In the three years since the Miller’s Crossing project was initiated in 2017, all works are now complete and water has filled the Miller’s wetland. See how one project was able to harness the drainage flows on the Swan Coastal Plain to improve a local constructed wetland.
The Miller’s Crossing diversion weir project is one of seven projects implemented under the ‘Better collaborative drainage management project’ otherwise known as the ‘Mayfield Project’ which was designed to reduce nutrients entering the Peel-Harvey Estuary. The Mayfield project is part of the Regional Estuaries Initiative administered by the Department of Water and Environmental Regulation (DWER).
The Miller’s Crossing sub-project in Hopeland saw a badly degraded farm crossing (Photo 1) on the Water Corporation’s “Dirk Brook Sub B Drain” that took flows from the upstream Karnet Brook to the Serpentine River upgraded to allow diversion of some of its flow to a wetland (Photo 2) that was constructed in early 2008.
Landowners: Karen and Bernie Miller (shown in Photo 3 with two of the project managers: Matt Giraudo and Neil Dixon), had been actively working for 15 years to improve the environment along the drain with wetland and drain revegetation works undertaken since 2004. The REI funding allowed engineering design to formalise sustainable diversion of flows from the drain to the wetland with the approval of the Water Corporation.
The designed works included construction of a culvert crossing of the drain with adjustable stopboards to inundate a small pool immediately upstream. Adjacent to the culvert was a floodway to take higher flood-flows past the structure and prevent scour. The third component of the works was a second culvert to divert water held in the upstream pool straight to the wetland (Photo 4).
The works were complete in June 2020 and the first rains in late June 2020 providing local runoff to the wetland, before more extensive follow-up rains on 14 July brought flow over the weir (Photo 5) and into the wetland (Photo 6) as shown in the two photos taken by landowner Karen Miller on 17 July 2020. The wetland now has 1.1m of water in it.
The two objectives of the project were to reduce nutrients in downstream flows with a secondary objective to increase the local environmental amenity of the wetland. For the flows in July 2020, a significant proportion of the phosphorous and nitrogen carried in the first flush of the event ended up in the wetland as planned. In addition, the local amenity around the wetland has been significantly improved with the hydroperiod of the wetland expected to be significantly extended into summer this year.
Miller’s Crossing construction
The project concept was a concept that Karen and Bernie had hoped see com to fruition a long time before the current project completion. In 2017, Matt Giraudo begun working with the Millers and installed a pump sampler in the drain (Photo 7) to take water samples and record water levels for flow. Flows in The Sub B Drain in August 2017 were the highest since 2005 with a peak flow of about 3.7m³/s, and this flow was used for the design of the new crossing.
A Project Proposal was developed by Matt Giraudo in November 2017 and after gaining approval from DWER and Water Corporation, a tender process began for the construction component of the project which commenced on 29 March 2019.
The construction process commenced with clearing out the old crossing before compacting the foundations and placing the new culvert (Photo 8). After the main culvert was in place, the diversion headwall (Photo 9) and culverts (Photo 10) were installed before backfilling (Photo 11). The final stage of the initial construction works were installing rock protection on upstream (Photo 12) and downstream (Photo 13) walls.
With the main works complete by mid-April 2019, there was a delay before the stopboards could be fabricated and then fitted. Some further modelling of the behaviour of flows across the weir and down the Sub B Drain was undertaken in April 2020 and a floodway was installed in June 2020 (Photo 14) with the stopboards and two gauge-boards installed just before (Photo 15).
This project is part of the Regional Estuaries Initiative and is supported by the State Government’s Royalties for Regions program.