Siphonic roof drainage comes out on top
Most downpipe systems have a very inefficient way of discharging water from roofs because water spirals down around the inner edges or walls of the downpipes in what is called annular flow (in the form of a ring), according to UniSA’s Professor of Sustainable Water Resources Engineering, Simon Beecham.
“At no time does the water ever exceed a third of the capacity of the downpipes,” Prof Beecham said. “The reason is that the downpipes are discharging two-thirds air and one-third water.
“Siphonic roof drainage works very differently. Rather than letting the water fall under gravity, which is what happens with traditional downpipes, it sucks the water off the roof at velocities up to seven metres per second. This means that instead of having downpipes every ten metres or so along the sides of large buildings, the water can be piped horizontally at roof level to almost any point in the building before being brought down to a single collection pipe at ground level,” Prof Beecham said.
“With gravity flow the water quickly reaches a maximum velocity that it can’t exceed (about two metres per second). Also, there is so much air in the vertical downpipes that many are needed to discharge the volume of air and water from buildings. Siphonic drainage replaces these downpipes with a single large pipe full of water that discharges at a very high flow rate.
“The siphonic system works like a domestic water supply. A lot of pressure or ‘head’ that has been built into the water supply system drives the flow of water out whenever a tap is turned on in any part of the house.”
The system self primes (to exclude air), which creates an operating head that sucks the water to any point in the building. Priming is achieved by placing a metal baffle just above the outlet to restrict air intake. Excluding air makes the flow more efficient because pressurised water flow can be generated.
“Priming can easily be explained by emptying a bath of water. When the plug is released, you can see a spiralling air core going down the drain. If you place your hand over that air core above the outlet, the spiralling disappears and the bath empties much quicker because full flow conditions have been initiated,” Prof Beecham said.
The design of siphonic drainage systems involves complex hydraulic analysis and Prof Beecham is the author of the software, called the Syfon program, which is now the most widely used software for siphonic roof drainage systems in Australia.
This revolutionary technology is employed at the Telstra Stadium in Sydney and at the Sydney, Hong Kong and the new Adelaide international airport terminal buildings. All of these buildings are characterised by elegant architectural form, largely because the architects have not had to deal with a multitude of downpipes.
Siphonic hydraulic drainage utilises the height of a building by converting that height into potential energy to drive the flow of water. It’s a sophisticated system that needs to be very carefully engineered by highly qualified hydraulic specialists with years of training.
The system is not designed for domestic use. It can only be used on significant roof structures because of the extremely complex hydraulics involved.
In addition to writing the Syfon program, Prof Beecham is chair of the Standards Australia committee charged with developing one of the world’s first standards on siphonic roof drainage.
“This involves research to increase our understanding of the very complex hydraulics involved in siphonic systems and UniSA is heavily engaged with that process. We have a strong working relationship with the largest manufacturer of this technology, Syfon Systems, which is co-funding our research at UniSA,” he said.
Prof Beecham is also the author of SWITCH and Switch2 water sensitive urban design software packages, which are recommended in Engineers Australia’s Australian Runoff Quality Manual.