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Ncwabeni Off-Channel Storage Dam

Background & Introduction

The Mzimkhulu Regional Water Supply Scheme, which forms part of the KwaZulu Natal’s Lower South Coast System, supplies water to the coastal region from Hibberdene to Margate, including Port Shepstone. In addition, the scheme also supplies many rural inland settlements such as Fairview, Kwa-Madlala, Louisiana, Bhoboyi, Murchison, KwaNdwalane, Izontsha, KwaMavundla, Gamalakhe, etc, with a total estimated present rural population size of about 140 000.

The water is presently sourced from non-regulated river flows in the Mzimkhulu River. The water is abstracted at the St. Helen’s Rock abstraction works near Port Shepstone and is pumped into the water treatment works. From there the water is distributed to the various user nodes.

The Southern KwaZulu-Natal Water Resources Pre-feasibility Study Phase 1, completed in 2002, concluded that during dry periods, the river flow is insufficient to meet the water requirements, even without provision for the release of the Ecological Reserve. The study recommended that, in order to provide for the water requirements for all users, including the Reserve, the construction of an off-channel storage (OCS) dam in one of the tributaries to the Mzimkhulu River, should be considered. The reservoir would be filled from its incremental catchment, supplemented by pumping from the Mzimkhulu River during times of high river flows. During times of low flows water would be released back into the Mzimkhulu River for abstraction downstream at the existing St. Helen’s Rock Abstraction Works.

The Southern KwaZulu-Natal Water Resources Pre-feasibility Study Phase 2, which was completed in February 2005, investigated numerous options with regard to the position of the potential OCS dams. Four competitive sites located about 20km north-west of Port Shepstone were selected as the most feasible potential OCS dam sites. Two of the sites (D2 and D2A) are located on the Ncwabeni River while the remaining two sites (D3 and D3A) are on the Gugamela River. Conceptual designs for the dam at these sites were undertaken as part of the Pre-feasibility study.

Following on the Pre-feasibility study a Reconnaissance Phase of the Mzimkhulu River Off-Channel Storage Pre-feasibility Study, completed in October 2007, re-assessed all four OCS dam options on the basis of more detailed hydrological modelling and updated information regarding water requirements, topographical surveys, geotechnical and flood hydrology data, which became available after completion of the Phase 2 Pre-feasibility Study. It was established that the D3 site on the Gugamela River and the D2A site on the Ncwabeni River were distinctly less favourable than the other two sites and were therefore not investigated any further.