The Western Cape Water Supply System (WCWSS) serves the City of Cape Town and
surrounding smaller towns and irrigators. It consists of infrastructure
components owned and operated by both the City of Cape Town and the Department
of Water and Sanitation.
The Department of Water and Sanitation undertook the Western Cape Water Supply
System Reconciliation Strategy Study which investigated a range of bulk water
supply schemes that could serve towards meeting the growing water requirements
that will need to be supplied from the WCWSS. These included options such as
desalination of sea water, water re-use, groundwater development and possible
surface water augmentation options along with water conservation and water
demand management. The Reconciliation Strategy Study identified the need for
augmentation of the WCWSS by 2019.
The Reconciliation Strategy Committee made the decision that the Department of
Water and Sanitation would investigate the surface water augmentation options
while the City of Cape Town would investigate the other options.
Following on the recommendations of the Reconciliation Strategy, the Department
of Water and Sanitation commissioned a detailed Feasibility Study to investigate
the identified surface water development options. The first phase involved a
pre-feasibility assessment of each of the six potential surface water
development options in order to select the two most viable options for further
investigation to feasibility study level in the second phase of the Feasibility
Study. The two most viable schemes identified were as follows:
- Berg River – Voelvlei Augmentation Scheme (BRVAS); and
- Breede – Berg (Michell’s Pass) Water Transfer Scheme, abbreviated as the Breede – Berg Transfer Scheme (BBTS).
Both schemes rely on the utilisation of the existing storage capacity in the
Voëlvlei Dam, and on the existing capacity of the City of Cape Town’s pipeline,
from their water treatment works at the dam, to their Plattekloof reservoir in
DESCRIPTION OF THE SCHEME
The Berg River – Voëlvlei Augmentation Scheme has been found to be the favourable
surface water intervention option, albeit only able to augment the Western Cape
Water Supply System by about 2 to 3 years. It is an option that will be
considered within the current planning horizon of the Western Cape
Reconciliation Strategy as the next possible surface water intervention. A
comparative analysis will be required between the preferred surface water
development option and the best of the other options investigated by the City of
Apart from having a favourable Unit Reference Value (a measure of unit cost), the
BRVAS also has the advantage that it is likely to be the only scheme that could
be implemented by about 2018/19 when the growth in water requirement may exceed
the existing yield of the system.
The proposed scheme would involve the pumped abstraction of winter water from the
Berg River, once the ecological water requirements of the river and the estuary
have been met. The ecological Reserve commensurate with a Category D River has
been allowed for in the system modelling of the scheme.
According to the Reserve for the Berg River Estuary the required stream flow into
the estuary during the summer months should vary between 0.6 and 0.9 m3/s. As
the present day inflows into the estuary are not gauged (although DWS has plans
to install a gauge), the present day inflow of 0.3 m3/s was estimated from the
gauged flows below Misverstand Dam, and from the downstream irrigation
allocations which will be metered in the near future. In order to provide the
required Reserve inflows to the estuary would require that additional releases
of between 0.3 m3/s and 0.6 m3/s should be made from Voëlvlei Dam, particularly
during the four summer months from December to March. Therefore, the
conservative assumption has been made in the system modelling of the proposed
scheme that an additional release of 0.5 m3/s should be made from Voëlvlei Dam
for the six summer months.
Two scheme options have been investigated, namely:
- Option 1 for a 4 m3/s pump station with a stepped-pump operating rule.
- Option 2 for 6 m3/s pump station with variable speed drives.
Location of the Berg River – Voelvlei Augmentation Scheme
CONCLUSIONS AND RECOMMENDATIONS
The following conclusions and recommendations are made in relation to the
potential implementation of the Berg River – Voelvlei Augmentation Scheme:
- The Berg River Reserve commensurate with a Category D River has been allowed for, and a recommended minimum summer low flow for the estuary of 0.6 m3/s, of which 0.5 m3/s (8 million m3/annum) would be provided out of releases from the Voëlvlei Dam to supplement the present day inflows (which are ungauged) into the estuary. The proposed scheme will need to comply with the requirements of the relevant legislation, including but not limited to: (a) National Environmental Management Act, (b) National Heritage Resources Act, and (c) National Water Act.
- The proposed Lorelei abstraction site is close to a bend on the Berg River which is favourable from a sedimentation management perspective. Geologically this is the only location of those considered at which any rock outcrop is evident for suitable founding conditions. From a hydraulic and geotechnical perspective this site is therefore recommended as the preferred location for the abstraction weir.
- The Lorelei site also has the shortest conveyance length of all abstraction site options considered and enables the rising main to the Voëlvlei Dam to be aligned such that the least possible impact is made on the Renosterveld within the Voëlvlei Conservancy.
- Of the two potential abstraction approaches investigated in detail, namely a 4 m3/s pump station with a stepped-pump operating rule, or a 6 m3/s pump station with variable speed drives, the former appears to be more easily implemented and operated, as well as offering a slightly higher resulting yield (23 versus 20 million m3/a). From an operational perspective, the 4 m3/s abstraction via a stepped-pumping operating rule is recommended.
- Suitably accurate survey information is available from the Feasibility Study for the purpose of undertaking detailed design of this scheme.
- Geotechnical conditions at the Lorelei site are generally favourable, and the weir design can be suitably accommodated at the proposed site. Machine excavation is expected to be possible along the pipeline route. Although there is potential for the use of excavated materials for backfilling, the final pipe type selection will influence the extent of selected fill material available in-situ.
- For the 6.3 km rising main, a 1700 mm diameter GRP is proposed for the 4 m3/s abstraction option and the same pipe type (1900 mm diameter) for the 6 m3/s option.
- The estimated capital cost of the 4 m3/s abstraction option is R277 million and that of the 6 m3/s option R 312 million including VAT. The corresponding Unit Reference Values are R1.52/m3 and R1.94/m3 respectively for a discount rate of 8% per annum, and based on the VAT exclusive costs. On the basis of the financial assessment, technical and environmental considerations, the 4 m3/s option is recommended.