Effects burial Borssele export cables

Transcription

1 Appendix A-18

2 Effects burial Borssele export cables Assessment of siltation in the EPZ intake due to dredging/dumping Client: TenneT TSO BV. Date: 20/06/2018 Clients Reference: 2018B Our Reference: WP2018_1104_R2r1 Status: Final

3 Title Effects burial Borssele export cables; Assessment of siltation in the EPZ intake due to dredging/dumping Client TenneT TSO BV. Reference WP2018_1104_R2r1 Keywords Western Scheldt, Borselle export cables, Offshore Wind Farm, Morphology, Sediment transports, siltation, EPZ, intake Summary The dredging of trenches and dumping of sediment along the cable trajectories, in particular the seaward part of the Western Scheldt Estuary, might trigger changes in sediment transport, with potential implications for the morphodynamic evolution of this region. In particular, concern exists about the possibility that (parts) of the sediment on the dumping sites will eventually end up in the nearby Cooling water intake of EPZ, meaning that an increased amount of sediment needs to be dredged from this channel. Tennet TSO requested WaterProof to determine the potential impact of the presence of trenches and dumping sites on the transport of cohesive sediment towards the EPZ cooling water intake. To address this impact, a detailed coupled SWAN-Delft3D model was setup, which accounts for flow and wave dynamics. Subsequently, this model was run for a period of 6 months. Moreover, two different seabed sediment fractions are applied. As a conservative maximum, results show that approx. 1-10% of the material dredged from the cable installation and dumped on the marshland area will deposit in the intake. Version Date Author Review & Approval R2r L.Perk/ A. Nnafie K. Koudstaal R2r L.Perk/ A. Nnafie K. Koudstaal WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 2/26

4 TABLE OF CONTENTS Table of contents Introduction Background Objectives Approach and report outline Cable design & project area Trenching and dumping location Hydrodynamics Water levels Flow velocities Wave climate Sediment composition Model setup and validation Model setup Mesh and boundary conditions Bathymetry Cooling water intake Sediment schematization Simulation period Model parameters Model Validation Model results Overview model simulations Results sediment 1, 25 µm Results sediment 2, 50 µm Summary of results Conclusions and Recommendations Conclusions Recommendations References WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 3/26

5 NTRODUCT ON 1.1 BACKGROUND The Dutch government has designated a number of areas in the Dutch North Sea for offshore wind farm development for the period until The first area to be developed is the Borssele Wind Farm Zone (BWFZ). The subsea power cables are to be buried into the seabed to protect them against external threats. In 2016, WaterProof Marine Consultancy & Services BV (WaterProof) has executed the Burial Depth Assessment (BDA) for the Borssele Offshore Windfarm (OWF) export cables. The four cables between the Borssele OWF and the landfall at Borssele will be buried in the seabed over a length of 67 kilometre. See Figure 1.1, where the trajectories of the Alpha (south) and Beta (north) cables are indicated by the red lines. In 2019 the Alpha cables will be installed, in 2018 the Beta cables. At landfall, the cables are situated next to the EPZ inlet, as shown in Figure 1.2. Figure 1.1: Bathymetric map of the study area. The Alpha (south) and Beta (north) Borssele export cables are indicated by the red lines. The black dots denote the distance KP in km with respect to the land fall at Borssele (KP 0). The cables extend over a distance of 67 km (KP 67). WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 4/26

6 1.2 OBJECTIVES The landfall of the Borssele export cables will be located just east of the EPZ inlet (see Figure 1.2 ). The cables will be buried approx. 1 m below the seabed by means of dry excavation (grab dredging). Concern exists about the possibility that part of the dredged sediment (mostly clay/mud) that will be temporarily placed next to the cables, would end up in the intake, with potential implication for the intake maintenance requirements and the water quality of the cooling water. The objectives of this study are to: 1. Quantify the amount of clay/mud that will deposit in the EPZ inlet, resulting from temporarily placing sediment (clay/mud) from dredging activities on the mudflats; 2. Assess the effect on suspended sediment concentrations at the intake. EPZ Outfall EPZ Intake Location of placed mud/clay from trench dredging on the mudflat Dumping locations for sand from trench dredging in the Honte Figure 1.2: Overview of the landfall of the Borssele export cables, EPZ intake and outfall location and location where dredged sediment will be placed (within 200 m from the cable trajectory). WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 5/26

7 1.3 APPROACH AND REPORT OUTLINE The outline below briefly summarizes the approach to the study. First, in Chapter 2, the project area and prospected dredging work are described. Next, in Chapter 3 a description is given of the numerical morphological model of the Scheldt Estuary and a more detailed model of the EPZ area. This overall model has been validated on hydrodynamics and morphodynamics. The validation results are described in detail in WaterProof (2018a). Chapter 4 presents the effects of the dredging works on the sediment infill in the EPZ intake and suspended sediment concentrations. Chapter 5 describes the conclusions and recommendations. WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 6/26

8 CABLE DES!GN & PROJECT AREA 2.1 TRENCHING AND DUMPING LOCATION The installation of the export cables will be executed by the contractor VBMS. The cable will be buried in the seabed using a trencher, which is capable to bury the cables at most 8.5m deep. The dredged sediment will be disposed along the trenches at a distance of maximum 200 m. Figure 2.1 Map showing an overview of the 2017 bathymetry, including the dredging locations used in the model simulations. The yellow, purple and green polygons represent the sills of Vlissingen, Honte and Borselle, respectively. At the landfall, the cables are located close to the EPZ inlet, as shown in Figure 1.2. The cables will be buried at a depth of minimal 1.0 m below the seabed. The trench will be dredged by means of a grab excavator. In the area between KP 0 and 0.7, this dredging depth is equivalent to a dredging volume of approx m 3 per cable, m³ in total. For the Alpha cables the dredged sediment will be placed at the eastern side of the cables, the dredged sediment from the Beta cables will be placed at the western side of the cables. WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 7/26

9 2.2 HYDRODYNAMICS Water levels The Scheldt estuary is a meso- to macro-tidal estuary in which the flow field is dominated by (semidiurnal) tidally induced currents, which are typically in the order of m/s (Wang et al., 2002). Measurements of the free surface elevation along the estuary show that the mean tidal range (difference between high and low water, averaged over one neap-spring tidal cycle) increases from about 3.8 m at the mouth to 5.2 m near Rupelmunde (89 km from Vlissingen), after which it decreases further landward. In Table 2.1 water level data for three locations near the project area is provided. Table 2.1: Water level data for three locations in the Western Scheldt. HW and LW levels are given in cm NAP (source: waterstanden, Rijkswaterstaat). Location Spring tide Average tide Neap tide HW- HW- HW- HW LW HW LW HW LW LW LW LW [cm] [cm] [cm] [cm] [cm] [cm] [cm] [cm] [cm] Westkapelle Vlissingen Hansweert Flow velocities Maximum flow velocities in the larger channels are approximately m/s during average tidal conditions, which increases to 3.0 m/s during spring tide in the deeper channels (Van Duren, 2009). These high velocities occur especially in the mouth of the estuary, in the Rede van Vlissingen and Honte channel. Inside the intake, flow velocities are substantially smaller when compared to those in the Honte with velocities ranging between 0.4 tot 0.2 m/s. On the salt marsh the flow velocities are also affected by breaking waves. WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 8/26

10 Figure 2.2: Maximum depth-average flow velocities during ebb (lower panel) and flood (upper panel) during spring tide (model results of the validated Delft3D model) Wave climate Waves are included in the model because they are important for the sediment transport, as they stir up sediment at the bed resulting in higher sediment concentrations, and the wave induced currents can significantly contribute to sediment transport. Time series of significant wave height (, in m), wave direction (, in degrees with respect to North) and peak wave period (, in s) collected by Rijkswaterstaat in the mouth area (at Deurlo station) between 2003 and 2015, are used to derive a representative wave WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 9/26

11 climate to force the model. From these time series a wind rose is constructed, which is presented in Figure 2.3. As can be seen from this figure, waves are coming mostly from the southwest and northwest. The southwesterly waves have a mean (averaged over the entire time period) significant wave height = 1 m, a mean peak wave period = 5.7 s and a mean wave direction = 250, and the northwesterly waves have = 0.9 m, = 7.1 s and = 345. Figure 2.3: Wave rose showing the probability of occurrence of wave events for different wave heights and directions. An example of the wave height distribution along the mouth of the estuary according to the validated numerical model with an imposed southwesterly offshore wave with conditions as described above is shown in Figure 2.4. The resulting wave height at the EPZ inlet is reduced to 0.6 m. Figure 2.4: Example of wave height distribution along the mouth of the estuary according to the numerical model (see Chapter 3) with an imposed southwesterly offshore wave with conditions = 1 m, = 5.7 s and = 250 WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 10/26

12 2.3 SEDIMENT COMPOSITION The bed of the Western Scheldt Estuary and its mouth consists of mainly medium to fine sand, with a grain size diameter D 50 ranging between 150 µm and 300 µm. See Figure 2.5, which shows a spatial distribution of the sediment grain size D 50 in the study area. Deeper areas are characterized by coarse sand, and shallower areas mostly contain finer sand. The EPZ inlet is located in and between mudflats, consisting of clay and fine sand (Deep BV, 2016). Cone penetration test at the site location indicate (VBMS, 2017) that the subsoil consists of medium dense clay and silt with undrained shear strengths of approx kpa. No information on the %fines or median grain size D 50 is available. Figure 2.5: Spatial distribution of D 50 (Geological service of the Netherlands, 2007). WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 11/26

13 MODEL SETUP AND VAL!DAT!ON 3.1 MODEL SETUP Mesh and boundary conditions Figure 3.1 shows the computational grid of the used Delft3D Scheldt model, which extends from Schelle to approx. 17 km offshore (marked by the red line in the figure). The grid size is about 50 to 200 m in the seaward part of the Scheldt Estuary, and it increases to 250 m in the more offshore areas. As boundary conditions, water levels are imposed at the offshore boundary, while combinations of water level and current (Riemann invariants) are imposed at the lateral boundaries. These boundary conditions are derived from the Simona Kuststrook fijn v6 coastal model. The imposed water level consists of 57 tidal constituents. At Schelle, a river discharge of 120 m 3 /s is imposed. Figure 3.1. Computational grid of Scheldt model. Since the resolution of the Scheldt model is too course for a detailed representation of the local bathymetric features in the area of interest, a detailed model of the latter area has been nested in the overall Scheldt model (see Figure 3.2). This nested model has a resolution of approx. 13 x 13 m at the EPZ inlet. The eastern boundary is located approx. 6 km east of the landfall. The western boundary is located about 7 km seawards of Vlissingen and consists of a water level timeseries, while discharge timeseries are used for the eastern boundary. WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 12/26

14 Figure 3.2. Mesh of the nested detailed model Bathymetry Bathymetrical data of the Western Scheldt (Vaklodingen) collected by Rijkswaterstaat in the years 2015 and 2017 are used in this study (see Figure 2.1). Due to lack of data in the most seaward parts of the model area, the 2014 bathymetrical data are used for these parts Cooling water intake Both the EPZ nuclear power plant and Coal power plant use water from the Western Scheldt for cooling purposes. In total both power plants use approx = 33 m³/s water (ARCADIS, 2012). This water is subtracted from the Western Scheldt at the cooling water intake and discharged in the outfall (locations, see Figure 1.2) Sediment schematization Unfortunately, no grain size distributions are available of the material dredged. Based on the Deep survey (Deep BV, 2016) and Cone Penetration tests as presented by VBMS (2017) it is expected the material is strongly silty/ clayey. Given the fact that relative high waves can approach this area and flow velocities can be above 0.4 to 0.5 m/s, the material should be well consistent. Dredging cohesive sediments can have a strong influence on the consistency of the material. Using grabtype dredging methods keeps the material more consistent than using a hydraulic dredging methods which fluidizes the material leading to much lower critical shear stresses for erosion than grab-type methods. Based on the information available and uncertainties on the effect of dredging on the sediment consistency, it is difficult to accurately schematize the sediment characteristics in the model. Thereto simulations have been performed for 2 cohesive sediment properties: WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 13/26

15 Table 3.1: Sediment properties applied Sediment D50 (µm) Fall vel. (mm/s) T crit erosion (N/m²) Dry bed density (Kg/m³) Erosion parameter (Kg/m²/s) Sediment Sediment The burial depth of the cable on the marshland area will be minimal 1.0 m, resulting in a volume of approx m 3 per cable, m³ in total for the two Alpha or Beta cables on the marshland area. Initially the minimal burial depth was 3.0 m, leading to a dredging volume of the two cables of approx m³, 5.5 times more than currently is expected to be dredged in the landfall area on the marshlands. The results presented in Chapter 4 are based on the dredging volume for cable installation at a depth 3 m below seabed. The effects for an installation depth of 1 m will be approx. 5.5 times less than calculated in this study Simulation period The simulation period employed in this study is chosen as follows. Because the hydrodynamic conditions (and related sediment transports), vary among the different neap-spring cycles (see Figure 3.3), first, a neap-spring cycle is determined where relatively high sediment transports occurs. To this end, the overall Scheldt model is run for a period of 5 months between 1/5/2018 and 1/10/2018. Results from the latter run are used to derive flow velocities and related sediment transports. Here, the assumption is used that sediment transport is proportional to (u 2 +v 2 ) 5/2 (Engelund and Hansen, 1967), with u and v the east- and northward components of the flow velocity. The computed transport, as well as its average over each neap-spring cycle (14.7 days) are presented in the lower panel of Figure 3.3 (blue and magenta lines, respectively). The latter figure reveals that the period between 5 and 20 august 2018 has the highest average transport. Therefore, this period is considered for conducting the simulations with the nested model. WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 14/26

16 Figure 3.3: Water levels (upper panel), transport equivalent ((u 2 +v 2 ) 5/2 ) and average sediment transports (lower panel) over a neap-spring cycle (magenta lines) for a 5 month period in (See also Appendix A.4) Model parameters The used model parameters for the nested detailed model are listed in Table 3.2. To fulfill the Courant stability criteria a time step of 1.5 seconds is chosen. A spatially non-uniform manning friction coefficient, ranging between 0.02 in the offshore area to in the Western Scheldt Estuary is applied. Coefficients for the horizontal eddy viscosity and diffusivity are fixed to the constant value of 1 and 10. The frequently applied cohesive sediment transport formulation of Partheniades-Krone, (Partheniades, 1965) is used. In the simulations no bedlevel updating is applied, the seabed level remains constant over time. To reduce computational time a morphological factor of 13 is applied, meaning that a period of approximately half a year is simulated. Effects of waves are included in the model, because they stir up sediment at the bed resulting in higher sediment concentrations, and thus increased transport. To model waves, the Delft3D nested flow model is coupled to the SWAN model, with a coupling time of 60 minutes. The southwesterly average wave conditions are used, thereby excluding waves coming from northwest. Specifically, a significant wave height = 1 m, a mean peak wave period = 5.7 s and a mean wave direction = 250 are imposed WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 15/26

17 at the offshore boundaries. The mesh of the nested model (Figure 3.2) is also used for the SWAN model. Note that extreme wave events are excluded from this study. Table 3.2: Overview model parameters. Parameter Timestep 2D/3D Value 1.5 seconds (Courant number 7-8 in Navigation channel and approx. 1 in shallow areas) 2D model Simulation period Neap-spring cycle + spinup period (15 days total in summer 2018) Sediment fractions 25 and 50 µm cohesive fractions in two separate morphostatic simulations Model bed level 2017 Roughness formulation Spatial variation of Manning values (0.02 at North sea up to in Western Scheldt Horizontal eddy viscosity 1.0 Horizontal eddy diffusivity 10.0 Sediment transport formulation Partheniades-Krone, Partheniades (1965) Morphological factor Waves 13 à to simulate a period of approx. 185 days (half year) Average wave conditions Hs = 1.0 m, Tp = 5.7 sec, Dir = 250 degrees 3.2 MODEL VALIDATION The overall Scheldt model has been validated against available water level measurements, discharge measurements and dredging volumes. Model results demonstrate that modelled water levels and discharges are in line with the measurements. Moreover, the simulated sedimentation and erosion patterns are in qualitative agreement with observations and the dredging volumes resulting from the morphodynamic simulations as have been considered in WaterProof (2018a) are comparable with actual volumes in the seaward part of the Western Scheldt Estuary. Base on the model validations as considered in WaterProof (2018a) it is concluded that this model is well suitable to assess the morphological effects of the Borssele export cable burial. For more information, see WaterProof (2018a). WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 16/26

18 MODEL RESULTS 4.1 OVERVIEW MODEL SIMULATIONS Model simulations are conducted whereby it is assumed that the overall bed level does not change during the total simulation time period (morphostatic). Only the dredged sediment that is placed along the cable trajectory is allowed to be eroded. In this way, the volume of sediment that erodes from the dumping site, and locations where this sediment deposits, can be modelled. As described before, two simulations have been conducted with two different cohesive sediment properties: Sediment 1: Tcrit erosion = 0.2 N/m² and fall velocity = 0.5 mm/s (D50 = 25 µm) Sediment 2: Tcrit erosion = 0.4 N/m² and fall velocity = 2.0 mm/s (D50 = 50 µm) The initial volume of sediment considered in the model, which is placed on the dumping area aside the cable trajectory, is approx m 3. The sediment is spread out over an area of approx. 700 m x 50 m with a layer thickness of 1.5 m, this is shown in Figure 4.1. Location of sediment at start of simulation Figure 4.1: Location and layer thickness of sediment from the dredged trench of the Beta cables. The sediment is placed between approx. KP 0.1 and KP 0.8. WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 17/26

19 4.2 RESULTS SEDIMENT 1, 25 µm In Figure 4.2 the sedimentation and erosion after a simulation period of 6 months is shown. After 6 months all sediment has been eroded from the initial location. Inside the cooling water intake the eroded material partly deposits. The layer thickness is on average approx m and maximum 0.12 m. In total approx. 500 m³ of sediment deposits inside the cooling water intake, this is 1% of the material which was dredged and placed alongside the Beta cables. Figure 4.2: An overview of the remaining amount of dumped sediments near the EPZ inlet, for the scenario with a grainsize of 25 µm. Out of the initial m 3, 500 m 3 has settled in the inlet. In the upper panel of Figure 4.3 the suspended sediment concentrations at three locations (see Figure 4.2) is plotted in time. The figure shows that in the first 2 weeks of the simulation at location 3 the concentrations can go up to 7 kg/m³. At the intake (location 1 and 2), the concentrations are much lower with maximum values upto 0.25 kg/m³, reducing to 0.02 kg/m³ in approx. 2 weeks. In the lower panel of Figure 4.3 the mass of sediment at the bed, with respect to the initial mass of sediment at location 3, is plotted. The figures shows that the amount of sediment available at location 3 reduces to zero in two weeks, meaning that all the placed sediment at this locations is eroded. At location 1 and 2 the strongest increase in available sediment at the bed is in the first 2 weeks. After 2 weeks the deposited sediment in the entrance to the intake is then eroded again (red line going down). Further inside the intake (location 2) the sedimentation rate slowly decreases. WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 18/26

20 Figure 4.3: Upper panel: time series of suspended sediment concentrations (kg/m 3 ) at three locations (see Figure 4.2). Lower panel: mass of sediment in the bed layer w.r.t. initial available mass in location RESULTS SEDIMENT 2, 50 µm In Figure 4.4 the sedimentation and erosion after a simulation period of 6 months is shown for sediment 2. After 6 months almost all sediment has been eroded from the initial location, only at the areas deeper than LAT -1 m some sediment remains. Inside the cooling water intake the eroded material partly deposits. The layer thickness after 6 months is on average approx m and maximum 0.35 m (in the mouth of the intake). In total approx m³ of sediment deposits inside the cooling water intake, this is 10% of the material which was dredged and placed alongside the Beta cables. WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 19/26

21 Figure 4.4: An overview of the remaining amount of dumped sediments near the EPZ inlet, for the scenario with a grainsize of 50 µm. Out of the initial m 3, 500 m 3 has settled in the inlet. In the upper panel of Figure 4.5 the suspended sediment concentrations at three locations (see Figure 4.4) is plotted in time. The figure shows that in the first 50 days of the simulation at location 3 the concentrations can go up to 0.9 kg/m³. At the intake (location 1 and 2), the concentrations are much lower with maximum values upto 0.07 kg/m³, reducing to 0.02 kg/m³ in approx. 50 days. In the lower panel of Figure 4.3 the mass of sediment at the bed, with respect to the initial mass of sediment at location 3, is plotted. The figures shows that the amount of sediment available at location 3 reduces to zero in approx. 50 days, meaning that all the placed sediment at this locations is eroded. At location 1 and 2 the available sediment at the bed increase strongest in the first 50 days. Compared to the simulation with 25 µm sediment, the 50 µm sediment is more difficult to erode; sediment that deposits in the mouth of the intake is not eroded anymore as was the case for the 25 µm sediment (compare red lines in Figure 4.3 and Figure 4.5) WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 20/26

22 Figure 4.5: Upper panel: time series of suspended sediment concentrations (kg/m 3 ) at three locations (see Figure 4.2). Lower panel: mass of sediment in the bed layer w.r.t. initial available mass in location EFFECTS OF DUMPING GROUNDS AT THE NORTHERN SIDE OF THE HONTE Apart from the dredging works on the saltmarsh also sediment will be dredged and dumped in the Honte. This sediment mainly consists of non-cohesive sands with a relative large grain size over 350 µm (see Figure 2.5). While dredging the trench trough the Honte the dredged sediment will be dumped in the dumping grounds located on the Northern side of the Honte channel (see Figure 1.2). These dumping grounds are located relatively close to the EPZ inlet channel; the dumping ground for the Alpha cables is located approximately 700 m form the mouth of the inlet channel, the dumping ground of the Beta cables approximately 400 m. The dumping grounds are for the largest part located at a depth larger than 30 m. In WaterProof (2018a) it has been studied how the dumped sediment will be transported after it is placed at the dumping grounds. In Figure 4.6 a map of the difference in bed level after 6 months between the present situation and the situation including the Alpha (top) and Beta (bottom) dredged trench and dumping grounds is shown. Red colors denote higher Alpha or Beta bed levels after 6 months, blue colors lower bed levels. The figure shows that after 6 months the dumping grounds are still present (still red colors remain after 6 months). Although some sediment has been transported out of the dumping ground, most sediment is still present at the dumping ground. The figure also shows that in the EPZ inlet channel or on the shallow parts in front of this inlet channel no changes in bedlevel occur (no red colors visible at these locations), meaning that the placed sand at the dumping grounds will not be transported toward the EPZ inlet. The main reason for this is that the sand is placed at large depths and is mainly transported in the flow direction and not in a direction towards the EPZ inlet. WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 21/26

23 Dumping location of dredged sand from the Alpha trench dredging in the Honte Dumping location of dredged sand from the Beta trench dredging in the Honte Figure 4.6: Map of difference in bed level after 6 months between present situation and situation including Alpha (top) and Beta (bottom) dredging/dumping for the Spijkerplaat region. Red colors denote higher Alpha or Beta bed levels after 6 months, blue colors lower bed levels. Model configuration: bathymetry of the year 2017 and D 50 =200 µm. WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 22/26

24 4.5 SUMMARY OF RESULTS A summary of the results for the considered cohesive sediment fractions is given in Table 2.1 Table 4.1: Overview of simulation results. Scenario 1: 25 µm Scenario 2: 50 µm loc 1: entrance loc 2: EPZ, intake loc 3: dump site loc 1: entrance loc 2: EPZ, intake loc 3: dump site Sediment thickness, initially Sediment thickness, after 185 days Maximum increase in sediment concentrations (kg/m³) 0 m 0 m 1.5 m 0 m 0 m 1.5 m 0.05 m 0.12 m 0 m 0.35 m 0.2 m 0 m Siltation volume in intake 500 m³ (1% of dredged sediment) 4200 m³ (10% of dredged sediment) The transport of non-cohesive sands from the dumping grounds in the Honte towards the EPZ inlet is minor to nil. WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 23/26

25 CONCLUS ONS AND RECOMMENDAT ONS 5.1 CONCLUSIONS From the simulations described in this report it can be concluded that: Depending on the type of sediment and consistency of the material approx. 1-10% of the total dredged and re-located sediment from the marshland area will end up in the inlet. In case of a cable installation depth of 3 m below seabed, based on the simulation results sedimentation in the intake of approx to 0.15 (averaged over intake) to maximum m occurs. Sediment concentrations in the cooling water will temporarily increase. Especially during and directly after dredging the increase in concentrations is highest. Depending on the type and consistency of the material dredged concentrations could increase to kg/m³ ( mg/l) at the EPZ intake As stated before, in this study simulations have been executed for the situation the cables are installed on the marshland at a depth of 3 m below seabed. In the latest design the cable installation depth has been reduced from 3 m to 1 m below seabed. This reduces the required dredging (and dumping) volumes on the marshland area with approximately a factor 5.5. It is expected that the resulting siltation inside the intake will also be a factor 5.5 lower than according to the simulations. Unfortunately no detailed information is available on the consistency and grain size distribution of the sediment present in the marshland area. The sediment properties combined with the dredging method, and related effect on sediment consistency, determine the erodibility and behavior of the material when eroded. In case more detail and certainty is required on the expected amount of sediment that will deposit in the cooling water intake it is recommended to execute a number of (small scale) laboratory experiments (see recommendations). In the present situation the seabed on the marshland area is believed to be relatively stable, no large changes in seabed level are visible from bathymetrical surveys (WaterProof, 2016). With a cable installation depth of 1 m the increase in bed level at the dumping location will also be relatively small (order 1 m). In case the dredging method will not influence the sediment consistency much (which is the case when grab dredging is applied), it is expected that the erodibility of the local material will not change much compared to the present situation. Because in the present situation the erodibility and related sediment transports from the marshland area are small, the sediment infill in the cooling water intake according to the simulation results are believed to be a conservative maximum. Based on the model simulations executed in WaterProof (2018a) it is concluded that no non-cohesive sands from the dumping grounds in the Honte will be transported into the EPZ inlet channel. WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 24/26

26 5.2 RECOMMENDATIONS As stated before, no detailed information is available on the consistency and grain size distribution of the sediment present in the marshland area. The sediment properties combined with the dredging method, and related effect on sediment consistency, determine the erodibility and behavior of the material when eroded. In case more detail and certainty is required on the expected amount of sediment that will deposit in the cooling water intake it is recommended to execute a number of (small scale) laboratory experiments on a number of sediment samples. Especially information on grain size analysis (including fraction < 63 µm), bulk density and critical shear stress (EROMES tests) gives better insight in the material present. WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 25/26

27 REFERENCES Deep BV., Geophysical and geotechnical site investigation survey Wind Op Zee export cable route. ARCADIS, Inpasbaarheid energie initiatieven sloegebied. Report B02024/CE0/0C9/000068/ws WaterProof, Seabed mobility based burial depth assessment Borssele. WP2015_01013_R1r5. ONL- TTB April, April, 2016 WaterProof, 2018a. Effects burial Borssele export cables. Assessment of morphological effects and sediment infill in the navigation channel due to dredging/ dumping. WP2018_1104R1r3, June 2018 WP2018_1104_R2r1_Potential_siltation_in_EPZ_inlet_Borssele_final Page 26/26