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Sedex Desalination (Pty) Ltd and SRK Consulting (South Africa) Pty Ltd Volwaterbaai, Northern Cape, South Africa

VOLWATERBAAI DESALINATION PLANT AND ASSOCIATED INFRASTRUCTURE, NORTHERN CAPE

Marine Modelling Specialist Study

Report 1127-001 REV.01

31 July 2014

Sedex Desalination (Pty) Ltd and SRK Consulting (South Africa) Pty Ltd Volwaterbaai, Northern Cape, South Africa

Prestedge Retief Dresner Wijnberg (Pty) Ltd 5

th Floor, Safmarine Quay, Clock Tower Precinct, Victoria & Alfred Waterfront

Cape Town, South Africa | PO Box 50023, Waterfront 8002 T: +27 21 418 3830

www.prdw.com

Cape Town, South Africa

Santiago, Chile

Perth, Australia

Seattle, USA



Impact Assessment

1127-001 Volwaterbaai Desalination Plant Marine Modelling Rev01 31July2014.docx

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

CONTENTS Page N°

1. INTRODUCTION 1

1.1 Background 1

1.2 Terms of Reference 2

1.3 Study Approach and Layout of Report 2

2. DISCHARGE CHARACTERISATION 3

2.1 Discharge Rates 3

2.2 Discharge Point 3

2.3 Concentrations and Required Dilutions 4

3. MODEL DESCRIPTION 7

3.1 Introduction 7

3.2 Wave model 7

3.3 Hydrodynamic model 7

4. MODEL SETUP 9

4.1 Regional Wave Model 9

4.1.1 Mesh and Bathymetry 9

4.1.2 Boundary Conditions 9

4.2 Coupled Hydrodynamic Model 10

4.2.1 Mesh and Bathymetry 10

4.2.2 Waves 13

4.2.3 Wind 13

4.2.4 Tides 14

4.2.5 Salinity 14

4.2.6 Bed Roughness 14

4.2.7 Vertical Eddy Dispersion 14

4.2.8 Model Calibration 14

4.2.9 Modelled Scenarios 17

5. RESULTS 19

5.1 Waves 19

5.2 Currents 21

5.3 Dispersion of Brine 24

5.3.1 Plant Capacity 6 million m3/annum 24


5.4 Dispersion of Co-discharges 35



6. INTERPRETATION OF RESULTS 44

6.1 Dispersion of Brine 44

6.2 Dispersion of Co-discharges 44

7. MITIGATION AND MONITORING 45

8. CONCLUSIONS 46

9. 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 (Hm0 = 1.95 m, TP = 11.9 s, ϴm = 232°,

water level = +0.44 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 Mm3/annum. Plots A, B

and C refer to Scenarios 5, 23 and 44, respectively. 25

Figure 5-5: Sample model results indicating typical increases in salinity near the bottom: 6 Mm3/annum. Plots A, B


Figure 5-6: Percentage of time that the increase in salinity exceeds 1 psu: 6 Mm3/annum 27

Figure 5-7: 99th

Percentile increase in salinity: 6 Mm3/annum 28

Figure 5-8: Median (50th

percentile) increase in salinity: 6 Mm3/annum 29

Figure 5-9: Sample model results indicating typical increases in salinity near the surface: 8 Mm3/annum. Plots A, B


Figure 5-10: Sample model results indicating typical increases in salinity near the bottom: 8 Mm3/annum. Plots A, B


Figure 5-11: Percentage of time that the increase in salinity exceeds 1 psu: 8 Mm3/annum. 32

Figure 5-12: 99th

Percentile increase in salinity: 8 Mm3/annum. 33

Figure 5-13: Median (50th

percentile) increase in salinity: 8 Mm3/annum. 34

Figure 5-14: Sample model results indicating typical dilution factors near the surface: 6 Mm3/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 Mm3/annum. Plots A, B and


Figure 5-16: Percentage of time that the number of dilutions does not exceed 30: 6 Mm3/annum. 38

Figure 5-17: 1st

Percentile number of dilutions: 6 Mm3/annum. 39

Figure 5-18: Sample model results indicating typical dilution factors near the surface: 8 Mm3/annum. Plots A, B and


Figure 5-19: Sample model results indicating typical dilution factors near the bottom: 8 Mm3/annum. Plots A, B and


Figure 5-20: Percentage of time that the number of dilutions does not exceed 30: 8 Mm3/annum. 42

Figure 5-21: 1st

Percentile number of dilutions: 8 Mm3/annum. 43

Sedex Desalination (Pty) Ltd and SRK Consulting (South Africa) Pty Ltd

Marine Modelling Specialist Study Page 1 of 47



Printed Document Uncontrolled




Impact Assessment

1. INTRODUCTION

1.1 Background

Sedex Minerals (Pty) Ltd (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) Ltd (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 m3/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






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 Ltd (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 Marine Modelling Specialist Study.

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.






2. DISCHARGE CHARACTERISATION

2.1 Discharge Rates

The proposed fresh/potable water output capacity of the desalination plant is 8 million m3/a, although an

alternative capacity of 6 million m3/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.

Table 2-1: Discharge rates

Annual product water

Discharge type Sea water

abstraction 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 41 085 16 428 24 657 0.285 4.04

Instantaneous 45 552 18 213 27 339 0.316 4.48

8 Average 54 780 21 904 32 876 0.381 5.38

Instantaneous 60 736 24 284 36 452 0.422 5.97

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 m3/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 31.02755°S, 17.68927°E, equivalent

to -125 150 m, -3 434 767 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.






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:






Table 2-2: Brine constituent concentrations, water quality guidelines and required dilutions

Constituent 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 11.67(b)

9.9(f)

9(m)

3

Chlorine mg/L 0.002(b)

0.002(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)

0.007(o)

11

1(j)

0.007(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 0.006(b)

Not available(h)

Not available(n)

Not available(p)

Low pH CIP solution (Hydrex 4503) mg/L 0.015(b)

Not available(h)

Not available(n)

Not available(p)

High pH CIP solution (Hydrex 4502) mg/L 0.015(b)

Not available(h)

Not available(n)

Not available(p)

Preservative SMBS (Hydrex 4301) mg/L 0.028(b)

Not available(h)

Not available(n)

Not available(p)

Notes

(a) Reference is email from Keith Turner, Royal HaskoningDHV, 20 June 2014. This represents a conservatively high salinity.

(b) Reference is email from Keith Turner, Royal HaskoningDHV, 12 June 2014.

(c) 2°C above intake temperature, reference is email from Drikus Janse van Rensburg, Frontier, 29 May 2014.

(d) 1 psu above background salinity, reference is email from Andrea Pulfrich, Pisces, 2 June 2014.

(e) 1°C above background temperature, South African Water Quality Guidelines for Coastal Marine (Department of Water Affairs

and Forestry, 1995).

(f) 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 0.002 mg/L, reference is email from Andrea Pulfrich, Pisces, 2 June 2014.

(h) No guideline available.

(i) ANZECC / Canadian guideline level, reference is email from Andrea Pulfrich, Pisces, 2 June 2014.

(j) World Bank guideline for effluents from thermal power plants, applies at the point of discharge, reference is email from Andrea

Pulfrich, Pisces, 2 June 2014.

(k) Reference is email from Keith Turner, Royal HaskoningDHV, 21 June 2014. This represents a high background salinity,

corresponding to the conservatively high brine salinity.

(l) Median value from 27 measurements at the site, data provided by Frontier.

(m) Median value from 21 samples at the site, data provided by Frontier.

(n) 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.






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 36.7 12.0 1027.9

Brine 66.0 14.0 1049.9






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 flux1 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 wind-

generated 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:

Refraction2 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

viscosity4 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






The time integration of the shallow water equations and the transport equations is performed using a semi-

implicit 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.






4. MODEL SETUP

4.1 Regional Wave Model

4.1.1 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 4.2.4. 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

4.1.2 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 (Hm0), peak

wave period (TP) and mean wave direction at TP (ϴm 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. Wind-

wave generation was not included in the regional wave model. Bottom friction was modelled with a

Nikuradse roughness of 0.04 m.






4.2 Coupled Hydrodynamic Model

4.2.1 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 1 200 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

1 0.1

2 0.1

3 0.2

4 0.3

5 0.3

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 Ltd. 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.






Figure 4-2: Mesh used in the 3D hydrodynamic model

Figure 4-3: Bathymetry used in the 3D hydrodynamic model






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






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 Hm0 of 1.95 m and a

maximum Hm0 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 4.2.9.

4.2.3 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






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 4.2.9.

4.2.4 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) 1.33 1.17 1.25

Mean High Water Springs (MHWS) 0.99 0.89 0.94

Mean High Water Neaps (MHWN) 0.48 0.41 0.44

Mean Level (ML) 0.17 0.13 0.15

Mean Low Water Neaps (MLWN) -0.15 -0.17 -0.16

Mean Low Eater Springs (MLWS) -0.65 -0.63 -0.64

Lowest Astronomical Tide (LAT) -0.93 -0.87 -0.90

4.2.5 Salinity

A background salinity of 36.7 psu was used in the model (see Table 2-2).

4.2.6 Bed Roughness

A bed roughness of 0.1 m was used in the model.

4.2.7 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 m2/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 0.0001 m2/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.

4.2.8 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.






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

+0.44 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.






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 (Hm0 = 1.95 m, TP = 11.9 s, ϴm = 232°, water level = +0.44 m MSL)






4.2.9 Modelled Scenarios

The discharge scenarios for each of the 6 million m3/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 m3/annum 8 million m

3/annum

Brine discharge rate (m3/s) 0.316 0.422

Discharge velocity (m/s) 4.48 5.97

Discharge Layer Bottom Bottom

Salinity (g/L) 66.0 66.0

Temperature (°C) 14.0 14.0

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.






Table 4-4: Summary of modelled environmental scenarios

Scenario Water Level

[+m MSL] Hm0 [m]

TP [s]

ϴm [degrees]

Wind Speed [m/s]

Wind Direction [degrees]

Probability of Occurrence [%]

002 -0.635 4.33 13.41 239.83 7.07 335 0.075%

003 -0.635 3.94 11.69 230.80 4.19 155 0.100%

004 -0.635 3.42 9.15 217.93 10.33 155 0.325%

005 -0.635 2.74 16.11 236.74 9.27 335 0.500%

006 -0.635 2.43 12.94 228.98 5.25 155 0.500%

007 -0.635 2.22 11.20 211.28 10.98 155 0.500%

008 -0.635 2.04 8.24 250.32 1.49 155 0.500%

009 -0.635 1.87 15.63 234.57 2.95 155 0.500%

010 -0.635 1.71 12.35 226.84 19.38 335 0.500%

011 -0.635 1.55 10.86 206.11 6.30 155 0.500%

012 -0.635 1.38 7.51 247.69 9.25 155 0.500%

013 -0.635 1.08 19.49 232.66 2.95 335 0.325%

014 -0.635 0.93 11.99 224.03 13.72 155 0.100%

015 -0.635 0.84 10.27 295.75 10.68 335 0.075%

016 -0.635 0.43 4.14 193.84 0.33 335 0.000%

017 0.146 7.68 14.59 244.05 7.49 155 0.000%

018 0.146 4.33 13.41 239.83 7.07 335 1.350%

019 0.146 3.94 11.69 230.80 4.19 155 1.800%

020 0.146 3.42 9.15 217.93 10.33 155 5.850%

021 0.146 2.74 16.11 236.74 9.27 335 9.000%

022 0.146 2.43 12.94 228.98 5.25 155 9.000%

023 0.146 2.22 11.20 211.28 10.98 155 9.000%

024 0.146 2.04 8.24 250.32 1.49 155 9.000%

025 0.146 1.87 15.63 234.57 2.95 155 9.000%

026 0.146 1.71 12.35 226.84 19.38 335 9.000%

027 0.146 1.55 10.86 206.11 6.30 155 9.000%

028 0.146 1.38 7.51 247.69 9.25 155 9.000%

029 0.146 1.08 19.49 232.66 2.95 335 5.850%

030 0.146 0.93 11.99 224.03 13.72 155 1.800%

031 0.146 0.84 10.27 295.75 10.68 335 1.350%

032 0.146 0.43 4.14 193.84 0.33 335 0.000%

033 0.936 7.68 14.59 244.05 7.49 155 0.000%

034 0.936 4.33 13.41 239.83 7.07 335 0.075%

035 0.936 3.94 11.69 230.80 4.19 155 0.100%

036 0.936 3.42 9.15 217.93 10.33 155 0.325%

037 0.936 2.74 16.11 236.74 9.27 335 0.500%

038 0.936 2.43 12.94 228.98 5.25 155 0.500%

039 0.936 2.22 11.20 211.28 10.98 155 0.500%

040 0.936 2.04 8.24 250.32 1.49 155 0.500%

041 0.936 1.87 15.63 234.57 2.95 155 0.500%

042 0.936 1.71 12.35 226.84 19.38 335 0.500%

043 0.936 1.55 10.86 206.11 6.30 155 0.500%

044 0.936 1.38 7.51 247.69 9.25 155 0.500%

045 0.936 1.08 19.49 232.66 2.95 335 0.325%

046 0.936 0.93 11.99 224.03 13.72 155 0.100%

047 0.936 0.84 10.27 295.75 10.68 335 0.075%

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