Future-proofing Western Australia’s south-west

The Southern Forests region around Manjimup and Pemberton is one of Western Australia’s most significant horticultural regions, however, climate change is expected to limit future water availability under current water management practices. To support the sustainability and expansion of the region, the Southern Forests Irrigation Scheme (SFIS) will provide a new, innovative way to secure reliable, good quality water supply all year round by improving water storage and distribution through a new dam, pipeline and pumping infrastructure. Jeremy Bower, CEO of the SF Irrigation Co-operative (SFIC), describes the project.

Western Australia’s Manjimup-Pemberton area is one of the state’s prime food hubs and is the largest economic contributor from irrigated agriculture. It is predicted, however, that this area will be one of the hardest hit by climate change, with models projecting rainfall could decline by up to 25 per cent in coming decades. In fact, the area is already receiving less rain than in the past.

Currently, farmers capture and store water in dams for irrigation during summer. However, rainfall and inflow to dams are highly variable in some areas, and water quality can be poor at the end of the irrigation season. As well, limited access to good dam sites in some locations has constrained the growth of the irrigation industry in the region. This current system of water capture also means that water cannot be shared with other growers and put it to its most productive use.

The Southern Forests region is one of Western Australia's most significant horticultural production regions.

To droughtproof the region’s horticultural water supply and to expand its potential as a supplier of quality fruit and vegetables to domestic and international markets, the SFIS is being developed by the SFIC in partnership with the State Government.

The SFIS was originally proposed under the previous State Government’s Water for Food program and offers the opportunity to supply water in a model that fits with the National Water Initiative and to stimulate economic investment. Its oversight moved to the Department of Primary Industries and Regional Development (DPIRD) under the current Labor Government, and in June 2017 the SFIC was established to lead the project development and will own and operate it once completed.

The scheme received funding from the State Government, from the Federal Government’s National Water Infrastructure Development Fund, and from local growers via the first round of water sales.

While on-stream or gully dams are commonly used to manage water in agricultural regions in Western Australia, the State Government wanted a more sustainable method that would protect the region’s agricultural industry, as well as the environment. In 2015-16, various options were assessed including building bigger dams in some of the subcatchments and approaching farmers to sell their excess water to those dams. However, these options proved to be unviable.

According to Jeremy, this led to the government taking inspiration from a solution in Tasmania.

“Tasmania has now built fifteen irrigation schemes where a major water source, such as a river, has been linked to a pipe network to deliver water to farms in certain districts. These schemes have been very successful in terms of increasing productivity and being sustainable. Tasmanian Irrigation assisted the Western Australian State Government to assess the potential for a scheme in the Southern Forests region, including completing a business case that clearly demonstrated that the region could support a similar scheme and would bring significant economic benefit to the south-west,” Jeremy said.

“Over the past three years, there have been extensive environmental surveys, catchment modelling and monitoring, and environmental flow studies to ensure first and foremost that the environment is not adversely affected by the scheme now and into the future.”

The SFIS takes account of the unique requirements and environment of the region and includes a reservoir that will be built about 3 km from the river source. Studies were completed for the location of the reservoir and pipe network, and expressions of interest for water purchase were sent to the local farming community to determine water demand and the likely scale of the scheme.

While demand totalled more than 11 GL of water, this was subsequently reduced to 9.3 GL by the Department of Water to reduce cost. Following this, reliability studies were done to ensure that the scheme would be sustainable over dry periods. The result was a 15 GL dam and approximately 250 km of pipeline to more than 90 farms.

“We have approximately 1.7 GL of water available for future sale, which if sold, would create a scheme that delivers water to over 100 farms in the district,” Jeremy said.

There have been some concerns around water trading but the rules and system being implemented for the SFIS do not permit water investor or speculator involvement. It is necessary to be a member of the SFIC to participate in this scheme, and to become a member people must be a landholder (lessee) or own a farm in the scheme district, with appropriate plans for agricultural use.

“It's taken a little bit for people to understand but being able to pipe water between properties is actually one of the most sustainable parts of a water scheme. If farmer A doesn’t require all their water allocation for the business for one year or even over a certain number of years, instead of that water sitting in an on-stream dam, or being lost out of the system, they could reduce costs and sell the water to a neighbour for agricultural use. This method ensures water is always put to its highest productive use and not wasted,” explained Jeremy.

“It's not about making money trading water, it’s recognising that water is becoming an increasingly scarce resource and we need to get smarter about how we manage it. It's about providing the most efficient system to ensure the provision of good quality, reliable water for a really important Western Australian food producing region.”

The proposed water supply system for the SFIS, developed after feedback from community and key stakeholders, is designed to minimise direct impact on the river and its riparian vegetation. For example, the headworks have been re-designed to use submersible pumps next to a deep pool created by a natural rock bar instead of building a weir across the river, which would have created a large inundation area flooding vegetation. In addition, the pump

stations have been separated to reduce the footprint at the river by around 80 per cent.

The harvest pump site at the river will use a set of five to seven submersible pumps housed in a concrete wet-well capable of pumping up to 200 ML per day. These pumps will be connected to a 3 ML balance tank at a boost-pump station located about 100 m from the river in a soundproof shed. The water will then be pumped via a set of seven to nine variablespeed-drive, end-suction pumps through a 3 km pipeline to the 15 GL reservoir in the Record Brook gully.

At the dam, a distribution pump set containing eight horizontal, variable-speed-drive pumps will be installed, which will move water to header tanks located at the highest points through separate northern and southern delivery lines. The water will then be gravity fed through a 250 km pipeline network to the SFIC farmers.

Jeremy said the proposed system requires significant pumping infrastructure as the scheme will only take water from the river during large flows mainly through the winter months and only when the minimum flow threshold (set by the Department of Water and Environmental Regulation) has been exceeded. These extraction rates are important to ensure that water is only taken within permitted river flow criteria so that environmental flow is not impacted.

“Submersible pumps have been used in Tasmanian schemes and have clearly demonstrated their low environmental impact and their water harvest capability when high river flow rates occur during narrow seasonal windows,” Jeremy said.

“This new design, with the submersibles and using a natural rock bar to provide adequate suction depth really reduces our footprint, as well as noise and visual impact.”

The SFIC has worked closely with river ecologists from DWER, as well as independent scientists and expert panels to ensure the minimum flow restrictions before pumping are correct and to ensure the river ecosystem is maintained.

To adhere to the strict pumping controls and to accurately measure and record stream flow for pump station control, the SFIS is investigating options for state-of-the-art monitoring equipment. The investigation is in the early stages, but the chosen equipment will have the level of accuracy required by DWER and is a condition of the water abstraction licence.

Working with farmers has been important throughout the design process, with the design and pipe network ultimately controlled by their requirements and their knowledge of the region.

“We are currently working with local landholders along the proposed construction corridor to finalise the alignment for the infrastructure. Farmers’ local knowledge has been used to complete the design. Knowing where there's a high point or a low point or a wet spot or something that is better to avoid is invaluable,” Jeremy said.

Before COVID-19 restrictions, the SFIC team was visiting landholders to ground-truth the alignment of the pipeline with farmers; consultations are now online, a process that is working well.

“We've got alignment maps drawn up and we’re sending those by email. Farmers are marking them up and we'll ground-truth them when restrictions are lifted,” Jeremy said.

Along with community consultations, other preliminary work is being done including completing preliminary design of the dam and second stage geotechnical work. Earlier this year, DPIRD tendered and awarded a contract to complete these works.

The SFIC planned to have these field investigations completed before the start of winter, but COVID-19 impacts on intrastate travel has lengthened the schedule. If these field and design tasks can be completed in the expected timeframe, the SFIC is expecting to release tender documents early in the first quarter of 2021.

Jeremy said two main tenders are expected to be advertised, with the project being run as design and construct contracts.

The project works will be split into two D&C contracts, with the dam works separate to the pumps and pipelines. The SFIC is currently looking at the various tender requirements, including the desired performance and duty cycles, but this process is subject to approvals and environmental reviews confirming sustainability of the scheme.

The SFIC is hoping to engage local suppliers and contractors, or those based in Western Australia. However, the two head contractors will need Federal Safety Accreditation, so key suppliers will need to engage with those tendering for the work and to be part of the bid teams.

The names of companies tendering for the work will be posted on the SFIC website so suppliers and subcontractors know who to approach. Ultimately, the contracts will be awarded to companies that can demonstrate they can complete works that will deliver the water and operate the scheme most efficiently. Once the head contractors are selected, notification will be posted on the website.

For more information about the Southern Forests Irrigation Scheme and to keep up to date with the project, visit the website.

Acknowledgment. Thanks to Pump Industry magazine for allowing Irrigation Australia journal to publish this article.

Lauren Cella, Pump Industry Magazine