisheries, habitat restoration and marine spatial planning where certain habitats may be prioritized.
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... major suppliers of environmental data and model results to EAGLE and at the same time acted as Øresundsbro Konsortiet's feedback adviser and information manager.All feedback management and advice came in time to be based upon the EAGLE system. In the words ofJensen & Lyngby (1999): 'The backbone of the environmental control and management during the construction phase [was] the environmental information system EAGLE.'The EAGLE system drew upon three databases. ...
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