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Rainwater  Harvesting Options For Commercial Buildings - Australia
 

Rainwater Harvesting Options For Commercial Buildings - Australia

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Rainwater Harvesting Options For Commercial Buildings - Australia

Rainwater Harvesting Options For Commercial Buildings - Australia

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    Rainwater  Harvesting Options For Commercial Buildings - Australia  Rainwater Harvesting Options For Commercial Buildings - Australia Document Transcript

    • Rainwater harvesting options for commercial buildings usingsiphonic roof drainage systems----Lessons for Building SurveyorsTerry Lucke1, Simon Beecham 1 and George Zillante 11 School of Natural and Built Environments, University of South Australia(terry.lucke@.unisa.edu.au)ABSTRACTWater conservation is an integral part of sustainable building practice and WaterSensitive Urban Design (WSUD). New building design priorities have beenestablished in Australia that focus on reducing the consumption of both energy andwater. The stormwater runoff from commercial buildings is one area in which muchpotential for improvement has been identified. Gone are the days where the onlyconcern with roof runoff was to ensure its rapid removal from the site. The need toharvest this precious resource has been recognised and new technologies are emergingto resolve this issue. Siphonic roof drainage is a relatively new building servicestechnology which has many benefits over conventional drainage systems. Buildingdesigners and architects are specifying siphonic roof drainage systems on anincreasing number of commercial and industrial buildings. For example, SydneyOlympic Stadium, the Norman Foster designed Chek Lap Kok airport in Hong Kongand the new International Terminal Buildings at Adelaide and Sydney airports allhave siphonic roof systems. The benefits of these systems include, ability to quicklydrain high intensity rainfall events, substantial cost reductions, virtual elimination ofunderground pipework and the opportunities for significant stormwater reuse options.This paper aims to provide an overview of the many benefits that are being realised byplanners, building designers, engineers, architects, surveyors, contractors and ownersby specifying a siphonic roof drainage system. Furthermore, it will examine the waterconservation and reuse options that are possible with siphonic drainage and comparethese to conventional roof and property drainage systems.KEYWORDSSiphonic roof drainage, stormwater harvesting, water reuse, building services designAIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 44using siphonic roof drainage systems- lessons for Building Surveyors’
    • INTRODUCTIONCommercial siphonic rainwater drainage systems were first developed by Ebeling andSommerhein in the early 1970s in Scandinavia (May, 1995). Since then, manythousands of new buildings worldwide have been designed incorporating siphonicroof drainage systems. The advantages of these systems over conventional roofdrainage systems are numerous and have much appeal for architects and designers.Siphonic Drainage is growing from a once “obscure curiosity” in Europe to anemerging market in the United States (Rattenbury, 2005). Because of the heightrequirements needed for siphonic roof drainage systems, the technology is only viablefor larger commercial buildings and structures over about four metres in height (Referto Plates 1 and 2) Plates 1 and 2 – Qantas Domestic Terminal in SydneySiphonic systems are designed to exclude air from the pipework and, once primed,cause the pipes to flow under pressure. Syphonic roof drainage systems have strategicadvantages over conventional systems, and particularly so in respect of their cost-effectiveness to quickly remove large volumes of rainwater safely and effectively(Brahmall and Saul, 1999). A major advantage of siphonic systems is the greatlyincreased driving head of water and consequent reduction in pipe diameter sizes. Thedriving head in this case is effectively the difference in level between the water in thegutter and the ultimate discharge point, which is usually near ground level.The increased driving head in siphonic systems offers much potential for stormwaterharvesting and reuse. Because the building’s total roof runoff is normally dischargedfrom only one or two downpipes with high velocity, the water can easily be directedto most places on a development site without the need for pumping. This means thatAIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 45using siphonic roof drainage systems- lessons for Building Surveyors’
    • rainwater tanks can be placed in a convenient location away from the buildingfootprint if so desired. The strategic location of stormwater collection tanks can thenfacilitate energy-free landscape irrigation or other reuse options.CONVENTIONAL ROOF DRAINAGEConventional roof drainage systems consist of a number of large diameter downpipeswhich connect the roof drainage to the underground stormwater drainage system.Conventional systems are designed to operate at atmospheric pressure (May, 1995).The amount of water that can enter the open-ended downpipes is dependent on thedepth of water in the gutter (H) and the outlet size (D). The type of flow is categorisedas either “weir” type or “orifice” type flow (May, 1995). Refer to Figure 1. Air Water H = head of water in gutter D = diameter of downpipe Figure 1 – Conventional Gutter Outlet and DownpipeResearch has shown that the water flowing in the downpipe in a conventional roofdrainage system is annular in nature (Wright, Jack and Swaffield, 2006). This meansthat the water spirals down the inner edges or walls of the pipe and there is a hollow,air-filled core down the centre of the water flow (Arthur, Wright and Swaffield,2005). The air that is drawn down by the water actually restricts the water dischargein a pipe to between one quarter and one third of the pipe cross section area. Thismeans that large diameter pipes are required to enable the gutters to drain quicklywithout risk of overflowing. These types of conventional roof drainage systems arevery inefficient and require extensive underground pipework systems (Figure 2).AIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 46using siphonic roof drainage systems- lessons for Building Surveyors’
    • Figure 2 – Conventional Roof Drainage SystemSIPHONIC DRAINAGESiphonic roof drainage systems are currently designed to operate with full bore flowwithout the need for any mechanical pumping either to prime or operate the system.The gutter outlets in siphonic roof drainage systems are specifically designed torestrict the inflow of air into the pipework and this allows a much greater volume ofwater to flow in the pipe. Because there is limited air in the pipework, the fallingwater generates a vacuum behind it which “sucks” the water into the gutter outlet.This vacuum effect around the outlet allows more water to be drawn into the piperesulting in much greater flow rates (Arthur and Swaffield, 2001).Unlike conventional roof drainage, in siphonic systems gutter outlet pipes are directedinto a horizontal collection pipe which often then flows into a single downpipe(Figure 3). This collection pipe usually runs at roof level close to the gutters collectingall the water flowing out of the gutter (Arthur and Swaffield, 2001). The total volumeof water collected then flows into the downpipe and is often discharged at a singleoutlet at ground level. This outlet can be located directly in the undergroundstormwater system or redirected to a rainwater tank for harvesting and reuse (Figure3).AIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 47using siphonic roof drainage systems- lessons for Building Surveyors’
    • Figure 3 – Siphonic Roof Drainage SystemBecause the roof runoff from siphonic systems is usually directed into a singledownpipe, the normally extensive, underground drainage pipe system is virtuallyeliminated. Besides the obvious benefit of lower excavation and pipe costs, siphonicsystems also minimise potential building damage associated with footing movementsin reactive soils. As there are generally no drainage pipes under the slab or parallel tofootings, soil heave problems caused by leaking pipes are also eliminated.Siphonic drainage systems are designed using pipe full flow conditions which assumeno air in the system. As such, most siphonic outlets are specially designed to reducethe amount of air entering the system. This is often achieved by means of a horizontalbaffle plate configuration at the siphonic outlet installed in the gutter floor. Thesebaffle plates restrict the formation of a vortex above the outlet which would suck airinto the system and break the siphon action. Other outlet configurations include a typeof “upturned dish” arrangement which forms an airlock to restrict the air entrainmentinto the flow (Figure 4).AIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 48using siphonic roof drainage systems- lessons for Building Surveyors’
    • Figure 4 – Typical Siphonic Outlet Showing Baffle PlateSYSTEM PRIMINGSiphonic rainwater drainage systems are designed to operate under both part-full andpipe-full conditions. The transition between these two states involves priming orunpriming of the system, both of which involve considerable air entrainment.Priming is the term used to describe the process where resistance to flow is sufficientto cause the pipe system to become full of water. It is the friction and form losseswhich are present in every pipe flow which resists the movement of the water andassists in the development of pipe-full flow conditions.The design of siphonic systems involves dynamic balancing of the pipe systems whenthey are flowing under pressure. Friction and form losses in pipes are proportional tothe square of the velocity of the fluid and are cumulative. This means that the furtherthe water travels, the more energy it loses and consequently the less volume of watercan flow. In order for the siphonic outlet flows to be balanced, smaller tail pipediameters are often used closer to the vertical downpipe to reduce the flow volumes tosimilar flows upstream.The priming sequence usually occurs in three phases. As the water level in the gutterincreases, so does the flow into the vertical tailpipe. This flow enters the lateralcollector pipe in a supercritical state and thereafter forms a hydraulic jump. As longas the subcritical depth downstream of the jump is less than the pipe diameter, theflow in the vertical stack remains annular, as for conventional gravity flow systems.As the flowrates increase, the subcritical depth downstream of the jump also increasesAIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 49using siphonic roof drainage systems- lessons for Building Surveyors’
    • until it eventually reaches the soffit of the pipe. The plug of air trapped upstream ofthe jump then moves downstream until it is expelled down through the vertical stack.At this point the system is said to be primed. The weight of the water in the fullflowing vertical downpipe leads to the generation of negative pressures in theupstream horizontal collector pipe.UniSA SIPHONIC DRAINAGE TESTING FACILITYAs part of an ongoing PhD research project in siphonic roof drainage systems titled,“The role of air entrainment in the performance of siphonic roof drainage systems”, a full scale siphonic drainage rig has been constructed in the hydraulics laboratory atthe University of South Australia (UniSA) Mawson Lakes Campus (Refer Plate 3).This research is being undertaken in collaboration with Syfon Systems of Melbourne,a leading Australian siphonic drainage company since 1992.The dimensions of theUniSA rig are 32m long by 6m high by 3m wide. This is, according to the literaturereviewed so far, the largest laboratory-based siphonic testing facility in the world.In order to allow visual observation of the flow patterns, perspex was chosen for thepipework material. Hydraulic calculations undertaken for this system usingcommercial siphonic software predicted that the expected maximum flow rate in thelaboratory will be 69 litres per second. This is equivalent to the rainfall of a 1 in 300year storm event falling on the entire roof of the hydraulics building in which themodel is housed. The maximum flowrate measured through the rig has been 70 litresper second which thoroughly agrees with the calculations. This demonstrates theaccuracy of the commercial software used in designing this system.AIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 50using siphonic roof drainage systems- lessons for Building Surveyors’
    • Plate 3 – UniSA Siphonic Drainage Testing Rig (Mezzanine floor shown is 6m above ground floor)OBSERVED FLOW PATTERNSResearch into siphonic roof drainage systems has been undertaken at Heriot WattUniversity in Edinburgh, Scotland. The principal researchers have been Swaffield,Wright and Arthur of the Heriot-Watt Drainage Research Group. They described theformation of hydraulic jumps within the pipework and the role they play in priming.Arthur and Swaffield identified three main effects that air entrainment has on systemperformance. Air affects system operating pressure, propagation velocity and frictionlosses. Their research has identified various interesting phenomena of siphonicdrainage which will be further investigated on the UniSA rig.The design of the testing facility at UniSA has evolved from studying the limitationsof existing experimental models identified in the research literature. The UniSA righas much larger pipe diameters than previously built models and has four gutteroutlets. At six metres high, the apparatus is considered to be representative of realsiphonic roof drainage systems in use.AIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 51using siphonic roof drainage systems- lessons for Building Surveyors’
    • OPPORTUNITIES FOR WATER RECYCLINGConventional roof drainage systems consist of a number of large diameter downpipeswhich connect the roof drainage to the underground stormwater drainage system. Asthe flow in these downpipes is annular, the flow volumes are relatively smallcompared to the downpipe diameters. This means that most of the potential energy ofroof gutter water is expended by the time it enters the underground drainage system.This results in low energy stormwater distributed along the entire length of theunderground drainage pipe system. In order to collect this water for re-use, some typeof collection pit and pumping system would normally be needed. This not onlyincreases system costs and energy consumption but also places landuse restrictions onthe building’s surrounding area.As previously discussed, the discharge from siphonic roof drainage systems is usuallyfrom a single, full-flowing downpipe at high velocity. This can generally enable thestormwater to be directed to any part of a development site, even to the highestelevated areas. Rainwater tanks or other collection devices can then be used to harvestand store the rainwater for later reuse. This stormwater can be utilised for manydifferent activities ranging from simple landscape irrigation to toilet flushing, vehiclewashing and other uses.Other areas of the research to be undertaken at UniSA include the effects of airentrainment on the performance of siphonic systems and investigation into thetransition area between siphonic downpipe outlets and underground drainage systems.The design of siphonic outlets to minimise air entrainment will also be investigated.BENEFITS FOR BUILDING AND BUILDING SURVEYINGAs discussed previously, the normally extensive, underground drainage pipe system isvirtually eliminated. Besides the obvious benefit of lower excavation and pipe costs,siphonic systems also minimise potential building damage associated with footingmovements in reactive soils. As there are generally no drainage pipes under the slabor parallel to footings, soil heave problems caused by leaking pipes are alsoeliminated.AIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 52using siphonic roof drainage systems- lessons for Building Surveyors’
    • In summary the major benefits of siphonic roof drainage systems are:• fewer downpipes are needed• smaller pipe diameters are needed• high flowrates for self cleansing• pipes can be laid without fall• pipework can be concealed in roof cavity (aesthetics)• single outlet means minimal ground excavation• building damage from leaking underground pipes minimised• significant material cost savings• single outlet enables stormwater harvesting and reuseCONCLUSIONSWater conservation is an integral part of sustainable building practice. New buildingdesign priorities have been established in Australia that focus on reducing theconsumption of both energy and water. Stormwater runoff from commercial buildingsis one area that has much potential for water harvesting and reuse. Figure 5 – Conceptual Siphonic Roof Drainage Reuse ConfigurationThe increased driving head of siphonic roof drainage systems compared toconventional systems offer many stormwater harvesting and reuse options. Becausethe building’s total roof runoff is normally discharged from a single downpipe withhigh velocity, the water can easily be directed to most places on a development sitewithout the need for pumping. This means that rainwater tanks can be placed in aconvenient location away from the building footprint if so desired. The strategiclocation of stormwater collection tanks can then facilitate energy-free garden wateringor other reuse options.AIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 53using siphonic roof drainage systems- lessons for Building Surveyors’
    • A full scale siphonic drainage rig has been constructed in the hydraulics laboratory atthe University of South Australia (UniSA) Mawson Lakes Campus andcomprehensive testing has been conducted. A method of measuring the harvestingpotential has been established.References:Arthur, S. and Swaffield, J.A. (2001). “Siphonic roof drainage: current understanding.” UrbanWater, 3, Taylor and Francis, pp. 43-52.Arthur, S., Wright, G.B., and Swaffield, J.A. (2005). “Operational performance of siphonicroof drainage systems.” Building and Environment 40, pp. 788-796.Bramhall, M.A., and Saul, A.J. (1999). “Hydraulic performance of siphonic rainwateroutlets.” Proceedings of the 8th international conference on urban stormwater drainage.Sydney. AustraliaMay, RWP, (1995), Design of conventional and siphonic roof drainage systems, PublicHealth Services in Buildings - Water Supply, Quality and Drainage, IWEM Conference,London.Rattenbury, John.M, (2005), Siphonic Roof Drainage: Where Is It Headed?, PM Enginneer,Plumbing and Architecture & Construction Groups, Troy, MI, USAWright, G.B., Jack, L.B., and Swaffield, J.A. (2006). “Investigation and numerical modellingof roof drainage systems under extreme events.” Building and Environment 41, pp.126-135.AIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 54using siphonic roof drainage systems- lessons for Building Surveyors’