Perspective EssayThinking outside the channel: Challenges and opportunities for protection and restoration of stream morphology in urbanizing catchments
Introduction
Contemporary stream management faces the challenge of restoring streams such that they “maintain or increase ecosystem goods and services while protecting downstream and coastal ecosystems” (Palmer et al., 2005), whilst also satisfying utilitarian functions such as flood and erosion protection (Gregory & Chin, 2002). The ultimate goal of stream restoration is to achieve these goals for least effort and cost. Nowhere is stream restoration more challenging than in streams that have a substantial portion of their catchments urbanized (Bernhardt and Palmer, 2007, Hatt et al., 2004, Vietz et al., 2014b, Walsh et al., 2012a). If current trends in population density continue then by 2030 urban land cover will increase by 1.2 million km2, nearly tripling the global urban land (Seto, Guneralp, & Hutyra, 2012). This means the planning for protection and restoration of streams in urbanizing catchments will require considerably greater effort and some thinking outside of current channel-based approaches.
Urban stormwater runoff is a highly effective geomorphic agent with increased magnitude, frequency and duration of disturbance flows (Burns, Fletcher, Walsh, Ladson, & Hatt, 2012). In concert with changed sediment supply and reduced floodplain interaction, streams in urban catchments often experience incision, enlargement, and homogenization of channel morphology (Vietz, Sammonds, et al., 2014). This not only has implications for infrastructure, but in conjunction with poor water quality these changes contribute to poor ecological condition (Walsh et al., 2005). The traditional management approach for many streams in urban catchments has been to address the symptoms by modifying the channel to cope with changes resulting from urban land use. This includes activities such as channelization, channel straightening, enlargement and armoring. In many cases this is still a valid approach given the value of urban land and the cost of flooding in cities. Channel reconstruction approaches to stream restoration are sometimes inevitable. It is reasonable to ask, however, whether the natural character of urban streams always has to be degraded in this way, and whether addressing the catchment-scale causes of stream degradation may provide greater opportunities for protecting or restoring channel morphology.
In this perspective paper we argue that application of the term ‘urban stream’ can sound the death knell for an otherwise naturally functioning stream. Rather, these streams should more appropriately be considered as streams affected by urban landuse and instead of ‘treating’ urban streams we should consider addressing the drivers of channel degradation. Addressing a driver of channel change to protect geomorphic form is not necessarily new thinking (e.g. stormwater management, Booth, 1991), but the novelty of this article lies in an investigation of the suite of opportunities that may be available. We, therefore, explore the opportunities for protecting and restoring streams in urban catchments by working with the drivers of change, rather than trying to combat change. We look outside the channel to opportunities for managing excess stormwater runoff, providing riparian buffer-space for streams, and managing sediment supply. We also consider the implications of former land uses (prior to urbanization) that may result in legacy channel morphology, and the role social and institutional drivers play in opportunities for stream protection and restoration.
The focus of this paper is on physical changes to streams (fluvial geomorphology) rather than on chemical and biological changes. Over the last two decades there has been increasing attention paid to the role channel morphology and geomorphic processes play in ecosystem health and how well restoration works endure (Grabowski et al., 2014, Newson, 2002). Failing to incorporate geomorphic processes as central to stream restoration has been suggested by some as a reason for the failings of current approaches (Elosegi et al., 2010, Newson and Large, 2006). However, achieving a geomorphologically functioning stream with appropriate rates of erosion and deposition in an urban setting, is only feasible if the significant stressors responsible for degradation are addressed.
Section snippets
Management responses to urbanization
Management responses to urbanization have been evolving from channelization, through to incorporating natural geomorphic features in channel reconstruction, and more recently, to protection and restoration (Chin and Gregory, 2009, Fletcher et al., 2014). For more than 5000 years streams flowing through population centers have been used for water supply and their channels managed to prevent inundation or erosion of usable land (Childe, 1950). Such an approach focused on modifying channels to be
Catchment-scale approaches
As a type of ‘stream restoration’ there is a movement toward activities in urban catchments that address the underlying causes of channel degradation. In particular there are calls to attenuate and reduce excess stormwater runoff to protect stream ecology (Bernhardt and Palmer, 2011, Burns et al., 2012, Fletcher et al., 2014, Walsh et al., 2005, Walsh et al., 2012a, Wenger et al., 2009) and stream geomorphology (Bledsoe, 2002, Booth, 2005, Hawley et al., 2011, Vietz et al., 2015).
Some challenges and opportunities
In the following section we explore stormwater management (4.1), providing riparian space (4.2), providing sediment to urbanizing streams (4.3), legacy impacts to streams (4.4) and social and institutional challenges (4.5). We discuss greenfield sites (where urbanization is just beginning), followed by brownfield sites (where much of the upstream catchment is already urbanized).
Catchment-scale activities are currently not considered ‘stream restoration’, despite their potential to improve the
Conclusion
The evolution of urban stream channel management is moving toward goals for dynamic and complex channels that are self-sustaining. In the urban catchment this is limited, if not unfeasible, unless managers and communities begin to address the causes of stream degradation at a catchment-scale. These causes include: reducing excess stormwater runoff through distributed stormwater harvesting (including use and infiltration of stormwater); ensuring streams have adequate riparian land for hydrologic
Acknowledgements
We would like to thank Bob Hawley for his review of an earlier version of this paper. Geoff Vietz was funded by the Cooperative Research Centre for Water Sensitive Cities, an Australian Government Initiative, formerly the Cities as Water Supply Catchments Program. Tim Fletcher is funded by an ARC Future Fellowship (FT100100144). Chris Walsh is funded by Melbourne Water through the Melbourne Waterway Research-Practice partnership. Finally, we thank Professor Wei-Ning Xiang and two anonymous
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