Cousins, W.J. 2015 Potential for rainwater harvesting to make Wellington liveable after a large earthquake. Lower Hutt, N.Z.: GNS Science. GNS Science report 2015/01 41 p.
Abstract: Metropolitan Wellington Region is vulnerable to large earthquakes. Of particular concern is a possible “gap” in water supply following a large earthquake on the Wellington Fault. The gap arises because restoration of bulk supply to reservoirs, or other emergency supply points, could take weeks to months, whereas water stored in reservoirs and at people’s homes is likely to be depleted within a few weeks. There are many ways of reducing, or even eliminating, the gap, including engineering measures such as developing new bulk-water sources, constructing large water storage reservoirs or lakes near Wellington, and upgrading old brittle pipes in the transmission networks, or taking water from unconventional sources like drainage water from tunnels, desalination, or rainwater harvesting. The potential benefits of one of the unconventional source, rainwater harvesting, are evaluated here. Wellington, on average, receives more than enough rainfall to meet the post-disaster emergency water demand of 15–20 litres per person per day. Variability in the daily rate of precipitation, however, can lead to water deficits lasting from days to weeks. Simplistic modelling using a family of four people, in a house with 100 m2 of roof collector area, a storage tank of 1200 m2, and seventy-seven years of daily rainfall data from Kelburn, Wellington, showed that the deficit was weakly dependent on the total annual rainfall, but strongly dependent on month-to-month variations. The month-to-month variations included both variations in monthly total rainfall and, more importantly, day-to-day variations within a month. Long dry spells in summer, for example, were a particular problem. Large-scale modelling involving the 75,000 Wellington people living more than 1km from their nearest reservoirs, demonstrated that having 300 litres per person (l/p) of rainwater storage would give acceptable security of emergency supply for most of the people most of the time, assuming total dependence on rainwater. Whole-city modelling, involving a Wellington Fault earthquake, showed that acceptable security of emergency supply for most of the people, most of the time, could be achieved with rainwater storage of either 200 l/p, or 1000 litres per building. In this case the emergency water was a combination of reservoir water, household personal supplies and the dedicated rainwater tanks. The above findings were for a scenario of median water deficiency. For an 85th percentile scenario, it was necessary to double the storage capacity to achieve a similar security of supply. Post-earthquake rainwater harvesting could have great benefits. It could be very difficult, however, to persuade large numbers of householders to install rainwater tanks, and so the greatest benefits are likely to be (a) to individuals who choose to install tanks of about 300 l/p so that they could exist on rainwater alone, and (b) for people who improvise storage facilities after the earthquake. (auth)
