Water Journal : Water Journal April 2011
refereed paper stormwater use water APRIL 2011 125 Abstract This paper outlines how harvesting of water from clustered house roofs in growth corridors is able to meet much, if not all, of the increased demand for water from growing towns and cities. The unique aspect of this principle is that water is harvested from all the roofs in a suburban subdivision via an independent pipe network that collects the water for further treatment as part of the town water supply. The regional roof water harvesting principle has been adopted for a growth corridor in Warrnambool, Victoria, providing a demonstration of how it can be successfully applied. Given that the population growth areas are concentrated along the coastal strip of Australia that typically has an annual rainfall of greater than 700mm, there is significant opportunity to use this principle. A Toolkit has been developed for those responsible for water resource planning to explore the opportunity of regional roof water harvesting in their towns. This paper also outlines the structure of the Toolkit and its power to explore opportunities to utilise this source of water in growing communities. Introduction Wannon Water is an urban water supply corporation serving the south-west of Victoria, including the provincial centre of Warrnambool, a fast-growing city currently in excess of a population of 28,000 facing increasing water demand. Localised climate change predictions also suggest increasing pressure on future availability of the existing water supply source. Growing towns and cities replace vacant land with roads, driveways, paved areas and roofs. From a water perspective, the net result is: • A 10-fold increase in runoff into the local rivers and streams; • Reduced volumes going to groundwater; • Creation of a 'heat island' effect; • An increased water demand; • An increased sewerage service demand. There is much discussion in the 'urban planning space' of how to make our cities more sustainable, with lower energy, water and ecological footprints while maintaining the living standards we have grown accustomed to. In many parts of Australia the harvesting of rainwater from the roofs of our growing urban areas can meet 100% of the annual demand of these new houses. Taking this component of water away from the 10-fold increase in runoff also reduces the adverse impact of development on the local rivers and streams. Rural Australia utilises and relies on rainwater from roofs for its daily existence, but larger towns and cities have a low dependence on roof water. Backyard rainwater tanks are slowly finding their way throughout suburbia, but to date this has had little bearing on reticulated demand. Rainwater tanks are also limited by storage capacity, with much of the water overflowing from the tank and lost, even during small rainfall events, and there is reluctance in urban situations to use such water for potable purposes without some form of disinfection. More recent developments have incorporated "Water Sensitive Urban Design" to reduce the peak flows and improve water quality, but have not addressed the better use of this resource. Regional roof water harvesting involves the construction of an independent roof water collection pipe network within the subdivision, in addition to the surface water (stormwater) network. The collected roof water can then be mixed with other raw water supplies before treatment, or be treated independently to meet drinking water standards. Either way, it contributes to the drinking water supply of the town or city. The Warrnambool Roof Water Harvesting Project involves the roof water being conveyed to an untreated water storage via a dedicated pipe to mix with other untreated water resources. It is then treated through the existing water treatment plant to become part of Warrnambool's drinking water supply. Numerous direct and indirect economic, environmental and social benefits to the local area have been identified, making this project a showcase of sustainability for the rest of the nation through the better use of available water resources and water-sensitive urban design. Current discussion about alternative water supply systems appears polarised into centralised versus decentralised models. This project provides a working example of a hybrid model, utilising a new source of water derived from a decentralised catchment, but linked to existing centralised storage, treatment and distribution. This innovative approach eliminates many of the public health risks associated with decentralised systems, and provides for coordination of the water supply demand cycle for regional towns and cities. P Wilson A Toolkit has been developed to explore the potential for regional roof water harvesting in local towns WARRNAMBOOL'S REGIONAL ROOF WATER HARVESTING Table 1: Water resource option comparison. Option Capital cost NPC $/ML# Ultimate yield per annum Groundwater resource development $7.81m $1,958 1500ML Regional roof water harvesting $11.03m $1,856 450ML Individual 5kL tanks $8.53m @ $5,482 210 ML* Notes: # The NPC is calculated over 34 years and divided by the discounted volume of water over that period (34 years to build out the catchment). @ If applied to all 3280 houses in the catchment. * Assumes all houses in the project's ultimate catchment (3280 houses) each have a tank that yields 70kL per year on average.
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