Rehabilitation of Gamtoos Conveyance System

The rehabilitation of the Gamtoos Conveyance System, which was a five year programme, was completed last year.

The system which was first constructed in the 1960’s is 64.7 kms long and conveys raw water from the Kouga Dam to Loerie Dam.The system comprises of open canals with a total length of 34.8 km, 7 covered canals with a total length of 3.0 km, 23 tunnels with a total length of 11.2 km and 29 siphons with a total length of 15.7 km.

Phase 1 and 2 of the Rehabilitation Project addressed the more urgent works required, prioritized by means of a risk analysis, and these are now complete.Raw WaterThe system currently transfers approximately 80 Ml/d (0.9 cumec) of raw water intoLoerie Dam where it is treated to potable water standards for use by the NelsonMandela Metropolis (NMM). This constitutes some 30% of the NMM’s potable waterdemand. Due to limitations within their distribution system, parts of the NMM canonly be serviced from this source. The Loerie Dam has a small catchment and aretention capacity of 24 days of current demand.

The dam is thus highly dependentupon this transfer to meet this portion of NMM’s potable water demand.In addition to providing raw water to NMM, the system is the sole source of raw water tothe villages of Hankey (1 Ml/d) and Patensie (0.5 Ml/d) located along the route of theconveyance system. These villages have their own treatment facilities.

Notwithstanding the importance of this inter-basin transfer, for the sustained economicdevelopment of the NMM, the major volumetric users of water from the system are theirrigators located within the Gamtoos River Valley, who are entirely dependent upon thesystem for their livelihood as the Gamtoos River water is brackish and not suitable forirrigation purposes. The valley has a GDP of some R800m/year and is a major sourceof agricultural products (citrus, vegetables and dairy products are supplied directly toretailers) within the region.

Challenges

According to Amelius; two great challenges stood out apart from the normal engineering and construction challenges of road, railway and river crossings and high ground water levels among others.

These were determining a risk assessment process and the other was carrying out the rehabilitation works on a ‘live system’, with work time limited to 2 week system shut-downs scheduled during times that demand was historically low.

As part of the Conditional Assessment of the conveyance system, a risk assessment process was followed. The real challenge was to develop methods and norms whereby the condition of a specific element of the conveyance system could be evaluated and “quantified” in order to obtain a level of probability of failure of the specific element.

Together with the potential impact of a partial or complete failure of the element on the water supply, provided the level of risk that could be associated with this particular portion of the conveyance system.  This enabled them not only to rank within similar groups of elements, for instance all reinforced concrete pipe siphons, but also to rank amongst various elements, for instance did tunnels pose a similar risk as pre-stressed concrete pipe siphons?  This provided the ability to prioritize the work and to provide a rational rehabilitation programme to the DWA.

Visual inspections played an important role to determine the condition of all elements and various criteria were applied to define the level of degradation.,In the case of pre-stressed concrete pipes, Eddy Current Scanning techniques had to be used to determine the inherent remaining strength and integrity of pipes within a siphon and therefore the probability of failure of this element.

Construction Review talked to one of the contractors involved in the project Peter Baxter of WK construction, a building and civil engineering construction company which carried out three of the contracts for the works. Contract No. W 10063 in which they carried out works on the rehabilitation of Line 16 siphon, Contract No. W 0082 involving rehabilitation of Bodker Tunnel/Siphon and Contract No. W 0084 – WTE which involved rehabilitation of De Koning Siphon.

As part of the construction phase, the major challenge was to carry out the rehabilitation work on a “live system”.  The only window of opportunity to perform certain critical tasks on the system itself was during the two, two-week dry periods the Gamtoos Irrigation Board (GIB) could offer. It was for instance quite a challenge for WK Construction to re-line a concrete lined pressure tunnel, 2.4m in diameter, with the launch pit seated at the base of an almost inaccessible valley, within this stringent time period. By an innovative approach, the full 140m length was relined by means of a 1.9m diameter steel liner within a period of two weeks.

Going forward all elements of the conveyance system will be closely monitored by the agent to the Department of Water Affairs (DWA), namely the Gamtoos Irrigation Board (GIB) and further remedial work will be initiated at the appropriate time.

Project Team

Consultants

Mbona Saunders and Wium (MSW – Pieter Maritzburg)

Contractors

Southern Pipe Contractors (SPC)

Shearwater Cerimele JV

WK Construction

Active Group Management Services AGMS – George)

Ursa Civils