L 220 - 2020-21 - Endeligt svar på spørgsmål 7: Spm., om ministeren vil oversende referater og mødemateriale fra de ESPOO-samrådsmøder mellem Danmark og Sverige om Lynetteholmprojektet, der har været afholdt i 2021, til transportministeren

Transportudvalget 2020-21
L 220
Offentligt

Memo

To:

Att.:

From:

Project

Date:

Topic:

Rambøll and

‘By og Havn’

Michael Lundgaard,

‘

By og Havn’

Bo Brahtz Christensen, DHI

11823523 Lynetteholm

12-04-2021

Additional assessments for ESPOO consultation

At the ESPOO consultation on 23 March 2021, the Swedish environmental authorities

indicated that they had difficulty getting an overview of the environmental impact of the

Swedish waters, as the prepared material is only available in Danish and focuses mainly

on Danish conditions. Therefore, it was agreed that a supplementary note should be drawn

up summarising the environmental assessments concerning the Swedish waters.

This note contains a number of clarifications and maps that provide better coverage of the

Swedish waters. The note has also been prepared in an English version so that the

Swedish authorities that are not entirely comfortable with the Danish environmental impact

reports can use the English version.

Environmental assessment of the hydromorphological quality of

the water bodies

Questions have been raised from the Swedish side as to whether the establishment of

Lynetteholm can impact the littoral transport, wave conditions and erosion of the Swedish

coasts.

Littoral transport and coastal erosion are primarily determined by the local wave conditions

and the current generated by the waves within the surf zone.

Waves in the Sound are wind-generated and thus determined by wind direction and wind

speed, as well as the free stretch over which the wind acts. Lynetteholm is located on the

western side of the Sound, far away from the Swedish coasts, and will therefore have no

significant impact on wind and wave conditions along the Swedish coast.

The following figures show simulated wave heights in the Sound in the model year 2018 as

follows:

•

Figure 1 shows the annual averaged significant wave height for current and future

conditions with Lynetteholm, respectively.

Figure 2shows the largest significant wave height in 2018 for current and future

conditions with Lynetteholm, respectively.

Figure 3shows the annual average change due to Lynetteholm.

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L 220 - 2020-21 - Endeligt svar på spørgsmål 7: Spm., om ministeren vil oversende referater og mødemateriale fra de ESPOO-samrådsmøder mellem Danmark og Sverige om Lynetteholmprojektet, der har været afholdt i 2021, til transportministeren

As can be seen from the plots, the differences are negligible. There is a slight shadow

effect (i.e., weakening of the waves) in the area south of Lynetteholm of

“Prøvestenen”

and

a minor reinforcement in the area north of Lynetteholm. The impact is seen to be only local.

No impact can be observed along the Swedish coast. Figure 4 shows the change of the

largest significant wave height occurring during 2018. Here the impact area is a little larger

but again entirely local in the area around Lynetteholm. Thus, it can be concluded that

Lynetteholm will not affect the wave conditions along the Swedish coasts and, therefore,

will not cause any changes in littoral transport. This means that Lynetteholm does not

induce erosion along Swedish coasts.

Figure 1

Annual average wave height. Top: present conditions. Bottom: future

conditions with Lynetteholm.

dhi notat - 12-04-2021 opfølgning på espoo-uk.docx / Bbc / 2021-04-12

Figure 2

Maximum significant wave height. Top: present conditions. Bottom: future

conditions with Lynetteholm.

dhi notat - 12-04-2021 opfølgning på espoo-uk.docx / Bbc / 2021-04-12

Figure 3

Change in annual average of significant wave heights.

Figure 4

Change of the maximum significant wave height.

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Sediment dispersion in connection with disposal of dredged

sediments in Køge Bay

Sediment dispersion calculations have been carried out in connection with the disposal of

dredged sediments in Køge Bay. The calculations are carried out with a coupled near-field

description, where the sediment disposal's movement towards the bottom is described by

a near-field model, which is transferred to the far-field model when density-driven effects

no longer determine the movement of the disposed sediment. The disposed material is

estimated to have a relatively high moisture content, which entails a large loss in connection

with the disposal itself, since the dry matter density is not large enough to send the

disposed material directly down to the bottom, which is why it instead settles as a sediment

cloud just above the bottom, from which it gradually deposit. It should be noted here that it

is assumed that the dry matter represents only 23% of the disposal volume in the model

calculations. Therefore, the relative loss from the disposal area will be reduced if the

volume of dry matter proves to be a larger part of the disposal material, reinforcing the

density-driven effect of the fall towards the bottom. Furthermore, the dry matter will tend to

be compressed in the lower part of the split barge hopper during the transport to the

disposal area, making it easier to deposit directly on the bottom. This is process is not taken

into account in the model calculations, which conservatively are based on the fact that the

disposal material is evenly mixed up in the hopper of the split barge during disposal.

In the report describing the disposal of dredged material, ref. /1/, in Chapter 4, current roses

are shown, which describe the current conditions on the two disposal areas at the seabed,

in the middle of the water column, and at the surface. Furthermore, it is shown how salinity

can vary over time in the two areas at the bottom, in the middle of the water column, and

at sea level. The Baltic Sea is a brackish water area, where water supply from rivers implies

a net transport of water to the north (from the Baltic Sea to the Kattegat). The net transport

takes place primarily in the upper part of the water column due to the interaction between

the heavier salty water from the Kattegat and the lighter brackish water from the Baltic Sea.

In the disposal areas, the net drift on the seabed is directed towards southwest. Strong

currents at the seabed in the two disposal areas are always directed to the southwest.

Therefore, the disposals primary advection and dispersion will be directed to the southwest,

as the sediment cloud is overlaid just above the seabed and advected with the current.

Therefore, the sediment clouds will most often move as shown in the figure below (Figure

5), which shows the trail of 6 disposal events with a time lag of one hour between each.

Figure 5

Example of the sediment dispersion associated with disposal of dredged

sediments.

dhi notat - 12-04-2021 opfølgning på espoo-uk.docx / Bbc / 2021-04-12

The deposition maps in ref. /1/ and the map in Figure 6show, as the above sediment plume,

that the material is primarily deposited on deeper water in the southwest direction. As the

advection and dispersion of the disposal material is associated with the bottom current, the

deposits take place mainly in the deeper water area southwest of the disposal sites. If

resuspended, the sediment will primarily move in the deeper channels in the direction

further into the Baltic Sea. Outside the area shown, the deposits are so thin that secondary

and tertiary resuspension is of no importance to the concentrations in the water column. If

sediment is resuspended in these areas, it will be dominated by the already present bottom

sediment and not the contribution from the disposal material.

The water depths in the Natura 2000 sites are shallow compared to the water depths of the

disposal areas. Therefore, it will primarily be in connection with the disposal release that

there may be a risk that parts of the sediment cloud may reach the Natura 2000 areas.

Here, maps showing duration of sediment concentrations exceeding 5 mg/l and a time

series from the model calculations extracted in the western part of the Natura 2000 area at

Falsterbo (see Figure 8 and Figure 8 show that it will occur only sporadically. The criterion

for a visible sediment plume is typically 2-5 mg/l.

Figure 6

Deposition maps due to disposal of dredged sediments, ref. /1/.

Figure 7

Duration of sediment concentrations exceeding 5 mg/l during the winter

months (October-March) using disposal site Ka.

dhi notat - 12-04-2021 opfølgning på espoo-uk.docx / Bbc / 2021-04-12

Figure 8

Time series of sediment concentrations at the bottom, in the middle of the

water column, and at the surface at point E 350,000 m and N 6,146,000 (UTM-

33) located in the outermost part of the Natura 2000 area at Falsterbo.

The bottom fauna also experiences shadow effect at night, and since the length of the day

is shorter in the winter season (when disposal is carried out), more than half of the sporadic

events will occur at times when there is no light supply anyway. As there is no significant

deposition in the Natura 2000 areas, and since any shadow effect occurs only sporadically,

briefly, and outside of the growing season, the disposal is not considered to have a tangible

impact on Natura 2000 areas.

Flow/blockage in the Sound

Lynetteholm is a land reclamation, which has as a consequence that the flow cross section

of the flow in the Sound is narrowed locally. The narrowing causes a local increase in flow

resistance and thus has a slight dampening effect on the dynamics, which manifests itself

in the calculated blockage. To change the frequency and amount of saltwater inlet to the

Baltic Sea, the project must have a real threshold effect, and the Lynetteholm project has

not. Hollænderdybet east of Middelgrunden is both deeper and wider than the Kongedybet

and will therefore continue to direct salt towards the Baltic Sea. The controlling flow margin

for the exchange of salt and water between the Baltic Sea and the Kattegat will

consequently be made up of the Drogden Threshold. Therefore, the general assessment

is that Lynetteholm will not change the frequency and amount of saltwater intrusions to the

Baltic Sea.

Lynetteholm differs from the Øresund Bridge project in that the impact is more local. The

Øresund Bridge stretches across the Sound in the Drogden Threshold area, where the

actual regulation of the water change occurs. Therefore, the Øresund Link could contribute

further to the threshold effect, thereby making it more difficult to exchange water and salt

between the Baltic Sea and the Kattegat via the Sound. This is not the case with

Lynetteholm.

Lynetteholm's impact on the current conditions in the Sound is described in ref. /2/ and the

following figures:

•

Figure 9 shows the annual mean of the average depth current calculated without

direction (gross current) for current conditions and future conditions with

Lynetteholm, respectively.

Figure 10 shows the most significant occurring maximum current (depth average)

during 2018 for current conditions and future conditions. The plot does not

•

dhi notat - 12-04-2021 opfølgning på espoo-uk.docx / Bbc / 2021-04-12

represent a snapshot because the maximum current rate will not occur

simultaneously everywhere. The plots show that the area around the Øresund Link

forms the regulating cross-section of the Sound since it is in this area that the

highest current speeds occur due to the Drogden Threshold and the narrower

cross-section.

•

Figure 11shows the change in the annual average current. It can be seen that there

is a reinforcement taking place in the area east of Lynetteholm, which extends into

Hollænderdybet, and a local weakening in the area just north and south of the land

reclamation.

Figure 12 shows the change in maximum current. The picture is the same as for

the current annual change, but the impact area is slightly larger. However, it should

be borne in mind that the maximum current relates to a short-term picture, while

the mean current indicates the more permanent impact.

•

Figure 9

Annual mean of the depth-integrated current calculated without direction

(gross current). Top: present conditions. Bottom: future conditions with

Lynetteholm.

dhi notat - 12-04-2021 opfølgning på espoo-uk.docx / Bbc / 2021-04-12

Figure 10

Maximum depth-integrated current in 2018. Top: present conditions. Bottom:

future conditions with Lynetteholm.

dhi notat - 12-04-2021 opfølgning på espoo-uk.docx / Bbc / 2021-04-12

Figure 11

Change in the annual mean current (mean depth-integrated gross current

change).

Figure 12

Modification of the mean depth-integrated maximum current.

Lynetteholm's impact on current conditions in the Sound is local seen in the Øresund scale.

Furthermore, the impact is not one-sided, as there are both areas with current amplification

and current relaxation. The impact is estimated to have a non-significant effect on the

exchange of water, salt and oxygen between the Kattegat and the Baltic Sea. Generally,

the assessment is therefore, that Lynetteholm will not change the frequency and amount

of saltwater intrusions to the Baltic Sea.

dhi notat - 12-04-2021 opfølgning på espoo-uk.docx / Bbc / 2021-04-12

References

/1/

/2/

DHI: ATR-11-klapning Køge bugt - spredningsberegninger. 16. december 2020.

Anlæg af Lynetteholm. VVM

–

Teknisk Baggrundsrapport nr. 1. Hydrauliske

undersøgelser. Endelig rapport version 1.6, 2. november 2020.

dhi notat - 12-04-2021 opfølgning på espoo-uk.docx / Bbc / 2021-04-12