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Innovating passive irrigation with the Monash Kerb Inlet

By intouch * posted 26-10-2016 10:23

  

In the search for a simple, low maintenance and cost-effective way to passively irrigate street trees, one council has broken the mould – creating their own in the process.

Source: City of Monash

The Monash Kerb Inlet (MKI) is a new passive irrigation design from City of Monash, University of Melbourne, Monash University and Melbourne Water, with funding from the Office of Living Victoria. Launched in a number of trial locations in 2013, the water sensitive urban design (WSUD) makes use of stormwater runoff to improve the drought tolerance of street trees, by delivering runoff to nature strips. 

Now, the research team has secured a second round of funding, this time from Melbourne Water, which they say will allow for further testing and refining of the design.

The anticipated benefits of the design also include reduced maintenance costs and a reduction in the volume of polluted runoff entering waterways.

The need for WSUD

As global temperatures and populations increase, the case for WSUD and green infrastructure is becoming clearer.

In cities, the ‘urban heat island effect’ – created when man-made infrastructure surfaces absorb and release the sun’s energy back into the urban environment – can be combatted with trees, which reduce surface and air temperatures by providing shade and through evapotranspiration. Earlier this year, the Federal Government flagged its intention to support the creation of more ‘urban canopies’.

At its October 2016 World Water Congress and Exhibition held in Brisbane, the International Water Association launched 17 principles for water wise cities. Recognising the destabilising impact of climate change and global warming, the recommendations included ‘enable regenerative water services’ and ‘modify and adapt urban materials to minimise environmental impact’.

A host of studies have confirmed the tangible and intangible benefits green cities provide, not the least of which are a better quality of life and improved human health outcomes.

The design

Source: City of Monash

Kanchana Withana, Acting Principal Design Engineer at City of Monash, says the project began when the team couldn’t find a passive street tree irrigation design that suited their needs.

“We investigated what was available, whether there are similar products in the market and what other council have done. We found the biggest problem was maintenance – they get clogged. To stop that, people use filters but those also get clogged. We knew the only way to do this is to make sure we only take the water in, but not the debris,” Withana says.

The council could have settled on a simple vertical grate in the face of the kerb – instead, they decided this was an opportunity to innovate, research and to push the boundaries of conventional thinking.

 Utilising his network of contacts, Withana approached Swinburne University’s Professor Saman Fernando, an expert in fluid dynamics. During an onsite visit, Fernando suggested an elegant solution: using aerofoil dynamics.

The design remodels the kerb face into an aerofoil‐type form with an inset grate.

A paper on the MKI design explains how it works:

“The MKl is designed with two shapes. The vertical aerofoil and the horizontal dip, which together cause acceleration of flow through the kerb. Flows and the debris will have a higher tangential velocity and will quickly pass the grated opening. Water passes through the grate when the flow depth increases in a similar manner to a weir,” the report reads.

The design also relies on a maintenance programme of street sweeping and cleaning of secondary filter baskets that collect the fine debris

The design was then tested.

“We made one out of sheet metal and plywood and did a test, and the flow patterns were exactly what Professor Fernando predicted,” Withana says.

The council then worked with a concrete manufacturer, further developed the surface profile into a complete 3D model with the entire cast kerb profile, created a mould and produced a completed design. There are currently 12 MKIs installed in the council, and Withana says they are working well – other councils have also showed interest in the design, with Maroondah City Council in the process of installing some of their own.

 The report outlines the cost of the MKI:

“The cost of installation is currently at $4000 – this is likely to reduce with a wider rollout of the product. The typical cost of loss of a mature tree is $40k and with a typical life span of 40 years, so there is considerable benefit in successfully passively irrigating our street trees.”

Although the team had hoped the MKI would require zero maintenance, it became clear that a secondary filter within the design was necessary to catch additional particles. It is anticipated this will need cleaning one to two times per year, although further testing is necessary. 

The team is also undertaking soil moisture testing of the nature strips and road pavement, to measure any potential changes beneath the pavement. 

Professional growth

Source: City of Monash

Withana encourages other councils to innovate, and push the boundaries.

“It was awesome, for me it’s one of the best things we’ve done. It was professionally satisfying to be one of the people delivering things, not just talking about it.

“This challenge provided us with the opportunity to look beyond the standard solutions. We wanted an out of the box solution therefore we looked elsewhere”

“During this process we learnt a lot, purely by working with experts that we wouldn’t normally get a chance to work with. We actually built things ourselves – getting the timber, cutting it up and digging the holes.

“I don’t think many councils do this kind of testing – but we thought what we want is not available, so why don’t we make it?”

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