by Dr Don Williams MRSV

This piece appears in the June 2023 edition of Science Victoria magazine. All issues can be read online for free at rsv.org.au/Science-Victoria.

Melbourne’s urban waterways provide a welcome contrast to an otherwise seemingly endless cityscape. Despite the changes that occurred after European settlement, these waterways still contain remnant ecosystems, which faintly echo the diversity they once displayed.

In addition to their contribution to the city’s environmental values, urban waterways also provide important social, economic, and aesthetic benefits. This article examines how science informs our understanding of how urban waterways have deteriorated and can guide efforts to protect and restore them.

The article focuses on waterways in the Melbourne metropolitan area and does not consider wetlands and estuaries. The Port Phillip and Western Port regions (Werribee, Maribyrnong, Yarra, Dandenong, and Westernport catchments) include 8,400 kilometres of waterways, of which more than 2,000 kilometres are affected by stormwater runoff from Melbourne’s built-up area.¹

In this article, ‘urban stormwater’ is water that runs off surfaces such as roads, roofs, and paved areas to stormwater drains. Drainage systems collect and discharge stormwater to surface waters, including rivers, creeks, lakes, estuaries, and the ocean. In Australia, polluted wastewater from households (toilets, bathrooms, kitchens) and industry is directed to a separate sewerage system. Wastewater is no longer discharged to Melbourne’s waterways.

The Past: Pre-European condition of Melbourne’s waterways

Before European settlement, Melbourne’s urban waterways were fed by runoff from catchments clad in natural vegetation. In these catchments, most rainfall that did not evaporate infiltrated the soil surface and entered streams by two processes: either via shallow pathways in permeable topsoil, or by percolation to groundwater, which released ‘baseflows’ to streams (baseflows are the sustained flows in streams during dry weather). Vegetation slowed the flow of rainfall across the soil surface and encouraged infiltration. Overland flows only reached streams after particularly large storms (which only occur on a few percent of days) and were a minor pathway for rainfall to enter streams. Collectively, these mechanisms moderated peak inflows to streams after rainfall and maintained baseflows during dry periods.² This natural hydrologic cycle slowed erosion.

Adjoining vegetation shaded waterways and stream beds and banks had not been subject to the massive physical modifications that occurred after European settlement.

These natural catchment and waterway conditions supported rich and diverse ecosystems. However, this changed abruptly after the foundation of Melbourne in 1835.

The Present

Melbourne’s astonishing growth from 1835 to the present has utterly transformed the city’s catchments and waterways. Continued urban development has led to thousands of square kilometres being stripped of vegetation and replaced by impermeable surfaces including roads, roofs, and paved areas. Formerly extensive wetlands have been drained. The ever-present drive to maximise the area of land available for development, combined with efforts to control flooding, have led to enormous physical modification of streams, which have often been converted to concrete-lined drains and, in the most extreme cases, been buried underground.

These changes to catchments and streams have completely transformed the conditions for waterway ecosystems. Urban streams are stressed by changed hydrologic regimes, increased pollutant loads and changed physical form.

Stormwater runoff from the impervious surfaces prevalent in urban areas is efficiently captured by drainage systems and quickly discharged to streams. This leads to frequent high flows in streams, after even modest rainfall. However, catchment infiltration is greatly reduced and baseflows are much smaller. Overall, stream hydrology is drastically changed, with more frequent and higher peak flows after rainfall, but much reduced baseflows. The enhanced peak flows exacerbate erosion and efficiently transport sediment to waterways.2

Stormwater from urban catchments includes pollutants such as suspended solids (i.e., solid particles suspended in the water column); metals, hydrocarbons and rubber shed from vehicles to roadways; detergents from vehicle washing; and fertilisers and herbicides from gardens. The suspended solids in urban runoff strongly bind pollutants such as metals and petroleum hydrocarbons and convey them to waterways.^2,3

The physical form of many urban streams has been greatly altered by engineering works, which can involve straightening and deepening of stream channels; replacing natural materials in beds and banks with hard impervious substances, such as concrete and rock armouring; and placing smaller streams in buried stormwater drains.

The changed hydrologic regime, increased sediment and pollutant inputs and altered physical conditions have, unsurprisingly, led to dramatic declines in the health of waterway ecosystems with, for example, macroinvertebrate communities in urbanised streams being reduced to a few tolerant taxa.⁴

In 1999, Victoria introduced some controls on the quality of stormwater discharged from urban development, based on scientific knowledge obtained in many countries, including Australia.^5,6 These controls focus on reducing the loads of suspended solids, phosphorus, and nitrogen in stormwater runoff (mandatory annual load reductions: at least 80% for suspended solids and at least 45% for both total phosphorus and total nitrogen).

Suspended solids harm aquatic ecosystems by reducing light transmission, smothering habitats, and carrying pollutants into waterways, meaning that reducing the quantity of solids entering waterways is a positive step. Reducing phosphorus and nitrogen loads in urban stormwater helps to reduce the risks of excessive algal growth in inland waterways and Port Phillip Bay (which receives stormwater runoff from Melbourne), respectively.⁷

While acknowledging the benefits of these controls, research shows that more comprehensive measures are needed to improve the protection of Melbourne’s urban waterways.²

The Future

Melbourne’s population continues to increase extraordinarily quickly, inevitably leading to further urbanisation of the city’s catchments. Continued urban expansion of Melbourne up to its Urban Growth Boundary would result in a further 900 kilometres of waterways being degraded by urban runoff if current stormwater drainage practices continue.¹ In the absence of comprehensive, science-based initiatives, further deterioration of Melbourne’s waterways is all too likely.

Fortunately, research has provided insights into how we can better protect the condition of our urban streams. A key finding is that the altered flow regime associated with catchment urbanisation is a significant factor in the degradation of stream ecosystems. Directly discharging stormwater flows from even small areas of an urban catchment (as little as 5%) can severely degrade stream ecology.^2,8,9

These findings led to the release of Victorian guidance in 2021, which sets out a two-tier flow reduction regime. This guidance applies stringent flow targets (that require stormwater flows from urban development to be greatly reduced by a combination of stormwater capture and infiltration, resulting in a more natural hydrologic regime) in catchments that drain to undisturbed high ecological value waterways, while less stringent flow controls apply in catchments draining to degraded waterways.¹⁰ The logic here is that stringent flow controls are justified to protect the valuable ecosystems in (relatively) intact waterways, whereas less rigorous controls prevent further deterioration of heavily degraded streams, which contain impoverished ecosystems. These flow controls apply in addition to the urban stormwater quality targets (i.e., the mandatory reductions in suspended solids, phosphorus, and nitrogen loads).

While the introduction of controls on urban stormwater quality and flow are important steps, we have to recognise that they are not cure-alls. Factors such as the reduction of vegetation in catchments, particularly that growing immediately adjacent to streams (‘riparian’ vegetation) and drastic physical modifications also harm the ecological health of urban streams.² These damaging changes constrain the improvements to ecological and social conditions that stormwater quality and flow controls can achieve in the dramatically modified waterways commonly found in Melbourne. If we want to maximise the benefits provided by urban waterways, a broader approach is required.

This will require a paradigm change, where our ambitions expand from protecting urban waterways from degradation, to recovering lost values by ‘naturalising’ degraded urban streams. This would involve, for example, restoring the beds and banks of waterways to more natural conditions and restoring as much vegetation as possible in their catchments, particularly riparian vegetation. Fortunately, several projects of this type are being planned or have recently commenced in Melbourne.¹¹ These projects must be guided by good science, which identifies the feasible ecological outcomes that can be achieved by an urban stream restoration project, integrates ecological and societal perspectives, and emphasises a multidisciplinary approach. As complete restoration of pre-European ecological values is unrealistic, a vital role of science is to identify the ecological improvements that are feasible. These findings will inform decision making about balancing ecological and social outcomes.¹²

We can expect that projects to rehabilitate urban waterways will face the typical trade off in natural resource management, where achieving increasingly ‘natural’ conditions requires a stronger set of interventions and greater costs. Good science can provide decision makers with information about these trade-offs.

Conclusions

The condition of Melbourne waterways declined severely after European settlement, as a result of massive modifications to catchments and the waterways themselves. The general decline of waterway condition includes deteriorated ecological values. Fortunately, science has identified the mechanisms responsible for these changes and also suggests how we could restore some of the waterway values we have lost.

Projects to restore urban streams will need to integrate science and insights from the community.¹³ Best results will be obtained by engaging a whole range of groups, including community groups, schools, the tertiary sector, citizen scientists and, importantly, learned societies such as the Royal Society of Victoria.

Dr. Don Williams MRSV worked for 30 years in the water quality management, wastewater regulation and water efficiency fields. Don then completed a PhD examining how planning laws influence the adoption of sustainable urban water practices. After the PhD, Don worked at Environment Protection Authority Victoria on regulating urban stormwater runoff.

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References:

1. Vietz, G. J., Rutherfurd, I. D., Walsh, C. J., Chee, Y. E. and Hatt, B. E. (2014). The unaccounted costs of conventional urban development: Protecting stream systems in an age of urban sprawl. In Proceedings of the Australian Stream Management Conference, Townsville (Vol. 31). DOI:10.13140/2.1.5029.6960.
2. Walsh, C. J., Leonard, A. W., Ladson, A. R. and Fletcher, T. D. (2004). Urban stormwater and the ecology of streams. Canberra: Cooperative Research Centre for Freshwater Ecology and Cooperative Research Centre for Catchment Hydrology.
3. Walker, T. B., Allison, R. A., Wong, T. H. F. and Wootton, R. M. (1999). Removal of suspended solids and associated pollutants by a CDS gross pollutant trap. Clayton: Cooperative Research Centre for Catchment Hydrology.
4. Walsh, C. J., Sharpe, A. K., Breen, P. F., and Sonneman, J. A. (2001). Effects of urbanization on streams of the Melbourne region, Victoria, Australia. I. Benthic macroinvertebrate communities. Freshwater Biology, 46 (4), 535-551.
5. Victorian Stormwater Committee. (1999). Urban stormwater: best-practice environmental management guidelines. Collingwood: CSIRO Publishing.
6. Mudgway, L. B. et al (1997), Best Practice Environmental Management Guidelines for Urban Stormwater Management, Report 97/7. Clayton: Cooperative Research Centre for Catchment Hydrology.
7. Environment Protection Authority Victoria (2019). Development of indicators and objectives for SEPP (Waters), Publication 1733. Carlton: Environment Protection Authority Victoria.
8. Walsh, C. J., Fletcher, T. D., & Burns, M. J. (2012). Urban stormwater runoff: a new class of environmental flow problem. PLOS ONE: doi.org/10.1371/journal.pone.0045814.
9. Ewert, J., O’Halloran, D., Lintern, A., Weber, T and McCarthy, D. (2020). Review of stormwater science, Publication 1919. Carlton: Environment Protection Authority Victoria.
10. Environment Protection Authority Victoria (2021). Urban stormwater management guidance, Publication 1739.1. Carlton: Environment Protection Authority Victoria.
11. Reimagining Your Creek Project, Melbourne Water melbournewater.com.au/services/projects/reimagining-your-creek-project.
12. Smith, R. F. et al (2016). Urban stream renovation: incorporating societal objectives to achieve ecological improvements. Freshwater Science, 35(1), 364-379. DOI: 10.1086/685096
13. Murphy, B. M., et al (2022). Closing the gap on wicked urban stream restoration problems: A framework to integrate science and community values. Freshwater Science, 41(3), 521-531. DOI: 10.1086/721134