There are six GPTs within Water Corporation’s main drainage system plus approximately 30% of Water Corporation inlet structures are fitted with safety screens or trash racks which also capture gross pollutants. Safety screens are primarily installed as a public safety measure to prevent people and animals entering drains. The screening function (and therefore treatment) is a balance between maintaining drainage water flow, safety and trapping debris. Insufficient or ill-timed maintenance of GPTs and screens can lead to blockage and flooding.

Light weight GPTs such as socks and nets are better suited to outlets of piped systems high in the catchment where they are less likely to result in wildlife entrapment or reduced drainage flows (and subsequent flooding).

Litter booms provide a simple physical barrier to the movement of floating material without major disruption to the water flow. They will trap all floating material including leaf litter and other organics. They provide a highly visual indication of gross pollutant load.

In living streams, vegetation can act as a physical barrier to litter transport; however litter persists in the environment and needs to be physically removed. Living streams also assist to improve water quality through the absorption of nutrients such as nitrogen and phosphorous.

The Drainage and Liveable Communities team is proposing to undertake a trial to better understand the impacts on operation and maintenance, hydraulic performance, water quality and capital and operational costs by installing various GPTs at inlets and at the outlet of Herdsman Lake. The purpose of these GPTS is to remove plastics and other gross pollutants that otherwise would be discharged to the ocean.

The trial consists on a number of activities that will be delivered during this financial year. These include the:
1. installation of two floating boom structures, one on the Main Drain (outlet) and one on a Branch Drain (inlet);
2. installation of one net structure on drainage assets contributing flows into Herdsman Lake; and
3. converting an open drain to a living stream in two locations adjacent to Herdsman Lake.

Scope of Work
GPTs are widely used by local councils locally and in the eastern states of Australia. This is because they are largely responsible for the implementation and management of stormwater quality and their drainage infrastructure. However, there seems to be little understanding on the performance of these devices on arterial drains. It’s understood that there are no standard methods or guidelines for testing GPTs in Australia.

Water Corporation would like a student to undertake a desktop study on the performance, maintenance and operational requirements that have been recorded by local councils in Perth and across Australia that have installed similar GPTs to those proposed to be installed at Herdsman Lake.

The performance, operation and maintenance requirement of GPTs are determined by the individual design characteristics, catchment characteristics and stormwater composition. This work needs to consider these factors via a literature review, material interaction with councils, and collection and analysis of information to characterise:

• performance of GPTs (composition of the ‘catch’)
• the GPTs catchments’ land use,
• gross pollutant sources,
• maintenance activities/ burden,
• impacts on the hydraulic function of drains where GPTs have been installed, and
• drainage water quality upstream and downstream of GPTs and/or living streams,

A report will need to be produced with all the findings, results and analysis at the end of the project.

This work will provide a point of comparison to the actual data that will be collected at Herdsman Lake once the GPTs are installed.