The need

The Central Coast has recently experienced the worst drought on record with 15 years of below average rainfall, resulting in a 50 per cent reduction in average stream flows between 1992 and 2007 in the region’s four major rivers – Ourimbah Creek, Wyong River, Mangrove Creek and Mooney Creek. These rivers currently supply a population of 300,000 people in the Gosford City and Wyong Shire local government areas.

The Mardi-Mangrove Link is being constructed as part of the Gosford City and Wyong Shire councils' long-term strategy to secure and sustain the Central Coast’s future water needs, WaterPlan 2050. First conceived in the 1970s, the project involves linking the Mardi and Mangrove Creek dams to increase the region’s current dam storage levels to a point where water restrictions can be eased.

The project

The Mardi-Mangrove Link project comprises an upgrade of the Wyong River to Mardi Dam water transfer system and the construction of the new Mardi Dam to Mangrove Creek Dam water transfer system.

A new 21.1 km, 1 m diameter pipeline, is being built as part of the project. It is comprised of two sections: a 19 km section which will have a capacity of 120 ML/d (with provision for future expansion to 160 ML/d) and a 2.1 km section which will have a capacity of 320 ML/d. The 2.1km section will connect the Wyong River to Mardi Dam, with the 19 km section extending from Mardi Dam to the eastern portal of Boomerang Creek Tunnel.

The pipeline will be constructed with mild steel cement-lined pipe with an 8mm wall thickness and a polyethylene (Sintakote) coating. According to pipe supplier Tyco Water, this steel has minimum yield and tensile strengths of 300 MPa and 400 MPa, and the pipe is pressure rated to 1,600 kPa.

Construction will involve four river crossings, 17 smaller tributary crossings, scour valves, section valves, air valves and a surge tank.

In addition to the pipeline, other components of the project include:

• Construction of a new Wyong River off-take structure, with a capacity of 500 ML/d;
• A new Wyong River Pumping Station with the capacity to pump 320 ML/d of water from Wyong River to Mardi Dam and provision for future expansion to 500 ML/d;
• A new Mardi-Mangrove Transfer Pumping Station with a capacity of 120 ML/d to pump water from Mardi Dam to Mangrove Creek Dam and provision for future expansion to 160 ML/day; and,
• Provision of pigging launching and receiving facilities and discharge water treatment ponds.

When operational, the Mardi-Mangrove Link pipeline will transfer water from Wyong River and Ourimbah Creek via Mardi Dam to Mangrove Creek dam and back.

The Gosford City and Wyong Shire councils have contributed a combined total of $40 million towards the Mardi-Mangrove Link, and the Australian Government is providing $80.3 million of funding through its Water Smart Program.

Pipeline construction

Wyong Shire Council – which is managing the Mardi-Mangrove Link project on behalf of the two participating councils – has employed a project management team made up of contractors and Gosford City and Wyong Shire Council staff for the design, construction and commissioning of the project.

New South Wales Public Works has been appointed to manage the GC21 construction contract with John Holland as the principal contractor and GHD as the principal design contractor. Approximately 1,000 staff and project contractors have worked on the project so far.

NSW Public Works Mardi Mangrove Link Construction Manager Peter Rainsford says that since the first sod was turned in March 2010, the project has encountered unique challenges.

“In the past I have worked on several water supply and sewer reticulation schemes, including construction of the Hunter Connection pipeline,” he says.

“The scale of the Mardi-Mangrove Link project and the size of the pipe are quite different to anything I have worked on before.

“In this project there are more complexities of scale in terms of handling such a large pipe, the larger fittings required, the depth of excavation required and the length of pipe to be laid.”

Mr Rainsford adds “As the project is in a larger scale than normal, there is also a greater need to source products from overseas. Many products of the required size are not available in Australia. Products have been sourced from countries such as Finland, Austria, Sweden and India.”

Safety precautions

Numerous precautions have been taken by contractor John Holland to ensure that its ‘no harm’ culture is upheld on the Mardi-Mangrove Link worksite. In addition to using a robust Safety Management Plan that focuses on planning, safe work practices and training, the company also ensures regular work site inspections are conducted, and that staff and contractors are prepared for incidents and emergencies.

Further precautions include installation of an ultra high frequency repeater station on Bumble Hill to enable radio communication between all project worksites (as mobile phone coverage is poor); strict traffic controls to ensure safety of vehicle traffic as well as workers; fencing of worksites to ensure the safety of livestock and members of the community; and, the requirement for all visitors to complete a site-specific induction program with John Holland prior to entering the worksite.

Engineering challenges

The Mardi-Mangrove project team has encountered various challenges during construction of the pipeline. One of these challenges was to determine the best way to handle the four river crossings required while taking into account engineering, environmental and stakeholder considerations.

It has been decided that three of the river crossings will be completed via pipe bridge. Horizontal directional drilling (HDD) will be used to install the fourth crossing, which is too wide to construct a pipe bridge. The HDD operation will involve the installation of a 260 m section of pipe under the Wyong River. According to Wyong Shire Council, this is the first time a pipe of this size, length and pressure rating has been installed in Australia using this technology.

“HDD is a better option than traditional methods such as microtunnelling as it is faster, safer, cheaper and less intrusive for residents,” Project Director Greg McDonald.

“HDD is also less disruptive to the local environment as the site has a significant stand of trees, animal burrows and a billabong on one side.”

A significant length of the Mardi-Mangrove Pipeline falls within an area that may be affected by subsidence from future mining activities. In line with the Mine Subsidence Board requirements, the pipeline needed to cater for maximum ground movements for strain of plus or minus 1.5 mm per metre and for tilt of plus or minus 2 mm per metre. To achieve this, the buried pipe was designed with extensive use of rubber ring-jointed pipe with deep sockets and maximum deflection of one degree. Pipe bridges required installation of both an expansion and rotational joint to withstand these ground movements.

With inclement weather throughout most of the project in 2010, sediment control has been a major issue. This has been overcome with implementation of control mechanisms, which are monitored and maintained. A constant surveillance of the three to five day weather forecast is also undertaken to avoid trenching in sensitive areas and to enable work sites to be secured.

The pipeline is located on a natural floodplain, creating inherent problems with drainage. A flexible construction program has been implemented to maximise construction in the floodplain during dry periods and in higher areas during wetter weather. Due to the frequent periods of inclement weather in 2010, this method was only partially successful.

Environmental challenges

Extensive environmental planning and investigations were undertaken to ensure project activities did not have a significant impact on the Yarramalong Valley environment.

A detailed Review of Environmental Factors document was produced, including environmental studies on the flora, fauna, aquatic, and cultural and heritage conditions in the Yarramalong Valley. A Construction Environmental Management Plan has also been implemented to provide detailed guidelines on managing environmental issues – from noise and dust control to preserving trees and platypus habitat. Senior ecologists were involved in the entire pipeline route marking process.

The final route was selected to avoid sensitive ecological areas and significant impact on local biodiversity, including threatened species and endangered ecological communities.

Protecting the flora and fauna

A platypus was spotted in Schofields Creek during construction. Work stopped so important steps could be taken to protect platypus burrows and any animals living nearby. This included the building of a ‘platypus run’ in the form of a 200 mm poly-pipe to allow any animals trapped by the temporary damming of the creek to escape. A toothless bucket was also used to excavate the creek bank so existing burrows would not be destroyed in the process.

Other flora and fauna management measures include:

• Engagement of specialists to provide guidance on sensitive areas and species;
• Inspection of all safely-accessible tree hollows before and after being felled;
• Installation of nest boxes;
• Collection and release of native animals encountered during construction into adjoining bushland;
• Excavation in stages to minimise ground exposure to erosion;
• Retention of large trees either side of the easement where possible; and,
• Monitoring of river levels during construction.

After construction is complete, platypus, fish, macroinvertebrates and streambank remediation works will be monitored and bank stabilisation measures – such as native plantings – will be conducted to strengthen river banks and restore habitats.

Respecting cultural heritage

An indigenous heritage study was undertaken to check if any important cultural items existed in areas where the pipeline was to be built. Aboriginal artefacts have been discovered during construction in the pipeline corridor. Work stopped in these locations and the sites were documented, reported and preserved in accordance with relevant State Government and Aboriginal community requirements.

Work also stopped when a section of road dating back to the early 1920s – called a ‘corduroy road’ – was uncovered on the pipeline route. Umwelt Australia and Conacher Environmental Group excavated the section and took photographs and measurements for archival records.

The future

Wyong Shire Council has said that following completion of the Mard-Mangrove Link – expected in mid-2011 – the councils will develop pipeline extensions to growth areas as required. Enhancements to the water supply system will also continue to be developed in accordance with WaterPlan 2050.