Using Rainwater Saves Money While Going Easy on the Earth

Forest and Bird, Issue 303, 1 February 2002, Page 48

Using Rainwater Saves Money While Going Easy on the Earth

t rains a lot up north — often as much as it does on the infamous We’t Coast. The problems of flash flooding and stormwater are much the greater in northern cities, however, with rains running across sealed streets and hardstanding, straight into the sea. North Shore City, which has already given a lead to engineers with alternative ways to handle stormwater, is now encouraging home-owners (and businesses) to save some of the rainwater in holding tanks and use it later. The argument goes that even a small rainwater tank can help prevent the the localised flooding and erosion that results from a small storm. Collecting rainwater can also significantly reduce demand on the city water supply. In the case of North Shore City, where the suburbs drain quickly to the sea, arresting stormwater also helps improve water quality at the beach. Approximately 65 percent of household water demand could be met by rainwater collected from the roof, according to the city council. It could be used in the laundry, for flushing the toilet and for watering the

garden. The share of the kitchen, which is only 10 percent of useage, and the bathroom (25 percent) would still come from mains water. If every household could install a 5000-litre rainwater tank, the city could halve water demand while significantly reducing beachwater pollution. The council also advocates using smaller tanks, such as rain barrels which hold 240 litres, to gather rainwater from smaller roofs (30 square metres). The installation of a 4500litre tank, collecting rainwater from a typical-sized roof, can reduce peak stormwater runoff by 20-35 percent. Outside its main library, in Takapuna, the council has a demonstration of what can be done, relatively cheaply. A 5000-litre tank takes rainwater run-off from the library complex and provides a source for gardeners maintaining the gardens in the city centre. The system cost $900 for the tank and $300 to install. The council has been offering help and grants for people prepared to introduce ways of using stormwater in

the city. These include installing rainwater tanks, the planting of ‘rain gardens’ where stormwater may

disperse, forming grassy swales and berms, and replacing impervious surfaces with permeable alternatives.