WESTERN REGIONAL WATER BALANCE

Transcription

1 WESTERN REGIONAL WATER BALANCE Greg Finlayson 1, Ryan Brotchie 1, Steven Roach 1, Lauren Mittiga 2, Abigail Farmer 3, John Chambers 3 1. GHD, Melbourne, VIC, Australia 2. Melbourne Water, Melbourne, VIC, Australia 3. Victorian Government, Melbourne, VIC, Australia ABSTRACT A MS Excel based tool was developed to model the current and future urban, rural and environmental water cycle for the Western Region of Greater Melbourne to inform the Victorian State Government s Water Future West strategy. Large population growth to 2050 will result in increases in water demands as well as pollutants entering waterways and Port Phillip Bay from both stormwater runoff and wastewater. Whole of system scenarios to reduce the demand for potable water from the Melbourne System, to improve environmental outcomes for waterways and the Bay and to improve reliability of supply to agriculture were investigated. INTRODUCTION The Water Industry and State Government in Victoria were investigating how to manage the water cycle into the future taking an integrated view of urban, rural and environmental water cycle planning. This paper summarises the development of a suite of tools to model the combined water, contaminant and energy balance for the whole of water cycle of the Western Region of Melbourne to the year The tool is being used to to inform the Water Industry s long term strategy for the Western Region of Melbourne. The partners to the project were the Victorian Government, Melbourne Water, City West Water, Western Water and Southern Rural Water. Background The study area was the Western region of Greater Melbourne. For the purposes of water planning at the time this area was defined to bound the Werribee catchment and the Maribyrnong catchment, two of the five major catchments in the Port Phillip and Westernport region of Victoria. The area covers about 4,179km 2 and is shown in Figure 1. Land use across the region is diverse. There are existing residential, commercial, industrial and agricultural land uses, a number of smaller periurban towns, as well as large areas of rural land which will become new urban areas over the next 35 years. Waste Water The urban areas in the region generate significant quantities of wastewater, and these will increase over time. There are several, including two major, inland Recycled Water Plants operated by Western Water. Some of the wastewater is treated for reuse locally, and some is treated and discharged to local waterways. The bulk of the wastewater is sent to the major wastewater treatment plant at Werribee, the Western Treatment Plant (WTP), where it is discharged into Port Philip Bay after treatment. WTP is operated by Melbourne Water and treats about half of Melbourne s sewage and reuses around 25 GL/yr of recycled water onsite for agriculture and provides conservation flows to the [Ramsar listed] Lake Borrie Wetlands. Water Supply The region includes two major rivers and their related tributaries -the Werribee and the Maribyrnong. Both of these river systems have a number of reservoirs which act as storages for water supply. These provide water for irrigation and other rural uses as well as and for urban water supply to many of the peri-urban towns. Many of the region s waterways are water-stresed, and their conditions range from pristine to degraded. Climate There is large climate variability across the region, including average annual rainfall ranging from less than 500 mm at the inner west to over 800 mm in the Macedon Ranges and over 1000 mm at the Werribee catchment headwaters. Climate change in the Western region of Greater Melbourne is projected to result in a hotter drier

2 climate. The key impacts of lower total rainfall and higher evaporation and evapotranspiration, from a regional water balance perspective, are reduced catchment runoff and reduced inflows to bulk water supply reservoirs, and higher urban and rural irrigation demands. Agriculture There are large agricultural enteprises in the region, including the two regulated Irrigation Districts at Bacchuss Marsh and Werribee, the onsite irrigation scheme at WTP, as well as irrigated agriculture, horticulture and viticulture distributed across the region. Summary Overall, the changes to the water cycle in the Western Region of Melbourne over the next 50 years will be significant. The investigation considered all elements of the water cycle, and compared the impact on the water balance, contaminant and energy balance for various regional options. APPROACH Whilst water balance work has been undertaken for a number of different areas, this study was unique in that it examines all of the following points in an integrated way: Supply sources modelled include rainwater, excess urban stormwater runoff, ground water, bulk water available from local surface water sources, wastewater and recycled water, and potable supplies from Melbourne Headworks and the Wonthaggi Desalination Plant. Demands modelled include residential and nonresidential (disaggregated by end-use), rural irrigation demands, farm dams, and urban irrigation demands to provide water for greening and environmental water requirements. Key to the analysis was the development of land use maps representing the level of urban development at different points in time out to These form the basis for estimating and modelling at a fine spatial resolution population and dwelling distributions, water demands, imperviousness and stormwater runoff volumes, etc. The project included development of a suites of tools to allow different supply and demand scenarios for the future to be explored relatively quickly. It includes long time series analysis to allow examination of the system over multi-year periods, whilst taking into account the daily and seasonal variations in various water supply sources and demands. Any 12 year climate series between can be selected. This allows the examination of periods which have been relatively dry, and periods which have been relatively wet, while maintaining a reasoned basis for multiyear analysis. It also allowed consideration of lot, precinct and regional scale initiatives, and allowed consideration of both real projects, where water is transferred or treated via proposed infrastructure, and substitution projects, where the use of an alternative water source in one location can free up water in another location. OUTCOMES In addition to developing the Water Balance, Pollutant Balance and Cost Benefit Analysis MS Excel tools, this project involved the use of these tools to analyse a number of regional scale whole of water cycle futures for the Western Region of Melbourne. Land use and development Key changes to land use and development between now and around 2050 to note are: The approach combines traditional water supply demand planning techniques with urban integrated water cycle management approaches, in a single analysis. Data was imported from REALM, rainwater harvesting, stormwater harvesting, recycled water plant water balance and irrigation demand models, as well as a range of other data and information sources. It includes spatial analysis which compares the different water balance outcomes across the region. The study area was broken into 27 different project units, such that a water balance is performed for each project unit. the footprint of urban development will increase from about 378km 2 to 644km 2 ; the population of the region is forecast to grow from about 768,000 people to 1,756,000 people. about 54% of new dwellings will result from greenfield development and 46% from brownfield and infill development. 31% of the existing building stock will turn over and is therefore potentially subject to any new development and/or building regulations.

3 Note that these estimates are all of course subject to ongoing change and represent a basis for consideration, rather than accurate prediction. Water balance The water balance analysis was undertaken for the current year, and for various WoWCM scenarios at the year The Figures below illustrate some of the types of information that is generated from the tool. Figure 4: Environmental supply and demand hydrographs (at key locations). Some of the key water balance results are detailed below. Unless otherwise indicated, volumes are reported as annual averages, modelled over either an average 12 year climate period or a dry 12 year climate period corresponding to the millennium drought. Some of the key water balance results for the current level of development are: Figure 1: Average annual results for 27 project units Figure 2: Average annual results for entire region There is currently an urban water demand of around 80 GL/yr. About 6 GL/yr is supplied by alternative water sources (i.e. rainwater, stormwater, recycled water), 25 GL/yr from local surface water and groundwater sources, and 49 GL/yr from the Melbourne potable water system. There is currently a rural water demand of around 72 GL/yr. About 32 GL/yr is supplied by recycled water (including around 25 GL/yr onsite at WTP), and about 40 GL/yr from local surface water and groundwater sources. The local surface water available for urban and rural supply decreases from 20 GL/yr and 56 GL/yr during an average climate period, to 11 GL/yr and 31 GL/yr during a drought period. Some of the key changes to the water balance between now and 2050 are: Figure 3: Cumulative supply and demand plots for both urban and rural, for 27 project units the urban water demand increases from 80 GL/yr to 160 GL/yr; the total demand for water, including both urban and rural, increases from 151 GL/yr to 235 GL/yr; excess urban stormwater runoff increases from 113 GL/yr to 172 GL/yr; wastewater generated increases from 53 GL/yr to 107 GL/yr; When considered in a city wide context, these numbers suggest that significant investments in augmentation will be required over the 50 year

4 period. The nature of this investment will depend on the water cycle options considered. An issue of particular note is that the amounts of additional wastewater and additional stormwater runoff, (when taken together), significantly exceed the demand for non-potable alternative water. So even highly efficient schemes to recycle and/or harvest will not fully manage the additional flows. Note that these results are the raw results which do not reflect incorporation of options which could allow recycling of waste water, or harvesting of stormwater. Such options would produce different Net results which were explored in the options analysis. Pollutant balance The water balance average annual results are imported into the pollutant balance tool to determine pollutant loads to the environment under different user defined environmental regulatory standards for stormwater and wastewater. The Figures below illustrate some of the types of information that is generated from the tool. Figure 5: Nitrogen load from stormwater runoff in all sub-catchments in the Western Region. Some of the key changes to pollutant loads entering receiving environments between now and 2050 are summarised below: total nitrogen entering local waterways from stormwater runoff (i.e. after stormwater harvesting, rainwater harvesting and additional WSUD treatment) increases by about 39% by total nitrogen entering local waterways from wastewater discharges (i.e. from local inland WWTPs) increases by about 600% by Note that the 2050 scenario for which results are presented corresponds to a traditional or conventional future servicing and regulatory scenario, projected into the future. These results reflect the assumption that current Best Practice Environmental Guidelines for Urban Stormwater (BPEM) water quality standards apply to all greenfield and brownfield development, but not infill development or redevelopment. It was also assumed that the current discharge licence concentration requirements of the wastewater treatment plants do not change, and that inland discharges to waterways occur even where licences may not exist. A range of alternative pollutant load scenarios arise for other input assumptions. The tool allows easy assessment of these alternatives. Options assessment Four regional scale WoWCM options for the Western region were analysed at the year 2050 using the various tools developed in this project. The options considered in this project were holistic WoWCM servicing strategies or scenarios for the entire region that comprises a suite of urban, rural and environmental infrastructure and noninfrastructure based WoWCM initiatives. These options were defined to balance supply and demand, and to explore the costs and benefits of different servicing WoWCM approaches, across the region. These options were: Conventional - A baseline representation of the WoWC, with conventional servicing approaches from Water Plan 3 (i.e. 2018). Business as usual The most likely WoWC scenario in the absence of the Water Future West strategy, based on planned water authority strategies. Environment An option that aims to achieve enhanced environmental outcomes. Agriculture - An option that aims to achieve increased agricultural production for the Western region. The different initiatives considered in these options included: Lot scale initiatives, such as rainwater harvesting to supply a variety of end-uses. Precinct scale initiatives, such as dual pipe recycling schemes supplied from local RWPs, from WTP, or from precinct scale stormwater harvesting and ASR schemes. Regional scale transfers, such as new irrigation pipelines. Development controls, such as stormwater quality standards for infill development and redevelopment. The outcomes of the analysis for these options are not presented or discussed in detail in this

5 presentation, as the outcomes of the project are still under consideration by project partners. Cost Benefit Analysis A cost benefit analysis tool was developed which combined NPV analysis of the physical elements with estimates of the key benefits and impacts. The results of this analysis are still subject to ongoing work as the project partners determine which options are attractive, but some observations can be made. These are: The Cost of dual pipe / rain tanks at houses is a key element in cost comparison as it amounts to thousands of dollars at every house. The question that arises here is how should these costs should be considered given the contribution they make to star ratings? One argument put forward is that they should be costed at the additional cost of these versus the lowest cost to achieve the star rating. This choice makes some difference to the rating of options. A key benefit of some options is the reduction of potable water needed from the wider network, and the reduction in the need to treat additional wastewater. A key input in the analysis is therefore the Long Run Marginal Cost (LRMC) of supply of potable water and treatment of waste water. As the amounts of water and wastewater are significant, the value of the LRMC becomes a key factor in the analysis, and the result is sensitive to what is adopted, within a reasonable range of numbers. There is work ongoing to provide more certainty on these inputs. There is a key question for some options over this long timeframe: how should possibility of future technology advances and improvement be considered? Two examples: current treatment technologies for recycling waste water and stormwater are likely to become cheaper to install and operate. Another: smart tanks and stormwater management which provide some combination of harvesting and flood/flow mitigation are likely to become reliable and mainstream. The economics analysis considered the ramifications of these and showed the result is sensitive to this input assumption. One benefit of interest is the reduction of flows and pollutants to water ways. However, such environmental benefits are hard to reliably and robustly quantify. It appears that an approach where a base case investment concept is developed DISCUSSION (one which provides a long term acceptable outcome), would be valuable to allow comparison with innovative alternatives. The following sections describes some of the observations from the project, noting that in some cases these confirmed the general received wisdom, and in other cases were somewhat counter intuitive. Observations Future urban development over the next 35 years will generate volumes of wastewater and storm water which in total will exceed the new demand for water in those new developments. However, the nature of water use within new suburbs means that to substitute a large proportion of the demand for potable water with alternative water sources is challenging without considering options such as treating stormwater to create potable water, or providing alternative water to end uses such as washing machines. One constraint is the overall increase in house density and house sizes on blocks. Less green space is present which reduces the proportion of substituable demand. The overall demand has been reduced, which of course is valuable, but the larger backyards also had value as a sink for stormwater and irrigated wastewater from dual pipe schemes, and this is potential is reduced. Significant additional loads of nutrients will enter the receiving environment of Port Philip Bay from Western Treatment Plant and from excess urban stormwater runoff and wastewater discharges directly to waterways unless the current treatment requirements for both stormwater and wastewater are made more stringent. Even where more stringent requirements are introduced, there may still be signficant increases in nutrients entering receiving environments. Major waterways in the area are currently water stressed (i.e. have too little water). However, as urban development continues in the growth areas, the additional excess urban stormwater runoff entering these waterways will exceed the required flows in the lower reaches of these major water ways. However, these will occur in short time frames which may not coincide with the environmental water requirements. From a pure water balance perspective, existing Irrigation Districts and irrigated land uses could be made more resilient through the use of the available alternative water sources. However, the economics and willingness to pay of this concept need further consideration.

6 Similarly, from a pure water balance perspective, the environmental water requirements of major waterways in the Werribee and Maribyrnong catchments could be supplied through provision of additional water from alternative sources or the substitution. The requirements for managing the discharge of both stormwater and treated wastewater, in both the waterways and the Port Phillip Bay are likely to be the driving consideration for determining the most appropriate options. This is a key observation from this work: the management of wastewater and stormwater are likely to be more significant drivers in strategic development than management of potable water supplies. Constraints There were a number of constraints on, or limitations of, the study that should be recognised. The first limitation is that the scope of the study was limited to a sub-region of the city. Two examples of where this caused issues are listed below: The main water source for the Western region is potable water transferred from Melbourne s water reservoirs in the North, East and South-East of Melbourne. The historical availability of water from these resources was included within the tool. However, the actual availability of water from these resources is dependant on the WoWCM scenarios or futures for the rest of Melbourne. The second is the Western region contains WTP which not only treats the majority of wastewater from the Western region but also the majority of wastewater from the North and Central regions of the city. Similarly to above, the projected increases in wastewater flows based on population growth outside the study area is taken into account in the tool. However, this is contingent on the amount of wastewater recycling that occurs in the North and Central regions. A third limitation, relating to the point above, is the difficulties in collating and combing outputs from different models that use different system boundaries. For example, REALM sub-catchments do not align with the stormwater sub-catchments that were used, and neither align exactly with the unique project unit boundaries adopted in this project. A fourth limitation is the use of MS Excel to develop such tools as described in this paper. The trade-off was between a widely accesible and useable tool that could be used by non-technical audiences, versus a more technically robust analytical tool or model. CONCLUSION The primary conclusion of this work is that there is now a sufficient level of analysis undertaken, and sufficient analytical tools available to decision makers, to inform long term strategy development for the integrated water cycle for the Western Region of Melbourne. The key issue is that value judgements now need to be made to develop a water strategy for the Western Region of Melbourne. While these tools provide data, they do not provide provide the value judgements to weigh up competing factors. However, the data is proving useful in policy development. ACKNOWLEDGMENT The authors would like to thank all project partners - Melbourne Water, City West Water, Western Water, Southern Rural Water, Victorian Government and project stakeholders. The second limitation is inherent in all IWCM or WoWCM strategies, and relates to the selection of units or areas for analysis. Administrative or operational boundaries (e.g. ABS statistical areas, local government areas and retailer boundaries) typically do not align with water system, wastewater system, or catchment boundaries. Similarly, boundaries delineating existing urban development from future urban development do not align with any of the above. This project adopted a hybrid approach that sought to balance different interests.

7 Figure 6: Western Regional Water Balance Study Area, Melbourne, VIC, Australia