khumani stockpile facility storm water …...assmang limited khumani extension environmental...

63 Wessel Road, Rivonia, 2128 PO Box 2597, Rivonia, 2128 South Africa

Tel: +27 (0) 11 803 5726 Fax: +27 (0) 11 803 5745 Web: www.gcs-sa.biz

GCS (Pty) Ltd. Reg No: 2004/000765/07 Est. 1987

Offices: Durban Johannesburg Lusaka Ostrava Pretoria Windhoek

Directors: AC Johnstone (Managing) PF Labuschagne AWC Marais S Pilane

Non-Executive Director: B Wilson-Jones

www.gcs-sa.biz

Khumani Stockpile Facility Storm Water Management Plan

Report

Version – 2

20 February 2015

Assmang Limited

GCS Project Number: 13-843

Client Reference: PN 13-843

Assmang Limited Khumani Extension Environmental Authorisation

13-843 26 January 2015 Page ii

Report Version – 2

20 February 2015

Assmang Limited

13-843

DOCUMENT ISSUE STATUS

Report Issue Version 2

GCS Reference Number GCS Ref – 13-843

Client Reference PN 13-843

Title Khumani Stockpile Facility Storm Water Management Plan

Name Signature Date

Author Hassen Khan

February 2015

Document Reviewer Karen King

February 2015

Director Alkie Marais February 2015

LEGAL NOTICE This report or any proportion thereof and any associated documentation remain the property of GCS until the mandator effects payment of all fees and disbursements due to GCS in terms of the GCS Conditions of Contract and Project Acceptance Form. Notwithstanding the aforesaid, any reproduction, duplication, copying, adaptation, editing, change, disclosure, publication, distribution, incorporation, modification, lending, transfer, sending, delivering, serving or broadcasting must be authorised in writing by GCS.


13-843 26 January 2015 Page iii

EXECUTIVE SUMMARY

GCS Water and Environment (Pty) Ltd (GCS) was appointed by Assmang Limited (Assmang) to

conduct a surface water assessment focussed on the impacts of the extension of a stockpile

facility on the already-approved Storm Water Management Plan (SWMP) at their Assmang

Khumani Iron Ore Mine in the Northern Cape Province of South Africa, situated approximately

30km south of the town of Kathu and 65 km north of Beeshoek Mine.

The hydrology studies are updates of the original hydrological study conducted by GCS in 2010

and by Knight Piesold (Pty) Ltd. (Knight Piesold) in 2005. The climate data used in this study

were obtained from the WR2005 database (WRC Surface Water Resources of South Africa,

Report numbers TT 380 to 382/08). In addition, the 24-hour peak rainfall depths, for various

return periods were obtained using the Design Rainfall Estimation software (Smithers, 2000).

A conceptual SWMP was drafted for the study area, which included the proposed stockpile

facility, the proposed low grade reclaim facility and the footprint area of a proposed tertiary

by-pass. The SWMP complies with the DWA Best Practice Guideline G1: Storm Water

Management (GCS, 2013) and General Notice 704 (GN 704) of the South African National

Water Act (Act 36 of 1998). Recommendations were made to separate dirty water areas from

clean water areas. Peak flows were calculated for each of the relevant sub-catchments using

the Rational Method. The calculated peak flows were used to calculate conceptual sizes of

proposed storm water infrastructure.

It was concluded that the existing storm water dam that has a footprint surface area of 11.4

ha should be used to contain dirty water. 900 m2 of the footprint area has been lined,

resulting to the current capacity of 1 350 m3 as a PCD. The calculated peak volume that will

be yielded from the proposed developments was 300 000 m3, a recommendation was made

that the current storm water dam should also be expanded such that it will have a maximum

capacity of 300 000 m3. In addition, the storm water dam should be operated empty so that

it is able to adequately store the required water volumes, in accordance with GN 704. If the

entire area (11.4ha) of the storm water dam is lined, and with a depth of 2.7m, the PCD will

have a capacity of 300 000m3 which is sufficient to meet the legislative standard. We

recommend that the entire storm water dam be lined to convert it to a PCD.


13-843 26 January 2015 Page iv

The National Environmental Management Water Act (NEMWA) classifies wastes from the Iron

industry as general waste. General waste is defined as waste that does not pose an immediate

hazard or threat to health or to the environment (NEMWA, 2014). Therefore it can be

concluded that the extended LGS can be unlined. It is; however, recommended that the

ground surface undergo a level of compaction before the extension of the LGS take place.

Assmang Limited Khumani Extension Environmental Authoristaion

13-843 26 January 2015 Page v

CONTENTS PAGE

1 INTRODUCTION .......................................................................................................................... 7

2 SCOPE OF WORK ........................................................................................................................ 7

3 METHODOLOGY ......................................................................................................................... 8

4 PREVIOUS HYDROLOGICAL ASSESSMENTS .................................................................................. 9

5 SITE DESCRIPTION ...................................................................................................................... 9

6 HYDROLOGY ............................................................................................................................. 11

6.1 MAP AND MAE ......................................................................................................................... 11 6.2 PEAK RAINFALL ........................................................................................................................... 11 6.3 MEAN ANNUAL RUNOFF (MAR) .................................................................................................... 12 6.4 NORMAL DRY WEATHER FLOW (NDWF) ......................................................................................... 13 6.5 DOWNSTREAM WATER USERS ....................................................................................................... 13

7 CONCEPTUAL STORM WATER MANAGEMENT PLAN ................................................................ 14

7.1 CONCERNS AND LIMITATIONS IN DEVELOPING A CONCEPTUAL SWMP .................................................. 15 7.2 SIMULATION OF RUNOFF AND FLOW FROM STUDY AREA ..................................................................... 16 7.3 PROPOSED STOCKPILE AND ASSOCIATED FACILITIES LAYOUT ................................................................. 17 7.4 DELINEATION OF CLEAN AND DIRTY WATER CATCHMENTS ................................................................... 19 7.5 STORM WATER DAM SIMULATION .................................................................................................. 19 7.6 FLOOD RUNOFF ........................................................................................................................... 20 7.7 PROPOSED STORM WATER INFRASTRUCTURE .................................................................................... 21 7.8 CONCEPTUAL INFRASTRUCTURE DESIGN ........................................................................................... 23

7.8.1 Existing storm water dam ............................................................................................... 23 7.8.2 Drainage Channel Sizing ................................................................................................. 23

8 CONCLUSION AND RECOMMENDATIONS ................................................................................. 25

9 REFERENCES ............................................................................................................................. 26

LIST OF FIGURES

Figure 5.1 Site Locality Map .......................................................................... 10 Figure 6.1 Runoff in Regional Rivers ................................................................ 12 Figure 6.2 Runoff into Local Rivers ................................................................. 12 Figure 7.1 Flow from Stockpile Areas .............................................................. 16 Figure 7.2 Runoff from Loading Bays and Product Handling Areas ............................ 17 Figure 7.3 Layout of Proposed New Development ................................................ 18 Figure 7.4 PCD Storage Simulation .................................................................. 20 Figure 7.5 Layout Indicating Proposed Storm Water Management Plan ...................... 22 Figure 7.7 Cross Section of the Proposed Earth Channel and Berm ........................... 23

LIST OF TABLES

Table 6.1 Mean Annual Rainfall and Evaporation in mm/month .............................. 11 Table 6.2 24-hour Design Rainfall Values ......................................................... 11 Table 7.1 Channel Sizing and Parameters ....................................................... 24


13-843 26 January 2015 Page vi

GLOSSARY OF TERMINOLOGY

Berm: A wall designed and constructed to change the direction of a natural surface water

flow path.

Catchment: That area from which any surface runoff will naturally drain to a specified point.

Clean water: Natural runoff water from a catchment area that has not been contaminated

through contact with known pollutants.

Dirty water: Water that has been, or could potentially become, contaminated through

contact with known pollutants.

Dirty water system: Any systems designed to collect, convey, contain, store or dispose of

dirty water.

Drainage channel: An artificial flow path designed to convey water.

Hydrology: The study of natural water cycles that include rainfall, evaporation and

transpiration and resulting surface flows.

Mean Annual Runoff (MAR): The average amount of water running over the land surface

during a given year.

Normal Dry Weather Flow (NDWF): The flow that is critical to the design and operation of

wastewater treatment plants which occurs when groundwater is at or near normal with no

surface runoff occurring.

WRSM: Water Resources Simulation Model

Pollution Control Dams (PCD): Specialised storage dams designed to prevent environmental

pollution by containing and storing dirty water runoff for safe disposal through evaporation

or by any other environmentally responsible process.

Process Water Dam (PWD): Specialised storage dams designed to store water for operational

and process purposes.

Runoff: Water that falls as rainfall and is not lost through evaporation, transpiration or deep

percolation into the ground. This water either does not penetrate soils but flows directly

across the soil surface, or re-emerges from local soils to flow on the surface along natural

flow paths or watercourses.

Watercourse: Watercourse refers to a river or spring; a natural channel in which water flows

regularly or intermittently; a wetland, lake or dam into which, or from which water flows

and any collection of water which the Minister may by notice in the Gazette, declare to be a

watercourse, and a reference to a watercourse includes, where relevant, its beds and banks

(National Water Act 1998 (Act 36 of 1998)).


13-843 26 January 2015

1 INTRODUCTION

GCS Water and Environment (Pty) Ltd (GCS) was appointed by Assmang Limited (Assmang) to

conduct a surface water assessment that focusses on updating the Storm Water Management

Plan (SWMP) in support of the proposed extension of the Low Grade Ore Stockpile Area (LGS),

the proposed low grade ore reclamation facility and tertiary by-pass at their Assmang

Khumani Iron Ore Mine. The Khumani Iron Ore Mine (Khumani) is situated in the Northern

Cape Province of South Africa, approximately 15km south of the town of Kathu and 65 km

north of Beeshoek Mine.

The Khumani project area is at an average altitude of approximately 1210m and located in

an arid semi-desert region. The Mean Annual Precipitation (MAP) is estimated at 357 mm,

while Mean Annual Symons Pan Evaporation(MAE) of 2351 mm is expected.

The existing (and approved) Low Grade Stockpile (LGS) covers 110.8 ha and it is proposed

that this area be expanded by a further 245.6 ha to a total surface area of 356.4 ha. The

previous SWMP study, conducted by GCS in 2010 (GCS Project Number: 10-070), identified

that storm water from the existing stockpile and associated facilities be channelled to an

existing storm water dam.

2 SCOPE OF WORK

The Scope of Work (SoW) for this study was defined as follows:

1. Project Initiation and Project Management

• Project Initiation meeting (Johannesburg)

• Site meeting

• Internal project management

2. Information sourcing / literature review

• Acquisition and assessment of any relevant, existing literature

3. Site Visit

• SWMP assessment


13-843 26 January 2015

4. Hydrology Review and Update

• Catchment characteristics and delineation

• Meteorological analysis

• Runoff calculations (Mean Annual Runoff (MAR), Normal Dry Weather Flow

(NDWF), 1 in 50- and 1 in 100-year floods)

5. Conceptual Storm Water Management Plan

• Delineate clean and dirty catchments

• Determine impact on MAR

• Conceptual SWMP Designs

6. Reporting

• Draft report

• Project close-out report

3 METHODOLOGY

The study initially included a desktop assessment of the hydrology of area of interest and

study of previous reports, data and literature pertaining to the area.

A site visit was conducted on the 27th of March 2014 to identify elements that could

potentially influence development of the site. This enabled an on-site assessment of normal

flow rates and factors that would influence the site hydrology and the effects of the proposed

changes on storm water runoff at the site. A meeting was held with Assmang.

Runoff from catchments was analysed using accepted techniques to downscale quaternary

catchment data. Generally-accepted algorithms and methodologies were used to determine

design floods at various points in the area and to estimate flood levels.

Software used in the study includes the following:

• ArcView10.1 for Geographic Information Systems (GIS) work and mapping, and

• Results of WRSM as published in WR2005 (Water Resources of South Africa; WRC

Reports TT 380 to 382/08), used for base-line runoff data.


13-843 26 January 2015

Climate data was obtained from the South African Weather Service (SAWS) and/or databases

of WR2005. The SWMP was conducted in accordance with the DWA BPG G1: Storm Water

Management (National Water Act, 1998 (Act No 36 of 1998), 1999).

4 PREVIOUS HYDROLOGICAL ASSESSMENTS

Previous hydrological studies that were undertaken for this site include:

• GCS in 2010 (GCS 10-070).

• Knight Piesold (2005)

Both these studies included MAE and MAP data that was obtained from the WR2005 Database.

In addition, 24-hour design rainfall was calculated by GCS (2010). A conceptual SWMP was

developed by GCS (2010) for infrastructure on the farms Parsons, Mokaning and King. This

updated SWMP addresses changes in infrastructure on Parsons only therefore the conceptual

SWMP for Mokaning and King is still relevant. Information from the previous SWMPs were used

as input in Section 7 of this report.

5 SITE DESCRIPTION

The Khumani Mine is situated at an average altitude of approximately 1210 mamsl. The site

is situated in a relative flat to hilly area with the overall slope in a north westerly direction.

Khumani falls within the summer rainfall region and is characterised by hot summers. Rain

falls in the form of occasional severe thunderstorms. The MAP for the area is 357.74 mm/year.

The winters are cold and dry. The mine is located in quaternary catchment D41J. Rivers and

streams receive only ephemeral flow after significant rainfall events. The non-perennial Ga-

Magara River flows from east to west across the site. This river then flows north into the

Kuruman River. Figure 5.1 shows the locality of the site.


13-843 26 January 2015

Figure 5.1 Site Locality Map


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6 HYDROLOGY

6.1 MAP and MAE

The climate data used in this study was obtained from the Water Resources of South Africa,

2005 Study (WR2005), and Water Research Commission Report Numbers TT 380 to 382/08.

The MAP calculated for this area is 357.74 mm while the MAE is 2351 mm. MAE is more than

five times greater than MAP. Which confirms the arid nature of the area.

Table 6.1 Mean Annual Rainfall and Evaporation in mm/month

Month Rainfall (mm) Evaporation (mm)

Oct 27.13 249.53

Nov 41.49 283.20

Dec 52.81 311.46

Jan 65.65 293.29

Feb 56.41 235.40

Mar 55.66 203.85

Apr 29.67 148.50

May 10.70 111.86

Jun 4.36 84.87

Jul 1.97 96.35

Aug 3.40 140.86

Sep 8.49 191.84

Mean Annual 357.74 2351.00

6.2 Peak Rainfall

The 24-hour peak rainfall depths were obtained for the site in order to calculate the design

flood peaks. These values were obtained using the Design Rainfall Estimation software

(Smithers, 2000). This software analyses gauged rain data in South Africa and tabulates peak

rainfall depths for various storm durations and return periods. The closest reliable rainfall

station to the Khumani area is the Olifantshoek station at Olifantshoek Dam, which has

rainfall and evaporation data records from December 1959 to September 2000. The 24-hour

design rainfall depths used are shown in Table 6.2:

Table 6.2 24-hour Design Rainfall Values

Station Name Olifantshoek

Station Number

Return Period (years) 2 5 10 20 50 100

Design Rainfall (mm) 45 64 77 91 109 124


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6.3 Mean Annual Runoff (MAR)

The runoff data for the quaternary catchment D41J was extracted from the WR2005 database.

The calculated MAR (for rivers in the region) can be seen in Figure 6.1.

Figure 6.1 Runoff in Regional Rivers This indicates an MAR equivalent to 0.42 mm of runoff over a wide area. It was, however,

considered that on a more local scale, higher values of runoff could be expected. This higher

runoff would not accumulate in rivers and streams, but would tend to infiltrate into the beds

of smaller watercourses. Down-scaled runoff would be expected to be distributed as shown

in Figure 6.2:

Figure 6.2 Runoff into Local Rivers

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13-843 26 January 2015

It is clear that runoff in this area is both rare and directly linked to abnormal rainfall events.

The local runoff simulated equates to 0.66 mm per annum runoff, with monthly peak values

in the order of 2.8 mm.

It would seem that these simulated runoff values don't agree with traditional methods used

for estimating peak floods. Simple Rational Method flood calculations indicate up to 27 mm

runoff in a 1.5 hour period that would produce a 1:50 year flood in the areas being developed.

In upstream clean water catchments this runoff is estimated at 24 mm in the same 1.5 hour

period. These higher flood runoff values are accepted for the following reasons:

1. Sandy soils in the region have a limited infiltration rate. It has been seen that with

extreme storm events, where rain falls with a high intensity, a surface layer within

the soil becomes saturated and further rain runs off.

2. It is noted that extreme storm events in the region are often extremely localised and

can occur over an area of 1 km2, or less.

3. Localised runoff peaks are not expected to be mobile and often tend to infiltrate into

the beds of watercourses rather than to flow downstream.

There is little data available on area reduction factors that apply in these extreme arid

conditions and for the purpose of this study, extreme point rainfall and runoff values

expected are applied over the full study area.

6.4 Normal Dry Weather Flow (NDWF)

The Ga Magara River and the tributaries within the existing and proposed mining development

are normally dry and only flow for comparatively short periods after significant rainfall events

(Knight Piesold, 2005). There is thus no NDWF that would be impacted by the proposed Low

Grade Stockpile (LGS) extension.

6.5 Downstream Water Users

There are no significant surface water users downstream of Khumani due to the unreliability

of flow in the Ga Magara River and its tributaries (Knight Piesold, 2005). Downstream users

(primarily the farming community) rely on groundwater abstraction for livestock watering

and domestic consumption (Knight Piesold, 2005). During a site visit on the 27th of March

2014, Khumani personnel confirmed that there are no significant surface water users

downstream of the mine area.


13-843 26 January 2015

7 CONCEPTUAL STORM WATER MANAGEMENT PLAN

A SWMP is a statutory requirement for mining and related activities in South Africa and is

defined by General Notice 704 and Regulation 77 of the National Water Act (Act 36 of 1988).

No water use licences in terms of this act will be granted without an approved SWMP. The

purpose of a SWMP is to prevent the pollution of water resources in and around mining areas,

or areas where mining related activity occurs. Regulations define a methodological approach

to preventing and/or containing pollution on mining sites, set design standards and specify

measures that must be taken to monitor and evaluate the efficacy of pollution control

measures that are implemented.

The basic principles of a SWMP include:

• Separation of clean and dirty water - clean water should, as far as possible, be kept

separate from dirty water. Water from clean water areas should be diverted away

from dirty water areas and should be allowed to pass through to downstream users.

Dirty water must be contained and captured on site.

• The design standard stipulated by GN704 is not that a 1 in 50-year flood should be

captured, but that the dam may not spill more than once every 50 years. Design

storage volumes are a function of peak storage requirements that often correspond to

abnormally wet conditions that continue for an extended period of time, and not to a

specific flood event.

• Containment of dirty water - reasonable measures must be taken to ensure that dirty

water is contained. All dirty water must be captured and transported in lined channels

(capable of containing 1:50-year design floods) to prevent the seepage of

contaminated water into groundwater resources. Dirty water runoff must be stored in

a PCD, where reasonable precautions are taken to prevent leaks or seepage.

• Reuse and recycling of dirty water - regulations stipulate a clear hierarchy of water

use. Firstly recycle any captured dirty water and minimise the import and use of clean

water resources. Excess water released from a dirty water area must be treated to a

standard agreed to by the regulator, Department of Water Affairs (DWA), and any plan

to treat and release excess water must be approved and licensed.


13-843 26 January 2015

• Preventing the pollution of water resources - exposure between water and potential

pollutants should be reduced to a minimum. Special precautions may be required to

prevent the transport of pollutants in water. Oil traps should be specified below

workshops, fuel depots and vehicle wash-bays to prevent the flow of hydrocarbons

into PCDs. Silt traps may be constructed where surface runoff is likely to lead to the

transport of suspended sediments and the like. For example, showers in a change-

room at a mine may yield heavy concentrations of dust that would reduce the

efficiency of a sewage treatment plant. Under similar circumstances, wash-water

should be separated from conventional water-borne sewage, and treated separately.

• Reducing dirty water areas - special attention should be paid to early rehabilitation of

mining and other dirty water areas to reduce the dirty water footprint area to an

absolute minimum. This will reduce the total volumes of dirty water and simplify the

final measures to be taken at mine closure. Part of any SWMP will include processes

that identify and implement opportunities to reduce the dirty water footprint areas.

A benefit of smaller dirty water footprint areas is that possible polluted runoff is

reduced, fewer drains are required and PCDs can often be smaller. (Smaller surface

area equates to cheaper and more effective storm water management)

7.1 Concerns and Limitations in Developing a Conceptual SWMP

Potentially polluted runoff from the larger area must (in terms of GN704) be captured and

contained in a PCD. This larger stockpile area will require a larger PCD and storm water

management is likely to become both more complex and more expensive.




concluded that the extended LGS can be unlined. Simulations used to compile this report have

assumed that no lining is provided below the LGS and that a large proportion of rainfall that

infiltrates into the LGS will seep into underlying soils and not report to the PCD. Simulations

also assumed evaporation over the full 11 hectare surface area of the current 'storm water

dam', but it is considered unlikely that this structure can function as a PCD.

Google Earth images of the site show that there are 2 pans in the study area which may be

calcrete lined depressions typically found in the region. Calcrete pans are generally not

considered to be environmentally sensitive and any water storage is transient.


13-843 26 January 2015

7.2 Simulation of Runoff and Flow from Study Area

In order to predict inflows into the PCD required for the proposed development and simulate

storage in the PCD it was necessary to simulate likely flows from different areas where runoff

reports to the current 'storm water dam'. These simulations were simplified by defining three

categories of runoff area and determining likely runoff or flow from those areas. Simulations

were based on:

Stockpile Areas:

These areas are dominated by stockpiles of low grade ore. Some rainfall is intercepted and

will evaporate off wet surface material, or from temporary ponds that form on the surface.

The majority of rainfall will infiltrate into stockpiled material where water is temporarily

stored in voids before seeping into underlying soils or leaching out from the base of stockpiles.

The following flow was simulated:

Figure 7.1 Flow from Stockpile Areas Product Handling and Transport Areas:

These areas include loading bays, reclaim facilities, conveyor systems and by-pass stockpiles.

Some of the surface contains smaller stockpiles, but the area is dominated by roadways and

other areas where soil surfaces are significantly compacted and infiltration into underlying

soils is restricted. Interception of rainfall and evaporation off compacted surfaces is

significant.

A large proportion of heavy rainfall will run directly off surfaces. Runoff from these areas is

simulated as follows:

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Runoff and Return Flows from LGS Areas


13-843 26 January 2015

Figure 7.2 Runoff from Loading Bays and Product Handling Areas Natural Runoff:

Areas exist where runoff is likely to duplicate the runoff expected from natural, undisturbed

catchment areas.

7.3 Proposed Stockpile and Associated Facilities Layout

The layout of the proposed LGS extension and associated infrastructure, (as provided by

Assmang) is shown in Error! Reference source not found.

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Figure 7.3 Layout of Proposed New Development


13-843 26 January 2015

7.4 Delineation of Clean and Dirty Water Catchments

Clean and dirty water areas were mapped out, based on topography and mine infrastructure.

The entire LGS area (including associated ore transport and handling facilities) was considered

to be a dirty area. Runoff from a catchment upstream of the stockpile was considered to be

clean, as indicated in Error! Reference source not found.. All clean water must be diverted

around the dirty water areas, as per the GN 704, to ensure that clean water never mixes with

dirty water.

7.5 Storm Water Dam Simulation

The storm water dam must meet GN704 design criteria to be considered as a PCD. To ensure

that the storm water dam in the project area will not spill more than once, on average, in 50

years, an Excel-based simulation was utilised. The basis of this calculation takes a simple

hydrological water balance of:

��

� ��

� ��

The rainfall and evaporation inputs were obtained from WR2005. Monthly storages were

simulated for a period of 85 years, as 85 years of data was available. Withdrawals from the

dam that would be used for dust suppression where assumed. Figure 7.4 shows the results of

the simulation, which indicates a PCD storage requirement of 159350 m3.


13-843 26 January 2015

Figure 7.4 PCD Storage Simulation

7.6 Flood Runoff

According to GN 704, the design standard is that a 1 in 50-year flood should be captured by

on-site PCDs, and that the PCDs may not spill more often than once in 50 year, on average.

The existing lined PCD is very small, but water will tend to overflow into the unlined portion

of the PCD.

Design storage volumes are a function of peak storage requirements that often correspond to

abnormally wet conditions that continue for an extended period of time. The volume of water

that seems likely to flow into the storage facility during an extreme flood event is, however

significant. Within the development area, local floods are estimated as a function of an

effective 36 mm per hour runoff and the various small sub-catchments. These floods report to

dirty water drainage systems and are directed to the PCD. The volume of water contained in

the flood hydrograph (0.5 hour time to peak) is estimated at 180 000 m3 and will report to the

PCD.

A dam simulation based on simulated monthly inflows indicates the need for a PCD that is

capable of storing in the order of 160 000 m3. The total PCD storage requirement is estimated

as 75% of simulated storage, plus 180 000 m3 that would accommodate the 1:50 year flood. A

total PCD storage capacity of 300 000 m3 is recommended.

0

50000

100000

150000

200000

250000

1

25

49

73

97

12

1

14

5

16

9

19

3

21

7

24

1

26

5

28

9

31

3

33

7

36

1

38

5

40

9

43

3

45

7

48

1

50

5

52

9

55

3

57

7

60

1

62

5

64

9

67

3

69

7

72

1

74

5

76

9

79

3

81

7

84

1

86

5

88

9

91

3

93

7

96

1

98

5

10

09

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e V

olu

me

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Simulation Month (based on 85 year record)

Simulated PCD Storage Requirements


13-843 26 January 2015

7.7 Proposed Storm Water Infrastructure

In order to ensure that all clean water from upstream of the LGS does not mix with dirty

water, a system of drainage channels and berms is proposed (See Figure 7.5).

Southern by-pass:

• Catchment Area 6.3 km2

• Design flood 56 m3/sec

• Excavated 1:400 slope

• 25m wide earth channel

• 1m flow depth

• Berm crest, 1.25 m above base

Western by-pass:

• Catchment Area 2.1 km2

• Design Flood 19 m3/sec

• Natural 1:133 slope

• 8m wide earth channel

• 0.75m flow depth

• Berm crest 1.0 m above base

The collected water should be channelled to neighbouring clean water stream. All berms were

standardised to similar dimensions, see Section 7.8.2 for conceptual drainage channel sizing.

The dirty water collected from the LGS and associated areas should be channelled to the PCD/

storm water dam (See Error! Reference source not found.).

A PCD with a capacity of 300 000 m3 is recommended.


13-843 26 January 2015

Figure 7.5 Layout Indicating Proposed Storm Water Management Plan


13-843 26 January 2015

7.8 Conceptual Infrastructure Design

This section provides a brief summary of the preliminary sizes of the proposed storm water

infrastructure. Please note that all infrastructures should be designed by a registered

professional engineer.

7.8.1 Existing storm water dam

The existing storm water dam should be upgraded to a PCD that is capable of storing 300 000

m3 and has a surface area of at least 11.11 hectares.

7.8.2 Drainage Channel Sizing

Collection of dirty water and diversion of clean water will be achieved through the use of

earth channels and berms (See Figure 7.6). The earth channels and drainage berms should be

designed such that water from clean water areas is diverted away from dirty water areas and

should be allowed to pass through to downstream users (NWA, 1998).

Figure 7.6 Cross Section of the Proposed Earth Channel and Berm Since the clean water flowing off the surrounding area is expected to be carrying sediments,

it is advisable that the drainage channel be designed with a gravel bed. The size of the

channel should be specified using the Manning’s Equation (See Equation 1) assuming a

Manning’s coefficient of 0.02222 for an excavated drainage channel with a gravel bed. Design

flow rates for 1:50-year storm water inflow into earth channel drains were calculated as

described in Section 6.3.


13-843 26 January 2015

The SANRAL Drainage Manual (SANRAL, 2007) recommends that the side slopes of an earth

channel have a ratio of 2:1, and for diversion purposes, a berm should also be constructed

with a slope of 2:1. When sizing the channels it was assumed that all channels will be

constructed in a trapezoidal shape, with a Manning’s coefficient of 0.02222 and a slope of 2

%. From the 1 in 50-year, 24-hour rainfall, the design flow was calculated at 2.90 m3/s. This

is the flow that the channels must be able to accommodate for a 1:50-year storm, without

spilling.

� � 80�"#√� (1)

Where:

Q = peak runoff for the area (m3/s)

d = depth of flow (m)

S = slope of the hydraulic grade line (m/m)

Based on Equation 1, the dimensions of the channel were calculated and are listed in Table

7.1. It was recommended that all channels should be constructed with same dimension since

this will be time saving during construction.

Table 7.1 Channel Sizing and Parameters

Parameter Value

Shape Trapezoidal

Base width 0.65 m

Side slopes 2:1 (V:H)

Flow depth (d) 0.65 m

Channel depth (+Freeboard) 0.8 m

Maximum flow velocity 2.9 m/s

Flow type at maximum velocity Supercritical


13-843 26 January 2015

8 CONCLUSION AND RECOMMENDATIONS

The study area is located in quaternary catchment area D41J, the MAP of the study area is

357.74 mm and the MAE of the study area is 2351 mm.

According to the Knight Piesold hydrology report of 2005 (Knight Piesold, 2005), there are no

significant surface water users downstream of the proposed stockpile facility development

owing to the unreliability of flow in the Ga Magara River and its tributaries. The downstream

farming community relies on groundwater abstraction for livestock watering and domestic

consumption. A site visit performed on the 27th of March 2014 confirmed that there are no

significant surface water users downstream of the area under consideration.

The SWMP recommends using the existing PCD and creating clean water drains around the

stockpile facility to ensure that clean water is diverted away from the stockpile facility. Dirty

water drains are also recommended to channel dirty water toward the existing PCD. From

the previous SWMP that was conducted by GCS (GCS, 2010), the surface area of the PCD

footprint was measured at 11.4 ha, however, currently only an area of 900 m2 is lined and its

capacity was calculated at 1 350 m3 (GCS, 2010). The calculated peak storage volume

required in a dedicated PCD is calculated at 300 000 m3. If the entire area (11.4ha) of the

storm water dam is lined, and with a depth of 2.7m, the PCD will have a capacity of 300

000m3 which is sufficient to meet the legislative standard. We recommend that the entire

storm water dam be lined to convert it to a PCD.




concluded that the extended LGS can be unlined. It is; however, recommended that the

ground surface undergo a level of compaction before the extension of the LGS take place.

It is recommended that detailed SWMP designs be undertaken by a registered professional

engineer.


13-843 26 January 2015

9 REFERENCES

GCS. (2010). SECTION A:STORM WATER MANAGEMENT PLAN. Johannesburg: GCS Water and Environment (Pty) Ltd.

NEMWA. (2014). National Environmental Management: Waste Amendment Act. Government Gazette.

NWA. (1998). NATIONAL WATER ACT (Act No 36 of 1998). Pretoria: Department of Water Affairs.

Regulations on use of water for mining and related activities aimed at the protection of water resources. (1999, June 4). Government Notice 704, 408(20119), 4.

Smithers, J. S. (2000). Design Rainfall Estimation. UKZN.

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