Another reason urban streams are stuffed: geomorphic history, challenges and opportunities1 Geoff Vietz1/2, Mike Stewardson1, Chris Walsh2, Tim Fletcher2 1 Department of Infrastructure Engineering, The University of Melbourne, Parkville, 3010. Email
[email protected] 2 Department of Resource Management and Geography, The University of Melbourne, Parkville, 3010
Key Points • Management of the physical form of urban streams has historically focused on flow efficiency and channel stability • Geomorphologic features and functions that may influence the ecological health of streams are rarely considered by management approaches • Protecting or reinstating channel complexity and geomorphic dynamism may assist in achieving urban stream health goals • The challenges associated with a geomorphologically-‐sensitive approach include: flashy hydrology, reduced sediment load, space limitations, legacy impacts, and social and institutional perceptions
Abstract Urbanisation influences a range of factors related to stream health, including the hydrologic regime and water quality. There is also a significant, but lesser known, impact on the physical form and functioning of stream channels. Most urban geomorphic research and management have focused on channel widening and deepening arising from a primary concern with flow efficiency and channel stability. However, changes in channel dimensions in themselves are unlikely to be primary drivers of change in biotic structure and function. Alongside efforts to address water quality and hydrologic stressors on stream biota and biological processes identifying geomorphic attributes, that are known to affect stream ecological structure and function in non-‐urban settings, may assist in achieving restoration goals in urban streams through both catchment and instream actions. This geomorphologically-‐sensitive approach to urban stream management requires a significantly greater understanding of the links between urbanisation and geomorphic features and functions, and recognition of the relevant constraints within the urban environment including: flashy hydrology, reduced sediment supply, limited space, legacy impacts and social and institutional perceptions.
Keywords: urbanisation, geomorphologic functioning, stream health, waterway management Introduction Healthy urban streams have been recognised as a fundamental prerequisite to achieving sustainable management of our cities and fulfilling our imperative to maintain healthy aquatic ecosystems for future generations (United Nations General Assembly, 1987). There are a number of excellent summaries on the effects of urbanisation on stream health (Walsh et al., 2005a; Gurnell et al., 2007), but these often ignore or only briefly touch on geomorphic impacts. While there is general consensus that urbanisation 1
Vietz, G. J., Stewardson, M. J., Walsh, C. J., Fletcher, T. D., 2012, Another reason urban streams are stuffed: geomorphic history, challenges and opportunities, in: Proceedings of the 6th Australian Stream Management Conference, 'Managing for extremes' (J. R. Grove, I. D. Rutherfurd, eds.), February 6-8, 2012, Canberra, Australia, pp. 110-115.
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results in channel enlargement, channel widening and channel deepening (e.g. Booth and Jackson, 1997; Chin, 2006; Grable and Harden, 2006) there is little evidence of impacts on instream features (e.g. bars and benches) and processes (e.g. sediment transport and storage). Though, some studies have demonstrated reduced physical habitat in urban streams (Davenport et al., 2001; Scholz and Booth, 2001). So whilst changes in mean channel geometry and physical habitat have been described, the underlying mechanisms and consequences have not, meaning that understanding of consequences for stream biota is very limited. In short, whilst there are conceptual models proposed to explain gross channel changes in response to urbanisation (e.g. Chin, 2006), the underpinning mechanisms for these changes are not well established and the consequences for geomorphic features or functions within river channels are poorly described. Given the limited understanding of important elements of physical form and function in urban streams, it is not surprising that little effort goes into restoring or protecting them. The precautionary principle is rarely applied to eco-‐geomorphic associations. We argue here for the need to move beyond the current urban stream management approaches to consider geomorphic form and functioning. We note, however, that we must significantly improve our knowledge of the impacts of urbanisation on a range of geomorphic features and functions comprising healthy streams. This paper provides a précis of the historical approach to the management of urban stream physical form, in Australia and internationally, as background to the current state of knowledge and practice. We identify elements of geomorphic management that are likely to be required if healthy streams are to be achieved, and describe five main challenges to retaining or reinstating geomorphic features associated with healthy urban, or peri-‐urban streams.
A focus on flow efficiency and channel stability: pre 2000 Research into urbanisation and geomorphology has almost solely focused on channel dimensions and bank stability, highlighting two overarching physical form management priorities: flow efficiency (i.e. to manage flood risk) and channel stability (i.e. to understand rates of erosion). This approach has raised concerns over the exclusive focus on human values (Florsheim et al., 2008). The focus on flow efficiency and stability could be traced back to the founding statement of the institute of civil engineers in 1830 whose intention was ‘to harness the great sources of power in nature for the use and convenience of man’ (Watson, 1988). Rivers were seen as disorganised systems that needed simplifying in order to replicate channels that were better understood (northern-‐hemisphere channels). It appears that Wilson (1946) was the first to propose working with rivers and natural processes, rather than ‘man versus river’ and it was not until the 1960s that the science of fluvial geomorphology matured in the US and later within Australia. In recent years, however, there has been increasing recognition that stream engineering should be underpinned by an understanding of geomorphology (Gilvear, 1999). Unfortunately, however, it appears that stream management professionals have not moved beyond the ‘big two’ objectives of flow efficiency and channel stability. A recent study of urban stream professionals and academics, conducted for Melbourne Water, revealed that the greatest driver for urban stream management was considered to be ‘stabilisation/asset protection’ (noted by 50% of respondents) (Zavadil, 2009). The third greatest driver (23%) was ‘flood mitigation’, following ‘recreation/amenity activities’ (33%). Despite the limited sample size (30 interviews), the study highlights that urban stream management priorities are still more closely aligned with achieving stability than with ecologically healthy streams.
Considering urban stream geomorphology Managing urban streams for flood mitigation and stability is unlikely to result in a healthy stream ecosystem. In simplistic terms we suggest there are two geomorphic attributes associated with streams
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capable of supporting biota: channel complexity (i.e. physical form diversity) and geomorphic dynamism (i.e. channel and sediment mobility). Channel complexity – driving hydraulic diversity -‐ can translate flow into an important ecological event such as a disturbance (Poff et al., 2010). It increases the diversity of hydraulic habitat available to species, particularly low velocity macroinvertebrates refuge, and provides a point from which they may subsequently disperse to recolonise the bed post-‐event (Rice et al., 2010). The more diverse the channel morphology the greater the resilience of the refuge habitat to a large range of flows, and the greater the diversity in flow depth and velocities (Booker and Dunbar, 2004). Geomorphic dynamism refers to the processes of erosion, transport and deposition of sediments as the stream mobilises the bed and/or bank sediments. In particular; bank erosion, mobile bed sediments, geomorphic features created by transient sediment storage (bars and benches) and planform migration (both erosional and depositional) are all important dynamic characteristics of natural geomorphic functioning. The niches and diversity provided by bank erosion have been cited as integral to the functioning of river ecosystems (Florsheim et al., 2008). In urban streams deposition tends to be rare (Grable & Harden, 2006) and as such bars, benches and sediment deposits (including coarse-‐grained substrates) are by observation often conspicuous by their absence from many urban streams. If you have made it to reading up to this point you are perhaps pondering that there is little hope for the health of our urban streams, particularly their physical form and functioning. Some current initiatives, however, aim to enhance our understanding and improve management strategies. The Cities as Water Supply Catchments project is investigating the role geomorphic form and function play in urban streams (Wong et al., 2011). This is in association with the main project goals of increasing the availability of water for use in cities and decreasing stormwater runoff to waterways. Research is being conducted to explicitly understand the link between directly connected imperviousness and geomorphic features which may assist in achieving ecological restoration goals. In terms of implementation in Victoria, Melbourne Water is developing a channel form and function position paper. This will outline a number of principles for incorporating geomorphic functioning into urban stream management including: the consideration of longitudinal and lateral connectivity, the intrinsic value of the physical form of streams (particularly when rare or threatened), and the role of planning in the management of riparian lands. These activities are expanding the realm of a physical form management with a geomorphologically-‐ sensitive approach more likely to influence the health of streams in the future. Relative to standard stream management approaches a geomorphologically-‐sensitive one needs to consider three main attributes: (1) the level of channel complexity, (2) the level of acceptable dynamism (planform and sediment movement), and (3) the amount of riparian land required to achieve these goals (Table 1).
Challenges and opportunities We suggest there are five main drivers of physical form degradation for urban streams discussed in the following paragraphs. While there may be opportunities to address these challenges the solutions are not yet fully understood. Flashy hydrology: Changes to the hydrologic regime due to urbanization have been well described (Walsh et al., 2005b; Gurnell et al., 2007), and it is the increased hydraulic efficiency of the urban drainage system which has been found to account for about 70 percent of the increase in runoff magnitude to receiving waters (Wong et al., 1997). Increases in the peak discharge, and of greater concern, the durations above erosional thresholds, have been found to be the most damaging to physical form (Coleman et al., 2005). Where possible the principle of ameliorating this driver of channel change should focus on the cause (urban runoff to streams) as described by Walsh et al. (2005b), rather than treating the symptoms in streams. With a focus on managing the urban water cycle in a more
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integrated and sustainable manner, including re-‐use of stormwater (Wong et al., 2011), it is critical that geomorphic considerations inform the design solutions. The tools for quantifying these components currently fall short and better guidance on the type of flow regime necessary to sustain the desired geomorphology is needed. Table 1. Approaches to managing streams and their geomorphic and ecological consideration
Traditional River Engineering
Geomorphologically-‐ Referenced River Engineering
Natural Channel Design
Geomorphologically-‐ sensitive aspirations
Channel complexity
Low – hard engineering to reduce roughness and increase stability
Medium – constructed features designed to resemble ‘natural’ stream with some ‘hard’ features designed for low resistance
Medium – some variable cross sectional features with a reliance on rock stabilisation
High – stream controlled geomorphic features such as benches, bars and variable sediment sizes which create resistance to flow
Allowable dynamism
None
Low – no stream adjustment, no mobile substrates, no erosion allowed
Medium – limited stream adjustment, limited mobile substrate sediments, erosion commonly addressed
Riparian space required
Very low
Low – no lateral flow engagement or adjustment
Medium – minimal lateral flow engagement and minor channel adjustment
Medium to High – stream adjusts dynamically within confined corridor, mobile substrates, acceptable rates of erosion Medium to High – engagement with actual or ‘internal’ floodplain to alleviate channel energy, provide adjustment corridor
Increasing geomorphic & ecological consideration à
Reduced sediment load: Coarse-‐grained sediment yield from an established urban catchment is generally considered to decrease (Bledsloe, 2002; Gurnell et al., 2007). Once the sediment supply is reduced, bank erosion is estimated to provide about two-‐thirds of the total sediment yield (Trimble, 1997). The limited sediment supply, coupled with increased sediment transport capacity (Bledsloe, 2002), particularly in the naturally supply-‐limited conditions of Australia, significantly reduce bedload sediments (Figure 1a). Sediment supply reductions are the least understood and potentially the greatest impediments to long-‐ term recovery, though opportunities exist in new or peri-‐urban developments where headwater sediment sources can be preserved and riparian land managed for migration. Limited riparian space: Floodplain engagement and lateral migration are important both geomorphically and ecologically (Coleman et al., 2005; Florsheim et al., 2008), but are reduced in urban catchments. Floodplain engagement is often undesirable in the urban environment, but it is important to recognise the geomorphic implications, namely, increased stream energy exerted within the channel. The altered flow regime in combination with channel modifications are a double-‐edged sword, driving channel degradation. The additional loss of riparian vegetation, and it’s binding and shading properties, has significant implications for the channel’s geomorphic integrity (Booth, 1991). Peri-‐urban areas or new developments provide the greatest opportunities for larger riparian buffers, or, in developed locations riparian space should be vigorously defended. The challenge is to allow for the greatest amount of ‘natural’ (stream controlled) adjustment and engagement within the often-‐significant constraints. Such urban riparian corridors may offer other benefits in terms of aesthetics improving urban livability. Legacy impacts: Urban stream degradation cannot always be entirely attributed to urbanisation and it is important to differentiate the relative, and often significant, role of the prior land uses (Figure 1b). The associated challenges include: distinguishing appropriate restoration goals; distinguishing natural reference conditions; and disentangling impacts both temporarally and spatially to understand futures
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responses. Some of the most susceptible stream types to these issues are ephemeral and ill-‐defined streams (e.g. ‘intact valley fills’ or ‘chain-‐of-‐ponds’ in greenfield sites). Focusing management and research efforts on relatively undisturbed but rapidly developing sites might avoid some of these challenges. b)
a)
Figure 1a) Impact of urbanisation reducing catchment-‐derived sediment supply coupled with increased sediment transport capacity leads to reduced bedload sediment, and b) Legacy impacts on urban stream geomorphology.
Social and institutional perceptions: In Australia during the mid 1900s rapid development of cities involved major drainage works with streams often piped or channelised (Brown et al., 2009). While attitudes are rapidly changing, suggestions of alterations to flow efficiency or channel stability will still face opposition within the community and the waterway management industry. The urban geomorphologist’s role is a particularly challenging endeavor, being to convince waterway managers and the community alike of the benefits of a geomorphologically-‐sensitive approach to urban stream management (and the important role of the floodplain in flood mitigation): these desires can co-‐exist with flooding and maintenance management if appropriately understood and implemented.
Conclusions Historically our management of urban streams, both in Australia and internationally, has focused on flow efficiency and channel stability, leading to degradation of physical form. If we are able address the catchment-‐scale perturbations to hydrology and water quality (the former of which has contributed to geomorphic impacts) will ecological response be constrained by the physical form and functioning? In addition to the importance of improving our knowledge in this realm there are a number of challenges faced by a geomorphologically-‐sensitive approach, including: flashy hydrology, reduced sediment supply, legacy impacts, lack of riparian space and the social and institutional perceptions. These challenges also provide opportunities in the current desire for more livable cities, particularly in Greenfield developments as the urban footprints expand. A better understanding will, if not reduce the risk, allow us to further quantify risks to humans and infrastructure resulting from greater consideration for the ecological and geomorphic functioning of streams.
Acknowledgements This research is funded by the Cities as Water Supply Catchments program, which is a collaborative research partnership between Monash University, The University of Melbourne, The University of
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Queensland, AECOM and a large number of industry partners. We thank James Grove for providing a review of this manuscript. References Bledsloe, B. P. (2002), Stream erosion potential and stormwater management strategies, Journal of Water Resources Planning and Management, 128(6), 451-‐455. Booker, D. J., and M. J. Dunbar (2004), Application of physical habitat simulation (PHABSIM) modelling to modified urban river channels, River Research and Applications, 20, 167-‐183. Booth, D. (1991), Urbanization and the natural drainage system -‐ Impacts, solutions, and prognoses, The Northwest Environmental Journal, 7, 93-‐118. Booth, D. B., and C. R. Jackson (1997), Urbanization of aquatic systems: degradation thresholds, stormwater detection, and the limits of mitigation, J. Am. Water Resour. Assoc., 33(5), 1077-‐1090. Brown, R. R., N. Keath, and T. H. F. Wong (2009), Urban water management in cities: historical, current and future regimes, Water Sci. Technol, 59(5), 847-‐855. Chin, A. (2006), Urban transformation of river landscapes in a global context, Geomorphol., 79, 460-‐487. Coleman, D., C. MacRae, and E. Stein (2005), Effect of increases in peak flows and imperviousness on the morphology of Southern California Streams, Southern California Coastal Water Research Project, Westminster, CA. Davenport, A. J., A. M. Gurnell, and P. D. Armitage (2001), Classifying urban rivers, Water Sci. Technol, 43(9), 147-‐ 155. Florsheim, J. L., J. F. Mount, and A. Chin (2008), Bank erosion as a desirable attribute of rivers, BioScience, 58(6), 519-‐529. Gilvear, D. J. (1999), Fluvial geomorphology and river engineering: future roles utilizing a fluvial hydrosystems framework, Geomorphol., 31, 229-‐245. Grable, J. L., and C. P. Harden (2006), Geomorphic response of an Appalachian Valley and Ridge stream to ubanization, Earth Surf. Process. Landf., 31, 1707-‐1720. Gurnell, A., A. Lee, and C. Souch (2007), Urban rivers: hydrology, geomorphology, ecology and opportunities for change, Geography compass, 1(5), 1118-‐1137. Poff, L. N., B. D. Richter, A. H. Arthington, S. E. Bunn, R. J. Naiman, E. Kendy, M. Acreman, C. Apse, B. P. Bledsloe, M. C. Freeman, J. Henriksen, R. B. Jacobson, J. G. Kennen, D. M. Merritt, J. H. O'Keefe, J. D. Olden, K. Rodgers, R. E. Tharme, and A. Warner (2010), The ecological limits of hydrologic alteration (ELOHA): a new framework for developing regional environmental flow standards, Freshw. Biol., 55, 147-‐170. Rice, S. P., J. Lancaster, and P. Kemp (2010), Experimentation at the interface of fluvial geomorphology, stream ecology and hydraulic engineering and the development of an effective, interdisciplinary river science, Earth Surf. Process. Landf., 35, 64-‐77. Scholz, J. G., and D. B. Booth (2001), Monitoring urban streams: Strategies and protocols for humid-‐region lowland systems, Environmental Monitoring and Assessment, 71, 143-‐164. Trimble, S. W. (1997), Contribution of stream channel erosion to sediment yield from an urbanizing watershed, Science, 278(5342), 1442-‐1444. United Nations General Assembly (1987), Report of the World Commission on Environment and Development: Our Common Future. Walsh, C. J., A. H. Roy, J. W. Feminella, P. D. Cottingham, P. M. Groffman, and R. P. Morgan (2005a), The urban stream syndrome: current knowledge and the search for a cure, J. N. Am. Benthol. Soc., 24(3), 706-‐723. Walsh, C. J., T. D. Fletcher, and A. R. Ladson (2005b), Stream restoration in urban catchments through redesigning stormwater systems: looking to the catchment to save the stream, J. N. Am. Benthol. Soc., 24(3), 690-‐705. Watson, G. (1988), The Civils -‐ the story of Institution of Civil Engineers, Thomas Telford Ltd. Wilson, C. M. (1946), River come closer to my door!, The Scientific Monthly, 62(2), 117-‐126. Wong, T. H. F., R. Allen, J. Beringer, R. R. Brown, V. Chaudhri, A. Deletic, T. D. Fletcher, W. Gernjak, L. Hodyl, C. Jakob, M. Reeder, N. Tapper, and C. J. Walsh (2011), Blueprint2011 -‐ Stormwater Management in a Water Sensitive City, 48 pp pp, Monash University, Clayton, VIC. Zavadil, E. (2009), Understanding how we manage urban streams, Report P109064_R01 by Alluvium for Melbourne Water, Melbourne, Australia.
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