Stormwater Harvesting and Reuse Systems: a Brisbane Experience

Stormwater Harvesting and Reuse Systems: A Brisbane Experience

Stormwater Harvesting and Reuse Systems:

A Brisbane Experience

Kieren Davis, Niloo Tara, Stefanie Stanley and Lee O’Connor

City Design, Brisbane City Council

Abstract

Council has identified significant learning outcomes from the planning, design, construction, operation, monitoring and maintenance of a number of stormwater harvesting and reuse (SHR) systems in Brisbane parks and commercial sites. A corporate guidelineis being developed to guide Development Assessment officers to the relevant policy and guidelines for implementation of SHR systems. A planning methodology has been trialled to enable the identification and prioritisation of SHR opportunities city wide. Methods have been developed for estimating water demands (irrigation), efficiency of the catchment supplying the water and storage requirements. Improvements to water balance modelling, stormwater treatment and site integration during the design phase have been documented. Opportunities, constraints and learning outcomes associated with the construction as well as the operation, maintenance and monitoring of these systems have also been documented.

Case studies are presented that encapsulate the best aspects of Brisbane’s Council owned and operated SHR systems. The case studies will also highlight how improvements to processes, tools and knowledge can improve the application of stormwater harvesting. These learning outcomes are relevant to SHR projects undertaken by local government and industry. Stormwater harvesting and reuse is one way that Council is working towards becoming Australia’s most sustainable and liveable WaterSmart City.

Key Words:

Stormwater Harvesting, Retrofit, Water Sensitive Urban Design, Reuse.

1.Introduction

Urban green space provides a number of direct and indirect financial, environmental and social benefits to the community. These benefits are dependant on green space vegetation being maintained during dry conditions. Drought induced water restrictions have adversely impacted Brisbane City Council’s (Council) ability to satisfy the irrigation demand for its parks and gardens and highlighted the need for cost-effectiveness and supply security when designing water supply strategies. Stormwater harvesting has the ability to satisfy this demand and provide a reliable and more sustainable supply option as well as a multitude of additional economic, social and environmental benefits.

The urbanisation of natural catchments has resulted in a dramatic change in the natural hydrology and water quality of Brisbane’s catchments and waterways. The large percentage of impervious area prevalent in many of the city’s catchments has resulted in an increase in runoff volumes and a decrease in stormwater quality. The end result is a modification to stream flow processes causing erosion and habitat degradation. It is now widely accepted that a direct relationship exists between stream degradation and impervious area.

1.1Why Stormwater Harvesting & Reuse?

Stormwater harvesting and reuse is often incorporated into water sensitive urban design (WSUD) strategies in urban developments (EPHC, NHMRC & NRMMC 2008). Another opportunity associated with stormwater harvesting is the potential to retrofit it when upgrading existing stormwater infrastructure. Implementation of stormwater harvesting has multiple benefits in the urban environment, including:

• Reducing demand for potable water or the need for carted water;
• Providing an alternative source of water for users that are susceptible to water restrictions (e.g. irrigation of parks and sports grounds);
• Providing treated water appropriate to the end use;
• Reducing stormwater flows, volume and frequency of runoff;
• Assisting mitigation of flood impacts;
• Reducing pollutant loads to waterways.

Stormwater harvesting provides an alternative source of water that can supplement or replace potable water and other alternatives at suitable sites. This paper presents stormwater harvesting projects in Brisbane, and learning outcomes gained from the design, construction, maintenance and monitoring phases of these projects, as well as reflections on the prioritisation and site selection process. With these experiences, future projects can be improved to better achieve multiple outcomes.

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Stormwater Harvesting and Reuse Systems: A Brisbane Experience

2.Policy and strategy

With the increasing popularity of stormwater harvesting as an alternative water source, government agencies, organisations and technical experts are working together to develop clear guidelines to inform decision makers and property developers.Council is currently strengthening policy and developing guidelines for application at the city-wide scale.

2.1BrisbaneCity Council’s Corporate Policies & Strategies

Brisbane City Council has an overarching vision for the city, Our Shared Vision - Living in Brisbane 2026(Brisbane City Council, 2009d). This document lists a number of City-Wide Outcomes that relate to each of eight vision themes. These City-Wide Outcomes will be achieved through the implementation of short and medium term plans, namely Council’s Corporate Plan 2008-2012(Brisbane City Council, 2009a) (currently under review) and the Water Space planning tool under the guidance of Council’s WCM001 Water Cycle Management Policy (WCM001)(Figure 1).

WCM001is a strategic document that aligns with Living in Brisbane 2026 and the WaterSmart City – Integrated Water Cycle Management Strategy for Brisbane(Brisbane City Council, 2010b). WCM001 outlines seven policy outcomes to manage Brisbane’s water resources in a sustainable and integrated manner. This document has been updated to reflect changes in policy requirements and is due for a major review in the 2010/11 financial year. Implementation of Stormwater Harvesting & Reuse (SHR) relates particularly to the following outcomes:

1. Create healthy waterway catchments.
2. Provide sustainable water sources for the City and local areas.
3. Potential to assist with minimising the impact of flooding on people and properties
4. Manage water to deliver economic prosperity.

A key component of the Water Space planning tool is the Water Futures Program. The Water Future Program evolved from the City-Wide Integrated Water Cycle Plan project to develop a series of individual and dynamic “Water Futures” based on the different core elements of integrated water cycle management (IWCM), one of which is Stormwater Harvesting (Brisbane City Council, 2009b). Once developed, the Stormwater Harvesting Water Future will contribute to the fulfilment of the concept of integrated water cycle planning for Brisbane City, by addressing the management actions identified in Brisbane City Council’s (Council’s) WaterSmart City – Integrated Water Cycle Management Strategy for Brisbane(Brisbane City Council, 2010b).

2.1.1‘WCM024 Stormwater Harvesting Corporate Guideline’

Water Resources Branch of Brisbane City Council, in conjunction with City Design, is developing the ‘WCM024 Stormwater Harvesting Corporate Guideline’ (WCM024), a corporate document for Stormwater Harvesting & Reuse (SHR) within Brisbane city to advise future development of both public and private infrastructure (Brisbane City Council, n.d.). It is intended that this document will be used for internal Council purposes.The objectives of this document are:

• To ensure Council officers are aware of how to respond to approaches from property owners with stormwater harvesting proposals including third party access;
• To ensure Council meets the requirements of Commonwealth and State legislation and policy obligations;
• To provide information to decision-makers so that a range of ecologically sustainable stormwater harvesting projects are developed across the City, which will be:
• Consistent with Council’s Vision for the City – Living in Brisbane 2026;
• Aligned with the strategies and initiatives in the Corporate Plan;
• Aligned with Council’s Water Cycle Management Policy WCM001.

To provide Council staff and contractors with a summary of legislation and the regulations required for stormwater harvesting applications or the implementation of stormwater harvesting projects.Harvesting the potential of stormwater Fact Sheet and Harvesting the potential of stormwater: A guide for groups exploring stormwater harvesting potential to maximise the potential to identify and deliver SHR projects through partnerships with business and industry, and linking SHR projects with fit for purpose end uses.Implementation of stormwater harvesting is supported through the guidelines and procedures as detailed in the ‘Draft Stormwater Harvesting Guidelines’ (Water by Design, 2009).

The document will provide Council staff and contractors with a summary of legislation and the regulations required for stormwater harvesting applications or the implementation of stormwater harvesting projects.

This document will provide information to decision-makers so that ranges of ecologically sustainable stormwater harvesting projects are developed across the City, which will be:

• Consistent with Council’s Vision for the City – Living in Brisbane 2026;
• Aligned with the strategies and initiatives in the Corporate Plan;

Aligned with Council’s Water Cycle Management Policy WCM001.

The WCM024 Stormwater Harvesting Corporate Guideline is in the final stages of development and is currently with our legal team for review. Final approvals and endorsement will then be sought from Council’s Executive Management Team.

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Stormwater Harvesting and Reuse Systems: A Brisbane Experience

Figure 1. Stormwater harvesting in the context of Brisbane City Council’s policies and strategies.

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Stormwater Harvesting and Reuse Systems: A Brisbane Experience

2.1.2Stormwater Harvesting Planning Methodology

The priority of sites for design and construction is a combination of economic, social and environmental criteria to meet multiple objectives. A decision making tool to reflect the multiple objective approach was required.

In response, Council developed a city-wide Stormwater Harvesting Prioritisation Methodology for the selection and prioritisation of stormwater harvesting sites for park and sporting field irrigation across Brisbane (City Design, 2009a). The methodology was intended to provide and prioritise a number of technically feasible sites to enable a cost and time efficient approach to detailed investigation. To achieve this, a two stage process was employed:

• Feasibility methodology (Stage 1): uses technical criteria to assist in identifying sites where SHR is feasible. The criteria are based on the limiting constraints of implementing an on-site SHR scheme.
• Prioritisation methodology (Stage 2): uses a multi-criteria analysis tool to prioritise feasible SHR sites. The criteria include financial, environmental and economic opportunities and constraints; these are weighted to reflect the relative importance of each category.

Stage 1 and Stage 2 of the Stormwater Harvesting Planning Methodology has been applied at a city-wide scale and is currently being revised.

The Stormwater Harvesting Prioritisation Methodology was originally developed as a stand alone process. It is recognised that an integrated approach that satisfies outcomes from a number of council program areas allows us to achieve multiple outcomes and social benefits. Projects are now being selected based on the potential to achieve multiple sustainable water outcomes for public infrastructure. The focus is more opportunistic involving integration with major infrastructure projects before the design phase and streamlining the process within Council’s operating environment. This is being undertaken using a consultative process within Council. A schedule of works is currently being developed with the view to designing and implementing projects within upcoming financial years.

3.Investigation

Investigations into the viability of stormwater harvesting whether it be for a specific water supply strategy for a park or the possible integration of SHR into an infrastructure project, centre largely on obtaining a comprehensive understanding of the site characteristics and client needs. All Council investigations have focussed on irrigation as the demand, primarily due to decreased risks and treatment costs. As such, a typical investigation consists of:

1. Initial site/project meeting with client and key stakeholders to determine project drivers and client needs
2. Investigate site characteristics and review opportunities and constraints
3. Undertake water balance modelling
4. Provide potential system layout and construction estimate.

Depending on client needs, the level of investigation can vary from informal meetings discussing the possibility of stormwater harvesting to undertaking feasibility studies which include conceptual layouts and preliminary construction estimates. Clarification is required to determine if the investigation process is being undertaken as part of a feasibility study or to justify budget expenditure on a potential future project. Potential opportunities and constraints associated with parallel projects should also be identified.

The technical feasibility of SHR schemes depends largely on a combination of demand, supply and constraints whether physical or financial. The interaction of these three variables will generally dictate a potential storage size and reliability (Figure 2). As a result the more information and data that can be gathered during the investigation phase the greater the confidence in potential layouts and cost. Missing a key constraint or opportunity during the investigation phase can often result in delays and cost increases during the design phase. Key site characteristics include:

• Topography: Can dictate potential layout options and diversion options. Flooding immunity can be problematic on flat sites.
• Climate:Climatic data including rainfall and evapotranspiration must be site specific as annual rainfall can vary by 400 mm in Brisbane.
• Existing and proposed infrastructure:Obtaining accurate site data including existing and proposed stormwater infrastructure and other services is always problematic.
• Soils:An indication of soil-storage capacity is essential for modelling and can prove constraining when faced with acid sulphate soils or contaminated land. Minor changes to irrigation scheduling can disproportionately affect model output.
• Catchment: Past, present and future land use, impervious area, constructed sewer overflows
• Existing practices. Existing irrigation scheduling, source of present supply, cost of alternative supply options.

Figure 2 Interaction and dependence of variables influencing storage size

Water balance modelling is undertaken using an integrated soil-water-storage (ISWS) model. The ISWS model is a physically-based (deterministic), continuous model that utilises a forward daily time step.The model can be structurally separated into two distinct models; a Soil-Water Balance that models fluctuations in the availability of water within the soil matrix, and a Storage Balance that models the fluctuations in stored water volume within a specified storage. Figure 3 below illustrates the extents of the ISWS model (shown in blue) in context with additional data and model outputs required for function.

Figure 3 Basic Structure of the Integrated Soil-Water-Storage Model

The model allows the user to reflect ‘best practice’ irrigation techniques by letting the soil dry out to a critical point before irrigating or represent other irrigation practices such as on a timer or certain days of the week. Site-specific climatic and catchment characteristics are utilised as inputs for a continuous hydrologic model with a daily time step as well as the ISWS model. The hydrologic model provides inflows to the Storage Balance, while the Soil-Water Balance uses climatic and Storage Balance inputs to maintain a desired vegetation standard. The dashed arrow from climatic characteristics to the storage balance model represents the possible influence of evaporation if an open storage is utilised.

Water balance modelling should focus primarily on client needs and information relevant for the design phase. The level of project funding will often dictate the reporting requirements, whether these include yield, pollutant removal, hydrologic restoration, cost per kL, reliability. Preferably, a minimum of 50 years of local climatic data should be usedhowever the length of the time seriescan be limited by the complexity of the model particularly when using Microsoft Excel. The Model for Urban Stormwater Improvement Conceptualisation (MUSIC) is the most frequently used hydrologic model.Existing hard or even anecdotal data relating to the temporal variation in demand or irrigation scheduling should be sourced where possible. Potential site layout and reporting requirements will often dictate model set-up as will additional water supply sources such as trickle top-up from mains. As a result, each model is distinctly different. Standardising models can become difficult and user error in models is common due to individual preference for set-up style and multiple references when using software such as excel.Maintaining a basic ‘no frills’ model to carry over to new projects is essential to minimising project time. However, a suitable system for reviewing models prior to design is an essential step in quality control.

Provision of conceptual layouts and preliminary construction estimates in the investigation stage is once again dependant on client needs. The benefits of the scheme are often as important to the client as the cost, highlighting the need to comprehensively understand client needs and quantify through the modelling process. The most cost-effective design ($/kL) often revolves around a lower reliability and utilising alternative supply (e.g. potable supply, groundwater) or externally carted water for top-up during extended dry periods.

Key Learning outcomes

1. Focus must be on client needs and objectives and understanding site characteristics.
2. Clarify the intended use of the investigation. Is it being used to justify expenditure?
3. The interaction between demand, supply and constraints largely dictates site feasibility.
4. Get site characteristics right. Minor changes to inputs such as demand and irrigation scheduling can disproportionately affect model output. Obtain hard or even anecdotal data wherever possible. Site specific climatic data for a minimum of 50 years is essential.
5. Models that account for soil-water balancing are essential when irrigation is the primary source of demand.
6. Utilise a basic ‘no frills’ to save project time, however control user error through a suitable review system. When modifying models ensure that reporting requirements are an output of the model and any benefits are quantified.
7. The most cost-effective designstend to be designed at a lower reliability (70-80%) and utilises trickle top-up from mains water.

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