Dual-duty rainwater harvesting: water supply and urban stream restoration

Dual-duty rainwater harvesting: water supply and urban stream restoration

An exciting epoch is before us where we are focused on transforming urban living to a higher symbiosis with nature. For now, and looking over the immediate horizon, the pursuit is for water sensitive cities with green spaces which encourage a modern lifestyle that is considerate of, connected with and dependent on the natural environment. Practical realisation of this vision is perpetuated by innovations in water sensitive urban design (WSUD). The main objective is to capture, treat and use stormwater at its source, of which rainwater harvesting is fundamental.

Rainwater harvesting is well-known as a decentralised water supply alternative or supplement to the centralised water supply services of municipalities. The majority of design and assessment of rainwater tanks is focused on the reliability of supply. Additionally, rainwater tanks can significantly improve urban hydrology by capturing, consuming and effectively removing excess urban runoff. In this dissertation, a new approach is introduced to assess the combined outcomes of rainwater tanks. Dual-duty rainwater tanks are designed to restore degraded aspects of urban hydrology which stream ecosystems are particularly vulnerable to, while providing an alternate water supply.

The dual-duty performance framework is applied to examine the implications of enabling environmental flows from rainwater tanks. Research questions are explored: will environmental flows improve dual-duty performance; are adaptive approaches for managing environmental flow superior to a fixed leaking approach; to what extent do environmental flows diminish water supply; can rainwater tanks significantly improve urban stream hydrology in isolation to WSUD or other stormwater management initiatives; and what are the realistic expectations of dual-duty performance across the spectrum of urban residential living in Australia.

To answer these questions, a mass-balance rainwater tank simulator UrbanTank © was created and alternate storage arrangements and operating conditions were studied including the conventional tank, where the sole purpose is to supply rainwater to households and environmental flows do not occur; the leaking tank, which trickle-releases environmental flow from a virtual chamber of fixed volume; and the adaptive tank where environmental flow storage is actively regulated by the severity of rainfall statistics, rainfall forecasts and/or a combination of both controls.

Also, to qualify simulation results outdoor water use was linked to climatic indices of daily rainfall and daily maximum temperature. Rainwater yield estimates were verified by independent field measurements, simulation and statistical analyses throughout Australia. To allow a comparative assessment of all tank alternatives, a method was developed to supplement the limited duration of rainfall forecast archives.

The results demonstrate environmental flows, regardless of the method of operation, significantly improved dual-duty performance; the increasing complexity of adaptive approaches for managing environmental flows was not justified by a significant improvement in dual-duty performance over the simpler leaking tank arrangement; when enabling environmental flows the water supply independence dropped by a marginal 2% while the environmental benefits increased by 33%; the leaking tank was able to achieve on average a 90% compliance with natural hydrology measured by a simplified version of the environmental benefit index, which demonstrates rainwater tank can be used in isolation to WSUD or other stormwater management initiatives; and results from leaking tanks are encouraging over the breadth of simulation scenarios studied.

The dissertation concludes by establishing a relationship between dimensionless fractions and the key performance metrics of supply independence and environmental benefit index. These relationships facilitate rapid assessment of the dual-duty performance of conventional and leaking rainwater tanks across the spectrum of urban residential living. Rapid estimates are based on rainfall statistics, which can be potentially determined at any location in Australia and for similar climates elsewhere; and the scope of parameters studied which comprise roof area
(100 m2 to 200 m2), tank volume (2.5 kL to 7.5 kL) and annual rainwater demand (44 kL/y to 176 kL/y).

This dissertation has introduced a dual-duty framework for the design and assessment of rainwater tanks with a focus on minimising the degradation and demand municipalities place on contiguous water resources. These contributions to research have broadening our scientific knowledge and it is hoped the outcomes will expedite the promotion of water sensitive cities.