Assessment of the impact of rainwater tanks and on site detention on urban run-off quantity and quality characteristics

Abstract

Stormwater run-off from urban developments, if left untreated can be detrimental to the quality of the receiving waters. To counteract the effects of urban development on the natural environment, holistic management strategies and treatment at the source have been introduced in Australia, in the form of catchment management authorities, legislation (e.g. Building And Sustainability IndeX) and design techniques, such as Water Sensitive Urban Design (WSUD). In practice, these principles result in lot scale (re)development with a Rainwater Tank (RWT), an On Site Detention (OSD) system and an infiltration or bio-retention system, with most of the overflows discharging to the existing drainage systems. It is argued in this thesis that the implementation of these systems on a lot scale often results in over design and can be considered costly for developers, thereby reducing the opportunity of (re) developments. OSD is currently installed only to control water quantity therefore, the question raised in this thesis is what effect does a RWT have on water quality and quantity discharges on a lot scale and how does this affect the discharges on a catchments scale. This study was based in Western Sydney, in particular Hawkesbury City Council (HCC), which is one of the fastest growing areas in Sydney and is part of the North-West growth sector. A developed catchment, with known drainage issues, and five RWT were selected within the Council area for the longitudinal cross-sectional water quality and quantity data collection. The results of this longitudinal cross-sectional investigation were utilised in a commercial modelling software package (XP-SWMM) for calibration and verification of a lot and catchment scale stormwater quality and quantity models. Testing of the collected water quality samples revealed that the overflow of a RWT had elevated numbers of microbes, and high concentrations of nutrients and some heavy metals. This contamination was speculated to be the effect of run-off and possibly biofilm growth at the air/liquid interface, flowing out of the tank. Furthermore, the data also indicated that RWT are more likely to exceed the drinking water guidelines for lead, Escherichia coli (E. coli) and Enterococci spp. after a storm event. The modelling of the lot scale showed reduction in discharges due to a RWT on-site, but the amount of reduction in the discharges was dependent on the end uses of the RWT. It also indicated that up to a 1-year Average Recurrence Interval (ARI) storm event could be stored within the RWT, provided the RWT is connected to multiple end uses. The lot scale water quality and quantity modelling on a lot scale showed minimal errors with the observed data. The catchment model indicated a 6% reduction in predicted run-off discharges to the receiving stream, if RWT are utilised throughout the catchment, but can increase in volume due to significantly reduced overland flooding. This shows that the gradual implementation of RWT through governmental incentives and (re) developments can have a notable impact on the run-off discharges from a catchment. It is concluded, as a result of these findings, that significant changes should be made in the relevant council legislation. These recommendations include strategies to assist in the implementation of WSUD on a catchment scale and development of RWT design guidelines. An OSD system can be replaced with a RWT for up to the 1-year ARI rainfall event. Further investigations should be conducted on the effect of bio-retention systems on the discharges from both a lot and catchment scale developments, and the contamination levels in the associated overflows from the RWT and filtration systems.

Date of Award	2012
Original language	English