In 1995, a study of the secure yield of the Tantawangalo-Kiah Water System that supplies Candelo, Wolumla, Merimbula, Tura Beach, Pambula and Pambula Beach, was conducted by the New South Wales Department of Commerce (DoC). It was determined that the available supply in certain years was lower than the assessed demand. A pipeline connecting the Bega River bore field to the Tantawangalo-Kiah water supply system was determined to be the most effective way of providing long-term security to this water supply.

The Bega to Yellow Pinch Dam Pipeline was proposed in August 2006 by the Bega Valley Shire Council and the design developed in conjunction with NSW Public Works.

In July 2007, the Australian Government’s Water for the Future Initiative, provided $10 million towards the project, as part of the Water Smart Australia Program, demonstrating their support and recognition of the project’s importance in securing the water supply for the southern half of Bega Valley Shire.

The objectives of the project are threefold. Firstly, the project is designed to improve the water supply security of the existing Tantawanglo-Kiah water supply system that supplies water to the urban areas in the south of the Bega Valley Shire including Candelo, Wolumla, Merimbula, Tura Beach, Pambula, Pambula Beach and Eden.

Secondly, the pipeline is designed to lessen extraction of water from Tantawanglo Creek and the Kiah Borefield during low stream flows. The project does this by connecting a new source of water to the system which allows water to be extracted during higher, less environmentally sensitive stream flows at Bega.

Finally, the project is designed to enhance operational flexibility in the supply system.

Contracted works

After an early contractor involvement process with three potential contractors followed by a competitive tendering process, Mitchell Water was selected to finalise aspects of the design and construct the pipeline in June 2010. Since then, the company has worked closely with Bega Valley Shire Council and the NSW Public Works Department (engaged as project managers by the Bega Valley Shire Council) in a GC21 contract arrangement to complete construction of the project on budget and on time while delivering a high standard of quality, safety and environmental performance.

Under the contract, Mitchell Water was responsible for the:

• Design development and construction of approximately 20km of 450mm diameter PN35 ductile iron cement lined pipe, as well as a 0.5 ML balance tank;
• Conversion of an existing Wolumla Village reservoir to a balance tank for an ultimate transfer of 20 ML/d of raw water from South Bega Reservoir site to the inlet pipe work into Yellow Pinch Dam (the South Bega pump station and pipeline is being constructed with the further provision for a future capacity upgrade);
• Design and construction of thrust-bored creek and road crossings, the above ground 375 mm diameter glass reinforced plastic bridged creek crossing at Frogs Hollow Bridge, and associated connective pipe work, thrust blocks and valves;
• Design and construction of fluoride and chlorine dosing equipment at South Bega Reservoir;
• Design and construction of a 1 ML reservoir at Wolumla including pump upgrade, rising main, delivery main, pressure reducing valve, chlorination and telemetry;
• Installation of pumping machinery for South Bega Pumping Station, including all associated controls and ancillary equipment – this includes all civil, mechanical and electrical requirements including but not limited to pumps, pipes, switchgear and controls for the pumps, telemetry and SCADA, level controls at the balance tanks and ancillary equipment; and,
• Supply and construction of thrust blocks and other nominated air, scour and stop valves as required for the 450 mm diameter pipeline.

Construction commenced on the Bega to Yellow Pinch Dam Pipeline in July 2010 and a total of 34 employees including subcontractors were engaged by Mitchell Water.

The project was constructed between the existing South Bega Reservoir and the Yellow Pinch Dam, following the Princes Highway, and is generally located within the road reserve for the majority of its route. In some areas the pipeline also traverses private property.

A detailed site-specific project safety plan was developed for implementation on the project. All personnel including subcontractors were required to participate in a site-specific occupational health and safety induction prior to commencement of site works. Safe work method statements were developed and implemented for each construction activity with daily and weekly compliance inspections undertaken across the site by site supervision and management personnel. Weekly toolbox meetings were also conducted to provide feedback on safety performance.

Flooding, erosive soils and mountain terrain

There have been many design and construction challenges on the project.

According to Mitchell Water Engineering Manager Lee Briggs “Simply finding a logical, constructible route for the pipeline given the steep terrain adjacent to the Merimbula to Bega roadway was a major engineering and survey achievement.

“The topography of the Bega region has caused many challenges for the company – in particular, the undulating nature of the land traversed by the pipeline, erosive soils, granite rock and extremely high rainfall. The 100-year flood event experienced during the construction phase has also been a challenge for the team,” says Mr Briggs.

Mr Briggs also says that locating the 1 ML concrete storage tank on the top saddle of a hill at Wolumla required some creative engineering. Steep cut batters required rock bolts and shotcreting to stabilise the embankment for the tank site.

Due to the erosive nature of soils and high rainfall, erosion control was critical. With this in mind, Mitchell Water implemented detailed sediment and erosion plans, with specialised operators and labourers employed to operate them. A program was developed to monitor the effectiveness of erosion control structures.

Between a rock and a hard place

During construction, Mitchell Water encountered a large quantity of hard rock. As Mitchell Water Construction Manager Wayne Slimmon explains “The rock was so hard that after an hour with a rock breaker all there was to show was a small bowl of dust and a ruined breaker mall point.”

Innovation was required to complete construction through the rock. “A rock breaking methodology including drilling and expansion materials eliminated the requirement for blasting and subsequent impacts to the environment and amenity,” said Mr Slimmon.

Environmental and cultural heritage management

Detailed flora and fauna management plans were developed and implemented to protect the numerous patches of sensitive roadside vegetation and habitat for wildlife during the construction phase, and the workforce undertook cultural heritage awareness training as part of the site-specific environmental induction prior to the commencement of works on the project.

What’s next for Mitchell Water?

Mitchell Water is now commencing work on 146 km of pipelines, a pumping station and a 120 ML earthen storage on the East Loddon Stock and Domestic Pipeline project north of Bendigo in Victoria.