Zitholele Consulting Reg. No. 2000/000392/07 PO Box 6002 Halfway House 1685, South Africa Building 1, Maxwell Office Park, Magwa Crescent West c/o Allandale Road & Maxwell Drive, Waterfall City, Midrand Tel + (27) 11 207 2060 Fax + (27) 86 674 6121 E-mail : [email protected]Directors: S Pillay (Managing Director); N Rajasakran (Director); Dr AM Van Niekerk (Director) DOCUMENT CONTROL SHEET Project Title: Conceptual Engineering Design Report for the Continuous Ashing at Kendal Power Station Project No: 12810 Document Ref. No: 12810-REP-ENG-001-Conceptual Design Report DOCUMENT APPROVAL ACTION FUNCTION NAME DATE SIGNATURE Prepared Project Engineer N Rajasakran 25-08-2014 Reviewed Project Reviewer D Jansen van Rensburg 25-08-2014 Approved Project Director S Pillay 25-08-2014 RECORD OF REVISIONS Date Revision Author Comments 05-05-2014 0 N Rajasakran Concept Design Report Issued for Comments. 25-08-2014 1 J Heera Updated Concept Design Report Issued for Comments.
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Zitholele Consulting
Reg. No. 2000/000392/07 PO Box 6002 Halfway House 1685, South Africa Building 1, Maxwell Office Park, Magwa Crescent West c/o Allandale Road & Maxwell Drive, Waterfall City, Midrand Tel + (27) 11 207 2060 Fax + (27) 86 674 6121 E-mail : [email protected]
Directors: S Pillay (Managing Director); N Rajasakran (Director); Dr AM Van Niekerk (Director)
DOCUMENT CONTROL SHEET
Project Title: Conceptual Engineering Design Report for the Continuous Ashing at Kendal Power Station
05-05-2014 0 N Rajasakran Concept Design Report Issued for Comments.
25-08-2014 1 J Heera Updated Concept Design Report Issued for Comments.
Zitholele Consulting
Reg. No. 2000/000392/07 PO Box 6002 Halfway House 1685, South Africa Building 1, Maxwell Office Park, Magwa Crescent West c/o Allandale Road & Maxwell Drive, Waterfall City, Midrand Tel + (27) 11 207 2060 Fax + (27) 86 674 6121 E-mail : [email protected]
Directors: S Pillay (Managing Director); N Rajasakran (Director); Dr AM Van Niekerk (Director)
REPORT ON
CONTINUOUS ASHING AT KENDAL POWER STATION
CONCEPTUAL ENGINEERING REPORT
Report No : 12810
Submitted to:
Eskom Holdings SOC Ltd
P O Box 1091 Johannesburg
2000
DISTRIBUTION:
2 Copies - Eskom Holdings SOC Ltd
1 Copy - Zitholele Consulting (Pty) Ltd – Library
05 September 2014 12810
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EXECUTIVE SUMMARY
Kendal power station was commissioned in the mid 1980’s, with a 40 year operating life. The initial
dry ash dump site was designed to have a capacity for the operating life with an eight year
contingency period. The life of the power station has since been upgraded to 60 years and with
some other contributing factors, such as the dry density and the load factor, the initial dry ash
dump is now under capacity. The power station is therefore expected to be decommissioned at
the end of 2053. This means that area to accommodate the additional ash generated during this
operational period will need to be extended. The area to the north of the existing facility will need
to be optimised in order to receive this ash. The Conceptual Engineering designs indicate that ash
may be accommodated here until early 2030 and thereafter an alternative site will need to be
licenced to receive ash up to the end of 2053. The supplementary site up to the end of 2053 is a
separate submission for Environmental Authorisation and is not addressed within this report.
Eskom has appointed Zitholele Consulting (Pty) Ltd (Zitholele) to start with the environmental
impact assessment (EIA) to extend the existing Kendal dry ash dump into the northerly direction.
Zitholele are also responsible for the conceptual engineering design for the options identified and
to recommend a preferred option. These engineering designs will be used to underpin and inform
the EIA.
The Conceptual Engineering Report discusses the following:
Continuation of the Ash Disposal Facility (ADF) with the extended footprint being lined in
compliance with DEA’s Norms and Standards as promulgated on 23 August 2013;
Design of pollution control dams and stormwater management infrastructure in compliance with
GN704
Diversion of a natural stream to accommodate the extended footprint of the ADF
Remedial works to an existing dam within Eskom’s property boundary but not part of their
water management system which addresses the mixing of flow from the final voids of the
adjacent mining operations.
Several options were considered for determining the go-forward option on the ADF and stormwater
management philosophy. The “piggyback” options described in both the determination of air space
on the ADF and stormwater management philosophy is not deemed feasible currently from a
mechanical perspective. This option was therefore not considered for implementation in this
report. Further investigation is currently underway here and will be reported on a separate project
that has been commissioned by Kendal Power Station.
Trade-off studies were conducted in order to determine the optimised go-forward scheme for the
continuation of the current Kendal Ash Dump and stormwater infrastructure. Environmental,
Technical and Financial aspects are considered in the trade-of studies when making the decision
on which option to proceed with to the next stage of design. Six options for the ADF layout and
nineteen options for the stormwater management philosophy were assessed. This was supported
by:
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Conceptual airspace modelling of each alternative;
A daily time-step water balance model over a 50 year period for run-off determination and PCD
sizing, as well as sizing of clean and dirty water conveyances. From the trade-off study,
Scenario 2 option 3B was identified at the preferred option
Conceptual development of dump infrastructure including surface drainage structures,
hydraulic structures, leachate and sub-surface drainage, lining and cover materials;
Cost estimation;
Trade-off study workshop.
From the trade-off study held the preferred option is identified as Option 2A, which is to divert the northern stream to get the maximum dump volume on the current site;
Option 2A provides a ashing capacity of 98 Mm3 at a density of 0.85t/m3
This provides a remaining life of 15 years from January 2015 until December 2029;
The preferred option as selected through the trade-off study analysis is based on a preliminary design. All fundamental aspects were addressed to inform the EIA study.
It is recommended to progress with the preferred option through a feasibility study and detailed design phase.
The requirements that are raised as issues and concerns through the EIA and stakeholder engagement process should be addressed during the feasibility and detailed design phase and the impacts should be mitigated through engineering solutions.
Based on the trade-off study work, it can be stated that the proposed facility extension is a feasible solution.
Following the workshops, it was proposed that Option 2A, for the ADF layout of diverting the
northern stream with no Piggy Backing, and Scenario 2 Option 3B, for the stormwater
management philosophy, be taken to the next stage of design. Scenario 2 being the scenario with
an optimum open ash area of 82 hectares, and Option 3B being the option where Dam 1 & 5 are
considered dirty, Dam 2, 3 and 4 are considered clean, dust suppression is done from Dam 5 to open ash
areas, irrigation is done from Dam 4 to rehabbed areas and irrigation is done from the existing Clean Water
Dam to the power station terrace. A Waste Management Licence (WML) and Water Use Licence
(WUL) need to be applied for the infrastructure and activities as described in the aforementioned
3.1.1 Option 1A: Minimum volume .............................................................. 6 3.1.2 Option 1B & 1C: Minimum volume with staged (1B) or concurrent
piggybacking (1C) .............................................................................. 7 3.1.3 Option 2A: Maximum volume ............................................................. 8 3.1.4 Option 2B & 2C: Maximum volume with staged (2B) or concurrent
piggybacking (2C) .............................................................................. 9 3.2 Trade-off study for ADF Preferred Option ...................................................... 10 3.3 Approach to the trade-off study ...................................................................... 10 3.4 Trade-off study .............................................................................................. 12 3.5 Results of the trade-off study ......................................................................... 13
5.2.1 Power Station Terrace ..................................................................... 22 5.2.1.1 Impacted Areas ................................................................................ 23 5.2.1.2 Clean Areas ..................................................................................... 23 5.2.2 Cross over plant ............................................................................... 24 5.2.3 Silt Traps .......................................................................................... 24 5.2.4 Dirty Water Dam............................................................................... 24 5.2.5 Emergency Dirty Water Dam ............................................................ 25 5.2.6 Clean Water Diversion Berm and Channel ....................................... 26 5.2.7 Clean Water Dam............................................................................. 26 5.2.8 Coal Stockyard Attenuation Basin .................................................... 26 5.2.9 E-dump Stormwater Management .................................................... 27 5.2.10 Pumps and Pipelines ....................................................................... 27
5.3 OBJECTIVES OF PROPOSED STORMWATER MANAGEMENT SYSTEM . 27
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5.4 MODELING APPROACH AND ASSUMPTIONS ............................................ 28 5.4.1 Rainfall data ..................................................................................... 28 5.4.2 Operating flows ................................................................................ 29 5.4.3 Description of catchment areas at ADF ............................................ 29 5.4.3.1 Open Ash Areas ............................................................................... 29 5.4.3.2 Rehabilitated Areas .......................................................................... 30 5.4.3.3 Transfer House F and Surrounding Areas ........................................ 30 5.4.3.4 Ash conveyor from E-dump .............................................................. 31 5.4.4 Stormwater runoff calculations ......................................................... 32
5.11.1 Sizing of “Clean” stormwater diversion drains and berms ................. 58 5.11.2 Sizing of “Dirty” stormwater conveyance drains ................................ 58
5.12 WATER BALANCE ........................................................................................ 60 5.13 OPERATIONAL REQUIREMENTS ................................................................ 60
5.13.1 Monitoring of Quality in Clean Water Dams ...................................... 61 5.13.2 Maintaining Open Ash Areas for Dust Suppression .......................... 61 5.13.3 Maintaining Silt Traps ....................................................................... 61
5.14 WAY FORWARD FOR STORMWATER MANAGEMENT .............................. 61 6 STREAM DIVERSION ...................................................................................... 62 7 PHASED APPROACH TO ADF ....................................................................... 63
8 FARM DAM SYSTEM ....................................................................................... 64
9 OPERATION AND MAINTENANCE PLAN ...................................................... 66 9.1 Code requirements in terms of SABS 0286 .................................................... 66
10 DISCUSSION .................................................................................................... 67 11 RECOMMENDATION AND CONCLUSION ..................................................... 68
Table 22: Results of Hydrology Calculations .................................................................... 50
Table 23: Output from the Three Dams Water Course Pre-Development Model .............. 51
Table 24: Output from the Farm Dam Pre-Development Model ........................................ 52
Table 25: "Dirty" Water Channels Concept Design Parameters ....................................... 60
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LIST OF APPENDICES
Appendix A: Geotechnical Report
Appendix B: Waste Classification Report
Appendix C: Conceptual Engineering Drawings
Appendix D: Water Balance Diagram
Appendix E: Trade-off study matrix
Appendix F: Tech Memo Selection for barrier System
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1 INTRODUCTION
Kendal power station was commissioned in the mid 1980’s, with a 40 year operating life. The initial
dry ash dump site was designed to have a capacity for the operating life with an eight year
contingency period. The life of the power station has since been upgraded to 60 years and with
some other contributing factors, such as the dry density and the load factor, the initial dry ash
dump is now under capacity. The power station is therefore expected to be decommissioned at
the end of 2053. This means that area to accommodate the additional ash generated during this
operational period will need to be extended. The area to the north of the existing facility will need
to be optimised in order to receive this ash. The Conceptual Engineering designs indicate that ash
may be accommodated here until early 2030 and thereafter an alternative site will need to be
licenced to receive ash up to the end of 2053. The supplementary site up to the end of 2053 is a
separate submission for Environmental Authorisation and is not addressed within this report.
Eskom has appointed Zitholele Consulting (Pty) Ltd (Zitholele) to start with the environmental
impact assessment (EIA) to extend the existing Kendal dry ash dump into the northerly direction.
Zitholele are also responsible for the conceptual engineering design for the options identified and
to recommend a preferred option. These engineering designs will be used to underpin and inform
the EIA.
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2 BASIS OF DESIGN
2.1 Assumptions and limitations
The following assumptions were made in developing the conceptual designs presented herewith;
The requirements for the clean and dirty water systems stipulated in Regulation 704 and
Regulation 1560 of the National Water Act, 1998 will be adhered too;
The Pollution Control Dam (PCD) will be a Stormwater Dam; classified in terms of the National
Water Act, 1998 and will operate empty;
The life of the Power station was taken up to the year 2053 excluding final rehabilitation and
closure.
Ashing on an extended lined facility will start in July 2017; this timeline provides for
Authorisation, Further design, tender and construction project steps will be required to
commission the extended facility.
Conveyor system will move radially around existing pivot point and can extend to the required
dimension
The existing area of the ash facility that has been rehabilitated is seen as a final rehabilitation
and no other additional rehabilitation is needed except in the areas where the piggyback
options may be placed.
The Volume of ash per annum stays constant through the whole life of power station.
All volumes, remaining life and timelines are subject to the dump models that are produced.
2.2 Ash characteristics
2.2.1 Grading and Specific gravity
The fly ash varies from silty sand to silty clay using a triangular soil classification chart (US corps of
Engineers). The grading curve exhibits a uniform particle size distribution. According to ASTM
D422-63:
Clay sized particle is larger than 1 micrometre and smaller than 5 micrometre.
Silt sized particle is larger than 5 micrometre and smaller than 75 micrometre
Sand sized particle is larger than 75 micrometre and 425 micrometre
Thus using the above mentioned envelopes the grading of ash are as follows:
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Table 1: Ash grading
Particle size Weathered fly ash Median Fly ash
Clay-sized (%) 5-77 16 14
Silt sized (%) 23-83 60 59
Sand sized (%) 0-64 30 27
D50 (µm) 3-120 23 27.5
Specific Gravity (Gs) 2-2.2 2.2
Data used from : J.S. Mahlaba et al. Fuel 90 (2011)
2.2.2 Stability
The Stability of the final ash dump was not investigated in this phase as the already rehabilitated
slopes are at 1V:5H which has been assessed through an observational approach as stable
slopes. There is however a concern for the advancing ash face which is at angle of repose of
1V:1.2H. On the site visit held on 9 April 2013, some cracks have already formed close to the edge
of the ash disposal facility.
There are measures to mitigate the stability concerns of the ash face on the base liner; a textured
geomembrane can be used, this will increase the interface friction angle between the critical friction
interfaces. Other measures such as terracing the natural ground can also be considered. Further
investigations during the detailed design will have to be done to confirm the stability of the
advancing face when it is placed on the liner.
2.2.3 Permeability
The permeability is largely dependent on the density of the ash on the facility. A value of
11.5 m/year for medium dense ash was assumed. This is the mean of 3 m/y (dense ash) to
20 m/year (loose ash) (Brackley et al, 1987) (6.34*10-7 m/sec). This is required for calculating
seepage pool to the leachate collection system.
2.3 Annual tonnages
The indicated annual tonnage of ash placed on the ash disposal facility is 5500 kt/ annum. This
information is from a previous report (Report nr: 11613601-10981-2) in which Golder Associates
Africa did the development of an industry waste management plan for Eskom where all the waste
type and quantities of all the power stations were considered.
The density of the ash is 850kg/m3 and thus the annual airspace required for the continuous
ashing facility is approximately 6.5 Mm3/ annum. Based on this the remaining life for the continuous
ashing site is determined.
Refer to Eskom document number: 240-71273834 for coal quality used and Eskom Consistent
Data Set (CDS) – 36-623 for remaining life and coal burn plan utilised for coal consumption values.
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2.4 Capacity requirements
The existing ash disposal facility was commissioned in the 1980’s for a 40 year life span and an 8
year contingency period. The operating life of the ash disposal facility has since then been
increased to 60 years and with a number of other design and material changes the existing dump
geometry is grossly under capacity.
The total additional capacity required for the ash disposal facility is 291 Mm3 from January 2015
until December 2058. With the current boundary and operating machinery limitations this capacity
will not be reached on the current site. The remaining area between the western and northern
streams does not have sufficient capacity to allow a new facility to be established. If the northern
stream is diverted, the continuous ash dump will only provide 98 Mm3 capacity, requiring a new
“30”year ADF of 193 Mm3. There are current investigations to identify a suitable site for the
remaining ash to be deposited. The size and commissioning date of this new site is dependent on
the Continuous ADF site capacity.
2.5 Dust suppression
The current approach is to use water from the three dam system (Dirty Water Dam, Emergency
Dirty Water Dam and Clean Water Dam) that is located on the Eastern side of the site and then via
irrigation, spray the exposed ash areas to minimise the mobilisation of the ash. Key operational
staff members in Eskom, are of the opinion that the current system is not fulfilling its intended
purpose and that the system will have to be modified so that the ash mobilisation is minimal.
There are a number of techniques and products that can be used such as:
Using a dust suppression chemical
Using a self-propelled spraying system; using pressure to propel itself forward
Upgrading the current system with better controls in place
50mm subsoil cover
The above mentioned options will have to be investigated further in the following phase of design
as there might be a lot of other innovative approaches that can be followed.
2.6 Stormwater Management
The management philosophy for the routing and capturing of stormwater is summarised as follows:
The separation of the runoff draining south-easterly towards the extended ash disposal facility
(i.e. from the area upslope of the ash dump) and runoff generated from within the footprint of
the extended ash dump;
The diversion of “clean” surface runoff generated from the upslope contributing catchments
away from the extended ash disposal facility, thereby isolating the ash dam as “dirty areas” in
accordance with the requirements GN 704 in terms of the National Water Act, 1998;
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Containment of all “dirty” surface runoff generated from within the “dirty” catchment,
conveyance and discharge into a dedicated pollution control dam sized in accordance with the
requirements GN 704 in terms of the National Water Act, 1998.
The current and proposed stormwater management philosophy is discussed in depth in Section 1
of this report.
2.7 Stream Diversion
The current extent of the ash dump is bordered by one perennial stream to the east and one non-
perennial stream to the west. The stream to the East flows in a north-westerly direction whilst the
stream to the West flows northerly. The two (2) streams converge north of the existing ash dump.
Flood management philosophy is as follows;
Diversion of the stream forming the eastern border of the ash dump in a northerly direction.
The diversion channel will be sized to match the discharge capacity of the existing clean water
dam spillway upstream of the culvert system across the district road adjoining the R555 and R50
national roads, and the stormwater runoff to the east below the culverts. The stream diversion
design will be carried out in accordance with the provision of the water use licence.
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3 ADF OPTIONS MODELLED
The Kendal Power Station Continuous Ashing project had to ensure the continued operations of
the Power Station when considering the options for the extended ashing facility. Any option that
involved the temporary stoppage of production was considered fatally flawed. The advancing face
of the current ashing operations is in a northerly direction and options were considered for this area
only as it did not involve major configuration changes of the conveyors and stackers. The vacant
area to the south of the existing ADF between the rehabilitated area and the National Road R545
was considered as being fatally flawed with respect to aforementioned conditions and was not
considered further as a feasible option.
Two broad options, and the respective sub-options, were considered in determining the air space
required for the extended facility. The broad options are as follows:
Option 1 Minimum Dump – The ADF is positioned between the two streams as previously
described.
Option 2 Maximum Dump – The positioning of the ADF requires the northern stream to be
diverted.
The sub-options for the above includes for a piggybacking option which may be done concurrently
with the current operations or done once the existing footprint is exhausted at the prevailing levels.
3.1 Description of options
3.1.1 Option 1A: Minimum volume
The minimum volume option stays within the original footprint area and is lined from the set
timeline of early 2017 as shown in the figure below. Physical parameters of the options are:
Total Footprint Area: 480 ha
Remaining dump volume: 32.5 Mm3
Remaining life: 5 years from January 2015
Maximum height: 60 m
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Lined area: 114 ha
Figure 1: Option 1A
3.1.2 Option 1B & 1C: Minimum volume with staged (1B) or concurrent piggybacking
(1C)
These options are have the same footprint and piggyback area, the definitive difference is the way
in which they are constructed and the specific areas that were lined. The physical parameters of
these options are as follows:
Option 1B & 1C
Total Footprint Area: 480 ha
Remaining dump volume: 47 Mm3
Remaining life: 7 years from January 2015
Maximum height: 80 m
Only Option 1B
Lined area: 275 ha
The method of construction for this option is the footprint will first be totally completed before the
piggyback can be constructed the piggyback area will also be lined in this option.
Only Option 1C
Lined area: 114 ha
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The method of construction for this option is the footprint area and the piggyback area will be
constructed concurrently by moving the conveyors back into the required position and not lining the
piggyback area.
Figure 2: Option 1B & 1C
3.1.3 Option 2A: Maximum volume
The maximum volume option falls outside the existing footprint and entails that the north eastern
stream be diverted up against the slope. The physical parameters are:
Total Footprint Area: 583 ha
Remaining dump volume: 98 Mm3 from January 2015
Remaining life: 15 years from January 2015
Maximum height: 60 m
Lined area:224 ha
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Figure 3: Option 2A
3.1.4 Option 2B & 2C: Maximum volume with staged (2B) or concurrent piggybacking
(2C)
These options again have the same footprint and piggyback area as well as the stream
diversion.The most significant difference is the way in which they are constructed and the specific
areas that were lined. The physical parameters of these options are as follows:
Option 2B & 2C
Total Footprint Area: 583 ha
Remaining dump volume: 114 Mm3
Remaining life: 18 years from January 2015
Maximum height: 80 m
Only Option 2B
Lined area: 385 ha
The method of construction for this option is the footprint will first be totally completed before the
piggyback can be constructed the piggyback area will also be lined in this option.
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Only Option 2C
Lined area: 224 ha
The method of construction for this option is the footprint area and the piggyback area will be
constructed concurrently by moving the conveyors back into the required position and not lining the
piggyback area.
Figure 4: Option 2B & 2C
3.2 Trade-off study for ADF Preferred Option
The objective of the trade-off study is to select a preferred option from those considered (refer 3.1),
with which to go forward to subsequent development stages of the project. Selection of an
alternative does not render it inflexible to improvement opportunities, but instead provides a broad
engineering framework for the development of the ash disposal facility.
3.3 Approach to the trade-off study
Six possible alternatives for the deposition of ash were conceptualised for consideration in the
trade-off study (refer 3.1).
Three broad criteria were selected for analysis of the options, namely:
Environmental influences;
Engineering aspects;
Financial considerations.
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Each of these criteria was given a weighting in terms of perceived importance or influence on the
project. Each criterion was also subdivided into sub-categories which were deemed to be relevant
to the project. The table below shows the overall criteria and weighting matrix used for evaluating
and comparing the options:
Table 2: Weightings for the options analysis workshop
ENGINEERING ENVIRONMENTAL
CONSIDERATIONS
FINANCIAL
50% 30.0% 20%
Airspace won 15.0% Level of impact of the
footprint size
15.0% Lowest Cost in terms of R/m3
70%
Does the airspace
model provide
sufficient capacity to
reach the required
timeline of 2020
1.0% Impact on the 30 year
scheme
15.0% Least Total Capital Spent
30%
What is the
complexity of the
operations for the
Spreader and Stacker
20.0% Significance of
encroachment on
current land uses and
natural habitat (Zone
of Influence)
15.0%
What is the
complexity of the
phase construction
5.0% Influence of proximity
to water course
20.0%
Complexity of
disposal facility
geometry
12.0% Complexity of
disposal facility
geometry for closure
5.0%
What is the size and
complexity of the
leachate collection
system
5.0% Level of impact that
the proposed option
has on the ground
water system
20.0%
What is the
complexity of the
Stormwater
management system
on the dump
7.0% Visual impact
assessment of post
closure landform
5.0%
What is the
complexity of the
proposed Stormwater
management system
around the dump
5.0% Impact of exposed
ash body on air
quality
5.0%
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What is the impact on
the proposed height
10.0%
What is the impact of
the required capping
system
10.0%
Impact on slope
erosion and resulting
sediment transport
10.0%
A trade-off study workshop was conducted on 9 May 2013 with the following representatives
present:
Table 3: Trade-off Study Workshop Delegates and Designations
ESKOM ZITHOLELE GOLDER
ASSOCIATES
NUT AIRSHED
Andre Kreuiter N Rajasakran D Hattingh Mina Cilliers Dr. Terri Bird
Tobile Bokee Virginia
Ramakuwela
C McLuckie
Eddie Setei Mathys Vosloo Dieli Mesoabi
Warren Kok Gerhard Coetzer
Jan-Dirk Brak Johan Jordaan
Andre Zinn
Francois Marias
Eddie van Wyk
Danie Brink
Warren Aken
3.4 Trade-off study
All delegates present participated in critically discussing each criteria and subcategory. The result
is that the ranking and rating matrix was modified and agreed to by the meeting.
Following the above process, each cell in the matrix was populated by robust debate between all
representatives and disciplines present. Financial criteria were not discussed at the trade-off
workshop because the workshop was seen as a qualitative workshop in where only the technical
feasibility and the environmental impacts were evaluated.
Financial comparison was conducted by analysing capital and closure costs associated with each
option and calculating the cost-benefit in terms of a rate – Rands paid per m3 airspace won or
R/m3. The total costs and the cost benefit rates were shown in the trade-off matrix and rated
accordingly. Costs were determined by measurements from the CADD models and using rates
obtained from previous work done on similar projects.
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3.5 Results of the trade-off study
The trade-off study matrix is shown in Appendix E. The results are shown in the table below:
Table 4: Results from the Trade-off matrix
OPTION DESCRIPTION ENVIRONMENTAL ENGINEERING FINANCIAL TOTAL
Weighting
30.0% 50% 20% 100.0%
Option 1A Minimum dump 3.6 Fatally Flawed 0 FF
Option 1B Minimum dump and lined piggybacking (staged piggybacking)
3.25 2.23 1.3 2.35
Option 1C Minimum dump no lined piggyback concurrent piggybacking
2.9 2.61 4.3 3.04
Option 2A Maximum dump 1.75 3.85 4 3.25
Option 2B Maximum dump and lined piggybacking (staged piggybacking)
1.65 2.23 1.7 1.95
Option 2C Maximum dump no lined piggyback concurrent piggybacking
1.5 2.71 4.7 2.75
Option 1A is the preferred option for the environmental influences criteria; this result is a defensible
outcome, as this option has the smallest influence on the surrounding environment. Option 1A is
deemed fatally flawed in the engineering aspects criterion as it does not meet the timeline required.
Option 2A is the preferred option from an engineering perspective. The financial consideration was
based on the principal of the best return on investment; option 2C is the preferred option in this
case.
Taking all these considerations into account and looking at the criteria as a whole, Option 2A is the
preferred option and is recommended to be taken forward to the next phase of design.
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4 ADF CONCEPTUAL DESIGN
4.1 Ash disposal
The ash is deposited onto the ash disposal facility by means of a conveyor system. The transfer
conveyors moves the ash from the power station to transfer house E, the emergency ash dump (E-
dump) is located just to the north of the transfer house and was initially designed to provide a
capacity of five days of ashing for emergencies such as maintenance to the overland conveyors
etc.
From transfer house E the ash is transported via the overland conveyors which cross a provincial
road and the north eastern stream to transfer house F. The extendable conveyors transfer the ash
from transfer house F to the shiftable conveyors. The extendable conveyors were initially designed
to extend in the direction of its current bearing as soon as the shiftable conveyors are
perpendicular to the extendable conveyors this method of deposition is called parallel shifting, but
this deposition strategy cannot be implemented due to the new boundary extents of the existing
area.
The shiftable conveyors are the stacker shiftable conveyor (Primary system) and the spreader
shiftable conveyor (Standby system). This is used to deposit the ash onto the ash disposal facility.
The current deposition strategy is to place ash only via radial shifting. In the figure below the layout
and naming for the conveyor system is shown.
Figure 5: Layout of conveyor system used to deposit ash
There are some limitations to these shiftable conveyor systems as the ash is only placed radially.
Some of the limitations are:
The maximum gradient the system can traverse is 1V:20H
As the conveyor cannot bend the advancing face as well as the final face position cannot have any kinks or bends as this meant that the conveyor had a bend in place
The maximum height the of the spreader system is approximately 45m and 62m for the stacker system
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The spreader system can only place a front stack where the stacker system can place a front stack and back stack.
Shift intervals needs to be kept to a minimum, between 4-6 months per shift.
The estimated time it would take to get all the authorisation in place as well as providing detailed
designs, tender and construction would take up to September 2015, During this time the ash facility
still needs to continually ash and thus there is still a certain amount of ashing that will take place
while the authorisation is being considered on virgin ground which would then be impractical to line
the area. Refer to drawings 13615205-100-D-D100 and D102 for Options 1A,B and C and
13615205-100-D-D101 and D103 for options 2A,B and C for the section that will be lined.
The deposition strategies are explained in the sections below, the same philosophy was followed
for the various maximum and minimum options numbered A, B and C thus where the deposition
strategy explanations were combined
4.1.1 Option 1A &2A: Minimum & Maximum volume deposition strategy
The Minimum & Maximum volume options will be constructed in the same manner as currently
used. The ash is deposited radially while the stacker and spreader systems are pivoting around its
own fixed point on the extendable conveyors. The split that is designed for is 80:20 split which
means the stacker system is depositing 80% of the ash and the spreader only 20%.
This is currently not the case as the split is closer to 60:40 resulting in the front spreader system
running away from the stacker system. This is a major problem as this means that the backup
system will run its route a lot faster than anticipated with the result that the backup system will
become redundant because there is no designated area left to ash. This would have to be taken
Step 3: The adjusted C-values for Dam 1 is increased by 3.5% to take leachate into account. Only
half the facility will be line as this meets the requirements of the prevailing legislation therefore an
amount of 3.5 percent was utilised opposed to a norm of 7 percent.
Step 4: Rainfall, catchment area and C-values are multiplied to obtain a volume of runoff for each
area and dam.
5.5 OPTIONS MODELLED
Nineteen (19) Options under three (3) scenarios were modelled. The table below describes each
of the options.
Table 10: Options modelled
OPTIONS MODELLED
SCENARIO 1: Minimum open ash working area = 63 hectares
SCENARIO 2: Optimum open ash working area = 82 hectares
SCENARIO 3: Piggyback open ash working area = 98 hectares
OPTION 1 OPTION 2 OPTION 3 OPTION 4
• Status Quo
• Proposed system – Five (5) new dams.
• Proposed system – Five (5) new dams.
• Proposed system – Five (5) new dams.
• All dams considered to be dirty. Spills from Dams 2, 3, 4 & 5 over flow to Dam 1.
• Dam 1 & 5 considered dirty. Dam 2, 3 and 4 considered clean.
• Dam 1 & 5 considered dirty. Dam 2, 3 and 4 considered clean.
• Dust suppression from • Dust suppression from • Dust suppression from
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Dam 1, DWD & EDWD. Dam 1, DWD & EDWD. Dam 1, DWD & EDWD.
• Spills from e-dump overflow to DWD.
• Spills from e-dump overflow to DWD.
• Spills from e-dump overflow to DWD.
• Upper catchment not bypassed. (Total catchment area = 330 hectares)
• Spills from Dam 5 over flow to Dam 1.
• Spills from Dam 5 over flow to Dam 1.
• No irrigation to rehabbed areas.
• Upper catchment not bypassed. (Total catchment area = 330 hectares)
• Upper catchment bypassed. (Total catchment area = 4 hectares)
• Irrigation from existing CWD to power station terrace. (20mm)
• Irrigation to rehabbed areas. (20mm)
• Irrigation to rehabbed areas. (20mm)
Dam capacities for Dams 2, 3 & 4 determined for 50 year storm event. Dam 2 = 166,000 m
3, Dam 3 =
57,000 m3, Dam 4 = 32,000
m3
Irrigation from existing CWD to power station terrace. (20mm)
Dam capacities for Dams 2, 3 & 4 sized to spill once in 50 years.
Resultant Dam 1 sized to GN704 to only spill once in 50 years.
Dam capacities for Dams 2, 3 & 4 sized to spill once in 50 years.
Resultant Dam 1 sized to GN704 to only spill once in 50 years.
Resultant Dam 1 sized to GN704 to only spill once in 50 years.
CWD becomes process dam, i.e. EDWD spills into CWD
OPTION 2a OPTION 3a OPTION 4a
EDWD spills into Dam 5, Dam 5 spills into Dam 1
EDWD spills into Dam 5, Dam 5 spills into Dam 1
CWD spills into Dam 5, Dam 5 spills into Dam 1
OPTION 2b OPTION 3b OPTION 4b
Water treatment plant at EDWD
Water treatment plant at EDWD
Water treatment plant at CWD
5.6 MODELLING RESULTS
The results of the modelling exercise are contained in Table 10 below.
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Table 11: Modelling results
OPTIONS
2A 2B 3A 3B 4A 4B
SCEN
AR
IO 1
Dam 1 57,600,000 371,500 10,623,000 312,000 22,388,500 390,000
Dam 2 166,000 166,000 282,000 282,000 282,000 282,000
Dam 3 57,000 57,000 76,000 76,000 76,000 76,000
Dam 4 32,000 32,000 32,000 32,000 32,000 32,000
Dam 5 76,000 76,000 76,000 76,000 76,000 76,000
WTP(ML/d
)ay)
0 12.25 0 2.75 0 8.75
SCEN
AR
IO 2
Dam 1 17,368,623 371,500 120,000 120,000 297,000 120,000
Dam 2 166,000 166,000 257,000 257,000 257,000 257,000
Dam 3 57,000 57,000 76,000 76,000 76,000 76,000
Dam 4 32,000 32,000 32,000 32,000 32,000 32,000
Dam 5 76,000 76,000 76,000 76,000 76,000 76,000
WTP(ML/d
)ay)
0 9.25 0 0 0 1
SCEN
AR
IO 3
Dam 1 1,150,000 371,500 111,000 111,000 200,000 111,000
Dam 2 166,000 166,000 257,000 257,000 257,000 257,000
Dam 3 57,000 57,000 76,000 76,000 76,000 76,000
Dam 4 32,000 32,000 32,000 32,000 32,000 32,000
Dam 5 76,000 76,000 76,000 76,000 76,000 76,000
WTP(ML/d
)ay)
0 8.5 0 0 0 1
5.7 TRADE-OFF ASSESSMENT
The objective of the trade-off study is to select a preferred option from those considered in Section
5.5, with which to go forward to subsequent development stages of the project. Selection of an
alternative does not render it inflexible to improvement opportunities, but instead provides a broad
engineering framework for the development of the ash disposal facility. The nineteen (19) possible
alternatives for the stormwater management system around the ADF were conceptualised for
consideration in the trade-off study. Three broad criteria were selected for analysis of the options,
namely:
Environmental and social influences;
Engineering aspects;
Financial considerations.
The following approach was adopted:
Each of the above criteria was given a weighting in terms of perceived importance or
influence on the project.
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Each criterion was also sub-divided into sub-categories which were deemed to be relevant
to the project. Table 12 below shows the overall criteria and weighting matrix used for
evaluating and comparing the options:
Table 12: Option Analysis Criteria and Weighting
Category Description Category Weight
Engineering (Overall Weight = 35%)
No and size of dams required 10.0%
What is the complexity of the proposed Stormwater management system
around the dump
7.5%
Ease of maintenance of stormwater management infrastructure 7.5%
Complexity of operational philosophy 12.5%
Experienced human resources to run facility (controlled release, water
treatment plant)
10.0%
Infrastructure requirements (more pipelines to divert spillages, erosion control
at multiple discharge points, etc.)
10.0%
Air space lost due to larger PCD requirements 12.5%
Management of excess water (dust suppression, irrigation and WTP
optimisation)
10.0%
Dam safety requirements due to higher dam walls 5.0%
Security risk to stormwater management equipment 5.0%
Construction and monitoring complexity to ensure clean and dirty water
separation
10.0%
Environmental (Overall Weight = 35%)
Encroachment on wetlands and floodlines 15.0%
Level of impact that the proposed option has on the surface water system 20.0%
Regulatory process risks 15.0%
Groundwater impacts 10.0%
Compliance with GN704 30.0%
Multiple points of discharge from dams to receiving waters 10.0%
Financial (Overall Weight = 30%)
Net Present Value (Rands) 100%
A workshop was convened with all stakeholders involved in this project. Delegates present at this
workshop participated in critically discussing each criteria and sub-category. The result is that the
ranking and rating matrix was modified and agreed to by the meeting.
Following the above process, each cell in the matrix was populated by robust debate between all
representatives and disciplines present. Financial criteria were not discussed at the trade-off
workshop because the workshop was seen as a qualitative workshop in where only the technical
feasibility and the environmental impacts were evaluated.
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Financial comparison was conducted by analysing net present value of the capital costs associated
with each option. The total costs and the cost benefit rates were shown in the trade-off matrix and
rated accordingly. Costs were determined by measurements from the CAD models and using rates
obtained from previous work done on similar projects.
The summary of the results of the rating and ranking workshop are shown in Table 13. Full details
for all criteria are given in the appendices.
Table 13: Results of Trade-off Study Workshop
OPTION DESCRIPTION ENVIRONMENTAL CONSIDERATIONS
TECHNICAL FINANCIAL
Weighting 40.0% 30% 30% Score Rank
SCENARIO 0 STATUS QUO
0.00 0.00 5.00 0.00 14
SCEN
AR
IO 1
OPTION 2A 2.95 0.00 0.10 0.00 14
OPTION 2B 2.95 1.98 0.10 1.80 13
OPTION 3A 3.85 0.00 0.10 0.00 14
OPTION 3B 3.70 2.28 0.10 2.19 10
OPTION 4A 3.90 0.00 0.10 0.00 14
OPTION 4B 3.60 2.38 0.10 2.18 11
SCEN
AR
IO 2
OPTION 2A 3.10 0.00 0.10 0.00 14
OPTION 2B 2.95 2.63 0.10 2.00 12
OPTION 3A 4.30 3.53 4.93 4.26 4
OPTION 3B 4.30 3.53 4.98 4.27 3
OPTION 4A 4.20 2.80 4.35 3.83 6
OPTION 4B 3.90 2.88 0.10 2.45 9
SCEN
AR
IO 3
OPTION 2A 4.20 0.00 1.92 0.00 14
OPTION 2B 4.20 2.63 0.10 2.50 8
OPTION 3A 4.60 3.33 4.95 4.32 2
OPTION 3B 4.60 3.33 5.00 4.34 1
OPTION 4A 4.30 3.25 4.83 4.14 5
OPTION 4B 4.30 3.08 0.10 2.67 7
Option 3B, Scenario 3 is the preferred option following the technical, environmental and financial
scoring. However, this Scenario assumes that piggybacking is feasible. This has not been proven
as yet and cannot be considered at this stage. If proven feasible in future, the proposed
infrastructure will need to be sized adequately to accommodate the potential flows during this
Scenario. Since Scenario 3 is the best case scenario, it is proven that the infrastructure
implemented under the other Scenarios will accommodate flow generated under Scenario 3.
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Taking all these considerations into account and looking at the criteria as a whole, Option 3B
Scenario 2 is the preferred option and is recommended to be taken forward to the next phase of
design.
5.8 PROPOSED INFRASTRUCTURE
Option 3B Scenario 2 is the preferred option following the Trade-off Assessment and will be taken
forward to the Conceptual Design Phase. The proposed infrastructure is detailed below. Layout
drawings of the proposed infrastructure for the options modelled as well as the preferred option are
attached to the appendices.
5.8.1 Pollution Control Dams
The two (2) existing pollution control dams, dirty water dam and emergency dirty water dam,
remain in place. Additional Dams 1 and 5 are proposed. The capacity of Dams 1 and 5 is 120 Mℓ
and 76 Mℓ (+ 2 days storage for dust suppression water) respectively. The performance (dam
levels) of these four pollution control dams for the fifty (50) year simulation period is shown on the
respective graphs below.
Figure 22: Dirty Water Dam Levels for Simulation Period
0%
20%
40%
60%
80%
100%
120%
16
-May
-63
16
-May
-66
16
-May
-69
16
-May
-72
16
-May
-75
16
-May
-78
16
-May
-81
16
-May
-84
16
-May
-87
16
-May
-90
16
-May
-93
16
-May
-96
16
-May
-99
16
-May
-02
16
-May
-05
16
-May
-08
16
-May
-11
Dam Level Dam Max Capacity
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0%
20%
40%
60%
80%
100%
120%
Dam Level Dam Max Capacity
Figure 23: Emergency Dirty Water Dam Levels for Simulation Period
Figure 24: Proposed Dam 1 Levels for Simulation Period
0%
20%
40%
60%
80%
100%
120%
Dam Level Dam Max Capacity
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Figure 25: Proposed Dam 5 Levels for Simulation Period
5.8.2 Clean Water Dams
In addition to the existing Clean Water Dam, three (3) more dams are proposed for clean water
containment. These dams will be operated on a controlled release principle which is based on the
receiving water quality. It is not the intention to impound clean water if not required provided that
the discharge quality is acceptable. If the water in these dams are deemed impacted, it will be
irrigated onto the areas that it emanated from or utilised in the power station water balance if
possible.
The following are the proposed Clean Water Dams:
Dam 2 257 Mℓ
Dam 3 76 Mℓ
Dam 4 32 Mℓ + two day storage for irrigation water
The performance (dam levels) of these four (including the existing Clean Water Dam) clean water
dams for the fifty (50) year simulation period is shown on the respective graphs below
0%
20%
40%
60%
80%
100%
120%
Dam Level Dam Max Capacity
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0%
20%
40%
60%
80%
100%
120%
16
-May
-63
16
-May
-66
16
-May
-69
16
-May
-72
16
-May
-75
16
-May
-78
16
-May
-81
16
-May
-84
16
-May
-87
16
-May
-90
16
-May
-93
16
-May
-96
16
-May
-99
16
-May
-02
16
-May
-05
16
-May
-08
16
-May
-11
Dam Level Dam Max Capacity
Figure 26: Existing Clean Water Dam Levels for Simulation Period
Figure 27: Proposed Dam 2 Levels for Simulation Period
0%
20%
40%
60%
80%
100%
120%
16
-May
-63
16
-May
-66
16
-May
-69
16
-May
-72
16
-May
-75
16
-May
-78
16
-May
-81
16
-May
-84
16
-May
-87
16
-May
-90
16
-May
-93
16
-May
-96
16
-May
-99
16
-May
-02
16
-May
-05
16
-May
-08
16
-May
-11
Dam Level Dam Max Capacity
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Figure 28: Proposed Dam 3 Levels for Simulation Period
Figure 29: Proposed Dam 4 Levels for Simulation Period
0%
20%
40%
60%
80%
100%
120%
Dam Level Dam Max Capacity
0%
20%
40%
60%
80%
100%
120%
Dam Level Dam Max Capacity
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5.8.3 Toe Paddocks
The slope available to facilitate self-cleansing velocities in the western dirty water channel
discharging to the pollution control Dam 1 could not be achieved. In order to prevent siltation within
the channels and reduce the required velocities, it is proposed that paddocks be constructed at the
toe of the advancing face to intercept run-off from the disposal facility and allow this to overflow to
the discharge channels. The temporary structures will facilitate siltation. It is envisaged that the
paddocks will be constructed from ash and will be located on top of the lined portion of the facility.
The paddocks will be covered over when dozing the side slope down to the final 1:5 slope for
rehabilitation of that section of the facility.
Figure 30: Positioning of Temporary Toe Paddocks
5.8.4 Storage Reservoirs
Apart from capturing runoff from its respective areas, dust suppression and irrigation water will be
stored in Dam 5 and Dam 4 respectively. It is proposed that additional two days storage be
allowed for in the capacity of these dams.
5.8.5 Conveyance infrastructure (pumps, pipelines and channels)
The proposed operational philosophy around stormwater management will involve the construction
of new infrastructure. Apart from the dams as mentioned in the previous sections of this report,
conveyance infrastructure will be required for the following reasons:
Conveyance of spills from one facility to the next;
Conveyance of dust suppression water from the relevant dams to the dedicated storage
reservoirs;
Conveyance of rehabilitated irrigation water from the relevant dams to the dedicated storage
reservoirs;
Dust suppression from storage reservoir to open ash area of the ADF;
Irrigation from storage reservoir to the rehabilitated area of the ADF;
Irrigation of the power station terrace grassed areas from the Clean Water Dam
CLEAN WATER CUT-OFF BERM
ACCESS ROAD
CHANNEL TO PCD
TEMPORARY TOE PADDOCK
CONSTRUCTED FROM ASH
ON TOP OF LINER
ADVANCING FACE OF ADF
REHABILITATED FACE OF ADF
ASH BODY
LINER UP TO DIRTY
WATER CHANNEL
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5.9 1 IN 100 YEAR FLOODLINES
5.9.1 Methodology
Delineation of Catchment Areas
The 0.5m interval contours were obtained from the topographical survey. The contours and 1 in 50
000 topographical maps were studied to divide the area into smaller sub-catchments. Water
courses, streams and rivers were identified using the maps. Two separate water courses were
identified, namely the Three Dams Water Course and the Farm Dam Water Course. The surface
area of each sub-catchment was determined as well as the ground cover and the average slopes
and distances of overland flow and average slopes of water courses. The surface area of the sub-
catchments are shown in Table 14.
Table 14: Sub-Catchment Surface Areas
Water Course Sub-catchment Surface Area (ha)
Three Dam System A 436
Three Dam System B 408
Farm Dam System C 3 617
Flood Hydrology
The Rational Method as described in the document Drainage Manual, Sixth Edition (2013) as
published by SANRAL (South African National Roads Agency Limited) was used to determine the
magnitude of the 1 in 100 year floods.
The peak flow rate is calculated with the following formula:
Where Q = peak flow (m3/s)
c = run-off coefficient (dimensionless)
i = average rainfall intensity over the catchment (mm/hour)
A = effective area of catchment (km2)
3.6 = conversion factor
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Rainfall data was obtained for the Cologne rainfall station (Number 0478008_W). The rainfall
record for this station is 97 years long. The rainfall station is located approximately 3.7km west of
Kendal Power Station. The Mean Annual Precipitation (MAP) was found to be 675 mm/year.
The Time of Concentration was determined for each sub-catchment in accordance with the
following formulas. Refer to Drainage Manual, Sixth Edition (2013):
For overland flow:
Where Tc = time of concentration (hours)
r = roughness coefficient = 0.4 for medium grass cover
L = length of the catchment from the boundary to where the flood needs to be
recorded (km)
S = slope of the catchment (m/m)
For defined water courses:
Where Tc = time of concentration (hours)
r = roughness coefficient = 0.4 for medium grass cover
L = length of the water course from the boundary to where the flood needs to be
recorded (km)
S = average slope of the catchment (m/m)
The total time of concentration is the sum of the time of concentration for overland flow plus the
time of concentration for water course flow, namely Tc1 plus Tc2. The time of concentration for
overland flow, flow in water courses and the total time of concentration is presented in Table 15.
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Table 15: Resulting Time of Concentration for the sub-catchments
Water Course Sub-catchment Calculated time
of
concentration
for overland
flow (minutes)
Calculated time
of
concentration
for flow in a
water course
(minutes)
Total Time of
Concentration
(minutes)
Three Dam
System
A 7 22 29
Three Dam
System
A and B
together
7 66 73
Farm Dam
System
C 3 173 176
The storm duration is taken as the total time of concentration. The rainfall intensity is determined
from Figure 3.8 in the Drainage Manual, Sixth Edition (2013) as published by SANRAL.
The resulting storm depths, storm durations and rainfall intensities are shown in Table 16.
Table 16: Resulting Storm Depths, Storm Durations and Rainfall Intensities
Water Course Sub-catchment Storm Depth
(mm)
Storm Duration
(hours)
Rainfall
Intensity, i
(mm/hrs)
Three Dam
System
A 75 0.49 152
Three Dam
System
A and B
together
130 1.22 106
Farm Dam
System
C 160 2.93 55
The Run-off coefficient (c) for each sub-catchment must now be calculated.
The recommended values are selected according to 3 characteristics of the catchment under
consideration, namely:-
a) Surface slope classification
b) Permeability classification
c) Vegetation classification
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The run-off coefficients with regard to surface slope classification are presented in Table 17.
Table 17: Run-off Coefficients with regard to Surface Slope Classifications
Description Run-off coefficient (cs) for rural
areas with an MAP between 600
and 900mm/year
Steep areas (slopes > 30%) 0.03
Hilly (10 to 30%) 0.08
Flat areas (3 to 10%) 0.16
Vleis and pans (slopes < 3%) 0.26
The run-off coefficients with regard to permeability are presented in Table 18.
Table 18: Run-off coefficients with regard to Permeability
Description Run-off coefficient (cp) for rural
areas with an MAP between 600
and 900mm/year
Very permeable 0.04
Permeable 0.08
Semi-impermeable 0.16
Impermeable 0.26
The run-off coefficients with regard to vegetation classification are presented in Table 19.
Table 19: Run-off coefficient with regard to Vegetation
Description Run-off coefficient (cv) for rural
areas with an MAP between 600
and 900mm/year
Thick bush and plantation 0.04
Light bush and farmlands 0.11
Grasslands 0.21
No vegetation 0.28
The overall c-value for a catchment, also known as the c1 is the sum of the c-values for the three
characteristics, as shown by the following equation:
Where c1 = Overall run-off coefficient for a catchment
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cs = Run-off coefficient with regard to slope
cp = Run-off coefficient with regard to permeability
cv = Run-off coefficient with regard to vegetation
The overall run-off coefficient c1 is multiplied by an adjustment factor with regard to the return
period, as presented in Table 20:
Table 20: Adjustment Factors for c1 with regard to Return Period:
Return Period (years) 2 5 10 20 50 100
Factor (Ft) for steep
and impermeable
catchments
0.75 0.80 0.85 0.90 0.95 1.00
Factor (Ft) for flat and
permeable
catchments
0.50 0.55 0.60 0.67 0.83 1.00
The magnitude of the 1 in 100 year flood can now be determined for exact locations within the two
water courses. Generally, the larger a catchment area contributing to the run-off at a specific point,
the larger the magnitude of the flood at that point will be.
The resulting c-values are presented in Table 21.
Table 21: Resulting c-values
Description c-value
cs 0.26
cp 0.12
cv 0.16
c1 0.54
The magnitude of the 1 in 100 year flood can now be determined for exact locations within water
courses, streams and rivers.
The results of the hydrology calculations are presented in Table 22.
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Table 22: Results of Hydrology Calculations
Water Course
Catchment or Sub-Catchment
Area (ha)
i (mm/hours)
c1-value Maximum 1 in 100 Year
Flood (m3/s)
Three Dam
System
A 436 152 0.54 99.3
Three Dam
System
A and B
together
408 106 0.54 134.4
Farm Dam
System
C 3 617 55 0.54 296.8
Hydraulic Simulation
The 0.5m interval contours were obtained from the topographical survey. The software Autodesk
Civil3D 2013 was used to extract longitudinal sections of the two water courses. Cross sections
were also extracted at intervals of approximately 250m. The length of typical cross sections was
approximately 1.2km.
The geometry was imported into the software HEC-RAS, a backwater model, which was developed
by the United States Army Corps of Engineers.
A Manning’s “n” hydraulic roughness value of 0.045 was assumed for all sections, which
corresponds to natural straight water courses.
Normal water flow depths were assumed for the upstream position and downstream position of the
river section. Normal flow depth here means the calculated water depth for a certain roughness, a
certain flow and a certain longitudinal slope. The software does the hydraulic calculations for each
cross section. The output from the software are flow depths, flow velocities, cross sectional area,
wetted perimeter etc. Four separate models were produced, namely:
Three Dam Water Course Pre-Development
Three Dam Water Course Post-Development
Farm Dam Water Course Pre-Development
Farm Dam Water Course Post-Development
The outputs from the Three Dams Water Course Pre-Development Model are presented in Table
23. An output longitudinal section of this model is presented in Figure 5-11.
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Table 23: Output from the Three Dams Water Course Pre-Development Model
River Station
(m)
Maximum Channel
Depth (m)
Velocity (m/s)
Cross Sectional
Area (m2)
Hydraulic Depth
(m)
5815.29 0.67 2.1 63.96 0.44
5692.63 0.86 0.82 164.91 0.67
5500.75 0.68 1.61 83.22 0.48
5264.58 1.19 0.93 144.28 0.72
4992.33 1.04 2.23 60.15 0.51
4714.41 1.66 1.44 93.22 1.12
4559.7 0.84 2.62 51.24 0.69
4380.86 1.99 0.61 221.22 1.61
4164.74 2.45 0.26 525.01 2.2
4084.26 2.44 0.21 632.53 2.29
4002.2 2.62 4.31 31.19 1.87
3916.36 4.82 0.14 948.05 3.57
3851.62 1.42 2.66 50.47 0.71
3775.49 3.14 0.24 561.42 2.48
3699.91 1.63 2.99 44.95 0.91
3628.32 6.86 0.25 540.42 3.19
3442.92 2.26 3.21 41.9 1.05
3333.43 1.75 3.84 35.03 0.76
3233.58 1.59 3.14 42.86 0.99
3133.43 1.88 4.21 31.92 0.96
3012.8 2.47 3.61 37.25 1.06
2901.17 2.16 3.1 43.36 1
2749.19 2 4.06 33.13 0.84
2564.16 1.13 2.75 48.81 0.77
2348.07 1.07 1.99 67.68 0.81
2142.4 1.32 2.38 56.5 0.63
1912.37 1.58 1.84 73.11 0.63
1698.17 1.11 1.8 74.66 0.77
1399.16 0.95 2.35 57.09 0.56
1000.06 0.86 1.31 102.35 0.64
742.81 0.99 1.08 124.55 0.7
501.76 0.8 1.98 67.88 0.49
222.11 0.89 1.1 122.2 0.63
0 1.63 1.84 72.86 0.34
A new diversion channel was designed for the Three Dams Water Course Post-Development
scenario. The new diversion channel will be lined with reno-mattresses and seeded with
indigenous grasses. The new diversion channel was designed to contain the 1 in 100 year flood
without overflowing. The minimum longitudinal slope of the new channel is 0.192% or 1 in 522
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(m/m). A Manning’s “n” hydraulic roughness value of 0.035 was assumed for the new channel,
which corresponds to long grass growing in the diversion channel. The normal flow depth in the
channel was calculated to be 2.9 m, and the critical depth is 1.8m deep. Flow will therefore be sub-
critical. A cross section of the channel is presented in Figure 5-12.
The outputs from the Farm Dam Water Course Pre-Development Model are presented in Table 24.
An output longitudinal section of this model is presented in 5-13Error! Reference source not
ound..
Table 24: Output from the Farm Dam Pre-Development Model
River Station
(m)
Maximum Channel
Depth (m)
Velocity (m/s)
Cross Sectional
Area (m2)
Hydraulic Depth
(m)
5336.07 1.97 2.92 101.48 0.87
5005.81 2.68 5.91 50.23 1.22
4695.89 3.3 1.62 183.1 1.21
4343.33 3.11 1.94 152.6 1.08
4004.85 3.04 1.1 270.8 1.2
3740.42 1.83 1.72 172.86 0.91
3406.65 1.23 1.84 161.25 0.8
3149.75 2.09 1.68 176.47 1.59
2839.08 1.82 1.7 174.68 1.34
2550.1 3.2 0.37 792.46 2.34
2227.47 3.19 0.27 1104.32 2.95
1977.23 2.84 2.34 126.64 1.16
1802.67 3.01 0.21 1420.39 2.6
1556.71 3.01 0.25 1170.21 2.79
1331.24 3 0.2 1452.15 2.79
1164.38 1.4 2.14 138.79 0.45
946.45 1.06 1.48 201.2 0.71
624.74 1.88 0.79 374.08 0.84
441 1.11 1.57 189.17 0.45
176.53 0.8 0.89 335.31 0.66
0 2.26 1.99 148.9 0.39
The wall of the Farm Dam will be lowered by 3 m in elevation for the Post-Development scenario.
The post-development output longitudinal section of this model is presented in Figure 5-13.
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5.9.2 Results
The resulting 1 in 100 year floodlines are shown in Figure 5-10.
Figure 31: 1 in 100 Year Floodlines Pre- and Post-Development
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Figure 32: Resulting Longitudinal section of Three Dams Water Course
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Figure 33: Cross Section of Diversion Channel
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Figure 34 Resulting Longitudinal Section of Farm Dam Water Course, Pre-Development
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Figure 35: Resulting Longitudinal Section of Farm Dam Water Course, Post-Development
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5.10 Suppression Abstraction Philosophy
The Stormwater Management Philosophy proposed below is for the preferred option, Option 3B,
Scenario 2. Impacted (dirty) stormwater will be contained in four (4) pollution control dams,
namely:
1. Dirty Water Dam (existing)
2. Emergency Dirty Water Dam (existing)
3. Dam 1 (proposed)
4. Dam 5 (proposed)
The operating philosophies of these dams are interlinked with respect to abstraction of water for
dust suppression and will need to be managed effectively to ensure peak performance. The
relationship between these dams is as follows:
The Emergency Dirty Water Dam will need to always have 55 Mℓ available storage capacity
therefore it should be given priority for dust suppression to maintain this volume.
If the Emergency Dirty Water Dam has the available storage available, then water for dust
suppression will have to be abstracted from either the Dirty Water Dam or Dam 1 (proposed).
Water will be abstracted from the dam with the highest volume by percentage of its storage
capacity.
Dam 5 is used as a storage reservoir for dust suppression. Water from the three (3) other
pollution control dams are pumped here for dust suppression.
5.11 Hydraulic Analysis
5.11.1 Sizing of “Clean” stormwater diversion drains and berms
The topography of the area earmarked for the extension of the existing ash dump has a natural rolling terrain
in a north-westerly direction. Consequently, “clean” surface runoff can only be expected from the contributing
sub-catchments located to the east of the study area. This resultant runoff will be managed as part of the
river/stream diversion discussed in section 1 of this report.
The river diversion will be sized to convey the peak discharge generated during a 1:100yr storm event from
the contributing catchment downstream of the spillway and the clean water dam spillway capacity of
100m3/s. The spillway is used as a control point from the contributing catchments upstream.
5.11.2 Sizing of “Dirty” stormwater conveyance drains
Surface runoff generated from within the footprint of the extended ash dump will ultimately report into toe-
drains running northerly along the western toe line. This system comprises of two (2) types of drains (Toeline
drain & Outlet Drain) in series leading into the new PCDs shown in figure below.
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Figure 36: Toe Line Drain
Figure 37: Outlet Drain
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Preliminary design parameters for the “dirty” stormwater drains are indicated in Table 25:
Table 25: "Dirty" Water Channels Concept Design Parameters
Design
Parameter
“Dirty” Water Channel
Toe line
Drain (D1)
Outlet Drain
(O1)
Channel Type Trapezoidal Trapezoidal
Lining Type Concrete Concrete
Friction
Calculation
Method
Manning’s
Formula
Manning’s
Formula
Flow Rate
Q(m3/s)
33 46
Bed Slope S
(m/m)
0.005 0.005
Manning’s N
(s/m1/3
)
0.016 0.016
Velocity V(m/s) 4.1(to be
optimised at
detailed
design stage)
4.4(to be
optimised at
detailed
design stage)
Side Slopes
(m/m)
0.5 0.5
Bottom Width
(m)
2 2.5
Normal Depth
(m)
1.5 1.7
5.12 WATER BALANCE
The Water Balance for the preferred option is shown in Appendix D. Normal
operating levels for the dams, both existing and proposed, are shown on the
graphs in Section 5.8.
5.13 OPERATIONAL REQUIREMENTS
In sizing the proposed infrastructure, several assumptions were made for the
operational philosophy surrounding the ADF and its infrastructure. These
assumptions need to be realised during operation in order to ensure the
performance of the new infrastructure.
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5.13.1 Monitoring of Quality in Clean Water Dams
The clean water dams, Dams 2, 3 and 4, have been sized not to spill more than
once in fifty years which takes into consideration irrigation onto the rehabilitated
areas. These dams will need to be monitored for water quality on a continuous
basis. If the water is deemed clean with respect to the discharge quality of the
receiving environment, then it may be released. However, if the quality does not
meet the discharge quality, then this water must be irrigated onto the rehabilitated
areas or utilised in the station water balance.
5.13.2 Maintaining Open Ash Areas for Dust Suppression
Option 3B Scenario 2 recommends an optimum open ash area of 82 hectares to
be maintained during operations, to balance the dirty water system using dust
suppression without the need for a Water Treatment Plant. The respective dams
have been sized accordingly. If significantly smaller areas are maintained, the
dams recommended is this report will be too small to ensure that we do not spill
more than once in fifty years from the pollution control dams.
5.13.3 Maintaining Silt Traps
The storage capacity of the proposed dams does not assume a continuous influx of
silt into it as it is equipped with a silt trap. If these silt traps are not maintained as
per their design requirements, the performance of the dams will be compromised.
The silt traps will be finalised during detailed design and the operations thereof
need to be communicated to the power station operators.
5.14 WAY FORWARD FOR STORMWATER MANAGEMENT
The trade-off study was conducted and finalised to inform the permitting process in
order to determine the optimised go-forward scheme for the implementation of
stormwater management infrastructure surrounding the extension of the current
Kendal Ash Dump. Option 3B Scenario 2 is the preferred option and conceptual
designs will need to be finalised for this.
It is also recommended that a Water Use Licence Application be made for the
water uses as described in this report.
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6 STREAM DIVERSION
The perennial stream located to the East of the existing ash dump currently runs
through the proposed footprint of Options 2A, 2B and 2C. Consequently, this
stream will be diverted to run northerly, parallel to the extended footprint of the ash
dump.
This stream serves as a receptor for discharge from the existing clean water dam
located up-slope. This dam serves as a primary source of flow for the stream. The
maximum discharge over the dam’s spillway is 100m3/s. The diversion channel will
be sized to cater for this flow, plus the runoff from the area below the spillway,
while incorporating an additional freeboard of 1m. A preliminary sizing has been
done; the bottom width of the stream diversion is 10m wide and a depth of 2m the
left and right side slopes are 1V:2.5 and 1V:3H respectively., on the lower left bank
side a berm is constructed to provide the 1m freeboard. In the figure below a
typical section is provided for the stream diversion. There are a number of aspects
listed below that needs to be investigated in the following phase of design to
provide an optimal solution for the stream diversion:
Investigating impact of stream diversion downstream of the stream diversion ,
Establishing similar vegetation in new stream
Erosion mitigation in initial stages
Probability of leachate of the existing facility migrating towards the stream
diversion
Design of this stream diversion channel will be carried out at a later stage of the
project, only a preliminary river section is provided in Figure 38
Figure 38: Proposed Stream Diversion
Berm constructed from suitable
excavated material
Reno-mattress lining
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7 PHASED APPROACH TO ADF
7.1.1 Option 1A & 2A
All the constraints where identified in terms of site boundaries, stream 100m lines
(Stream diversion lines for 2A) machinery and equipment used. Within these
options the maximum footprint was identified within these constrains. And a
maximum height was determined to be 60m.
For Option 1A the capacity of the site gives a remaining life of 5 years from
January 2015. This gives us time to 2019. If construction of the liner only finishes
at the end of 2015 the remaining life is only 4 years which means it would be
impractical to have phase development for this option
For Option 2A the capacity gives a remaining life of 15 years from January 2015.
This is triple the capacity of option 1A. It was assumed that the minimum cell life for
this option would be 5 years. Thus after construction there is only life for 11 years.
Thus a two cell approach is considered on which Cell 1 will have a life of six years
and Cell 2 a life of five years.
7.1.2 Option 1B & 2B
These options have the same footprints as option 1A and 2A therefore their
footprint cells will be the same. As the idea here is to do staged piggybacking the
piggybacked area is seen as another lined cell on its own. Option 2B has a
remaining life of 18 years which provides another three years of capacity. The
piggyback cell will only have a capacity of three years and will also involve moving
the suitable conveyor systems regularly if the piggyback is constructed solely
which can be an operational flaw.
7.1.3 Options 1C & 2C
These options again have the same footprint as option 1A and 2A, but due to the
piggyback section being constructed concurrently the piggyback area will not be
lined and the majority of it will be placed on the existing dump as the shiftable
conveyor system will be moved back and then covered with topsoil and grassed.
Option 1 C will have the same footprint cell and will thus have a capacity of 5
years, this means that 2 years for this option will be placed on the existing footprint
which will then be topsoiled and grassed.
Option 2C will also have the same footprint as Option 2A and thus have two
footprint cells with a capacity for 11 years. Thus 5 years of ashing will take place
on the existing ash dump and will then be topsoiled and grassed.
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8 FARM DAM SYSTEM
To the west of the existing ADF, there exists a farm dam on Eskom’s property
which does not form part of the water management philosophy. The dam is
currently used by a farmer to irrigate two centre pivots, located on the footprint of
the proposed extension. His activities will cease once the ADF is extended and the
dam will no longer be required to serve its intended purpose. However, the dam
sustains a wetland located at the toe of the dam wall and will pose a significant
environmental impact if removed in its entirety. The wetland is sustained via
seepage through the dam wall, which was not designed or constructed to
acceptable engineering standards and poses a risk of failure.
The height of the dam wall poses a significant institutional challenge for Eskom as
the top water level reaches at times the level of the final open cast coal mine voids
located adjacent to the farm dam. This is not ideal as clean water flow enters the
final voids when the level in the dam is high, resulting in contamination of the clean
water in the dam. However, the mine may decant uncontrollably into the farm dam.
The latter should be addressed by the mining house as it is outside the control of
Eskom.
The farm dam is in-stream downstream of a diverted watercourse created by the
mine in order to undertake open cast mining on its original course.
The following work is proposed in order to make this system environmentally
acceptable:
New earth dam wall to be built to prevent overflow into mining voids and
vice-versa
Existing dam wall to be removed
Engineered seepage from the dam to downstream of wall taken into
consideration for wetland sustainability
Upstream approach channel and outlet channel to dam to be lined using
reno mattress – flat gradients
Channel designed for the 1:2 year stormflow velocities
It should be noted that a portion of the upstream approach channel falls within the
property of Side Minerals. This is portion 1 of Leeuwfontein 219 IR. Side Minerals
(or Shanduka Coal) are the mining right holders of the Lakeside and Stuart
Collieries which are no longer operational.
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The proposed works are shown on the figure below.
Figure 39: Proposed changes at Farm Dam System
Reline stream diversion with
reno mattress
Lower dam wall to
below final void
decant level
Final voids
Reline dam outlet channel
with reno mattress
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9 OPERATION AND MAINTENANCE PLAN
There is currently an operational manual in place for the existing Kendal facility
which provides insight into all current activities on site.
9.1 Code requirements in terms of SABS 0286
The Following needs to be in accordance with SABS 0286
Management
Operational phase appointment
Facility audit
Hazard classification
Operating manual
Operation of the ash dump
Operation of silt trap/s and pollution control dam
Monitoring and maintenance requirements
Rehabilitation and environmental considerations
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10 DISCUSSION
Trade-off studies were conducted in order to determine the optimised go-forward
scheme for the expansion of the current Kendal Ash Dump and stormwater
infrastructure. Six options for the ADF and nineteen options for the stormwater
management philosophy were assessed. This was supported by:
Conceptual airspace modelling of each alternative;
A daily time-step model over a 50 year period for run-off determination and
PCD sizing, as well as sizing of clean and dirty water conveyances;
Conceptual development of dump infrastructure including surface drainage
structures, hydraulic structures, leachate and sub-surface drainage, lining and
cover materials;
Cost estimation;
Trade-off study workshop.
From the trade-off study held the preferred layout option is identified as Option 2A and the preferred water balance option is identified as Scenario 2, Option 3B
Option 2A provides a ashing capacity of 98 Mm3 at a density of 0.85t/m3
This provides a remaining life of 15 years from January until 2029
The preferred option as selected through the trade-off study analysis is based on a preliminary design. All fundamental aspects were addressed to inform the EIA study.
It is recommended to progress with the preferred option through a basic and detailed design phase.
The requirements that are raised as issues and concerns through the EIA and stakeholder engagement process should be addressed during the basic and detailed design phase and the impacts should be mitigated through engineering solutions.
Based on the trade-off study work, it can be stated that the proposed facility extension is a feasible solution.
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11 RECOMMENDATION AND CONCLUSION
Several options were considered for determining the go-forward option on the ADF
and stormwater management philosophy. The “piggyback” options described in
both the determination of air space on the ADF and stormwater management
philosophy are not deemed feasible currently from a mechanical perspective. This
option was therefore not considered for implementation in this report. Further
investigation is currently underway here and will be reported on a separate project
that has been commissioned by Kendal Power Station.
The go-forward option as proposed for the ADF layout is Option 2A and for the
stormwater management philosophy is Scenario 2 Option 3B. A Waste
Management Licence (WML) and Water Use Licence (WUL) need to be applied for
the infrastructure and activities as described in the aforementioned options.