APPENDIX 18 - gcs-sa.biz · APPENDIX 18 HYDROLOGICAL STUDY . 63 Wessel Road, Rivonia, 2128 PO Box...

APPENDIX 18

HYDROLOGICAL STUDY

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

Hydrological Assessment Report for the proposed ZAC Mngeni Shaft

Version – Final Version 4

28 June 2019

GCS Project Number: 17-1187

Zululand Anthracite Colliery Mngeni Shaft Hydrological Assessment

17-1187 28 June 2019 Page ii

Report

Version – Final Version 4

28 June 2019

Zululand Anthracite Colliery

17-1187

DOCUMENT ISSUE STATUS

Report Issue Final Version 4

GCS Reference Number 17-1187

Client Reference Zululand Anthracite Colliery

Title Mngeni Shaft: Hydrological Analysis

Author 2 Lungile Lembede

25 April 2019

Document Reviewer Henri Botha

27 April 2019

Director Pieter Labuschagne

27 June 2019

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.


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DECLARATION OF INDEPENDENCE

GCS (Pty) Ltd (GCS) was appointed by Zululand Anthracite Colliery (ZAC) to conduct this specialist

hydrological study and to act as the independent hydrological specialist. GCS performed the work in

an objective manner, even if this results in views and findings that are not favourable. GCS has the

expertise in conducting the specialist investigation and has no conflict of interest in undertaking this

study. This report presents the findings of the investigations which include the activities set out in

the scope of work.

EXECUTIVE SUMMARY GCS Water and Environment (Pty) Ltd. (GCS) was appointed by Zululand Anthracite Colliery (ZAC) to

provide a hydrological assessment for the proposed Mngeni shaft. The study is intended to supplement

a Water Use License Application (WULA). The site is in quaternary catchment W22J within the Usutu-

Mhlatuze Water Management Area (WMA) 6.

It was agreed that the hydrological assessment supplied information on the following aspects:

• Baseline hydrology,

• Conceptual stormwater management plan,

• Floodline analysis,

• Water balance,

• Risk identification and mitigation, and

• The final technical report discussing the above.

Baseline Climate and Hydrology

The ZAC site has a Mean Annual Precipitation (MAP) of 911 mm and Mean Annual Evaporation (MAE)

of 1 500 mm. Natural catchment runoff amounts to approximately 44 mm, which represents

approximately 4.8% of incident rainfall. The Mngeni shaft is situated near the Mfolozi River, within

small first order non-perennial catchments of the Mfolozi River.

Conceptual Stormwater Management Plan

A SWMP aims to ensure that runoff from the site will not culminate in off-site pollution, add sediments

to any watercourse, or result in damage to properties located downstream of any stormwater

discharge. The SWMP needs to comply with regulations of the National Water Act and should meet

Human Settlement Planning and Design Guidelines of the Council for Scientific and Industrial Research

(CSIR) (CSIR, 2005). The United States Environmental Protection Agency (EPA) SWMM (Storm Water

Management Model) was used to size proposed stormwater management infrastructure that is

https://en.wikipedia.org/wiki/United_States_Environmental_Protection_Agency


17-1187 28 June 2019 Page iv

designed to attenuate the peak discharge that is anticipated as a result of existing impervious areas

(Rossman, 2010).

The client supplied a KML of the planned mining areas, which was used to develop the conceptual

storm water management plan.

The access road in Mngeni shaft falls within the exclusion zone. However, a GN704 exemption is not

triggered as a water use lice (Section 21 (c) and (i)) will be applied for by ZAC mine.

It is recommended that the Mngeni shaft PCD be moved to a lower elevation or be constructed so

that the maximum surface water elevation of the PCD is 169 mamsl.

It is recommended that further optimisation of the conceptual engineering designs be undertaken

to ensure optimal pollution control and keep the dirty areas as small as possible.

Floodline Analysis

The exclusion zone, which indicates the furthest of either the 1:50 and 1:100-year flood-lines or a

100m buffer from the river that passes the proposed ZAC developments, was calculated and

represented on a site map. The Mngeni access roads fall within the exclusion zone. GN704 exemption

is not required as a water use licence application will be applied for. The water use licence has higher

authority than the regulations.

Water Balance

The annual water requirement/use for the Mngeni shaft is 190 454 m3/annum. Potable water is

supplied from the Umfolozi River.

Risk Identification and Mitigation

A risk assessment was conducted to identify the environmental risks posed by the proposed mining

infrastructure on water resources and mitigation measures that can be taken to reduce impacts during

the construction, operational and rehabilitation phases of the proposed developments.

During the construction phase:

• Silt fences and silt traps should be used to minimize impact on receiving water bodies,

• Minimise footprint of construction area to decrease the runoff generated,

• Quick clean-up of any hydrocarbon spills.

Operational period of the mine:

• Storm water management and maintenance of storm water infrastructure to minimise impact

on downstream water bodies,

Rehabilitation phase:


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• The risks and mitigation measures identified during the construction phase are applicable in

the decommissioning phase. However, in this instance, the risks are triggered by the

demolition rather than the construction of infrastructure.

Recommendations

It was recommended that:

• Engineering designs should consider recommendations made in this report.

• The proposed stormwater infrastructure should be inspected frequently and maintained;

• Water quality monitoring should be undertaken in order to effectively manage water quality

at the site. The quality of the water discharged to the natural environment must comply with

the standards set by DWS;

• A portion of the access road in Mngeni shaft falls within the exclusion zone, storm water

infrastructure recommendations as shown in the conceptual storm water management plan

will mitigate this.

• The Mngeni shaft area requires a clean water diversion channel;

• Measurement of water flow should be done at the outlets of water infrastructure to ascertain

more accurate volumes for the water balance; and

• For the risks identified, it was recommended that the proposed mitigation measures be

implemented in order to ensure minimal impact on receiving waterbodies.


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CONTENTS PAGE

1 INTRODUCTION .......................................................................................................................... 1

2 SCOPE OF WORK ........................................................................................................................ 3

3 METHODOLOGY ......................................................................................................................... 4

3.1 INFORMATION SOURCING / LITERATURE REVIEW ................................................................................. 4 3.2 BASELINE HYDROLOGY .................................................................................................................... 4 3.3 CONCEPTUAL SWMP ..................................................................................................................... 4 3.4 FLOODLINES ................................................................................................................................. 4 3.5 WATER BALANCE ........................................................................................................................... 5 3.6 RISK IDENTIFICATION AND MITIGATION .............................................................................................. 5

4 LEGISLATION AND GUIDELINES ................................................................................................... 7

4.1 THE NATIONAL WATER ACT (ACT NO. 36, 1998) ............................................................................... 7 4.2 GN704 ....................................................................................................................................... 8 4.3 BEST PRACTICE GUIDELINE: G2 WATER AND SALT BALANCES ................................................................. 8

5 BASELINE HYDROLOGY ............................................................................................................... 9

5.1 RAINFALL ..................................................................................................................................... 9 5.2 EVAPORATION ............................................................................................................................... 9 5.3 RUNOFF ..................................................................................................................................... 10

6 CONCEPTUAL STORMWATER MANAGEMENT PLAN ................................................................. 12

6.1 DELINEATION OF CLEAN AND DIRTY WATER AREAS .............................................................................. 12 6.2 PROPOSED STORMWATER MANAGEMENT PLAN MEASURES .................................................................. 14 6.3 DESIGN FLOOD PEAKS ................................................................................................................... 15 6.4 SIZING OF INFRASTRUCTURE ........................................................................................................... 15

7 FLOODLINE ANALYSIS ............................................................................................................... 17

7.1 PEAK FLOWS ............................................................................................................................... 17 7.2 FLOODLINE RESULTS ..................................................................................................................... 17

8 WATER BALANCE ...................................................................................................................... 21

8.1 OPERATIONAL PHILOSOPHY AND PFD .............................................................................................. 21

9 RISK IDENTIFICATION AND MITIGATION ................................................................................... 26

9.1 CONSTRUCTION PHASE ................................................................................................................. 26 9.2 OPERATIONAL PHASE ................................................................................................................... 27 9.3 DECOMMISSIONING PHASE ............................................................................................................ 27

10 CONCLUSIONS .......................................................................................................................... 29

10.1 BASELINE CLIMATE AND HYDROLOGY ............................................................................................... 29 10.2 CONCEPTUAL STORMWATER MANAGEMENT PLAN ............................................................................. 29 10.3 FLOODLINE ANALYSIS ................................................................................................................... 29 10.4 WATER BALANCE ......................................................................................................................... 29 10.5 RISK IDENTIFICATION AND MITIGATION ............................................................................................ 29

11 RECOMMENDATIONS ............................................................................................................... 31

11.1 CONCEPTUAL STORMWATER MANAGEMENT PLAN ............................................................................. 31 11.2 FLOODLINE ANALYSIS ................................................................................................................... 31 11.3 WATER BALANCE ......................................................................................................................... 31 11.4 RISK IDENTIFICATION AND MITIGATION ............................................................................................ 31

12 REFERENCES ............................................................................................................................. 33


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LIST OF TABLES

Table 3-1: Summary of peak flow methods ......................................................... 4 Table 3-2: Risk Assessment Rating Matrix .............................................................. 6 Table 3-3: Risk Assessment Significant Values ........................................................ 6 Table 6-1: 50-year design flood peaks ................................................................. 15 Table 6-2: Storm water infrastructure sizing ......................................................... 15 Table 6-3: PCD Sizing for the 1:50 year return period .............................................. 16 Table 7-1: Design peak flows for the delineated catchments ..................................... 17 Table 8-1: Mngeni Shaft Annual Water Balance ...................................................... 23 Table 8-2: Mngeni Shaft Monthly Water Balance .................................................... 24 Table 8-3: Mngeni Shaft Daily Water Balance ........................................................ 25 Table 9-1: Risk Identification and Mitigation for Mngeni shaft .................................... 28

LIST OF FIGURES Figure 1-1: Locality map of the ZAC Mngeni shaft .................................................... 2 Figure 5-1: Rainfall Distribution for the ZAC site ..................................................... 9 Figure 5-2: S-pan Evaporation for Quaternary Catchment W22J .................................. 10 Figure 5-3: Naturalised runoff for W22J catchment ................................................ 11 Figure 6-1: Proposed stormwater management plan for the Mngeni shaft...................... 13 Figure 7-3: Exclusion zone for Mngeni shaft .......................................................... 19 Figure 7-4 Mngeni Shaft flood lines .................................................................... 20 Figure 8-1: Mngeni shaft Process Flow Diagram ...................................................... 22

LIST OF ACRONYMS

Acronym Description

CSWMP Conceptual storm water management plan

DEM Digital Elevation Model

DWS Department of Water and Sanitation

GCS GCS Water and Environment (Pty) Ltd.

GN704 General Notice 704

IWULA Integrated Water Use Licence Application

MAE Mean annual evaporation

MAR Mean Annual Runoff

MIPI Midgley and Pitman

NWA National Water Act, 1998 (Act No. 36 of 1998)

PCD Pollution Control Dam

PFD Process flow diagram

SDF Standard design flood

TDS Total dissolved solids

WMA Water Management Area

WR2012 Water Resources of South Africa 2012

ha Hectare

m³ Cubic Metres


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1 INTRODUCTION

GCS Water and Environment (Pty) Ltd. (GCS) was appointed by Zululand Anthracite Colliery

(ZAC) to undertake a hydrological analysis for the proposed Mngeni Shaft for the purpose of a

Water Use License Application (WULA). ZAC is situated in quaternary catchment W22J (refer

to Figure 1-1) which falls under the Usutu-Mhlatuze Water Management Area (WMA 6).

For the WULA, the hydrological requirements addressed by this report include a floodline

analysis, a conceptual stormwater management plan (CSWMP), a static water balance and a

risk identification and mitigation analysis.


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Figure 1-1: Locality map of the ZAC Mngeni shaft


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2 SCOPE OF WORK

The objective of the study was to assess the proposed Mngeni shaft plans to ensure

compliance with the National Water act (act 36 of 1997) (NWA) for the proposed

developments. This was done by undertaking the following:

1. Information sourcing and literature review that included:

• Acquisition and an assessment of existing literature, and

• Investigation and summary of legislative and policy frameworks relating to

the relevant surface water resource management.

2. Updating of the Hydrology- the following was undertaken:

• A desktop evaluation of climate based on WR2012 data (WRC, 2015); and

• Mean Annual Runoff (MAR) and Mean Annual Evaporation (MAE) for the site.

3. A conceptual SWMP that included:

• Identification of clean and dirty catchments;

• Determination of stormwater flows and volumes was undertaken;

• Indication and explanations of the placement of channels and stormwater

attenuation infrastructure were offered; and

• Conceptual designs were undertaken and illustrated using GIS software for

the infrastructure.

4. A Flood line Analysis:

• Determination of the 1:50 and 1:100-year flood peak runoff; and

• Lateral extent of the 1:50 and 1:100-year flood lines.

5. Static Water Balance:

• Water requirements based on the operational requirements; and

• Water Balance as per the Department of Water and Sanitation (DWS) Best

Practice Guidelines (BPG 2) Water and Salt Balance (DWA, 2006).

6. Risk Identification and Mitigation:

• Description of all surface water impacts and proposed mitigation measures,

using GCS’ standard EIA Risk and Mitigation methodology.

7. A report which includes details of the abovementioned points, conclusions and

recommendations was compiled.


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3 METHODOLOGY

The following details the methodology used for the hydrological study of the Mngeni shaft.

3.1 Information Sourcing / Literature Review

A desktop assessment of the area of interest was carried out, including a review of existing

data pertaining to the W22J quaternary catchment.

An investigation and summary of legislative and policy frameworks relating to the relevant

surface water resource management was undertaken. The legislation and guidelines can be

viewed in chapter 4.

3.2 Baseline Hydrology

Baseline hydrology was determined from WR2012 data. Climate data were obtained from the

Nongoma weather station; 0374264W (DWS, 2018) and from the WR2012 database (WRC,

2015).

3.3 Conceptual SWMP

The conceptual SWMP was designed using the proposed infrastructure layout plans, reviewing

the topographical data in order to delineate areas of clean and dirty water and to contain

dirty water on site. Measures to be taken to achieve an efficient and legally-compliant

conceptual SWMP were compiled. The conceptual SWMP was designed to comply with

regulations of General Notice 704 of the South African National Water Act (Act 36 of 1998)

(NWA, 1998). The conceptual SWMP was modelled using the Environmental Protection Agency

Storm Water Management Model to determine runoff and infrastructure sizing.

3.4 Floodlines

The HEC-RAS model was used to simulate flooding potential in rivers close to the Mngeni shaft

site. The HEC-RAS model simulates total energy of water by applying basic principles of mass,

continuity and momentum as well as roughness factors between all cross sections (US Army

Corps of Engineers, 1995). A height is calculated at each cross-section, which represents the

level to which water will rise at that section, given the potential peak flows. This was

calculated for the 1:50 year and 1:100-year flows on river sections.

Three methods were used to calculate peak flows for the ZAC study site. These are the

Rational Method, Standard Design Flood (SDF) and the Regional Maximum Flood (RMF)

methods. A brief description of each of the peak flow methods are summarised in Table 3-1,

below.

Table 3-1: Summary of peak flow methods


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Rational Method

The rational method was developed in the mid-19th century and is one of the most widely

used methods for the calculation of peak flows for small catchments (< 15 km2). The formula

indicates that Q = CIA, where I is the rainfall intensity, A is the upstream runoff area and C

is the runoff coefficient. Q is the peak flow. There are 3 alternatives to the Rational Method

which differ on the methodology used to calculate rainfall intensities. The first alternative

(RM1) uses the depth-duration frequency relationships approach, the second uses the

modified Hershfield equation while the third alternative uses the Design Rainfall software for

South Africa (SANRAL, 2013).

Empirical Methods

Empirical methods such as the Midgley and Pitman (MIPI) and Regional Maximum Floods (RMF)

are based on correlation between peak flows and some catchment characteristics. Regional

parameters are then mapped out for South Africa. These methods are mostly suitable for

medium to large catchments (SANRAL, 2013).

3.5 Water Balance

A Process Flow Diagram (PFD) was set up using information obtained from the client that

provided insight into the flow linkages within the present infrastructure. This information

was then used to develop temporal (annual, monthly and daily) water balances

The water balance was based on techniques used by the DWS (Department of Water and

Sanitation) Best Practice Guidelines (BPG) G2: Water and Salt balances (DWA, 2006).

3.6 Risk Identification and Mitigation

Risk assessment was conducted to identify the risks that the development may pose to the

receiving environment. This was undertaken using a risk matrix where the various impacts

are scored according to qualitative measures of the extent, magnitude, duration, reversibility

and probability of the risk occurring (see Table 3-2 and Table 3-3). The risk assessment was

undertaken based on potential risks that could be encountered. The results from the risk

assessment are described in Table 9-1.


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Table 3-2: Risk Assessment Rating Matrix

Status of Impact

+: Positive (A benefit to the receiving environment)

N: Neutral (No cost or benefit to the receiving environment)

-: Negative (A cost to the receiving environment)

Magnitude:=M Duration:=D

10: Very high/don’t know 5: Permanent

8: High 4: Long-term (ceases with the operational life)

6: Moderate 3: Medium-term (5-15 years)

4: Low 2: Short-term (0-5 years)

2: Minor 1: Immediate

0: Not applicable/none/negligible 0: Not applicable/none/negligible

Scale:=S Probability:=P

5: International 5: Definite/don’t know

4: National 4: Highly probable

3: Regional 3: Medium probability

2: Local 2: Low probability

1: Site only 1: Improbable

0: Not applicable/none/negligible 0: Not applicable/none/negligible

Table 3-3: Risk Assessment Significant Values

The maximum value that can be achieved is 100 Significance Points (SP). Environmental effects were rated as follows:

Significance Environmental Significance Points Colour Code

High (positive) >60 H

Medium (positive) 30 to 60 M

Low (positive) <30 L

Neutral 0 N

Low (negative) >-30 L

Medium (negative)

-30 to -60 M

High (negative) <-60 H


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4 LEGISLATION AND GUIDELINES

4.1 The National Water Act (Act No. 36, 1998)

As mentioned, the SWMP assessment aims to demonstrate compliance with the National

Water Act (Act No. 36, 1998) (NWA).

The National Water Act provides the broad legal framework for water resources management.

The requirements of the Act have to be implemented. Details about implementation are

outlined in regulations that are issued by the DWS (previously the Department of Water Affairs

and Forestry, DWAF) and published in the Government Gazette.

• Government Notice No. 1352, 12 November 1999, National Water Act, 1998 (No. 36

of 1998): Regulations requiring that a water use be registered.

The NWA stipulates that the following tasks should be undertaken;

• Separate clean and dirty water systems:

o Demarcation of dirty water footprint areas;

o Delineation of upstream catchment areas that would naturally drain into

dirty water areas;

o Estimation of peak flood runoff from relevant catchments;

o Design of drains, diversion channels and berms to prevent clean water from

entering dirty water areas.

• Control and contain dirty water runoff:

o Design of drains and berms to prevent dirty water from leaving dirty water

areas;

o Design dams that will not spill more than once in 50 years, on average, to

contain dirty water runoff.

• Prevent or reduce pollution to water resources:

o Prevent or reduce the pollution of water resources;

o Evaluate processes and adapt systems to minimize the contact between

potential pollutants and water resources.


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4.2 GN704

General Notice 704 and Regulation 77 of the National Water Act (Act 36 of 1988) regulates

the use of water for mining and related activities and aims to protect water resources.

Regulation 4(b) of GN704 reads; ‘No person in control of a mine or activity may - except in

relation to a matter contemplated in regulation 10, carry on any underground or opencast

mining, prospecting or any other operation or activity under or within the 1:50-year flood

line or within a horizontal distance of 100 meters from any watercourse or estuary, whichever

is the greatest’. Furthermore, GN704 defines an activity as: “the operation and the use of

mineral loading and off-loading zones, transport facilities and mineral storage yards, whether

situated at the mine or not, out of which process any residue is derived, stored, stockpiled,

accumulated, dumped, disposed of or transported”. The area within this designation is

termed the Exclusion Zone.

GN704 further specifies that every person in control of a mine or activity must- design,

construct, maintain and operate any dirty water system at the mine or activity so that it is

not likely to spill into any clean water system more than (on average) once in 50 years.

According to (Department of Water Affairs and Forestry, 2000): “Should an exemption from

any requirements of these regulation imply the necessity for a water use licence, the person

in control of a mine or activity need only to apply for a water use licence, i.e. a water use

licence has higher authority than the regulations. However, the following clause needs to be

incorporated into the water use licence: In terms of the conditions of this licence, the Licence

Holder is exempted from the clause (specific regulation) of the regulations on use of water

for mining and related activities aimed at the protection of water resources (GN704).”

4.3 Best Practice Guideline: G2 Water and Salt Balances

The water balance was based on techniques used by the DWS Best Practice Guidelines (BPG)

G2: Water and Salt balances (DWA, 2006).


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5 BASELINE HYDROLOGY

Climate parameters were calculated from data obtained from the WR2012 database for

quaternary catchment W22J, within which the ZAC site is located (WR, 2012). Quaternary

catchment W22J falls within the Usuthu-Mhlatuze Water Management Area (WMA 6).

5.1 Rainfall

Rainfall for the site is based on 73 years of record at Nongoma weather station (0374264W)

(DWS) and historical records indicate a long-term average of approximately 911 mm per

annum (refer to Figure 5-1). ZAC receives summer rainfall with dry winters.

The weather station is situated approximately 30 km from the study area. WR 2012 suggests

a MAP of 722 mm for quaternary catchment W22J. The weather station represents more site-

specific rainfall, while WR 2012 reports on the average rainfall for the entire quaternary

catchment. The 90-year WR2012 record is more variable and may be more influenced by

climate change. Actual site rainfall may fall between these two extremes, but the local

station is considered to better represent site rainfall.

Figure 5-1: Rainfall Distribution for the ZAC site

5.2 Evaporation

The ZAC area experiences on average 1 500 mm of evaporation per annum. With Evaporation

rates being highest during the summer months and lowest during winter (refer to Figure 5-2).


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Figure 5-2: S-pan Evaporation for Quaternary Catchment W22J

5.3 Runoff

Average runoff from unmodified catchments in quaternary catchment W22J is recorded in

WR2012 as being equivalent to 44 mm per annum over the surface area (WRC, 2015). This is

approximately 4.8% of the 911 mm MAP. Given the higher than catchment average rainfall

expected at the site, local runoff could potentially be as high as 70 mm per annum. Local

rainfall and runoff will need to be confirmed by local measurement. The monthly natural

average runoff for Quaternary Catchment W22J is distributed as shown in Figure 5-3.


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Figure 5-3: Naturalised runoff for W22J catchment


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6 CONCEPTUAL STORMWATER MANAGEMENT PLAN

The conceptual stormwater management plan was developed to manage the runoff to GN 704

standards for the Mngeni shaft (refer to Figure 6-1).

6.1 Delineation of clean and dirty water areas

Clean and dirty water areas were delineated, with an emphasis on separating clean and dirty

water areas. The water areas were delineated using topography and mine plans provided by

the client.


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Figure 6-1: Proposed stormwater management plan for the Mngeni shaft


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6.2 Proposed stormwater management plan measures

The dirty areas were grouped into one unit to simply the stormwater management plan. The

dirty areas consist of the following areas:

• Adit;

• Salvage yard, hard park, explosives delivery bay, parking area;

• Workshop, wash bay, cable shop, diesel bay, delivery yard, mining store and visitors

parking; and

• Stockpile and brake test ramp.

A sump was included to pump water into the PCD. The recommended PCD locality is shown in

Figure 6-1. Moving the maximum water surface elevation in the PCD to 169 mamsl (metres

above sea level) will facilitate gravitational flow. This will mitigate the need to pump water

from the dirty water runoff areas to the PCD and water can be transferred under gravity.

A stream flow diversion channel is required to divert clean water to the north-west of the adit

to the outlet point shown in Figure 6-1.


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6.3 Design flood peaks

The rational method was used to calculate the design flood peaks for the relevant delineated

areas. This is one of the most widely-used methods of peak flow calculation and the study site

characteristics fit the criteria for the use of this method (SANRAL, 2013). The SWMP is based

on future land cover conditions that will cause an increase in impervious areas, thus the best

flood peak determination method for the purpose of this study is the rational method as this

method incorporates a runoff factor that is based on surface slope, perviousness (ability of

water to infiltrate into the ground) of the site and vegetation.

Within the delineated water areas, it was required to calculate design flood peaks to size the

proposed drains with a return period of 50 years, as recommended by GN704.

The flood peaks under natural and modified conditions are summarised in Table 6-1.

Table 6-1: 50-year design flood peaks

Catchment 1:50 year peak runoff (m3/s)

Mngeni shaft

Mngeni Clean Sub catchments 40.78

Dirty areas 0.005

6.4 Sizing of infrastructure

The proposed stormwater controls include stream diversions as well as transporting dirty water

runoff to the PCD. Table 6-2 summarises the sizing required for the recommended

infrastructure. The SANRAL Drainage Manual (SANRAL, 2007) recommends earth channel side

slopes of 2:1, and for diversion purposes, a berm should also be constructed with a slope of

2:1

The PCD at the Mngeni shaft will receive water from the underground de-watering as well.

Table 6-2: Storm water infrastructure sizing

Infrastructure Shape

Side slope

(m/m) Depth (m)

Bottom Width (m)

Mngeni Shaft

Clean water diversion berm Trapezoidal 8 1 1

Dirty water areas Circular N/A 1 N/A


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The PCD was sized using the same assumptions as the water balance for plant water demand,

dust suppression as well as ground water values. The PCD was sized so as to ensure that no

spillage would occur during a 50-year period. Circular pipes were recommended as the dirty

water needs to be conveyed through a clean water catchment. The sizing of the PCD is

summarised in Table 6-3.

Table 6-3: PCD Sizing for the 1:50 year return period

PCD Volume (m3) Abstraction assumed (m3/d)

Mngeni 131 800 985

The volume of the sump needs to accommodate the 1:50-year peak runoff event for the

delineated dirty sub-catchment (refer to Table 6-1). The time of concentration was multiplied

by the peak discharge to obtain the minimum required sump volume using the equation:

𝑉𝑜𝑙𝑢𝑚𝑒 (𝑚3) = 𝑃𝑒𝑎𝑘 𝑑𝑖𝑠𝑐ℎ𝑎𝑟𝑔𝑒 (𝑚3𝑠−1) ∗ 𝑇𝑐(𝑠)

The minimum volume required sump was calculated as 814 m3.


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7 FLOODLINE ANALYSIS

7.1 Peak Flows

The flood peak flows for the Mngeni shaft were calculated using the Rational, MIPI and the

RMF methods. The Rational and MIPI methods have peak flows of the same order of magnitude

which implies these peaks are realistic for the study site. The Rational method peaks were

selected for use in HEC-RAS for smaller catchments because this method is generally more

appropriate for small catchments (i.e. catchments less than 15 km2). The MIPI method was

used for larger sub catchments. The calculated peak flows are summarised in Table 7-1.

Table 7-1: Design peak flows for the delineated catchments

Catchment Area

(km2)

Method

Rational MIPI RMF

1:50 year 1:100

year 1:50 year

1:100

year 1:50 year

1:100

year

m3/s

Mngeni Shaft

N1 2 600 1 447.85 1 809.09 1 660.34 2 101.69 3 996.58 5 005.33

N2 96.96 350.40 466.05 247.95 313.86 536.00 671.29

N3 1.95 41.97 55.82 28.55 36.14 60.47 75.73

7.2 Floodline Results

According to the NWA (1998), no person may carry out any underground and opencast mining

activity within the 1:50 year floodline or within 100 m from a watercourse, whichever is

greatest.

Floodlines were calculated for the river sections within close proximity of the Mngeni shaft.

Floodlines were calculated for flood events of the 1:50 and 1:100-year return period. A 100 m

buffer was also created. An exclusion zone was then created, which is the furthers of either

the 1:50 year floodlines (refer to Error! Reference source not found. and the 100 m buffer

(refer to Error! Reference source not found.).


17-1187 28 June 2019 Page 18

The access road in the Mngeni shaft falls within the exclusion zone. This activity refers to

transport facilities but does not trigger GN704 exemption since a water use licence will be

applied for. However, the following clause needs to be incorporated into the water use

licence: In terms of the conditions of this licence, the Licence Holder is exempted from the

clause (Regulation 4(a)- Restrictions on Locality) of the regulations on use of water for mining

and related activities aimed at the protection of water resources (GN704).


17-1187 28 June 2019 Page 19

Figure 7-1: Exclusion zone for Mngeni shaft


17-1187 28 June 2019 Page 20

Figure 7-2 Mngeni Shaft flood lines


17-1187 28 June 2019 Page 21

8 WATER BALANCE

8.1 Operational Philosophy and PFD

To set up a water balance model, a Process Flow Diagram (PFD) was drafted to create insight

into all water-linked flows within the Mngeni shaft operations. The data for the water balance

was obtained as follows:

• The water volumes from the underground workings were sourced from the

Hydrogeological report (GCS, 2019). The hydrogeological report quoted an average of

100 m3/day for groundwater dewatering operations and this value was used to

represent the anticipated volume of dewatering.

• Metered data within the water reticulation system was provided by the client. The

data from January 2018 – December 2018 to calculate water use in the ZAC mine to

provide an estimation for expected water uses in the Mngeni shaft. This includes data

pertaining to:

o Plant water use;

o Total abstractions from the Mfolozi River;

o Total water use in the offices and change house;

o Water use in the Ngwabe shaft

o Sewage works

Assumptions made for the water balance:

• Plant and dust suppression volumes were based on excess water volumes,

• Potable water is abstracted from the Mfolozi river and treated for consumption.

• The volume of water that is routed to sewage works from the offices and change house

was assumed as 80% of water supplied to the offices and change house.


17-1187 28 June 2019 Page 22

Figure 8-1: Mngeni shaft Process Flow Diagram

Mngeni Adit WorkshopOffices and

change house Stockpile

PCD

SupplyRainfall Runoff RunoffRunoff

Rainfall

Evaporation

Dust suppression

Plant

Rainfall

Undergroundworkings

Ngwabeshaft

Coal Wash

100 m3/d

De

-wat

erin

g

106 m3/d 104 m3/d 23 m3/d

4 m3/d

54 m3/d 145 m3/d 10 m3/d

10 m

3/d

7 m3/d 2 m3/d 7 m3/d90 m3/d 4 m3/d

27 m3/d53 m3/d

Sewage works

113 m3/d

72 m3/d


17-1187 28 June 2019 Page 23

Table 8-1: Mngeni Shaft Annual Water Balance

Facility Name Water In Water Out Balance

Mngeni Shaft Water Circuit/stream

Quantity

(m3/a) Water Circuit/stream

Quantity

(m3/a)

From: Groundwater dewatering 36 500.00 To: PCD 38 686.00

From: Rainfall 2 571.76 To: evaporation and losses 385.76

Total 39 071.76 Total 39 071.76 0

From: Supply 41 410.00 To: PCD 37 959.52

From: Runoff 690.52 To: Consumption losses 4 141.00

Total 42 100.52 Total 42 100.52 0

From: Supply 32 700.00 To: Sewerage works 26 160.00

From: Runoff 2 471.41 To: Consumption losses 654.00

To: PCD 8 357.41

Total 35 171.41 Total 35 171.41 0

From: Umfolozi River 74 110.00 To: Offices and change house 32 700.00

To: Workshop 41 410.00

Total 74 110.00 Total 74 110.00 0

From: Adit 38 686.00 To: Evaporation 19 289.18

From: Workshop 37 959.52 To: Dust suppression 53 281.72

From: Offices and change house 8 357.41 To: Plant 4 012.00

From: Rainfall 9 965.77 To: Ngwabe shaft 19 863.00

From: Stockpile 1 477.21

Total 96 445.90 Total 96 445.90 0

From: PCD 4 012.00 To: Coal wash 4 012.00

Total 4 012.00 Total 4 012.00 0

Total In 290 911.60 Total Out 290 911.60 0

Plant

Annual Water Balance for Mngeni Shaft

Adit

Workshop

Offices and change house

PCD

Potable water supply


17-1187 28 June 2019 Page 24

Table 8-2: Mngeni Shaft Monthly Water Balance


17-1187 28 June 2019 Page 25

Table 8-3: Mngeni Shaft Daily Water Balance

Facility Name Water In Water Out Balance

Mngeni Shaft Water Circuit/stream

Quantity

(m3/d) Water Circuit/stream

Quantity

(m3/d)

From: Groundwater dewatering 100.00 To: PCD 105.99

From: Rainfall 7.05 To: evaporation and losses 1.06

Total 107.05 Total 107.05 0

From: Supply 113.45 To: PCD 104.00

From: Runoff 1.89 To: Consumption losses 11.35

Total 115.34 Total 115.34 0

From: Supply 89.59 To: Sewerage works 71.67

From: Runoff 6.77 To: Consumption losses 1.79

To: PCD 22.90

Total 96.36 Total 96.36 0

From: Umfolozi River 203.04 To: Offices and change house 89.59

To: Workshop 113.45

Total 203.04 Total 203.04 0

From: Adit 105.99 To: Evaporation 52.85

From: Workshop 104.00 To: Dust suppression 145.98

From: Offices and change house 22.90 To: Plant 10.99

From: Rainfall 27.30 To: Ngwabe shaft 54.42

From: Stockpile 4.05

Total 264.24 Total 264.24 0

From: PCD 10.99 To: Coal wash 10.99

Total 10.99 Total 10.99 0

Total In 797.02 Total Out 797.02 0

Plant

Daily Water Balance for Mngeni Shaft

Adit

Workshop

Offices and change house

Potable water supply

PCD


17-1187 28 June 2019 Page 26

9 RISK IDENTIFICATION AND MITIGATION

The following potential surface water impacts, or risks were identified for the construction,

operational and rehabilitation phases of the proposed Mngeni shaft:

9.1 Construction Phase

Impact:

• Increased runoff and flow velocities resulting from soil compaction due to movement

of vehicles and machinery. This will lead to accelerated erosion and the siltation of

nearby watercourses.

Mitigation:

• Keep vehicle movement to designated access roads to avoid spreading the impact to

wider areas; and

• Ensure flow velocities are managed along the roads by implementation of flow

break/diversion berms, etc.

Impact:

• Soil erosion may result from the destabilisation of soils by earthworks involving the

construction of dirty water dams and other infrastructure and this subsequently leads

to sediment transport to downstream river reaches.

Mitigation:

• Construct silt traps to stop sediments from reaching nearby watercourses.

Impact:

• Reduction of Water Quality resulting from hydrocarbon spills including grease, oils and

other pollutants which are washed away by overland flow and cause pollution in

nearby watercourses.

Mitigation:

• Minimise spills and keep vehicles away from the watercourse and conduct quick clean-

ups when spills occur. Used oils and grease should be disposed of by accredited

vendors.


17-1187 28 June 2019 Page 27

9.2 Operational Phase

Impact

• Water discharging from the site could lead to river bank erosion.

Mitigation

• Implement bank stabilisation and engineering design of water discharge points.

Impact:

• Reduction of water quality due to pollutants and spillages from moving vehicles which

are washed into nearby watercourses.

Mitigation:

• All dirty water should be contained as indicated in the stormwater management plan.

• Regular inspection and correct maintenance of stormwater infrastructure.

9.3 Decommissioning Phase

The decommissioning phase is anticipated to have similar risks and mitigation measures as the

construction phase. However, in this instance, instead of construction, the demolition of

infrastructure may trigger risk.


17-1187 28 June 2019 Page 28

Table 9-1: Risk Identification and Mitigation for Mngeni shaft

POTENTIAL ENVIRONMENTAL IMPACT APPLICABLE AREA ACTIVITY

ENVIRONMENTAL SIGNIFICANCE

BEFORE MITIGATION RECOMMENDED

MITIGATION MEASURES

ENVIRONMENTAL SIGNIFICANCE

AFTER MITIGATION

M D S P

TO

TA

L

ST

AT

US

SP M D S P

TO

TA

L

ST

AT

US

SP

CONSTRUCTION PHASE

Soil compaction may be caused by the

movement of heavy machinery which

may result in increased runoff, soil

erosion and siltation

Mngeni Adit Movement of heavy

machinery 8 4 2 3 42 - M

Restrict vehicle movement

to designated access roads 4 2 2 2 16 - L

Hydrocarbon spills from site vehicles

pollute local water resources Mine vehicle routes

Leakage form Mine

Vehicles and machinery 4 4 2 4 40 - M

Minimise areas where spills

might occur; capture and

contain runoff from these

areas; install an oil and

grease trap at the discharge

into the PCD/Sump; safely

dispose of captured

pollutants

4 2 2 3 24 - L

Soil disturbance during construction of

new infrastructure including the

construction of dirty dams

Mngeni Adit Construction of

infrastructure 2 2 1 5 25 - L

Construct silt traps to stop

sediments from reaching

nearby watercourses

2 1 1 4 16 - L

OPERATIONAL PHASE

Increased runoff or stormflow from the

site could lead to river bank erosion as

well as pollution of downstream water

bodies

Receiving surface

water bodies

Increasing impervious

areas at the mine site 2 5 2 3 27 - L

Correct storm water

management 2 5 1 1 8 - L

Polluted decant may cause pollution of

water in the receiving water body Adit Sump/Storage 6 4 2 4 48 - M

Decant water should be

captured and contained in

the sump, and disposed of

in an approved manner

2 1 1 1 4 - L


17-1187 28 June 2019 Page 29

10 CONCLUSIONS

The following section describes the conclusions derived from the results determined within

the various sections of this study.

10.1 Baseline Climate and Hydrology

Rainfall for Nongoma weather station, which represents rainfall for the ZAC site indicates a

long-term average of approximately 911 mm per annum. Evaporation data for quaternary

catchment W22J amounted to 1 500 mm. Approximately 44 mm of surface runoff is expected

from the study site, which is approximately 5% of incidental rainfall.

10.2 Conceptual Stormwater Management Plan

The SWMP included the delineation of clean and dirty water catchments. The dirty water was

collected and diverted away from clean water catchments using berms. Clean water flow at

the Mngeni shaft will need to be diverted.

10.3 Floodline Analysis

An exclusion zone was created for the Mngeni shaft. The exclusion zone consists of the 1: 50-

year floodline for the and a 100 m buffer from the river. The proposed developments in both

sites fall within the exclusion zone. The access roads fall within the exclusion zone in the

Mngeni shaft area but GN704 exemption is not triggered since a water use licence will be

applied for. A water use licence has higher authority than the regulations set out in GN704.

10.4 Water balance

The annual water requirement/use for the Mngeni shaft is 190 453.69 m3/annum. The dirty

water is stored in the PCD, which has been designed not to spill (on average) once in 50 years.

Potable water is sourced from the Umfolozi River.


During the construction phase, the potential risks were identified as erosion, siltation and

sedimentation as a result of the movement of heavy vehicles, machinery and construction.

Hydrocarbon spills, along with oils and grease were also identifed as a potential risk. The

mitigation measures that can be implemented include restricting vehicles to the designated

access roads, the installation of silt, oil and grease traps and conducting emergency clean-ups

in the event of pollutant spillages.


17-1187 28 June 2019 Page 30

For the operational phase, bank erosion and the pollution of other water sources were

identified as potential risks. These risks may be mitigated by ensuring that bank stabilizers

are in place, along with engineering designs at water discharge points. According to the

geohydrological report, no decant is anticipated from the Mngeni shaft. The decommissioning

phase is anticipated to have the same risks and mitigation measures as the construction phase.


17-1187 28 June 2019 Page 31

11 RECOMMENDATIONS

The following recommendations have been made based on the outcomes of this study:

11.1 Conceptual Stormwater Management Plan

• The PCD at Mngeni should be moved to a lower elevation to mitigate pumping or dug

to a depth where the maximum surface water elevation of the PCD is 169 masl.

• Conduct regular inspection and maintenance of storm water infrastructure.

• Employee training including conducting of training of personnel who are responsible

of implementing activities identified above.

11.2 Floodline Analysis

The access road falls within the exclusion zone, which poses inundation risk upon flood events.

It is recommended that for the water use licence application, the licence holder specify the

exemption from GN704 in Mngeni shaft access roads as a result of the water use licence

application.

11.3 Water Balance

It is recommended that water flow meters be installed so as to monitor water usage. Water

meters should be installed on the abstraction from the PCD as well as the pump from the

underground area. It is also recommended that water from mine workings be recycled or used

in the plant if the plant water requirement is not met. Alternatively, excess water may be

treated either actively with the use of water treatment plants or passively through the use of

phytoremediation or artificial wetlands. It is recommended that further studies are done to

optimise the method used to manage the excess water on site. The water balance should be

updated once measured volumes are available.


During the construction phase:

• Silt fences and silt traps should be used during construction to minimize impact on

receiving water bodies,

• Minimise footprint of construction area to decrease the runoff generated,

• Quick clean-up of any hydrocarbon spills.

Operational period of the mine:

• Storm water management and maintenance of storm water infrastructure to minimise

impact on downstream water bodies,


17-1187 28 June 2019 Page 32

Rehabilitation stage:

• The mitigation measures specified during the construction phase are applicable for

the decommissioning phase.

Zululand Anthracite Colliery Mngeni Shaft Hydrlogical Assessment

17-1187 28 June 2019 Page 33

12 REFERENCES

Alexander, J. (2002). The Standard Design Flood. South African Institution of Engineers, 26-30. CSIR, 2005. Guidelines for Human Settlement Planning and Design: Volume 2, Pretoria: CSIR Building and Construction Technology. Department of Water Affairs and Forestry, 2000. Operational Guideline No. M6.1. Guideline document for the implementation of regulations on use of water for mining and related activities aimed at the protection of water resources. Second Edition.

DWA. (2006). Best Practice Guidelines for Water Resources Protection in the South African

Mining Industry. BPG G2: Water and Salt Balances. Pretoria: DWA.

DWS. (1998). The South African National Water Act (36 of 1998). DWS.

GCS. 2018. Hydrogeological Investigation for the ZAC New Mngeni Adit and Deep E Open Cast Mine. Private Report for ZAC. Report No: 17-1189. Report Date: July 2018.

SANRAL. (2013). South African Drainage Manual. Pretoria: SANRAL. Rossman, L.A. (2010). Storm water management model user’s manual version 5.0. United States Environmental Protection Agency. Retrieved from: http://www.owp.csus.edu/LIDTool/Content/PDF/SWMM5Manual.pdf US Army Corps of Engineers. (1995). HEC-RAS Hydraulic Modelling Software. Version 4.1. California. WRC. (2015). Water Resources of South Africa, 2012 Study (WR2012). Retrieved from: http://www.waterresourceswr2012.co.za/resource-centre/

http://www.owp.csus.edu/LIDTool/Content/PDF/SWMM5Manual.pdf

http://www.waterresourceswr2012.co.za/resource-centre/

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