Peel Main Drain Swales project

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 that would adsorb and retain much of the phosphorous.  The first year of operation began in October 2022 and ran until the drain flows became too low in January 2023.

For the 2023 operation, learnings from the previous year were used and a series of design modifications made:

• Setting-up three operational phases:
  • I        High flow [August 2023]
  • II      Medium Flow [Sept-Oct 2023]
  • III     Low flow [Nov2023-Jan2024]
• Installing high tailwater stopboards (Figure 2) to set deeper water in the Swales (Figure 3) that would allow an early start to Phase I and extend our understanding of the Medium Flow type scenario from 2022,
• Plan on diverting all PMD flow through the Swales in a ‘Low flow’ phase (III),
• Add levees to the sloping Swale#6 to mimic the function of pools (Figure 4),
• Reduce the monitoring frequency to ‘fortnightly’ as 2022 sampling showed weekly was not necessary for this operation,
• Made changes to stopboards to make them easy to swap between phases.

Figure 1 ─ Flow over diversion weir ─ 02-August-2023

We were lucky this year due to the quick thinking of our construction contractor (Charlie Catalano), who worked on the weekend for us (04 June) prior to the first heavy winter rainfalls to get our earthworks and timber structures complete.

Phase I operation began on 15 August, with the change to Phase II on 05 September and the change to Phase III on 24 October, with the final sampling and season-end expected around the end of the calendar year.

Figure 2 ─ Tailwater stopboard in Swale #3 ─ 24 August 2023

Figure 3 ─ Flow in Swale #1 looking downstream ─ 08 September 2023

Figure 4 ─ Swale#6 downstream ‘riffle’ structures ─ September 2023

2023 Operation results

Once sampling has been completed with the cessation of flows in PMD, the results will be collated and analysed before writing a report explaining the 2023 operational findings; likely finished in March 2024.  It is expected that learnings from this year will facilitate the design of a monitoring program for 2024, which will hopefully commence in August 2024.

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