The journey to a more sustainable society includes revisiting stormwater management as part of an integrated system.
Water cycle management has been dominated by the centralised and separate management of water supply, sanitation and stormwater runoff since the mid-1800s. This centralised paradigm of management has served Australia well over the last 100 years. However, reliance on centralised water sources and sinks has been challenged by population growth, droughts, climate change, a requirement for liveable settlements and the need to protect of important ecosystems. A requirement to act in a more sustainable manner is recognised and the centralised management paradigm has been inflexible and resistant to the necessary changes required for sustainable water management.
Water Sensitive Urban Design (WSUD) and Integrated Water Cycle Management (IWCM) have emerged as real alternative or supplementary approaches to an emerging water cycle management paradigm which requires linked actions at multiple scales. A new momentum of activity is emerging across the Australian water sector to embrace spatial and temporal variance of water cycle responses in metropolises.
Replacement costs of urban water cycle infrastructure in Australia are greater than $50 billion (circa 2005) and increase as infrastructure ages. These costs will rise further as cities increase density and expand in response to population growth. It is possible to use spatially distributed and innovative source control or multiple scale approaches to supplement deteriorating service levels provided by traditional urban water infrastructure.
Stormwater drainage has historically developed as a stand-alone system to meet a single objective – rapid disposal of storm water runoff via pipe networks to avoid local flooding – this “end of pipe” approach may include a retarding basin or constructed wetland at the discharge point of a drainage network. This approach has been codified in Australian engineering practice which has narrowed perceptions of management possibilities and has created inertia. However, stormwater management is an essential component of integrated water cycle management in which stormwater runoff is a valuable resource, a means to enhance the amenity of urban settlements and for repair of remnant natural environments.
There will also be many ecosystem responses within the urban catchment. The Figure below outlines how the response of an urban catchment can be investigated as a traditional “end of pipe” problem or as a distributed scale catchment problem. Unfortunately, most analyses and models (both ecological and infrastructure related) operate at this traditional “end of pipe” scale. This method does not account for the true complexity of the catchment and provides only “net effect” information. Information in this form is difficult, if not impossible, to deconstruct to the necessary detail to provide insights into upstream processes and usually precludes the effectiveness of decentralised solutions.
Small pieces of critical information may be glossed over or hidden amongst the mass of other information in lumped catchment models. Put simply, responsive solutions cannot be formulated to non-responsive data. How can decisions on countering deteriorating ecosystem health, design of stormwater infrastructure or risk of flooding be made if there is only non-responsive data to infer decisions from? The rush to solutions fosters the oversimplification of such notions as sustainable development and ecosystem health, and favors the tendency to ignore the complexity of natural systems. A distributed scale approach will be required for strategies that foster ecosystem health and diversity solutions.
Analysis using Systems Frameworks of Big Data (see www.urbanwatercyclesolutions.com) that includes spatial and temporal detail has revealed substantial “hidden opportunities” and challenges a range of normative assumptions about efficacy of solutions for stormwater management.
Stormwater peak discharges within sub-catchment and catchments are dependent on cumulative runoff volumes. All runoff events commence with smaller discharges from local surfaces – such as roofs. It also follows that the effectiveness of distributed storage measures (such as rainwater tanks, bio-retention and restoring soil moisture) increase with catchment area – this is not a linear relationship with greater whole of catchment impacts exhibited by storages located in the upper reaches of waterways. Thus it is illusionary to make direct comparisons between the volume of stormwater storage required at the “end of pipe” and storages distributed throughout a catchment. Traditional “lumped catchment” stormwater models cannot account for the full impact of distributed storage measures. Similarly peak stormwater discharges at the building scale do not indicate catchment scale peak discharges for storage measures.
This evolving approach to stormwater management must be integrated into the holistic concept of integrated water cycle management to capture the synergistic benefits and otherwise hidden opportunities for sustainable cities. This understanding and subsequent development of policy will require a systems approach and first principles understanding of the behaviour of urban water cycle systems – this understanding has been hitherto obscured or even hidden by codified design approaches that provide a narrow view of stormwater management as a drainage function or recipe.
Rational Method or design storm methods of determining peak discharges are not a robust approach to design of volume sensitive systems – such as WSUD and IWCM. Continuous simulation may be the only reliable method to determine the “most likely” impact of volume sensitive systems on flood events and infrastructure. In addition, it is important to evaluate the joint probability of water use and climate processes. Moreover, the commonplace use of average assumptions applied at a centralised scale is likely to create substantial errors in understanding of the performance stormwater management opportunities.
The dominant focus on conveyance of stormwater has, more recently, been paralleled by a necessary focus on the quality of stormwater discharging from our cities in response to mounting community concern about impacts of urban areas on waterways and associated ecosystems. These objectives are codified by the recently published guideline Australian Runoff Quality and supported by the MUSIC model. Nevertheless, stormwater drainage and quality management has emerged as separate codified approaches that generate separate infrastructure responses that have a mostly centralized focus. This can result in costly redundant or competing infrastructure solutions.
It can be argued that the industry is now operating with four separate streams of water cycle management that are presented as integrated systems by ultimate addition following otherwise traditional design processes. Genuine IWCM approaches remain as hard won exceptions rather than normal practice. An integrated systems approach can provide significant benefits for costs, aesthetics, energy, liveability and be more sustainable. However, guidelines and policies are required to foster these integrated approaches and the current genre of codified approaches or recipes can act to stifle innovation.
Importantly, integrated designs can make hydrology of urban systems respond in a similar way to the original natural systems and assist with internalizing water security within cities. These so called non-traditional approaches can provide significant advantages in remote, greenfield, indigenous and sensitive environments – and are being retrofitted into existing systems to counter the impacts of evolving urban form. They assist with augmenting the capacity of aging infrastructure, improving urban salinity, amenity and restoration of urban catchments.
These more flexible approaches are increasingly required in response to urban landscapes that are also constantly changing with urban renewal and expansion a natural part of city development. This evolving urban form presents itself as a “moving target” for urban water solutions, and is an additional layer of uncertainty. The evolving urban form includes changing population densities which will impact on infrastructure performance and provision, may potentially change ecosystem response and presents as substantial financial risk for large scale centralised infrastructure. Timely intervention using distributed solutions can play an important role in countering uncertainty about water supply, drought, flood risks and climate variability.