Untitled Page
DWA Home Page  
 
Untitled Page

WESTERN CAPE WATER SUPPLY SYSTEM AUGMENTATION PROJECT

INTRODUCTION

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 Cape Town.

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 Cape Town.

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:

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. Suitably accurate survey information is available from the Feasibility Study for the purpose of undertaking detailed design of this scheme.

  6. 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.

  7. 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.

  8. 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.

 

Untitled Document
This site is best viewed with Internet Explorer 7.0 (or newer) and Mozilla Firefox 32.0 (or newer) at 1024x768 screen resolution.Click here to download Internet Explorer 7 or click here to download Firefox 32.
DWS Sitemap
If you experience any challenges regarding this site, please contact the Webmaster.