Peel MD Swales project

The concept of this project is to divert phosphorous-rich waters of the Peel Main Drain (PMD) through a series of six clay-lined swales that would adsorb and retain much of the phosphorous.  Its full title is the ‘Peel Main Drain Off-line water treatment pilot project’.  Primary partners in the project were PHCC, DWER and Water Corporation.  After a lengthy approvals, construction and testing process, operation began in October 2022 and ran until the drain flows almost ceased in January 2023.

For more background on the Peel Main Drain Project please see : https://peel-harvey.org.au/peel-main-drain-swales-project/

Figure 1 ─ Flow over diversion weir ─ November 2022

Figure 2 ─ Flow in Swale #1 looking downstream ─ November 2022

Components of the project included a diversion weir (Figure 1), offtake structure, six clay-lined swales of 230m long by 13m wide (Figure 2), tail-water structures to control the depth of water in each swale (Figure 3) and a tailwater pond to collect treated water before it is fed back into Peel MD.  Water monitoring for the project included measurement of flows in PeelMD and each swale, plus water quality (WQ) sampling of the inlet water and individual swale outflows as well as the downstream receiving environment.

Figure 3 ─ Tailwater levees and structures ─ November 2022

2022 Operation

Results of this initial operating period in late 2022, were very encouraging and have allowed us to evolve the design to allow for higher flow-rates, deeper water in the swales and increased treatment volumes for 2023.  This will result in treating a larger proportion of the PMD flow.

The retention of Total Phosphorous (TP) in individual swales during the 2022 operating period varied between 27% up to 86%.  The high retention rates were seen in swales with only low flows and a long detention time of water, and the lower percentage for swales with high flow rates and short detention time.  By increasing the depth of water in specific swales, a high flow rate can be treated in the swales by keeping the detention time relatively long. 

Figure 4 below shows the impact of the clay-lined swales over the operational period with all swales reducing the concentration of TP compared to the inflow TP from Peel Main Drain (PMD).  The figure also lists the average flow-rate through each swale and its water depth. 

Midway through the 2022 operational period, Swale#1 was modified to test a theory of retention behaviour.  The swale had its water depth increased to almost 200mm and a high flow of 10L/s was passed through it.  While the percentage retention of phosphorous of the total flow was not as much as had been hoped (only 27%), the retention by mass was 2-3 times that of any of the other swales with their higher retention percentages.

Figure 4 ─ Total Phosphorous (TP) concentrations at each swale outlet

A report explaining the detailed results of the 2022 operation is available from PHCC.

2023 Operation

For the operational period during 2023, it is proposed to increase our understanding of how to maximise the phosphorous retention using the Serpentine floodplain clays; particularly using deeper water and higher flow rates.   The following summarises the proposed operational parameters of the modified swales which will allow the science behind the system to be quantified:

• 2023 operation period will begin as soon as the impact of backwater from Serpentine River is low enough to have no impact on operations.
• The operation will be split up into three ‘phases’: High flow, medium flow and low flow.
• Beginning operation in August 2023 with ‘Phase I’ when the flows are still high and a large flow-rate can be drawn from PMD.  Phase II will be similar to the 2022 program, but with deeper water in the swales.  The program will switch to the low-flow ‘Phase III’ in December when all PMD flows will be diverted through the six swales till flows cease.
• Each Phase will have stopboards designed to manage the flows available with that Phase. 
• Sampling in 2023 will be reduced to fortnightly as changes in WQ between weekly samples were found to be negligible.
• The sloping Swale#6 will have riffles placed in it to mimic possible solutions to drainage elsewhere.

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