The Peel Main Drain carries runoff from rural and semi-rural parts of Mundijong, Mardella, Hopeland and Baldivis into the Peel-Harvey estuary via the Serpentine River. It flows between the Serpentine River and the Kwinana Freeway south of Karnup Rd, and joins the river at a point just south of the heavy haulage inspection bay on the Freeway. Towards the south of the Peel Main Drain we have constructed a swale system to explore some of the factors that influence water quality in the Serpentine River.  The swale system can be seen on Google at https://maps.app.goo.gl/pGM5Y8UiEJV373Dg7 

The concept of this project is to divert phosphorous-rich waters of the Peel Main Drain (PMD) (Figure 1) through a series of six clay-lined swales (Figure 2) that would adsorb and retain much of the phosphorous.  The first year of operation began in October 2022 and ran for 12 weeks, while the second year began in August 2023 and ran for 16 weeks.

Both years operation showed that the swale system achieved significant reduction in phosphorous concentrations in water released downstream.

Current Operation 2024

For the 2024 operational period, we decided to operate for the whole flow period, trying to maximise the amount of flow passing through the swales for treatment and being diverted away from the PMD.  To do this, a new set of stopboards were designed that could easily be slotted in and out of the headwater pool inlets and set at different heights dependant on water levels in the PMD.  Similar sets of stopboards were built and installed in the tailwater end to change the depth of water in the swales themselves.

  • After a long and very dry summer/autumn, flow in the PMD began on 04 June 2024, with all flow being diverted through the swales and no water flowing over the weir.  During this period flow through each of the flat swales (#1 ─ #5) was about 20 L/s, with the depth of water being about 80 mm over the stopboard inlet.  With the sloping swale (#6), there was little flow due to backwater effect from levees installed to hold back water and designed to operate at higher water levels.
  • A series of rainfall events began on 17 July, with significant flows over the weir (peaking at 1100 L/s over the weir) and about 1000 L/s entering the Swales though not initially able to discharge due to high water levels in PMD (Figure 3) preventing flow returning back to the drain temporarily.
  • On 30 July, stopboards were installed to control flows into and water depths in the swales (Figure 4). With higher flows in PMD, deeper water in the swales helps to treat larger loads of phosphorus while keeping the flow velocities in the swales low.
  • Monitoring the system is on-going, with telemetered continuous flow measurements and fortnightly water quality sampling being undertaken for Total Phosphorous.
  • Due to the deterioration in the health of local wetlands adjacent to the swales (Figure 5), we are currently investigating water depth and water quality in key wetlands and the changes over the year.  We are hoping to be able to supplement the wetlands with treated water from the swales to help them survive the drying climate that severely impacts on both their flora and fauna.

For the rest of the 2024 operational period through to June 2025, we plan to:

  • Operate the swales with stopboards through until there is a drop in PMD water levels expected in late October, before removing the stopboards and running the system through until flows stop, likely in late December 2024.
  • Continue fortnightly sampling in the swale system until flows stop and also sample wetlands on a monthly basis through the first half of 2025 (including continuous monitoring of water levels).

Figure 1 ─ Flow over diversion weir ─ 04-August-2024

Figure 2 ─ Inundation in swale #5 looking upstream ─ 01 August 2024

Figure 3 ─ Inundated tailwater pool and downstream levees following heavy rainfall ─ 01 August 2024

Figure 4 ─ Flow entering swale #1 ─ 04 August 2024

Figure 5 ─ Sampling wetland [PMD006] by Michelle Mullarkey─ 30 July 2024

This project is part of the Regional Estuaries Initiative and is supported by the State Government’s Royalties for Regions program.   #WAEstuaries

We acknowledge the Noongar people as Traditional Custodians of this land and pay our respects to all Elders past and present