VOLWATERBAAI DESALINATION PLANT AND ASSOCIATED INFRASTRUCTURE, NORTHERN CAPE

Transcription

1 INFRASTRUCTURE, NORTHERN CAPE Report REV July 2014 Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Volwaterbaai, Northern Cape, South Africa

2 INFRASTRUCTURE, NORTHERN CAPE Impact Assessment 31 July 2014 REV. TYPE DATE EXECUTED CHECK APPROVED CLIENT DESCRIPTION / COMMENTS 00 A 04 July 14 PMH/SAL SAL SAL Draft for comment 01 C 31 July 2014 PMH/SAL SAL Updated in response to comments from client TYPE OF ISSUE: (A) Draft (B) To bid or proposal (C) For Approval (D) Approved (E) Void Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Volwaterbaai, Northern Cape, South Africa Prestedge Retief Dresner Wijnberg (Pty) 5 th Floor, Safmarine Quay, Clock Tower Precinct, Victoria & Alfred Waterfront Cape Town, South Africa PO Box 50023, Waterfront 8002 T: Cape Town, South Africa Santiago, Chile Perth, Australia Seattle, USA

3 CONTENTS Page N 1. INTRODUCTION Background Terms of Reference Study Approach and Layout of Report 2 2. DISCHARGE CHARACTERISATION Discharge Rates Discharge Point Concentrations and Required Dilutions 4 3. MODEL DESCRIPTION Introduction Wave model Hydrodynamic model 7 4. MODEL SETUP Regional Wave Model Mesh and Bathymetry Boundary Conditions Coupled Hydrodynamic Model Mesh and Bathymetry Waves Wind Tides Salinity Bed Roughness Vertical Eddy Dispersion Model Calibration Modelled Scenarios RESULTS Waves Currents Dispersion of Brine Plant Capacity 6 million m 3 /annum Plant Capacity 8 million m 3 /annum Dispersion of Co-discharges Plant Capacity 6 million m 3 /annum Plant Capacity 8 million m 3 /annum INTERPRETATION OF RESULTS Dispersion of Brine 44

4 6.2 Dispersion of Co-discharges MITIGATION AND MONITORING CONCLUSIONS REFERENCES 47 TABLES Page N Table 2-1: Discharge rates 3 Table 2-2: Brine constituent concentrations, water quality guidelines and required dilutions 5 Table 2-3: Water densities 6 Table 4-1: Thicknesses of sigma-layers used in the 3D hydrodynamic model 10 Table 4-2: Predicted tidal water levels at Volwaterbaai interpolated from known levels at Saldanha Bay and Port Nolloth (SANHO, 2014) 14 Table 4-3: Summary of modelled discharge scenarios 17 Table 4-4: Summary of modelled environmental scenarios 18 Table 5-1: Summary of modelled environmental scenarios presented in Section 5 19 FIGURES Page N Figure 1-1: Locality plan showing mine, pipeline route and desalination plant (SRK Consulting, 2014) 1 Figure 2-1: Brine discharge point is at the end of the pipeline indicated by the green line (from WSP Drawing 4830SKK018 Rev A) 4 Figure 4-1: Mesh and bathymetry used in the regional wave model 9 Figure 4-2: Mesh used in the 3D hydrodynamic model 11 Figure 4-3: Bathymetry used in the 3D hydrodynamic model 11 Figure 4-4: Detail of bathymetry in discharge gulley: Mean Low Water Springs 12 Figure 4-5: Detail of bathymetry in discharge gulley: Mean High Water Springs 12 Figure 4-6: Wave rose and exceedance plot of modelled wave conditions offshore of Volwaterbaai in a water depth of -22 m MSL) 13 Figure 4-7: Qualitative description of turbulent processes in the discharge gulley during average conditions (Royal Haskoning DHV, 2013). 16 Figure 4-8: Modelled currents in the discharge gulley (H m0 = 1.95 m, T P = 11.9 s, ϴ m = 232, water level = m MSL) 16 Figure 5-1: Sample model results indicating typical wave refraction. Plots A, B and C refer to Scenarios 5, 23 and 44, respectively. 20 Figure 5-2: Sample model results indicating typical surface currents. Plots A, B and C refer to Scenarios 5, 23 and 44, respectively. 22 Figure 5-3: Sample model results indicating typical bottom currents. Plots A, B and C refer to Scenarios 5, 23 and 44, respectively. 23 Figure 5-4: Sample model results indicating typical increases in salinity near the surface: 6 Mm 3 /annum. Plots A, B and C refer to Scenarios 5, 23 and 44, respectively. 25

5 Figure 5-5: Sample model results indicating typical increases in salinity near the bottom: 6 Mm 3 /annum. Plots A, B and C refer to Scenarios 5, 23 and 44, respectively. 26 Figure 5-6: Percentage of time that the increase in salinity exceeds 1 psu: 6 Mm 3 /annum 27 Figure 5-7: 99 th Percentile increase in salinity: 6 Mm 3 /annum 28 Figure 5-8: Median (50 th percentile) increase in salinity: 6 Mm 3 /annum 29 Figure 5-9: Sample model results indicating typical increases in salinity near the surface: 8 Mm 3 /annum. Plots A, B and C refer to Scenarios 5, 23 and 44, respectively. 30 Figure 5-10: Sample model results indicating typical increases in salinity near the bottom: 8 Mm 3 /annum. Plots A, B and C refer to Scenarios 5, 23 and 44, respectively. 31 Figure 5-11: Percentage of time that the increase in salinity exceeds 1 psu: 8 Mm 3 /annum. 32 Figure 5-12: 99 th Percentile increase in salinity: 8 Mm 3 /annum. 33 Figure 5-13: Median (50 th percentile) increase in salinity: 8 Mm 3 /annum. 34 Figure 5-14: Sample model results indicating typical dilution factors near the surface: 6 Mm 3 /annum. Plots A, B and C refer to Scenarios 5, 23 and 44, respectively. 36 Figure 5-15: Sample model results indicating typical dilution factors near the bottom: 6 Mm 3 /annum. Plots A, B and C refer to Scenarios 5, 23 and 44, respectively. 37 Figure 5-16: Percentage of time that the number of dilutions does not exceed 30: 6 Mm 3 /annum. 38 Figure 5-17: 1 st Percentile number of dilutions: 6 Mm 3 /annum. 39 Figure 5-18: Sample model results indicating typical dilution factors near the surface: 8 Mm 3 /annum. Plots A, B and C refer to Scenarios 5, 23 and 44, respectively. 40 Figure 5-19: Sample model results indicating typical dilution factors near the bottom: 8 Mm 3 /annum. Plots A, B and C refer to Scenarios 5, 23 and 44, respectively. 41 Figure 5-20: Percentage of time that the number of dilutions does not exceed 30: 8 Mm 3 /annum. 42 Figure 5-21: 1 st Percentile number of dilutions: 8 Mm 3 /annum. 43

6 Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Impact Assessment 1. INTRODUCTION 1.1 Background Sedex Minerals (Pty) (Sedex Minerals) proposes to develop the Zandkopsdrift Rare Earth Element mine on the remainder of Farm Zandkopsdrift 537, and portion 2 of Zandkopsdrift 537 in the Northern Cape Province. Due to the shortage of water resources in the area, Sedex Desalination (Pty) (Sedex Desalination), a subsidiary of Sedex Minerals, was established to develop a seawater desalination plant to provide water for the proposed mine. For the purposes of this study, two possible desalination plant output fresh/potable water capacities have been considered: 6 and 8 million m 3 /annum. The desalination plant will be located at Volwaterbaai on Farm Strandfontein 559, on the west coast of the Northern Cape Province. From there, water will be pumped via a pipeline to the mine, as shown in Figure 1-1. Figure 1-1: Locality plan showing mine, pipeline route and desalination plant (SRK Consulting, 2014) One of the key environmental issues is that the abstraction of seawater and the discharge of treated brine (and potential co-discharges) into the ocean may result in impacts on marine biota in a sacrificial area characterised by elevated salinity levels and the presence of co-discharges. This impact could be Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 1 of 47

7 exacerbated should local bathymetry and inadequate design of discharge infrastructure promote the accumulation of brine, rather than rapid mixing and dispersion. SRK Consulting (South Africa) Pty (SRK) has been appointed by Sedex Desalination to undertake Scoping and Environmental Impact Reporting (S&EIR, also referred to as the EIA) process for the desalination plant, required in terms of the National Environmental Management Act 107 of 1998, as amended (NEMA). PRDW has been appointed by SRK to undertake the. 1.2 Terms of Reference The purpose of the Marine Modelling Study is to inform the identification and assessment of impacts by the Marine and Coastal Ecology Specialists. The following Terms of Reference are applicable to the Marine Modelling Study: Determine and describe the baseline physical coastal processes including waves, currents and tides; Undertake a desktop assessment of coastal processes and dispersion characteristics at the proposed site of the desalination plant, intake and discharge points and provide guidance on the expected environmental issues and possible fatal flaws early on in the project; Undertake the required numerical modelling to evaluate the dispersion of brine from the desalination plant and associated impacts; Provide an interpretation of the outputs/findings of the modelling studies to inform the assessment of impacts on marine ecology by the Marine Ecologists; Provide recommendations for mitigation and monitoring of impacts; Assist the EIA team in responding to any comments received from stakeholders as they relate to physical marine impacts; and Provide technical input required for the submission of applications to the Department of Environmental Affairs (DEA) in terms of the Integrated Coastal Management Act (ICMA). 1.3 Study Approach and Layout of Report The first task was to characterise the discharge, i.e. discharge rates, discharge point and constituent concentrations. The water quality guidelines were then used calculate the required dilutions for each constituent, as described in Section 2. Numerical modelling was used to simulate both the physical coastal processes at the site including waves, currents and water levels, as well as the dispersion and dilution of the brine and associated co-discharges. A description of the models is provided in Section 3, while the setup of the models is described in Section 4. The model results are provided in Section 5, followed by an interpretation of the results in Section 6. Mitigation and monitoring options are described in Section 7 and conclusions follow in Section 8. Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 2 of 47

8 2. DISCHARGE CHARACTERISATION 2.1 Discharge Rates The proposed fresh/potable water output capacity of the desalination plant is 8 million m 3 /a, although an alternative capacity of 6 million m 3 /a was also assessed in this study. The discharge rates for the various input and outlet streams were provided by Sedex and are tabulated below. Annual product water Discharge type Sea water abstraction Table 2-1: Discharge rates Product water produced Total brine discharged Discharge velocity for port exit diameter = 0.3 m (Mm 3 /a) [m 3 /d] [m 3 /d] [m 3 /d] [m 3 /s] [m/s] 6 Average Instantaneous Average Instantaneous In Table 2-1 the average discharge assumes that the desalination plant operates 100% of the time, while the instantaneous discharge assumes it operates 90% of the time. The instantaneous discharge rate is higher and thus represents the worst case scenario for dispersion. Thus only the instantaneous discharge rate was modelled for both the 6 and 8 million m 3 /a scenarios. 2.2 Discharge Point The discharge point assessed is as shown in WSP Drawing 4830SKK018 Rev A (Royal Haskoning DHV, 2013), as shown in Figure 2-1. The discharge coordinates are S, E, equivalent to m, m in WG19 coordinates. The seabed level at the discharge point is approximately -1.2 m relative to Mean Sea Level (MSL), i.e. 1.2 m below MSL. The discharge is through a single port located directly above the seabed and is directed horizontally offshore (Royal Haskoning DHV, 2013). The recommended port exit velocity for brine diffuser ports is 4 to 6 m/s (Bleninger & Jirka, 2010). To meet this recommendation for the range of discharge rates considered, the port exit diameter has been selected by PRDW as 0.3 m (see Table 2-1). The model results are thus only applicable to a discharge via a single port with an exit velocity of 4 to 6 m/s. Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 3 of 47

9 Figure 2-1: Brine discharge point is at the end of the pipeline indicated by the green line (from WSP Drawing 4830SKK018 Rev A) 2.3 Concentrations and Required Dilutions In addition to elevated salinity the brine discharge contains a number of other chemicals used in the desalination process, referred to as co-discharges. The concentrations of each of the constituents in the brine discharge are given in Table 2-2. In this study total dissolved solids (TDS) is effectively the same as salinity and the units are equivalent - either gram per litre (g/l) or practical salinity units (psu). Also shown in Table 2-2 are the required dilutions to meet the water quality guidelines, which are calculated as follows: Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 4 of 47

10 Constituent Table 2-2: Brine constituent concentrations, water quality guidelines and required dilutions Unit Concentration in Brine Discharge to Sea Water Quality Guideline Background Concentration in Sea (Intake Concentration) Required Dilution TDS / Salinity g/l or psu 66.0 (a) 37.7 (d) 36.7 (k) 29 Temperature C 14 (c) 13 (e) 12 (l) 2 Suspended solids mg/l (b) 9.9 (f) 9 (m) 3 Chlorine mg/l (b) (g) 0 1 Sodium Metabisulphite (SMBS) mg/l 3.14 (a) Not available (h) Not available (n) Not available (p) Ferric chloride mg/l (as Fe) 3.33 (b) 0.3( (i) (o) 11 1 (j) (o) 3 Anionic polymer (alternative to ferric chloride) mg/l 1.67 (b) Not available (h) Not available (n) Not available (p) Phosphonate mg/l 4.7 (b) Not available (h) Not available (n) Not available (p) Peroxyacetic acid (Hydrex 4203) mg/l (b) Not available (h) Not available (n) Not available (p) Low ph CIP solution (Hydrex 4503) mg/l (b) Not available (h) Not available (n) Not available (p) High ph CIP solution (Hydrex 4502) mg/l (b) Not available (h) Not available (n) Not available (p) Preservative SMBS (Hydrex 4301) mg/l (b) Not available (h) Not available (n) Not available (p) Notes (a) Reference is from Keith Turner, Royal HaskoningDHV, 20 June This represents a conservatively high salinity. (b) Reference is from Keith Turner, Royal HaskoningDHV, 12 June (c) 2 C above intake temperature, reference is from Drikus Janse van Rensburg, Frontier, 29 May (d) 1 psu above background salinity, reference is from Andrea Pulfrich, Pisces, 2 June (e) (f) 1 C above background temperature, South African Water Quality Guidelines for Coastal Marine (Department of Water Affairs and Forestry, 1995). 10% above background suspended solids concentration, South African Water Quality Guidelines for Coastal Marine (Department of Water Affairs and Forestry, 1995). (g) No S A guideline, a conservative trigger value is mg/l, reference is from Andrea Pulfrich, Pisces, 2 June (h) No guideline available. (i) ANZECC / Canadian guideline level, reference is from Andrea Pulfrich, Pisces, 2 June (j) (k) (l) (m) (n) World Bank guideline for effluents from thermal power plants, applies at the point of discharge, reference is from Andrea Pulfrich, Pisces, 2 June Reference is from Keith Turner, Royal HaskoningDHV, 21 June This represents a high background salinity, corresponding to the conservatively high brine salinity. Median value from 27 measurements at the site, data provided by Frontier. Median value from 21 samples at the site, data provided by Frontier. Data not available, but is likely to be very low. (o) Median iron as Fe dissolved measured at the site (Royal Haskoning DHV, 2013). (p) Required dilution cannot be calculated since no guideline is available. Salinity has the largest required dilution of 29. For a number of constituents no guideline value was available and thus the required dilution could not be calculated. In this study all the co-discharges have been modelled as a generic conservative tracer released with the brine and the results have been presented as the dilutions of this tracer after discharge into the sea. This will enable the impact of the co-discharges to be assessed as part of the Marine Ecology Specialist Study, assuming that the required dilution can be estimated. Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 5 of 47

11 The densities of the background seawater and the brine are given in Table 2-3. It is seen that the brine is significantly more dense than the seawater and will thus tend to sink towards the seabed, unless exposed to strong vertical and horizontal mixing. It is commented that the salinities of both the background sweater and the brine are conservatively high. Table 2-3: Water densities TDS/Salinity Temperature Density [g/l] [ C ] [kg/m 3 ] Background seawater Brine Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 6 of 47

12 3. MODEL DESCRIPTION 3.1 Introduction The dilution of effluents after discharge into the sea is often separated into the near-field and the far-field. The near-field is the region typically less than 100 m from the discharge point where the dilution is influenced by the jet momentum and the buoyancy flux 1 of the discharge. Thereafter the dilution is dominated by the ambient currents and turbulence and this region is referred to as the far-field. Because of the difference in time and space scales, different models are typically applied to the near- and far-fields. However, in this study a fine resolution (1 m element size) three-dimensional hydrodynamic model coupled to a wave model has been used to simulate both the near-field and the far-field dispersion of the brine. A regional wave model was used to transform the offshore wave conditions to the nearshore at the Volwaterbaai site. The nearshore wave conditions were used to characterise the wave climate at Volwaterbaai and to serve as an input to the local coupled hydrodynamic model. The coupled hydrodynamic model was used to model the dispersion of the brine and co-discharges under the influence of water levels, waves and wind stress on the water surface. The wave model and hydrodynamic models are described in this section. 3.2 Wave model The MIKE by DHI Spectral Waves Flexible Mesh model was used for wave refraction modelling. The application of the model is described in the User Manual (DHI, 2013a), while full details of the physical processes being simulated and the numerical solution techniques are described in the Scientific Documentation (DHI, 2013b). The model simulates the growth, decay and transformation of windgenerated waves and swell in offshore and coastal areas using unstructured meshes. In this study the parametric quasi-stationary formulation was used. The discretisation of the governing equation in geographical and spectral space is performed using cell-centred finite volume method. In the geographical domain, an unstructured flexible mesh comprising triangles is used. In this study the model included the following physical phenomena: Refraction 2 and shoaling 3 due to depth variations Dissipation due to bottom friction Dissipation due to depth-induced wave breaking. 3.3 Hydrodynamic model The three-dimensional (3D) MIKE 3 Flow Flexible Mesh Model was used for the near- and far-field modelling. The application of the model is described in the User Manual (DHI, 2013c), while full details of the physical processes being simulated and the numerical solution techniques are described in the Scientific Documentation (DHI, 2013d). The model is based on the numerical solution of the three-dimensional incompressible Reynolds averaged Navier-Stokes equations invoking the assumptions of Boussinesq and of hydrostatic pressure. The model consists of the continuity, momentum, temperature, salinity and density equations. Horizontal eddy viscosity 4 is modelled with the Smagorinsky formulation. 1 The force due to the difference in density between the effluent and the ambient water 2 Wave refraction is the change in wave angle and wave height due to varying water depths 3 The increase in wave height when waves enter shallower water 4 The turbulent transfer of momentum by eddies in the horizontal direction Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 7 of 47

13 The time integration of the shallow water equations and the transport equations is performed using a semiimplicit scheme, where the horizontal terms are treated explicitly and the vertical terms are treated implicitly. In the vertical direction a structured mesh, based on a sigma coordinate transformation is used, while the geometrical flexibility of the unstructured flexible mesh comprising triangles or rectangles is utilised in the horizontal plane. MIKE 3 Flow Flexible Mesh Model includes the following physical phenomena: Currents due to wind stress on the water surface Currents due to waves: the second order stresses due to breaking of short period waves are included using the radiation stresses computed in the spectral wave model Currents due to density gradients Bottom friction Flooding and drying Effluent sources, including both the volume and momentum of the source discharge. Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 8 of 47

14 4. MODEL SETUP 4.1 Regional Wave Model Mesh and Bathymetry The mesh and bathymetry used for the regional wave model are shown in Figure 4-1. The model extends approximately 15 km offshore to a depth of approximately -120 m relative to Chart Datum (CD). The relationship between CD and MSL is given in Section The mesh comprised triangular elements with a resolution varying between approximately 750 m at the offshore boundary to 150 m at the boundary of the 3D hydrodynamic model. The bathymetry was constructed using available bathymetric data from the MIKE by DHI CMAP Electronic Charts (DHI, 2013e). Figure 4-1: Mesh and bathymetry used in the regional wave model Boundary Conditions A dataset of hindcast wave data was available from the National Centers for Environmental Prediction (NCEP) Multi Reanalysis Database (NCEP, 2012). The database includes significant wave height (H m0 ), peak wave period (T P ) and mean wave direction at T P (ϴ m at Tp ) at 3-hourly intervals for the period of February 2005 to January 2014 on a 0.5 degree spatial grid. The closest node to Volwaterbaai is located at 31 S 17.5 E, which is 20 km west of Volwaterbaai. The 9 year dataset was applied along the offshore boundary and was transformed to the Volwaterbaai site to a depth of approximately -22 m MSL. Windwave generation was not included in the regional wave model. Bottom friction was modelled with a Nikuradse roughness of 0.04 m. Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 9 of 47

15 4.2 Coupled Hydrodynamic Model Mesh and Bathymetry The mesh used for the 3D hydrodynamic model is presented in Figure 4-2, while the bathymetry is presented in Figure 4-3. The mesh extends approximately m offshore to a depth of approximately -22 m MSL. The mesh comprised both triangular and quadrangular elements with resolutions ranging from 70 m at the model boundary to 1 m in the gulley in which the proposed discharge point is located. In the vertical domain, the mesh was constructed of non-equidistant sigma layers. The layer thicknesses are presented in Table 4-1, with Layer 1 referring to the bottom layer and Layer 5 referring to the surface layer. Table 4-1: Thicknesses of sigma-layers used in the 3D hydrodynamic model Layer Thickness factor The bathymetry was constructed using available bathymetric data from the MIKE by DHI CMAP Electronic Charts (DHI, 2013e) as well as topographic measurements of the gulley and profiles taken during a diver survey of Volwaterbaai (as per WSP Drawing 4830SKK018 Rev A). The bathymetry of the discharge gulley was also inferred using available satellite images and photographs taken during a site visit at spring low tide by Peter Schroeder from Frontier Rare Earths. Detailed views of the model bathymetry around the gulley in which the proposed discharge point is located are presented in Figure 4-4 and Figure 4-5 at water levels of Mean Low Water Springs (MLWS) and Mean High Water Springs (MWHS), respectively. It is noted that the available bathymetric data at the site is limited and the bathymetry applied in the model is thus indicative. Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 10 of 47

16 Figure 4-2: Mesh used in the 3D hydrodynamic model Figure 4-3: Bathymetry used in the 3D hydrodynamic model Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 11 of 47

17 Figure 4-4: Detail of bathymetry in discharge gulley: Mean Low Water Springs Figure 4-5: Detail of bathymetry in discharge gulley: Mean High Water Springs Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 12 of 47

18 4.2.2 Waves Wave-driven currents were included in the hydrodynamic model through an online coupling with the spectral wave model. The input wave conditions for the coupled hydrodynamic model were obtained from the regional wave model discussed in Section 4.1. The regional wave model was used to transform the hindcast wave data to the boundary of the coupled hydrodynamic model at a depth of -22 m MSL. The wave rose and exceedance plots of the transformed 9 year dataset are presented in Figure 4-6. The red line in the exceedance plot shows the percentage of time that the wave height will be less than the wave heights shown on the x-axis. The waves at Volwaterbaai are observed to be predominantly south westerly with a median H m0 of 1.95 m and a maximum H m0 of 7.68 m. Figure 4-6: Wave rose and exceedance plot of modelled wave conditions offshore of Volwaterbaai in a water depth of -22 m MSL) The transformed wave climate described above was used to determine a set of representative wave conditions for Volwaterbaai to be applied at the boundary of the coupled hydrodynamic model. This set of representative wave conditions is further discussed in Section Wind A dataset of hindcast wind data was also available through NCEP at the same location as the wave data (31 S 17.5 E, as presented in Section 4.1.2). The dataset comprises wind speed and direction at 3-hourly Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 13 of 47

19 intervals. A set of representative wind conditions was included in the hydrodynamic model in order to generate wind-driven currents. The set of representative wind conditions is further discussed in Section Tides Due to the remote nature of the site, predicted tidal water levels were not available at Volwaterbaai. Therefore, predicted tidal water levels were interpolated between known levels at Saldanha Bay (approximately 225 km to the south) and Port Nolloth (approximately 212 km to the north) as published by the South African Navy Hydrographic Office (SANHO, 2014). The interpolated levels are presented in Table 4-2. These levels are given relative to Mean Sea Level (MSL). At Volwaterbaai MSL is approximately 0.90 m above Chart Datum (CD) which is also the level of Lowest Astronomical Tide (LAT). Table 4-2: Predicted tidal water levels at Volwaterbaai interpolated from known levels at Saldanha Bay and Port Nolloth (SANHO, 2014) Tide Port Nolloth Saldanha Bay Volwaterbaai [+m MSL] [+m MSL] [+m MSL] Highest Astronomical Tide (HAT) Mean High Water Springs (MHWS) Mean High Water Neaps (MHWN) Mean Level (ML) Mean Low Water Neaps (MLWN) Mean Low Eater Springs (MLWS) Lowest Astronomical Tide (LAT) Salinity A background salinity of 36.7 psu was used in the model (see Table 2-2) Bed Roughness A bed roughness of 0.1 m was used in the model Vertical Eddy Dispersion The wave-driven currents included in the hydrodynamic model are phase-averaged and as such do not resolve the turbulent rush of water up and down the shore face due to individual breaking waves (swash). However, the wave set-up due to wave breaking is included in the model through the online coupling between the hydrodynamic and wave models and the vertical eddy viscosity was adjusted to account for vertical mixing caused by wave turbulence. The enhanced vertical mixing caused by wave turbulence in the surf zone was modelled by setting both the vertical eddy viscosity and vertical dispersion coefficients to 0.01 m 2 /s within the surf zone. This value is consistent with measured eddy viscosity coefficients in the surf zone (Jimenez, et al., 1996). Outside the surf zone, vertical eddy viscosity and dispersion coefficients of m 2 /s were used. This approach allowed for strong vertical mixing within the surf zone where wave-induced turbulence is prominent, and reduced vertical mixing offshore of the surf zone Model Calibration No measurements of waves, currents, water levels and dispersion have been undertaken at the proposed discharge location, due to the extreme difficulty in undertaking these measurements within the high energy surf-zone. Due to the lack of local measurements no quantitative calibration of the model was possible. Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 14 of 47

20 Therefore, the modelling procedure was based on PRDW s extensive experience in applying the MIKE suite of models for similar projects, many of which were calibrated to available measurements. These models include more than 60 wave refraction studies and more than 30 hydrodynamic studies. A qualitative description of surface current patterns in the discharge gulley was provided by WSP, as presented in Figure 4-7 (Royal Haskoning DHV, 2013). The description presents the mass flux of water during average wave conditions and indicates a larger flux north of the outer rock than on the southern side, resulting in a clockwise current circulation of the gulley. In an effort to validate the model to these observations, the model was run with median wave conditions at m MSL. The modelled currents in the gulley are presented in Figure 4-8. Similar to the qualitative description, the modelled currents indicate a larger influx north of the outer rock. The clockwise circulation of the gulley is also observed. Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 15 of 47

21 Figure 4-7: Qualitative description of turbulent processes in the discharge gulley during average conditions (Royal Haskoning DHV, 2013). Figure 4-8: Modelled currents in the discharge gulley (H m0 = 1.95 m, T P = 11.9 s, ϴ m = 232, water level = m MSL) Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 16 of 47

22 4.2.9 Modelled Scenarios The discharge scenarios for each of the 6 million m 3 /annum and 8 million m 3 /annum plant capacity scenarios are presented in Table 4-3. Table 4-3: Summary of modelled discharge scenarios Parameter Plant Capacity Scenario 6 million m 3 /annum 8 million m 3 /annum Brine discharge rate (m 3 /s) Discharge velocity (m/s) Discharge Layer Bottom Bottom Salinity (g/l) Temperature ( C) In order to model the dispersion of the brine under a range of environmental conditions, 46 scenarios were developed from an analysis of the wave, wind and water level conditions at Volwaterbaai. Water level, wave and wind conditions were combined based on engineering judgement to generate a set of environmental scenarios that is representative of the environmental conditions present at Volwaterbaai. The list of the 46 environmental scenarios is presented in Table 4-4, which also indicates the probability of occurrence of each scenario. For each of the scenarios, the steady state solution of the brine dispersion was modelled to determine the salinity concentration. Each of the environmental scenarios were modelled with each of the plant capacity scenarios described in Table 4-3, resulting in a total of 92 modelled scenarios. Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 17 of 47

23 Scenario Water Level [+m MSL] Table 4-4: Summary of modelled environmental scenarios H m0 [m] T P [s] ϴ m [degrees] Wind Speed [m/s] Wind Direction [degrees] Probability of Occurrence [%] % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % Sedex Desalination (Pty) and SRK Consulting (South Africa) Pty Page 18 of 47