MILJÖREDOVISNING BILAGA 5

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1 MILJÖREDOVISNING BILAGA 5 MODELLING OF SEDIMENT SPILL IN SWEDEN DOCUMENT ID: W-PE-EIA-PSE-REP EN-06

2 Intended for Nord Stream 2 AG Date September, 2016 Document number W-PE-EIA-PSE-REP EN-06 NORD STREAM 2 MODELLING OF SEDIMENT SPILL IN SWEDEN

3 NORD STREAM 2 Modelling of sediment spill in Sweden Revision 06 Date Document ID Ref W-PE-EIA-PSE-REP EN / PO Revision Record: Revision Date Made by Checked Approved by Approved by (discipline lead) (Project Manager) EAB/HSN JAN HSN MIBR EAB JAN HSN MIBR JAN JLA HSN MIBR JAN JLA HSN MIBR JAN JLA HSN MIBR JAN JLA HSN MIBR by Ramboll Hannemanns Allé 53 DK-2300 Copenhagen S Denmark T F

4 CONTENTS 1. INTRODUCTION 1 2. SEDIMENT CHARACTERISTICS Trenching locations Rock placement locations 8 3. SPILL SCENARIOS MODEL RESULTS Trenching Rock placement SUMMARY OF RESULTS CONCLUSIONS REFERENCES 40 Document No.: 9TW-9TPE-EIA-PSE-REP EN-06

5 ABBREVIATION LIST ADCP Acoustic Doppler Current Profiler DHI Danish Hydraulic Institute EIA Environmental Impact Assessment ES Environmental Study NSP Nord Stream pipeline system NSP2 Nord Stream 2 pipeline system MIKE 3 three-dimensional modelling suite MIKE 3 HD hydrodynamic module of MIKE 3 MIKE 3 PT particle tracking module of MIKE 3 SGU Sveriges geologiska undersökning (Geological Survey of Sweden) Document No.: 9TW-9TPE-EIA-PSE-REP EN-06

6 1 1. INTRODUCTION The Environmental Study (ES) for the Nord Stream 2 pipeline (NSP2) requires forecasting of the sediment spill expected during seabed intervention works in order to evaluate the corresponding impact on the marine environment. This report documents the model input and the results of the simulations that have been selected as representative scenarios. The seabed intervention works used in the model scenarios is based on specifications from NSP2 with possible positions and volumes for seabed intervention works, /8/ /9/. The modelling of sediment spreading from seabed intervention work (trenching and rock placement) is based on a worst case scenario, therefore it is considered to be a conservative approximation, and the extent of impacts will most likely be much smaller in reality. The expected seabed intervention works is graphically presented in Figure 1-1, Figure 1-2 and Figure 1-3. The locations are listed in Table 2-3, Table 2-5 and Table 2-6. Figure 1-1: Map of Swedish waters with planned trenching locations.

7 2 Figure 1-2: Map of Swedish waters with planned rock placement locations.

8 3 Rock Placement Figure 1-3: Map of Swedish waters with planned spot gravel locations. The purpose of this report is to disseminate model results among contributors to the Swedish ES and the Espoo EIA. Consequently, it focuses on the presentation of results. Background information on the hydrographic basis and the fundamental assumptions and common methodology for Finland, Sweden and Denmark are only briefly summarised in this report. An indepth description is found in /2/. An overview of the input to the model simulations are given in Table 1-1. Table 1-1: Summary of input to model simulations Scenario Trenching Rock placement Locations Southeast of Hoburgs Bank and Norra Midsjöbanken East and northeast of Gotland and east of Hoburgs Bank Route NSP2 line B NSP2 line B Activity Post-lay trenching Rock placement Method Plough Fall-pipe Speed 300 m/hr 20,000 T/day Scope 72.4 km 125 locations (+79 spot gravel locations) Volume 6.2 m 3 /m (448,390 m 3 ) 518,479 m 3 Duration 10 days 49 days Spill 2% (6467 T) 5-16 kg/s <0.5% (1372T) Parameters Sediment Sediment kg/s Weather Normal, winter, summer Normal, winter, summer

9 4 This report contains a brief description of the seabed intervention works that form the basis for the scenarios the works include both post-lay trenching and rock placement. The modelling is carried out for one pipeline, and it is assumed that the impact from seabed intervention works for the other pipeline is similar or less to the pipeline that is modelled. Seabed interventions for line B is used for the modelling. A full description of the seabed intervention works (construction scenarios) is found in /3/. The interrelationship between this report and the related background documents is illustrated in Figure 1-4. Numerical modelling: Methodology and assumptions /2/ The fundamental assumptions and methodology common for Finland, Sweden and Denmark Numerical modelling: Overview of scenarios /3/ Full description of scenarios in Finland, Sweden and Denmark This Report -Brief summary of above reports -Results Figure 1-4: Interrelation between this report and background documents.

10 5 2. SEDIMENT CHARACTERISTICS This section presents an analysis of the sediment samples and the model input for the seabed intervention works planned in Swedish EEZ. The characterisation of sediment to be spilled in Sweden is based on sediment classification maps from the Geological Survey of Sweden (Sveriges geologiska undersökning SGU), /5/. The classes applied in this context are: 1. Mud, clay and gyttja; 2. Fine sand to pebbles; 3. Clay of the Baltic Ice Lake; 4. Glacial deposits; 5. Mixed sediments. Each sediment class is characterised on the basis of sediment distribution analyses of samples within each of the classified areas. For example, all grain size distributions within areas classified as gyttja, fine sand to pebbles are grouped in one class and a characteristic grain size distribution is estimated. The characteristic grain size distributions are shown on Figure 2-1. For full details on the metrology and the detailed estimation, please see /2/. Figure 2-1: Average values for SGU sediment types. Vertical lines are the mean values from Table 2-1. The grain size distributions are divided in six (6) different particle fractions, which will be used in the modelling: very fine sand ( mm), coarse silt ( mm), medium silt ( mm), fine silt ( mm), very fine silt ( mm) and clay (<0.004 mm). For each of the SGU sediment types the amount (percentage) of sediment within each fraction is determined on the basis of the curves shown in Figure 2-1. The sediment types are characterised by these percentages as e.g. type 1 mud, clay and gyttja contains 46% within the clay fraction and 7% within the fine sand fraction, while type 2 fine sand to pebbles contains 3% within the clay fraction and 16% within the fine sand fraction. The full description of the percentage of the particle fractions for each sediment type can be found in /2/. The coarser sediment fractions (coarser than mm) are not taken into consideration in this context because they will settle in proximity to the seabed intervention works and therefore will not influence the environment outside the immediate work area.

11 6 Table 2-1 shows the grain size ranges for each of the fractions used in the modelling. The representative logarithmic mean grain sizes are also shown in the table. Settling velocities are estimated for each fraction in /2/ on the basis of average water properties for the Baltic Proper. Settling velocities are also given in Table 2-1. Table 2-1: Grain size range Sediment spill class Lower grain size limit (mm) Upper grain size limit (mm) Mean (ln) grain size within range (mm) Settling velocity (m/s) Very fine sand Coarse silt Medium silt Fine silt Very fine silt Clay The water content and content of dry matter have been estimated in /2/. The results are given in Table 2-2. For full details, please see /2/. Table 2-2: Average water/dry content Sweden,/1/ Sediment type Dry content (%) Water content (%) Mud, clay and gyttja Fine sand to pebbles Clay of the Baltic Ice Lake Glacial deposits - - Mixed sediments - - 1) Estimated based on experience as water content in sample results was considered underestimated. 2.1 Trenching locations Figure 2-2 depicts the trenching sections and the corresponding sediment types. The numbers from 1 to 10 above the lines in the figure refers to trenching section 1-10 (TR1-TR10) referred to in Table 2-3. The numbers below the lines refer to the numbering of trenching sections received from NSP2 of which six sections are in Swedish waters. Each overall trenching section is split up to minor sections for use in the modelling on basis of the sediment types along the overall section. The trenching sections TR1 - TR5 used in the modelling is one continuous line but is this way split up into five different trenching sections based on the sediment types.

12 7 Figure 2-2: Numbering of trenching sections and sediment classification map. The sediment characteristics for the 10 trenching sections are described in Table 2-3. Table 2-3: Trenching sections and sediment classification. Trenching section Length (m) TR C3 SGU sediment type Clay of the Baltic Ice Lake Detailed description Clay of the Baltic Ice Lake, the Yoldia Sea and the Ancylus Lake TR S2 Fine sand to pebbles Coarse aleurite ( mm) TR G4 Glacial deposits (till) Glacial deposits (till) TR S2 Fine sand to pebbles Coarse aleurite ( mm) TR C3 TR S2 Clay of the Baltic Ice Lake Clay of the Baltic Ice Lake Clay of the Baltic Ice Lake, the Yoldia Sea and the Ancylus Lake Clay of the Baltic Ice Lake, the Yoldia Sea and the Ancylus Lake TR S2 Fine sand to pebbles Coarse aleurite ( mm) TR S2 Fine sand to pebbles Coarse aleurite ( mm) TR S2 Fine sand to pebbles Coarse aleurite ( mm) TR S2 Fine sand to pebbles Coarse aleurite ( mm) For each trenching section (source), the resulting spill rates for the six particle fractions are defined as per Table 2-4.

13 8 Table 2-4: Trenching calculated spill rates per sediment fraction in kg/s. Source ID contains sediment codes as given in Table 2-2. Source ID Very fine sand (kg/s) Coarse silt (kg/s) Medium silt (kg/s) Fine silt (kg/s) Very fine silt (kg/s) Clay (kg/s) Total fines (kg/s) TR TR TR TR TR TR TR TR TR TR The spill is assumed to be confined to a height of 5 m above the seabed based on considerations of the plough size used for trenching, /4/. Assuming a trenching speed of 300 m/h, trenching operations will cover a period of 10 days (240 hours). This does not include any time for relocation of the equipment, which is considered conservative. The trenching amount is assumed 6.2 m 3 /m. Spill rates are calculated on basis of trenching speed [m 3 /s], density of the specific sediment type [kg/m 3 ], the spill percent (2%), the dry matter content in the specific sediment type and the proportion of the fraction in the specific sediment type, /2/. 2.2 Rock placement locations Figure 2-3 depicts the rock placement locations and corresponding sediment types, and Figure 2-4 shows the spot gravel sections and corresponding sediment types. For the spot gravel sections it is assumed that the total volume will be divided into equal rock berms of 1250 m 3. This leads to 18 berms for the 22,852 m 3 for section 1, 35 berms for the 43,601 m 3 for section 2, 16 berms for the m 3 for section 3, 1 berm for sections 4-7 and 6 berms for the 7,218 m 3 for section 8. In Figure 2-4, the sections are hence shown in red while the berm number is shown in black.

14 9 Figure 2-3: Locations for rock placement and sediment classification map. Figure 2-4: Locations for spot gravel and sediment classification map. For each rock placement location (source) the distribution between the six sediment fractions are defined as per Table 2-5.

15 10 Table 2-5: Rock placement for seabed intervention works - calculated spill rates in kg/s and berm volumes in m 3 Type Berm number F1 F2 F3 F4 F5 F6 Volume berm (m 3 ) B B B B Pre-lay B B B B B B B B B B B B B B B B B B B B Post-lay, 2nd phase B B B B B B B B B B B B B B B B B B

16 11 Type Berm Number F1 F2 F3 F4 F5 F6 Volume berm (m 3 ) B B B B B B B B B B B B B B B B B Post-lay, 2nd phase B B B B B B B B B B B B B B B B B B B B B B

17 12 Type Berm number F1 F2 F3 F4 F5 F6 Volume berm (m 3 ) B B B B B B B B B B B B B Post-lay, 3rd phase B B B B B B B B B B B B B B B B B B B B Crossings B B B B B From generic scenario B

18 13 Table 2-6: Rock placement for spot gravel works calculated spill rates in kg/s and berm volumes in m 3. Volume Berm Spot gravel F1 F2 F3 F4 F5 F6 Berm number section (m 3 ) Berm Spot gravel F1 F2 F3 F4 F5 F6 Volume

19 14 number section Berm (m 3 ) Pipeline crossing Tie-in The spill is assumed to be confined to a height of 2 m above the seabed, /2/. Assuming a rock placement speed of t/d, rock placement operations will cover a period of 49 days (1161 hours). This does not include any time for relocation of the equipment, which is considered conservative.

20 15 Spill rates are calculated on the basis of guidelines in /7/ assuming that 30% of the rock volume contribute to the spilling, a velocity of the falling rock inside a tube of 1.44 m/s and 10% of the total energy will cause a resuspension of sediments.

21 16 3. SPILL SCENARIOS The environmental modelling is carried out using actual hindcast hydrographic scenarios. This means that the modelling is carried out for a real design period as opposed to an artificial situation. The challenge of this approach is defining a representative design period for this purpose. The representative design period should within a short period of time (approximately one month) reflect conditions that are typically seen and that characterise long-term data. The scenario periods are chosen to represent different current conditions (for transport modelling) and different stratification conditions. On the basis of current, salinity and temperature time series from the hydrodynamic model, scenario periods are selected to represent: Summer, typical (calm conditions / weak currents and high stratification); Normal conditions (average currents and stratification for an entire year); Winter, typical (rough conditions / strong currents and low stratification). The terms summer, normal and winter reflect the fact that relatively calm summer conditions result in reduced particle spreading over a small area close to the point where a substance is released, whereas relatively rough winter conditions cause transport further away from the point of release under larger dilution. When a particle (or a quantity of dissolved substance) is released, it is transported by the ambient water. The net transport (linear distance between the start and the end of the trajectory) is in the following chosen to be the measure for characterising the above-mentioned hydrographic scenario periods in relation to transport and spreading. A time series of monitoring data and model data for the locations shown in Figure 3-1 has been analysed for a three-year period starting in autumn Measured current velocities are acquired from acoustic Doppler current profiler (ADCP) measurements from the NSP monitoring program at locations P5 and P6. Time series of modelled current are extracted from the hydrodynamic model positions (P1 to P4) shown in Figure 3-1. Temperature and salinity data are extracted from positions P1 to P6. Figure 3-1: Positions of time series data from the MIKE 3 hydrodynamic model and ADCP measurements. The black line represents the position of NSP2.

22 17 On the basis of time series from the six positions shown on Figure 3-1, three scenario periods are chosen as described above to represent different hydrographic conditions with respect to particle transport and temperature/salinity stratification. The same scenario periods are used throughout the model domain, i.e. the same periods are used in the EIAs of all countries. On the basis of evaluations of accumulated frequency diagrams for particle transport distances and evaluations of isopleth plots for salinity and temperature stratifications, the following scenario periods are chosen: Normal conditions April 2010: Represents average current conditions with average particle transport capacity and average temperature and salinity stratification. Calm conditions (summer) June 2010: Represents calm current conditions with little particle transport capacity and relatively strong temperature and salinity stratification. Rough conditions (winter) November 2010: Represents relatively strong current conditions with high particle transport capacity and relatively low temperature and salinity stratification (but without ice coverage). Further details about the choice of periods can be found in the modelling basis report, /2/. The hydrographic forcing (current fields) is simulated by a calibrated MIKE 3 hydrodynamic (HD) model. For detailed information on the set-up and calibration of the hydrodynamic model, see /4/.

23 18 4. MODEL RESULTS This chapter presents the results of the modelling of the seabed intervention works. The results are presented as two-dimensional maps. The height of the sediment release is modelled in the simulations as 2 m above the seabed for rock placement and 5 m above the seabed for trenching. Due to the location of the release and because sediment is settling through the water column, the highest sediment concentrations are found near the seabed. Therefore all results related to suspended sediment are based on an average of the lower 10 m of the water column. The results for the concentrations of suspended sediments are shown in relation to different threshold concentrations. The thresholds of 5, 10 and 25 mg /l are used based on biological/ecotoxikological thresholds values. Considerations have been given to different organisms when choosing the thresholds. In literature however, fish is the receptor with the lowest thresholds for suspended sediment which has been used; 5mg / l have been found to affect cod eggs ability to flow (after 96 hours), 10 mg / l is a usual threshold value used for the avoidance behavior of fish and 25 mg / l is a common threshold used for impacts of clams and salmon. The maps show the following parameters: The maximum concentration of suspended sediment occurring under the entire simulation period. The maximum concentration is not exceeded at any time during the seabed intervention works. Duration of exceedance of 5, 10 and 25 mg/l. This is the accumulated period of time during which the concentration of suspended material exceeds 5, 10 and 25 mg/l during either trenching or rock placement. The duration is expressed in hours. Durations are not shown on maps for 25 mg/l, because the areas are small and not clearly visible on a map. The maximum duration of 5, 10 and 25 mg/l is shown in a table in the results summary. Sedimentation, which is expressed as g/m 2. The corresponding thickness depends on the density, which again is dependent on the consolidation of the material. Considering fluffy sediment with a dry matter content of 100 kg/m 3 means that a 1 mm thickness corresponds to a sedimentation of 100 g/m 2. A higher degree of consolidation (consequently higher density) corresponds to a thinner layer thickness for the same sedimentation, e.g. 100 g/m 2. The results for trenching are presented in Figure 4-1 to Figure 4-12 in section 4.1, while the results for rock placement are presented in Figure 4-13 to Figure 4-18 in section 4.2. The figures presenting the results are listed in Table 4-1. Table 4-1: Overview of modelling results Seabed intervention work Normal Summer Winter Trenching Rock placem ent Maximum concentration of suspended sediment (mg/l) Figure 4-1 Figure 4-2 Figure 4-3 Duration (hours) of exceeding threshold concentration 5 mg/l Figure 4-4 Figure 4-5 Figure 4-6 Duration (hours)of exceeding threshold concentration 10 mg/l Figure 4-7 Figure 4-8 Figure 4-9 Sedimentation (g/m 2 ) Figure 4-10 Figure 4-11 Figure 4-12 Maximum concentration of suspended sediment (mg/l) Figure 4-13 Figure 4-14 Figure 4-15 Duration (hours) of exceeding threshold concentration 5 mg/l N/A 1 N/A 1 N/A 1

24 19 Duration (hours) of exceeding threshold concentration 10 mg/l N/A 1 N/A 1 N/A 1 Sedimentation (g/m 2 ) Figure 4-16 Figure 4-17 Figure Concentrations are only exceeded in very small areas, which are not clearly visible when plotted on a map. None of the small areas are located within a protected area. Therefore the plots are omitted. 4.1 Trenching Figure 4-1: Maximum concentration of suspended sediment for trenching under normal hydrographic conditions. Figure 4-1 shows the highest concentrations observed during the trenching operations under normal conditions.

25 20 Figure 4-2: Maximum concentration of suspended sediment for trenching under typical summer conditions. Figure 4-2 shows the highest concentrations observed during the trenching operations under typical summer conditions.

26 21 Figure 4-3: Maximum concentration of suspended sediment for trenching under typical winter conditions. Figure 4-3 shows the highest concentrations observed during the trenching operations under typical winter conditions.

27 22 Figure 4-4: Duration of exceeding 5 mg/l for trenching under normal hydrographic conditions. Figure 4-4 shows the duration for which the concentration threshold of 5 mg/l is exceeded during the trenching operations under normal conditions.

28 23 Figure 4-5: Duration of exceeding 5 mg/l for trenching under typical summer conditions. Figure 4-5 shows the duration for which the concentration threshold of 5 mg/l is exceeded during the trenching operations under typical summer conditions.

29 24 Figure 4-6: Duration of exceeding 5 mg/l for trenching under typical winter conditions. Figure 4-6 shows the duration for which the concentration threshold of 5 mg/l is exceeded during the trenching operations under typical winter conditions.

30 25 Figure 4-7: Duration of exceeding 10 mg/l for trenching under normal hydrographic conditions. Figure 4-7 shows the duration for which the concentration threshold of 10 mg/l is exceeded during the trenching operations under normal conditions.

31 26 Figure 4-8: Duration of exceeding 10 mg/l for trenching under typical summer conditions. Figure 4-8 shows the duration for which the concentration threshold of 10 mg/l is exceeded during the trenching operations under typical summer conditions.

32 27 Figure 4-9: Duration of exceeding 10 mg/l for trenching under typical winter conditions. Figure 4-9 shows the duration for which the concentration threshold of 10 mg/l is exceeded during the trenching operations under typical winter conditions.

33 28 Figure 4-10: Sedimentation of released material due to trenching under normal conditions. Figure 4-10 shows sedimentation after the last time step of the simulation under normal conditions.

34 29 Figure 4-11: Sedimentation of released material due to trenching under typical summer conditions. Figure 4-11 shows sedimentation after the last time step of the simulation under typical summer conditions.

35 30 Figure 4-12: Sedimentation of released material due to trenching under typical winter conditions. Figure 4-12 shows sedimentation after the last time step of the simulation under typical winter conditions.

36 Rock placement Figure 4-13: Maximum concentration of suspended sediment for rock placement under normal hydrographic conditions. Figure 4-13 shows the highest concentrations observed during the rock placement operations under normal conditions in the northern area with rock placement operations shown with a red square in the overview map. This is the area with the largest impact from the rock placement operations, and concentrations are generally lower or equal in the areas longer towards south where rock placement is undertaken (shown with black squares).

37 32 Figure 4-14: Maximum concentration of suspended sediment for rock placement under typical summer conditions. Figure 4-14 shows the highest concentrations observed during the rock placement operations under typical summer conditions in the northern area with rock placement operations shown with a red square in the overview map. This is the area with the largest impact from the rock placement operations, and concentrations are generally lower or equal in the areas longer towards south where rock placement is undertaken (shown with black squares).

38 33 Figure 4-15: Maximum concentration of suspended sediment for rock placement under typical winter conditions. Figure 4-15 shows the highest concentrations observed during the rock placement operations under typical winter conditions in the northern area with rock placement operations shown with a red square in the overview map. This is the area with the largest impact from the rock placement operations, and concentrations are generally lower or equal in the areas longer towards south where rock placement is undertaken (shown with black squares).

39 34 Figure 4-16: Sedimentation of released material due to rock placement under normal conditions. Figure 4-16 shows sedimentation after the last time step of the simulation under normal conditions in the northern area with rock placement operations shown with a square in the overview map. This is the area with the largest impact from the rock placement operations, and sedimentation is generally lower or equal in the areas longer towards south where rock placement is also undertaken.

40 35 Figure 4-17: Sedimentation of released material due to rock placement under typical summer conditions. Figure 4-17 shows sedimentation after the last time step of the simulation under typical summer conditions in the northern area with rock placement operations shown with a square in the overview map. This is the area with the largest impact from the rock placement operations, and sedimentation is generally lower or equal in the areas longer towards south where rock placement is also undertaken.

41 36 Figure 4-18: Sedimentation of released material due to rock placement under typical winter conditions. Figure 4-18 shows sedimentation after the last time step of the simulation under typical winter conditions in the northern area with rock placement operations shown with a square in the overview map. This is the area with the largest impact from the rock placement operations, and sedimentation is generally lower or equal in the areas longer towards south where rock placement is also undertaken.

42 37 5. SUMMARY OF RESULTS This section presents a summary of the modelling results for the trenching and rock placement simulations. The summary contains: Areas where concentrations of 5 mg/l, 10 mg/l and 25 mg/l are exceeded. Maximum duration of exceedance of 5 mg/l, 10 mg/l and 25 mg/l. Areas where sedimentation of 10, 100, 200, 1000 and 1500 g/m 2 are exceeded. Maximum concentration (mg/l) at specific distances from the pipelines. Maximum sedimentation (g/m2) at specific distances from the pipelines. The values presented are overall maximum values, based on conservative volumes for seabed intervention works, covering all three hydrographic scenarios normal, summer and winter. This means that for each period the largest value of the three scenarios has been used. The corresponding tables for the individual hydrographic scenarios are presented in Appendix 1. Table 5-1: Size of areas where threshold concentrations are exceeded Seabed intervention works maximum of hydrographic scenarios Route section Suspended sediment Area with concentration > 5 mg/l > 10 mg/l > 25 mg/l km tonnes km 2 km 2 km 2 Trenching Rock placement 125 locations (+79 spot gravel locations) <0.02 Total SE The smallest grid size (modelling setup) have an area of km 2 Table 5-2: Maximum duration of exceeding threshold concentrations Seabed intervention works maximum of hydrographic scenarios Route section Suspended sediment Max duration with concentration > 5 mg/l > 10 mg/l > 25 mg/l km tonnes hours hours hours Trenching Rock placement 125 locations (+79 spot gravel locations) <0.5 The smallest grid size (modelling setup) have an area of km 2

43 38 Table 5-3: Size of areas where sedimentation values are exceeded Seabed intervention works maximum of hydrographic scenarios Route section Suspended sediment Area with sedimentation > > 10 > 100 > 200 g/m 2 g/m 2 g/m g/m 2 > 1500 g/m 2 km tonnes km 2 km 2 km 2 km 2 km 2 Trenching <0.02 <0.02 Rock placement 125 locations (+79 spot gravel locations) <0.02 Total SE <0.06 <0.04 The smallest grid size (modelling setup) have an area of km 2 Table 5-4: Maximum concentration in specific distances from pipelines Seabed intervention works maximum of hydrographic scenarios Route section Suspended sediment Max concentration in specific distances from pipelines 200 m 500 m 1000 m Km tonnes mg/l mg/l mg/l Trenching Rock placement 125 locations (+79 spot gravel locations) The smallest grid size (modelling setup) have an area of km 2 Table 5-5: Maximum sedimentation in specific distances from pipelines Seabed intervention works maximum of hydrographic scenarios Route section Suspended sediment Max sedimentation in specific distances from pipelines 200 m 500 m 1000 m Km tonnes g/m 2 g/m 2 g/m 2 Trenching Rock placement 125 locations (+79 spot gravel locations) The smallest grid size (modelling setup) have an area of km 2

44 39 6. CONCLUSIONS The trenching scenarios show areas with maximum concentrations of suspended sediment above 5 mg/l, 10 mg/l and also 25 mg/l, depending on the hydrographic conditions and based on conservative volumes for seabed intervention works. For the trenching scenarios for average and summer conditions, the areas of impact are quite close to the pipeline route. However, for the winter conditions the sediment spreads to a larger area away from the vicinity of the pipeline route. The extent of the area over which the sediment is spread has been calculated and summarised in Table 5-1 for the three critical threshold concentrations. From Table 5-1 it is obvious that the trenching works during the winter period will have the largest impact due to high concentrations and large spreading of the sediments. Sedimentation is limited to the area very close to the pipeline route. For the rock placement scenarios, the maximum concentration of suspended sediment in a distance of 200 m from the pipeline is 12 mg/l, see Table 5-4. As shown in Figure 4-13 to Figure 4-15 and in Table 5-1 concentrations above 10 mg/l are limited to a very small area and only found right on the pipeline route. Sedimentation is at a maximum 0.8 kg/m 2 at a distance of 200 m from the pipeline route after the end of the trenching scenarios. After the rock placement simulations sedimentation is not exceeding 1.1 kg/m 2 at any location. The corresponding thickness depends on the density, which again is dependent on the consolidation of the material. Considering fluffy sediment with a dry matter content of 100 kg/m 3 a 1 mm thickness corresponds to sedimentation of 100 g/m 2. A higher degree of consolidation (consequently higher density) corresponds to a thinner layer thickness for the same sedimentation, e.g. 100 g/m 2. Comparing the results to the modelling performed in connection with the NSP project, the areas where a concentration of 10 mg/l is exceeded is larger, especially for the winter scenario where the area is 134 km 2 against only 70 km 2 in NSP. However, the duration of exceedance of 10 mg/l is generally lower than for the NSP. The larger area is mainly caused by a more detailed approach in the NSP2 modelling, where sediment characteristics are described in larger detail and the current field is described in larger detail. This means that in the NSP2 modelling the fine sediment fractions are described in more detail and with a larger range of settling velocities. Furthermore, the more detailed current field will tend to describe the large currents better and thereby also increasing the re-suspension of already settled material in the modelling. All this may bring concentrations above 10 mg/l to larger distances than seen in the NSP modelling. When comparing to NSP measurements, very low concentrations were seen during the monitoring campaign. This should be seen as a consequence of the specific hydrographic conditions under which the works was carried out, whereas the larger concentrations in the modelling is a consequence of the conservative input data and due to the modelling aiming at describing some worst case hydrographic scenarios. Also the duration of large concentrations should be considered as the concentration of e.g. 10 mg/l is only exceeded for a very short time at each location which limits the effect and the possibility for caching the highest concentrations during monitoring.

45 40 7. REFERENCES /1/ Nord Stream, 2007, Seabed sediment and fauna investigation, Nord Stream Environmental Baseline Survey in the Swedish EEZ, Contractor: Geological Survey of Sweden. ENV_S_BL_SED_IN_2007 /2/ Nord Stream 2. Numerical Modelling: Methodology and assumptions. Ramboll, August Doc. no. W-PE-EIA-POF-MEM EN-02 /3/ Nord Stream 2. Numerical Modelling: Overview of Scenarios. Ramboll, September Doc. no. W-PE-EIA-POF-MEM EN-02 /4/ Nord Stream 2 Project in the Baltic, Hydrographic basis for spill assessment, Data from enhanced DHI hydrographic model, The Rock Manual. The use of rock in hydraulic engineering (2nd edition) /5/ Western and central Baltic sea compiled from: -Repecka, M. & Cato, I., 1999: "Bottom Sediment Map of the Central Baltic Sea". Geological Survey of Lithuania (LGT) Series of Marine Geological Maps No. 1 and Geological Survey of Sweden (SGU). Series of Geological Maps Ba No. 54. /6/ Curt Fredén (editor), 1994: "Berg och jord". Sveriges National atlas, SNA Förlag, Stockholm, 208 pp. /7/ CIRIA, CUR and CETMEF, 2007, The Rock Manual - The use of rock in hydraulic engineering, C683, CIRIA, London. /8/ Nord Stream 2. Input data for EIA 1, Sweden. Saipem, , Rev. 2. /9/ Nord Stream 2. Input data for initiation of EIA Doc. No. W-EN-OFP-POF- MEM-800-EIA001EN-01

46 41 APPENDIX 1 RESULT TABLES FOR NORMAL, SUMMER AND WINTER HYDROGRAPHY

47 42 Normal hydrography This section presents a summary of the modelling results for normal hydrography Seabed intervention works normal conditions Route section Suspended sediment Area with concentration > 5 mg/l > 10 mg/l > 25 mg/l km tonnes km 2 km 2 km 2 Trenching Rock placement 125 locations (+79 spot gravel locations) <0.02 Total SE The smallest grid size (modelling setup) have an area of km 2 Seabed intervention works normal conditions Route section Suspended sediment Max duration with concentration > 5 mg/l > 10 mg/l > 25 mg/l km tonnes hours hours hours Trenching Rock placement 125 locations (+79 spot gravel locations) The smallest grid size (modelling setup) have an area of km <0.5

48 43 Seabed intervention works normal conditions Route section Suspended sediment Area with sedimentation > > 10 > 100 > 200 g/m 2 g/m 2 g/m g/m 2 > 1500 g/m 2 km tonnes km 2 km 2 km 2 km 2 km 2 Trenching <0.02 <0.02 Rock placement 125 locations (+79 spot gravel locations) <0.02 Total SE <0.02 The smallest grid size (modelling setup) have an area of km 2 Seabed intervention works normal conditions Route section Suspended sediment Max concentration in specific distances from pipelines 200 m 500 m 1000 m Km tonnes mg/l mg/l mg/l Trenching Rock placement 125 locations (+79 spot gravel locations) The smallest grid size (modelling setup) have an area of km Seabed intervention works normal conditions Route section Suspended sediment Max sedimentation in specific distances from pipelines 200 m 500 m 1000 m Km tonnes g/m 2 g/m 2 g/m 2 Trenching Rock placement 125 locations (+79 spot gravel locations) The smallest grid size (modelling setup) have an area of km

49 44 Summer hydrography This section presents a summary of the modelling results for summer hydrography. Seabed intervention works summer conditions Route section Suspended sediment Area with concentration > 5 mg/l > 10 mg/l > 25 mg/l km tonnes km 2 km 2 km 2 Trenching Rock placement 125 locations (+79 spot gravel locations) <0.02 Total SE The smallest grid size (modelling setup) have an area of km 2 Seabed intervention works summer conditions Route section Suspended sediment Max duration with concentration > 5 mg/l > 10 mg/l > 25 mg/l km tonnes hours hours hours Trenching Rock placement 125 locations (+79 spot gravel locations) <0.5 The smallest grid size (modelling setup) have an area of km 2

50 45 Seabed intervention works summer conditions Route section Suspended sediment Area with sedimentation > > 10 > 100 > 200 g/m 2 g/m 2 g/m g/m 2 > 1500 g/m 2 km tonnes km 2 km 2 km 2 km 2 km 2 Trenching <0.02 <0.02 Rock placement 125 locations (+79 spot gravel locations) <0.02 <0.02 Total SE <0.02 <0.02 The smallest grid size (modelling setup) have an area of km 2 Seabed intervention works summer conditions Route section Suspended sediment Max concentration in specific distances from pipelines 200 m 500 m 1000 m Km tonnes mg/l mg/l mg/l Trenching Rock placement 125 locations (+79 spot gravel locations) The smallest grid size (modelling setup) have an area of km Seabed intervention works summer conditions Route section Suspended sediment Max sedimentation in specific distances from pipelines 200 m 500 m 1000 m Km tonnes g/m 2 g/m 2 g/m 2 Trenching Rock placement 125 locations (+79 spot gravel locations) The smallest grid size (modelling setup) have an area of km

51 46 Winter hydrography This section presents a summary of the modelling results for winter hydrography. Seabed intervention works winter conditions Route section Suspended sediment Area with concentration > 5 mg/l > 10 mg/l > 25 mg/l km tonnes km 2 km 2 km 2 Trenching Rock placement 125 locations (+79 spot gravel locations) <0.02 Total SE The smallest grid size (modelling setup) have an area of km 2 Seabed intervention works winter conditions Route section Suspended sediment Max duration with concentration > 5 mg/l > 10 mg/l > 25 mg/l km tonnes hours hours hours Trenching Rock placement 125 locations (+79 spot gravel locations) The smallest grid size (modelling setup) have an area of km 2

52 47 Seabed intervention works winter conditions Route section Suspended sediment Area with sedimentation > > 10 > 100 > 200 g/m 2 g/m 2 g/m g/m 2 > 1500 g/m 2 km tonnes km 2 km 2 km 2 km 2 km 2 Trenching <0.02 <0.02 Rock placement 125 locations (+79 spot gravel locations) <0.02 <0.02 Total SE <0.02 <0.02 The smallest grid size (modelling setup) have an area of km 2 Seabed intervention works winter conditions Route section Suspended sediment Max concentration in specific distances from pipelines 200 m 500 m 1000 m Km tonnes mg/l mg/l mg/l Trenching Rock placement 125 locations (+79 spot gravel locations) The smallest grid size (modelling setup) have an area of km Seabed intervention works winter conditions Route section Suspended sediment Max sedimentation in specific distances from pipelines 200 m 500 m 1000 m Km tonnes g/m 2 g/m 2 g/m 2 Trenching Rock placement 125 locations (+79 spot gravel locations) The smallest grid size (modelling setup) have an area of km