Rainwater Tanks for On-site Detention in Urban Developments in Western Sydney: An Overview

Transcription

1 Rainwater Tanks for On-site Detention in Urban Developments in Western Sydney: An Overview M van der Sterren 1, A Rahman 1, G Barker 2, G Ryan 2 and S Shrestha 1 1 School of Engineering, University of Western Sydney 2 Barker Ryan Consulting Pty Ltd, Sydney @student.uws.edu.au Abstract Increases in population, pollution and environmental controls require government authorities to review policies towards sustainable water management. A holistic approach is increasingly becoming popular to integrate water demand, supply and stormwater management in urban developments. This is likely to affect current on-site detention and rainwater tank practices. On-site detention, a common feature of modern urban drainage systems, is to cope with under capacity street drainage and for flood and erosion control purposes. Rainwater tanks though primarily used for water supply can be used for on-site detention. Some councils and government authorities have combined on-site detention systems and rainwater tanks to achieve an overall cost saving. This paper presents a brief overview of the on-site detention and retention practices adopted in greater Western Sydney. It has been found that policies differ significantly for different councils. Recent studies have focused on the use of on-site retention instead of on-site detention, which has raised the question on the scale of modelling. A research project has been set up to compare the flow characteristics and water quality of conventional on-site detention system with a rainwater tank fitted with appropriate WSUD device on redeveloped properties in Western Sydney to enhance the applicability of total water cycle management. Introduction Increased urbanisation affects urban water cycle in many different ways, for example by increasing the volume and velocity of stormwater runoff and demand of mains water. These effects have been attempted to be addressed in the past by separate government authorities. The philosophy behind Total Water Cycle Management (TWCM) requires the cooperation between these different authorities. TWCM attempts to decrease water demand and stormwater runoff by using a holistic water sustainable design practices (Chanan & Woods, 2006). This holistic water cycle management approach has also been designed to reduce the higher pollutant wash off from urban areas. In the past various methods have been recommended to minimise the effects of increased urbanisation on urban water cycle. These methods include BASIX requirements (NSWDP, 2007), rainwater tanks, detention, water sensitive urban design and water cycle management. Introduced by the NSW Government, BASIX, the Building Sustainability Index, ensures homes are designed to use less potable water and be responsible for fewer greenhouse gas emissions by setting energy and water reduction targets for house and units (NSWDP, 2007). The shift towards a TWCM requires a change in policy and design methods. This paper focuses on on-site detention and rainwater tanks as components of Water Sensitive Urban Design.

2 On-Site Detention (OSD) system is used to reduce peak outflows from developed sites to manage urban flooding. The use of OSD systems on a lot scale has been implemented in Australia since The OSD system was first implemented by Ku-ring-gai Council, closely followed by Wollongong City Council (O Loughlin et al., 1995). Since then many councils in Greater Sydney have implemented OSD system. In recent years, rainwater tank has become very popular mainly as an alternative source of freshwater. However, rainwater tank can also be used for OSD purpose. In 2004, 17% of the household in Australia source water from rainwater tanks (Trewin, 2007). Rainwater tanks are used in new developments as well as in redevelopment projects. There has been some research on the effects of rainwater tanks being used as on-site detention in urban catchments. This paper reviews current OSD and on-site retention (OSR) practices in general with a particular emphasis on rainwater tanks. This also identifies future research needs in this field. On-site Detentions System in Greater Western Sydney The detention basins were initially introduced to reduce the runoff from new development sites as a flood control measure. These detention basins could not be implemented in built-up areas due to space limitation as detention basins generally cover a large area (O Loughlin et al., 1995; UPRCT, 2005). With these restrictions in mind, source control was developed. This resulted in the development of detention on a lot scale. The Upper Parramatta River Catchment Trust (UPRCT) has conducted significant research on OSD systems and has developed design methods that are used to control the outflow, size of basin and size of orifice for a subject site (UPRCT, 2005). With the introduction of Sustainable Cities and Development and Best Management Practices (BMP) in the mid 1990 s, and the increasing concern of stormwater quality, OSD became a research focus (e.g. Butler & Parkinson, 1997; Urbanos & Stahre, 1993). The Stormwater Committee Victoria noted that an integrated approach directed at managing the volume and the rate of catchment run-off, the quality of the runoff and the habitats are necessary for supporting a healthy aquatic community (Stormwater Committee Victoria, 1999). This increase in research was further enhanced by the development of Water Sensitive Urban Design in the early nineties. This resulted in a change towards on site retention (OSR) in combination with OSD. Since 1991, the UPRCT has conducted stormwater modelling works using XP-RAFTS model for 100 year average recurrence interval (ARI) flow which resulted in a permissible site discharge (PSD) and site storage requirement (SSR) (UPRCT, 2005). These requirements are used to design the OSD system, which generally results in very large detention tanks. Some Councils have followed the lead by UPRCT and conducted modelling to determine PSD. Penrith City Council, for example, has conducted a simulation, which resulted in different PSDs for different areas of the Council. On the other hand, Councils such as the Blue Mountains City Council (BMCC) and Hawkesbury City Council (HCC) have not conducted such modelling, and use the pre-development run-off as the constraint to design the OSD system (Hawkesbury City Council, 2000; Blue Mountain City Council, 2005). Furthermore, Hawkesbury City Council and Blue Mountain City Council do not have a significant local catchment flooding problem and have therefore not implemented the UPRCT requirements. Hawkesbury City Council does have some flooding problems due to the Hawkesbury River and South Creek; however, these cannot be resolved with the use of

3 TCWM or OSD in Hawkesbury City Council itself. It requires upstream catchments, to address downstream effects. Hawkesbury City Council uses OSD to reduce the run-off from new developments or redevelopments to ensure that the stormwater drainage infrastructure is not undersized. Blue Mountain City Council, on the other hand, uses the OSD basins to control erosion as the waterways are steep and increased run-off increases the erosion. This independent approach has caused each Council to have different requirements for their OSD Policy or Development Control Plan (DCP). As each council has their own DCP in regards to stormwater, designers face differing regulations on a daily basis. This inconsistency has been a problem and has been highlighted by O Loughlin et al. (1995) as one of the areas in need of research. The suggestion of Councils combining their efforts to create one set of rules and regulations is partly implemented by UPRCT, however, Blacktown City Council (BCC), Baulkham Hills Shire Council (BHSC) and Holroyd City Council (HC) have included their own requirements above the UPRCT requirements (Blacktown City Council, 2005; Baulkham Hills Shire Council, 2004; Holroyd City Council, 2003). In addition to these varying regulations, concerns with respect to the requirements of OSD systems have been expressed. One concern being that OSD does not resolve the issue of flooding; it attempts not to worsen the existing flooding from local catchments (Smith, 1994). Another issue is the continuous maintenance that is required for such a system (O Loughlin et al., 1995). Attempts have been made to resolve these and other problems. One solution is the requirement of a maintenance schedule to be submitted with a Construction Certificate or Occupation Certificate application. This, however, does not guarantee of an adequately functional system or adequate maintenance of the system. On-site Retention Versus On-site Detention As discussed previously, current research has focussed on the use of OSR in combination with OSD. As a result of the drought, rainwater tanks as an OSR system have become a major research focus. Some councils, such as Blue Mountain City Council and Hawkesbury City Council, agree that the OSD system can be stacked on top of the BASIX requirement. As BASIX is a state government requirement, councils have to comply with this. It is however acceptable to stack the OSD requirements from the local councils on top of the BASIX requirements. This method results in rainwater tanks with three outlets, one for use down the bottom of the tank, one for orifice discharge half way up the tank, and the third outlet is an overflow at the top of the tank. Developers and clients from Barker Ryan Consulting have indicated that they would prefer investing in a large tank and use it for storage rather discharging stormwater to the drainage system. Some changes have been made to legislation. UPRCT has developed the fourth edition of the Stormwater Handbook to include OSR and WSUD, which has not been widely accepted by all involved Councils. This fourth edition has been based on research conducted recently on the use of OSR techniques by Coombes et al. (2001) and Cardno Willing (UPRCT, 2005). Coombes et al. (2001) focussed their study on the UPRCT. The study compared the outflow of OSD, OSR, and a combination of these two systems. It was assumed that the rainwater tank was connected to the laundry, toilets and outdoor areas and the OSD system was

4 designed according to the UPRCT handbook. The results showed that the airspace in a rainwater tank can be used as a credit for OSD. This credit varies for each development type and ranges from 32 to 65% depending on the design of the system. Coombes et al. (2001) also found that on the lot scale the OSD systems reduced the peak discharge as required, but the OSR only reduced the volume of discharge, the peak flows remained the same. Coombes et al. (2001) argued that peak discharges at the lot scale had little or no bearing on the floods at a catchment scale, as flooding is a volume driven process. However, a management measure that may reduce peak discharges at the lot scale but also reduces flood volumes can make an important contribution to reduce flooding. Joliffe (1997) and Argue (1997) have both been promoting the use of OSR since Herrmann and Schmida (1999), Andoh and Declerck (1999), Argue and Scott (2000) and Vaes and Berlamont (2001) discuss that the reduction of peak discharge on lot scale with OSR is not significant. Argue and Scott (2000) indicated that with a large scale model, the OSD and OSR systems produce a similar hydrographs. They agree that the peak discharge on a lot scale is larger for OSR then OSD, but in medium large catchments, the cumulative effect of volume reduction, under OSR, obliterates the effect of high peak discharges delivered by individual sites. This would indicate that the scale of the model would significantly change the outcome of the study. Coombes et al. (2001) acknowledge that the scale of the model has an impact on the results of the model. Coombes et al. (2001) suggest to model the OSD systems on a large catchment and scatter the OSD and OSR systems as current modelling practices are to model a single entity at the centroid of the catchment and this might incur misleading results. Beecham et al. (2005) agree with the catchment wide modelling, as not all the results from this type of analysis are obvious and special characteristics of the catchment and drainage system are important factors. It can therefore be said that modelling of OSR and OSD should be conducted on a catchment wide basis. The added bonus of the use of OSR is that there is a significant monetary saving to both citizens and government authorities. OSR will supplement water supply and reduce the need for upgrades in the water supply network (Barry and Coombes, 2006). However to ensure a continuous water supply through the OSR system a detailed design is required (Barry & Coombes, 2006; Villareal, 2005). Methods into designing such systems have not been completely developed or applied to Council DCPs. Barry and Coombes (2006) indicate that the design of such a system is highly depended on the spatial rainfall, water demand and topup rate and therefore can be difficult to design. Their results indicate that specialist knowledge and computer programs would be required. This would, however, create some issues in the industry and with Councils due to the lack of knowledge, monetary constraints and current industry practices. The majority of the previous research conducted would not be directly applicable to some councils, such as Hawkesbury City Council or Blue Mountains City Council. Most of the research is based on the UPRCT requirements and these requirements are not applicable to many other councils needs. The results of OSD versus OSR could be significantly different when applied to different design criteria. The above studies also do not take both WSUD and OSR into consideration. The overflows from the OSR are directly discharged to the stormwater drainage. It can therefore be said that future research should focus on the WSUD and OSR combination, with the overflow of the OSR connected (directly or indirectly) to the WSUD device.

5 Water Quality The water quality of OSD and OSR has also been under scrutiny. In regards to the water quality of rainwater tanks, a number of contrasting results have been reported. Evans et al. (2006) reviewed the current literature on water quality of rainwater tanks, and found that a clear consensus on the quality and health risks associated with rainwater has not been reached. Furthermore, research has also indicated, however, that the internal processes in the rainwater tank, such as sedimentation and micro-layer flocculation, can increase the water quality (Spinks et al., 2003). Most Councils do not permit water use from a rainwater tank for drinking (UPRCT, 2005; Hawkesbury City Council, 2000; Blue Mountain City Council, 2005) even though the New South Wales Department of Health does not prohibit its use. Cuncliff (1998) discussed how to maintain a water tank to increase the water quality. As the rainwater tank would be used on a daily basis, the chance of adequate maintenance is higher, as it is not out of sight, out of mind, as an OSD basin or OSD tank might be. One of the most utilised water quality improvement systems is a first flush device. A first flush device diverts the first part of the runoff (e.g first five minutes of runoff) away from the rainwater tank. Griffin et al. (1980) showed that the first 30% runoff generated by a storm has a higher contaminant concentration and contains nearly 70% of the total pollutant loads. Bucheli et al. (1998) and Förster (1999) found that the first 2mm will cause the first flush on a number of different roof types. The remaining storm will have a lower contaminant concentration and could be treated within the rainwater tank itself, by the processes described above. Goonetilleke et al. (2005) disagrees as the first flush is never precisely defined and they argue that the pollutant load, rather than the concentration is the governing factor for the design. Furthermore the diverted runoff is directly discharged to existing stormwater drainage. It is therefore suggested that the first flush from a rainwater tank is further defined for general designs and diverted to a WSUD device to further improve water quality of the discharge from the site. UPRCT (2005) has introduced a dual orifice to ensure that the smaller storms are contained in the OSD basins, to ensure that the water quality of receiving streams does not deteriorate during minor storms. Vaes and Berlamont (2001) showed that the reduction in peak flow and volume during a minor storm (5 year ARI) can be achieved. It can be concluded that the installation of a OSR system would reduce the peak flows during the minor storms and when installed in combination with a WSUD device, can increase the water quality of the receiving streams during these storms. On-site detention, on the other hand, only removes the gross pollutants by using a mesh screen placed before the orifice (UPRCT, 2005). In most cases, a pollutant filter is added to the design to reduce the contaminants below the required percentages. It can therefore be concluded that the rainwater tanks have a greater potential in removing the pollutants from the stormwater, especially in regards to the volume of runoff and the reduction of sediments.

6 Scope of Further Research Hawkesbury City Council has been developing a new DCP in cooperation with Barker Ryan Consulting to ensure that the use of rainwater tanks and WSUD is promoted within the Council. The investigation showed that previous research on the rainwater tanks with WSUD s elements might not be applicable to some Councils. As both Argue & Scott (2000) and Coombes et al. (2001) have based the OSD design on UPRCT requirements. As discussed above, these do not apply to either Blue Mountains City Council or Hawkesbury City Council. Furthermore, the water quality improvements resulting from a WSUD device with rainwater tank are likely to be far greater than the use of a single OSD system. A research project has been set up to compare the flow characteristics and water quality of conventional OSD system with a rainwater tank fitted with appropriate WSUD device on redeveloped properties in Western Sydney. This research will aim to provide a generic design method for developments and to determine the water quality improvement of rainwater tanks in combination with a WSUD device in comparison with existing OSD practices. This research will be carried out jointly by University of Western Sydney, various local councils and Barker Ryan Consulting in Western Sydney. Conclusions This paper presents a brief overview on the on-site detention and retention practices adopted in greater Western Sydney. It has been found that different approaches are adopted by different Councils to reflect existing land use, drainage system and catchment characteristics which have resulted in different methods for the design of OSD system. Recent studies have focused on the use of OSR instead of OSD, which has raised the question on the scale of modelling and method of design and implementation. A research project has been set up to compare the flow characteristics and water quality of conventional OSD system with a rainwater tank fitted with appropriate WSUD device on redeveloped properties in Western Sydney. This research is being carried out jointly by University of Western Sydney, various local councils and Barker Ryan Consulting to standardise the design methods of OSD and OSR to enhance the applicability of the techniques. References Andoh, R.Y.G., & Declerck, C., 1999, Source Control and Distributed Storage a Cost Effective approach to Urban Drainage for the New Millennium, Joliff, I.B. & Ball, J.E. (ed), Proceedings of the 8 th International Conference on Urban Storm Drainage, Sydney Hilton Hotel, Sydney, Institution of Engineers Australia. Argue, J.R., & Scott, P., 2000, On-Site Stormwater Retention (OSR) in Residential Catchments: A Better option? 40 th Annual Conference New South Wales Floodplain Management Authorities, Parramatta. Argue, J.R., 1997, On-Site Retention and Use (OSRU) The new Kid on the Stormwater Block, Proceedings of Future Directions for Australian Soil and Water Management, 9-12 th September 1997, Brisbane.

7 Barry, M.E., & Coombes, P.J., 2006, Optimisation of mains trickle top up supply to rainwater tanks in an urban setting, Australian Journal of Water Resources, 10(3), pg Baulkham Hills Shire Council, 2004, Development Control Plan No.5: Dual Occupancy, Baulkham Hills Shire Council, Castle Hill. Beecham, S, Kandasamy, J. & Trinh, D., 2005, Modelling on Site Detention on a catchment wide basis, Urban Water Journal, 2(1), pg Blacktown City Council, 2005, Engineering Guide for Development 2005, Blacktown City Council, Blacktown. Blue Mountains City Council, 2005, Better Living Development Control Plan, Blue Mountains City Council, Katoomba. Bucheli, T.D., Müller, S.R., Heberle, S. & Schwarzenbach, R.P., 1998, Occurrence and behaviour of pesticides in rainwater, roof runoff and artificial stormwater infiltration, Journal of Environment, Science and Technology, 32(22), pg Butler, D., & Parkinson, J., 1997, Towards Sustainable Drainage, Water, Science & Technology, 35(9), pg Chanan, A., & Woods, P, 2006, Introducing total water cycle management in Sydney: a Kogarah Council initiative, Desalination, 187 (1-3), pg Coombes, P.J., Frost, A., Kuczera, G., 2001, Impact of Rainwater Tanks and On-Site Detention Options on Stormwater Management in the Upper Parramatta River Catchment, Department of Civil, Surveying and Environmental Engineering, University of Newcastle, August Cuncliff, D.A., 1998, Guidance on the use of rainwater tanks, National Environmental Health Forum Monographs, Water Series Number 3, Public and Environmental health Service, Rundle Mall, Australia. Evans, C.A., Coombes, P.J., & Dustan, R.H., 2006, Wind, Rain and Bacteria: The effect of weather on the microbial composition of roof-harvested rainwater, Water Research, 40(1), pg Förster, J. 1999, Variability of roof runoff quality, Journal of Water, Science and Technology, 39(5), pg Goonetilleke, A, Thomas, E., Ginn, S. & Gilbert, D. 2005, Understanding the role of land use in urban stormwater quality management, Journal of Environmental Management, 74(1), pg Griffin, M.G., Clifford, R. & Grizzard, T.J., 1980, Efficient design of stormwater holding basins used for water quality protection, Water Research, 14(10), pg

8 Hawkesbury City Council, 2000, Development Control Plan 2000, Hawkesbury City Council, Windsor. Herrmann, T. & Schmida, U, 1999, Rainwater utilisation in Germany: efficiency, dimensioning, hydraulic and environmental aspects, Urban Water 1(4), pg Holroyd City Council, 2003, Revised On Site Detention Policy, Holroyd City Council, Holroyd. Joliffe, I. (1997), Roof Tanks A Role in Stormwater Management, Proceedings of Future Directions for Australian Soil and Water Management, 9-12 th September 1997, Brisbane. New South Wales Department of Planning (NSWDP) (2007). The Building Sustainability Index (BASIX). O Loughlin, G., Beecham, S., Lees, S., Rose, L., & Nicholas, D., 1995, On-Site Stormwater Detention Systems in Sydney, Water Science & Technology, 32(1), pg Smith, K, 1994, On Site Detention: Is it the most effective way to resolve flooding problems?, Proceedings of the 2 nd Annual Conference Soil and Water Management for Urban Development, 6-7 September 1994, Sydney. Spinks, A.T., Coombes, P.J., Dunstan, R.H. and Kuczera, G., 2003, Water Quality Treatment Processes in Domestic Rainwater Harvesting Systems:, Proceedings of the 28 th International Hydrology and Water Resources Symposium, November 2003, Wollongong, Australia, Institution of Engineers, Australia. Stormwater Committee Victoria, 1999, Urban Stormwater Best Management Practice Environmental Management Guidelines, Collingwood, CSIRO. Trewin, D., 2007, 2007 Year Book Australia, Number 89, Australian Bureau of Statistics, Canberra, source: accessed: 02/04/07. Upper Parramatta River Catchment Trust, 2005, On-Site Stormwater Detention Handbook, 4 th Edition, UPRCT, Parramatta, Australia. Urbanos, B & Stahre, P, 1993, Urban Stormwater Best Management Practices and Detention for Water Quality, Drainage and CSO management, New Jersey, Prentice Hall. Vaes, G & Berlamont, J, 2001, The effect of rainwater storage tanks on design storms, Urban Water, 3(4), pg Villarreal, E.L. & Dixon, A., 2005, Analysis of rainwater collection system for domestic water supply in Ringdansen, Norrköping, Sweden, Building and Environment, 40(9), pg