Urban Rivers: Hydrology, Geomorphology, Ecology and Opportunities for Change
Abstract
This article describes how urban development impacts on the processes that control river geomorphology and influence ecology. At the catchment scale, urban development transforms the hydrological system through construction of impervious surfaces and stormwater drainage systems. River water and sediment quality also are affected by stormwater and waste water drainage and by point and diffuse inputs of pollutants. Within the river channel network, widespread river engineering improves floodwater conveyance but imposes major changes in river network and channel characteristics. The results of an analysis of Urban River Survey data from 143 channel reaches in three European rivers (the River Tame, UK; the River Emscher, Germany; and the River Botic, Czech Republic) are presented to demonstrate the strong influence of river channel engineering on channel structure, physical habitat features and vegetation patterns. This analysis also shows the surprisingly varied character of urban rivers and thus their differential potential to respond to rehabilitation efforts. Because the success of river rehabilitation efforts depend not only on a scientific understanding of form and process within urban river systems but also on the acceptance and support of urban communities and integration within urban design and planning, the article briefly explores the coupling of natural and social science approaches to drive a more sustainable future for rivers in cities.
Introduction
In their ‘natural’ state, rivers and their floodplains are composed of a complex, ever-changing, hierarchical mosaic of patches with different hydrological, geomorphological and ecological characteristics (Frissell et al. 1986). Changes in the mosaic in time and space are driven by river flow disturbances and sediment movements acting across longitudinal (upstream-downstream), lateral (channel-floodplain) and vertical (surface-subsurface) climatic, hydrological and biogeochemical gradients (Ward 1998; Ward et al. 2002).
The form and biodiversity of river systems depend on disturbance and recovery along the environmental gradients that they encompass. River flows erode, transport and deposit sediment to determine landforms along the river's course (Church 2002). The flow regime is also the key driver of river ecosystems (Bunn and Arthington 2002; Poff et al. 1997; Tockner et al. 2000). Bunn and Arthington (2002) suggest that this reflects four groups of mechanisms that link hydrology and aquatic ecology: flow determines physical habitats; flow drives longitudinal and lateral connectivity within the river's corridor; aquatic species have evolved life history strategies in response to the natural flow regime; and altered flow regimes often support the invasion and establishment of exotic species. Vegetation is a further key factor that interacts with the river's flow and sediment regimes to stabilise and reinforce landforms, accelerate sedimentation and induce rapid changes in the mosaic of river and riparian habitats (Bennett and Simon 2004; Gurnell 2007; Gurnell and Petts 2002; Hupp and Osterkamp 1996).
Thus, natural river landscapes are characterised by dynamic mosaics of habitat patches arranged along environmental gradients, sculpted and connected by flow and sediment transport disturbances, and in many cases reinforced and accentuated by vegetation growth. Urban development imposes enormous changes on the form and function of river systems. Catchment hydrology, river flow and sediment regimes are transformed by the construction of impervious surfaces and stormwater drainage systems. River water and sediment quality are affected by stormwater and waste water drainage and by point and diffuse inputs of pollutants. Widespread river channel engineering improves the conveyance of floodwaters but imposes major changes in the characteristics of the river network. In many circumstances, river engineering has removed the connectivity of flows, sediment movements and organisms between the river and floodplain and has severely constrained river channel dynamics (e.g. Figure 1) whereas longitudinal connectivity has usually been increased. In short, urban development changes all key processes that drive river corridor form, dynamics and biocomplexity. These changes are elaborated in Figure 2, illustrating how urban development affects catchment and river network functioning at all spatial scales from the entire catchment, through major sectors of the river network (i.e. sections of river channel between major channel junctions, typically several kilometres in length) and shorter reaches (i.e. sections of river channel of particular physical characters within sectors and typically several hundred metres in length) to individual habitats within the channel and its margins (typically several tens of metres in length). As a result, urban rivers tend to be severely morphologically and ecologically degraded. Walsh et al. (2005) call this the ‘urban stream syndrome’.
Hypothetical changes induced by urban development, channel reinforcement and channel enlargement on flow complexity and connectivity at three different flow stages.
The important properties at catchment, sector and reach scales (green text) and their modification within urban catchments (red text) that control the nature and dynamics of physical habitats within reaches of river corridor (black text in box in lower right corner).
This article develops the ideas presented in 1, 2by considering four themes. First, it evaluates the impact of urban development on the processes that control river characteristics and their geomorphological and ecological responses. Second, it presents the results of analysis of a large data set to illustrate how river channel modifications in urban catchments affect channel structure and connectivity. Third, it describes some examples of urban river rehabilitation and enhancement to illustrate the potential to reinstate physical structure and, in some cases, connectivity. Finally, the article explores the integration of natural and social science approaches to drive a more sustainable future for rivers in cities.
Controlling Processes
water and sediment quality
Urban development is one of the leading causes of water quality degradation in surface waters (US Environmental Protection Agency 2000). Runoff from urbanised surfaces, as well as municipal and industrial discharges, results in increased loading of nutrients (Grimm and Sheibley 2005; Groffman et al. 2002), metals (Gobel et al. 2006; Neal and Robson 2000), pesticides (Chevreuiel et al. 1999; Coupe et al. 2000; US Geological Survey's National Water Quality Assessment Program 2007), organic contaminants (petroleum products, polychlorinated biphenyls, polycyclic aromatic hydrocarbons, etc.) (Eganhouse et al. 1981; Foster et al. 2000; Latimer and Quinn 1998) and other contaminants, and also has significant effects on the thermal regimes of urban rivers (LeBlanc et al. 1997). The effects of urban development on water quality tend to be more variable than hydrological or geomorphological effects, depending on the nature of urban land use (residential versus commercial/industrial), the presence of water treatment plants, illegal discharge connections, effluent or combined sewer overflows, landfills, failing septic systems and the extent of stormwater drainage (Paul and Meyer 2001).
Many effects on urban river water quality are transient, associated with discharges from a sewerage overflow or the first flush of stormwater. In the UK, for example, more than 35% of the annual pollutant discharge comes from combined sewers and storm drains in less than 3% of the time (Faulkner et al. 2000; Heaney and Huber 1984). However, given the pathways of many contaminants (notably metals, phosphorus, solvents, petroleum products) are also in particulate-associated forms (Macklin et al. 2006; Martin and Meybeck 1979), longer-term effects on water quality are often mediated through river sediments.
The concentration, storage and transport of contaminants in urban streams is complex – related to sediment texture, organic content and prevailing pH and Eh conditions (Hudson-Edwards et al. 2005; Souch et al. 2002). Generally, higher sediment metal concentrations are found in areas of low velocity where fine sediments and organic particles concentrate (Rhoads and Cahill 1999). Studies on organisms suggest that often the greatest effects are due to the ingestion of fine sediments and organic matter from the riverbed rather than the overlying water (House et al. 1993).
river flow and sediment transport regimes
The major impacts of urban development on river flows have long been recognised. Early analyses of changing river flow regimes within individual catchments demonstrated increases in the total quantity of runoff and in the frequency of flooding; changes in the shape of storm hydrographs, particularly a decrease in time to peak and an increase in peak discharge; reduction or removal of contrasts in seasonal runoff responses to precipitation; and frequently an overall decrease in baseflow (e.g. Hollis 1974, 1975). Indeed these changes are so fundamental that a distinct discipline of ‘urban hydrology’ has been recognised (e.g. Akan and Houghtalen 2003; Lazaro 1979), and numerous studies continue to identify the remarkably consistent transformation of hydrological regime that results from urban development (e.g. Olivera and DeFee 2007; White and Greer 2006). These changes in river flow regime have been attributed to (i) the construction of impervious surfaces and the compaction of pervious surfaces, which promote rapid surface runoff from rainfall and (ii) the efficient drainage of surface runoff through storm sewers to the river network. Indeed, the percentage urban land cover, percentage impervious cover or percentage connected impervious cover within a catchment provide simple but very effective predictors of changes in streamflow characteristics resulting from urban development (Akan and Houghtalen 2003; Anderson 1999).
Transformation of the river flow regime by urban development is accompanied by transformation in the supply and transport of sediment. For example, Wolman (1967) demonstrated how building activity leads to an order of magnitude increase in sediment yield per unit area per year but that sediment yields drop to extremely low levels once urban development is complete. The reduction in sediment yield reflects sealing of the catchment surface with erosion-resistant materials and also the widespread reinforcement of river channel banks and bed. However, where channels are not stabilised, channel erosion can be an important sediment source for urban rivers. Trimble (1997) estimated that channel erosion provides about two-thirds of the total sediment yield from the urban catchment of the San Diego Creek, California.
geomorphology
Wolman (1967) also recognised the broad implications of changes in river flows and sediment transport for urban river form and process. He envisaged a cycle of channel aggradation during building construction followed by scour and bank erosion once the urban area was fully developed. Hammer (1972) also described stream channel enlargement following urban development. A recent analysis of information drawn from over 100 studies conducted across the world (Chin 2006) confirms these early ideas but also demonstrates the wide variability in stream channel responses. Many individual studies have revealed how and where urban rivers may adjust. The results of some of these studies are summarised below.
Channel enlargement is a characteristic response to the changed flow and sediment regime following urban development (Booth and Henshaw 2001; McBride and Booth 2005; Neller 1988; Pizzuto et al. 2000), particularly in humid and temperate environments (Chin 2006), and is the reason why bank and bed reinforcement are widely used along urban rivers where space is limited and infrastructure and buildings are susceptible to undermining. However, the degree and style of enlargement varies greatly. While enlargement did not involve bed incision in the study by Pizzuto et al. (2000), Booth and Henshaw (2001) observed channel incision ranging from <20 mm to 1 m per year across a sample of previously forested catchments in western Washington, USA. Channels with the greatest susceptibility to incision were characterised by a pre-development hydrology dominated by subsurface drainage; an erosion-susceptible substrate; moderate to high channel gradients; and an absence of natural or artificial grade controls. While channel incision and enlargement characterise urban rivers, Henshaw and Booth (2000) found that channels in their Puget Sound study stabilised following 10 to 20 years of stable urban catchment land use. Neller (1988) considered that enlarging urban river channels in Armidale, New South Wales, Australia, were not inherently unstable but were simply accommodating the changed flow and sediment transport regime following urban development. These observations suggest that reinforcement of urban river channels may not be necessary, particularly where the river has space for adjustment.
Pizzuto et al. (2000) investigated further geomorphological characteristics of urban rivers by comparing paired rural and urban gravel-bed systems. They found that urban channel enlargement in their study area, Pennsylvania, USA, was achieved by an increase in width rather than depth and that both pool depths and sinuosity were lower in the urban rivers but there was no difference in channel gradient. They estimated that the bankfull discharge accommodated by the urban channels was 131% higher than the rural channels. There was no difference in median bed material grain size but a secondary mode of sand to pebble-sized particles appeared to have been selectively removed from the urban channel beds. Similarity in the calibre of the coarser fraction of the bed material between rural and urban sites suggests bed armouring, which may explain why the urban channels increased width rather than depth to accommodate the urban discharge regime. Nevertheless, the authors estimated that bankfull shear stresses were sufficient for significant bed-load transport at both rural and urban sites and inferred that sediment supply probably remained significant despite the extensive cover of erosion-resistant impervious surfaces. Finkenbine et al. (2000) also noted selective removal of the finer fraction of bed material from urban stream beds in their study area around Vancouver, Canada. In particular, the reduction in intragravel fines in their urban streams appeared to have the ecologically beneficial effect of supporting higher values of intragravel dissolved oxygen.
Building on earlier research (Gregory 2002), Chin and Gregory (2005) developed a geomorphological classification of urban river channels, which relates direct human channel management activities to geomorphological forms, adjustments and stream channel hazards and is designed as a management tool. Six geomorphological types of channel are identified: (1) near-natural channels; (2) adjusting channels that could recover or (3) are unlikely to recover without intervention, such as where enlargement and incision have occurred; engineered channels, which are differentiated in terms of those that are (4) partly channelised but capable of recovery, (5) channels that are heavily engineered in constrained locations that inhibit alternative management methods, and (6) channels that are culverted. Stream channel hazards are particularly associated with channel types 2, 3 and 4 and include, for example, changed flood frequency, flood drainage, scour or aggradation in relation to both natural and human-made features; trash accumulation; and changes in aquatic or bank vegetation.
ecology
Given the enormous contrasts in flow and sediment regime, water and sediment quality, and geomorphology of urban streams in comparison with their rural counterparts, it is scarcely surprising that researchers have found significant degradation in their ecology.
The species and abundance of fish, benthic invertebrates and algae have all been used as indicators of the biological health of rivers. For example, Karr (1981) developed an Index of Biotic Integrity (IBI) for use in Illinois and Indiana, USA, based on properties of the fish population, including abundance, species richness and composition, condition, reproduction, etc. The index has been fine-tuned for application in many parts of the USA and features in many assessments of the biotic integrity of urban rivers. In Wisconsin, USA, Wang et al. (2003) found that the amount of connected (by storm sewers) impervious cover explained 59% in the variance of fish species, 39% of the variance in fish density, and 32% of the variance in IBI. In a threshold zone, between 8% and 12% connected impervious cover, small adjustments in urban development were reflected in large changes in stream condition. Areas of connected impervious cover located close to the survey point had more influence on biotic integrity than similar areas located more remotely within the catchment, suggesting that riparian buffer zones help to ameliorate the impacts of urban development. Similarly in Ohio, USA, Miltner et al. (2004) found a significant decline in the IBI when the impervious cover exceeded 13.8%. Where biological integrity was maintained despite higher levels of urban land use, the floodplain and riparian zone were relatively undeveloped, confirming the importance of a riparian buffer to protect the stream. In contrast, in catchments undergoing building development, a decline in the IBI could occur when total urban land use was as low as 4%, indicating the deleterious effects of poorly managed construction sites.
Using an IBI based on measures of the benthic invertebrate community, Morley and Karr (2002) also demonstrated an overall decline in stream biological condition as the percentage of urban land cover increased. Their observations that hydrological and stream substrate alteration both influence biological condition complements the emphasis on connected impervious area in the study by Wang et al. (2003) and indicates the importance of hydraulic-substrate conditions for benthic invertebrates. More recently, analysis of IBIs based on benthic invertebrates have illustrated that parameters of the river flow regime and land use along the river margins provide more informative indicators of stream health than measures of impervious area (Booth et al. 2004) and that there are significant associations between the amount and distribution of both urban and forested land and the biotic integrity of streams (Alberti et al. 2007).
The importance of a riparian buffer, particularly when comprised of forest, was investigated by Roy et al. (2006), who found that increases in richness and abundance of sensitive fish species were only associated with higher riparian forest cover when there was also a relatively stable and coarse river bed. This illustrates complex interactions between altered urban flow and sediment regimes and the characteristics and management of the river corridor. It is also insufficient to have a good riparian buffer zone if it does not interact freely with the river channel. Finkenbine et al. (2000) noted the lack of large wood in urban streams in Vancouver, British Columbia, and suggested that a healthy buffer zone with abundant large wood can stabilise stream banks. Large wood within rivers is an indicator of lightly managed or unmanaged riparian woodland and good connectivity between river channel and riparian zone. Among the numerous benefits of large wood for river ecosystems are bed sediment sorting and an increase in hydraulic and physical habitat complexity in the river channel and its margins (Gregory et al. 2003). Therefore, it appears that river channel – floodplain connectivity and dynamics are as important to urban rivers as they are to their rural counterparts.
sustainable urban drainage systems
Many of the studies discussed in the preceding sections use percentage impervious area or percentage connected impervious cover as indicators of the degree of urban development within a catchment. The concept of connected impervious area is becoming increasingly important as urban development incorporates more sustainable urban drainage systems (SUDS). There are numerous approaches to sustainable urban drainage that can be applied at different scales, in different combinations and with varying frequency across a catchment (for examples, see France 2002; Minnesota Stormwater Steering Committee 2006; for discussion of experiences in implementing SUDS, see Mitchell 2006). The general aims of SUDS are to slow down the rate of movement of stormwater towards the river network, to contribute to reinstating levels of infiltration and groundwater recharge that had existed prior to urban development and also to retain sediment and treat both sediment and water within local storage areas using natural bioremediation. Studies suggest that in many circumstances these aims can be achieved, although the impact on flood protection varies with scheme design and local environmental context (e.g. Fletcher et al. 2007). Urban impacts on receiving rivers in relation to flow and sediment regimes, water and sediment quality are likely to be far less pronounced in catchments where SUDS are implemented widely, particularly where urban woodland and riparian buffers are incorporated in the design. Walsh et al. (2007) provide some evidence for the beneficial effects of SUDS through their evaluation of relationships between benthic invertebrates, riparian forest cover and catchment urban development in the Yarra River, Victoria, Australia. They concluded that although riparian forest cover may have a beneficial influence on the richness of some macroinvertebrate taxa, sensitive taxa are more strongly affected by conventionally drained urban development. Thus, the introduction of SUDS and associated disconnection of impervious surfaces are likely to have a more beneficial effect on indicators of stream biological integrity than riparian vegetation improvements alone.
Physical Habitat Characteristics of Urban River Channels
By affecting river flows and sediment transport, urban development affects the size, form and dynamics of river channels. Where river channel change is seen to be a management problem as well as a risk, urban rivers are often reinforced with concrete, sheet-piling, gabions, etc. Engineered channel forms and reinforcement are also implemented to improve flood conveyance as part of flood management practice. The ecology of urban rivers is affected by changes in water and sediment quantity and quality and also by channel and riparian zone characteristics. In the previous section, the importance of a riparian buffer that supports physical habitat complexity and river-margin connectivity has been established. Therefore, improved urban river management needs to be based on a sound understanding of associations between management activities and the properties of urban rivers and their riparian zones – a theme that is explored in this section.
River habitat surveys are used to characterise rivers and their corridors (e.g. Australian River Assessment System 2007; Environment Agency 2003; US Department of Agriculture 1998; Vermont Agency of Natural Resources 2004) and in many cases to support river classifications, ranging from single indices, such as the UK Environment Agency's Habitat Quality Assessment and Habitat Modification Score derived from its River Habitat Survey (Raven et al. 1998), to hierarchical frameworks such as that proposed by Brierley and Fryirs (2005). Only a small number of habitat surveys and classifications have been developed specifically for urban rivers (e.g. Anderson 1999; Boitsidis and Gurnell 2004; Boitsidis et al., 2006; Chin and Gregory 2005; Davenport et al. 2004; Taylor et al. 2005).
Analysis of information collected using the Urban River Survey (URS; Boitsidis and Gurnell 2004) on 143 reaches of three European river catchments (the River Tame, UK; the River Emscher, Germany; and the River Botic, Czech Republic) illustrates the variety of physical habitats that can be found in urban rivers. Because many urban channels are reinforced and also have modified cross-profiles and/or planforms, the URS is applied to 500 m length river reaches of a single-engineering type. The ‘engineering type’ is defined by the combination of reinforcement, channel cross profile and planform along the reach as a whole (Table 1). Within each urban river reach, URS measurements record properties, such as channel dimensions, bed sediment calibre and reinforcement, bedforms, flow types, vegetation, artificial features, channel bank sediment calibre, reinforcement type and materials, natural and artificial bank profile types, bank face and bank top vegetation, and associated habitat features, along with floodplain land use immediately adjacent to the river (Boitsidis and Gurnell 2004).
| Planform | Cross section | Reinforcement |
|---|---|---|
| Straight (engineered straight) | Enlarged (cross section made substantially larger than a naturally adjusted channel would be at the same site) | Full (both banks and bed) |
| Meandering (engineered sinuous) | Resectioned (cross section reshaped to a more efficient trapezoidal form) | Both banks |
| Recovering (engineered straight or sinuous but showing significant planform readjustment induced by fluvial processes) | Two-stage (cross section includes a large flood channel with an inset smaller channel to accommodate non-flood flows) | Bed and one bank |
| Seminatural (no obvious sign of engineering of the planform) | Cleaned (flow resistance reduced through removal of roughness elements such as trees and shrubs and minor morphological irregularities) | One bank |
| Restored (cross profile form designed as part of a restoration scheme) | Bed | |
| Seminatural (cross profile form shows no obvious signs of human modification) | No reinforcement |
The data set derived from URS surveys of 143 urban river reaches was analysed using principal components analysis (PCA), which reduces measurements of a large number of variables, many of which may be highly correlated, to independent variable dimensions termed principal components (PCs) that are key linear combinations of the original variables (Griffith and Amrhein 1997). Figure 3 summarises the results of this analysis: providing an interpretation of the first two PCs; showing how the 143 survey reaches plot in relation to these PCs; and how the reaches also display distinct groupings across the graph according to their habitat characteristics and level/type of engineering (cross profile, planform, reinforcement).
A simplified interpretation of variations in engineering (cross profile, planform, reinforcement) of 143 urban river reaches from three European catchments in relation to their connectivity, complexity and habitat diversity and also their marginal tree and aquatic vegetation cover. The diagram is developed from a Principal Components Analysis of Urban River Survey data for the 143 reaches. It plots the position of the 143 reaches according to their scores on PC1 (horizontal axis) and PC2 (vertical axis), which together explain 40% of the variation in the original multivariate data set. The orange dots show the plotting positions for each of the 143 reaches in relation to PC1 and PC2. The text boxes describe the cross profile, planform, level of reinforcement and habitat characteristics of reaches located within them. The arrows and axis labels interpret the broad nature and direction of the PC gradients.
The degree to which the PCs reflected variations in the original variables was interpreted from the loadings of the individual variables on each PC. These loadings showed that PC1 describes a gradient from heavy channel modification and reinforcement (e.g. extensive bed reinforcement; artificial bank profiles stabilised by solid bank protection materials such as concrete, brick, laid stone or sheet piling) to low levels of modification and reinforcement (e.g. extensive and diverse natural bank profiles with no reinforcement). In terms of physical habitats, PC1 describes a gradient from low to high diversity of physical habitats (e.g. increasing numbers of channel bed form types, flow types, natural bank profile types, tree features and also increasing bank vegetation complexity). PC2 describes a gradient from channels with a high proportion of bed and bank protection, particularly solid bank protection such as concrete, brick, laid stone or sheet piling to channels with a high cover of in-channel vegetation, particularly rooted submerged aquatic plants and emergent plants. It also describes a gradient from continuous tree lined, reinforced channels with significant patches of rapidly flowing water to channels with few trees and significant areas of relatively lower flow velocity. Thus, PC2 defines the well-established negative association between tree shading and aquatic vegetation and also indicates the impact of channel-choking by aquatic vegetation on flow velocities and depths.
Overall, the results of the PCA demonstrate clear inverse associations between reinforcement and enlargement of channels and their physical habitat complexity and connectivity, and also between tree-lined margins and the cover of aquatic vegetation. Seminatural channels display by far the highest number and diversity of flow, channel and bank habitats and complexity of vegetation. However, among the managed channels, there is a clear contrast between the low habitat abundance and diversity of fully reinforced or both-bank reinforced channels and the higher habitat abundance and diversity of less reinforced channels or those with no reinforcement. Reinforced channels have particularly low abundance and diversity of physical habitats when they are also enlarged and straightened. Large, straight, fully reinforced channels are completely disconnected from their riparian zones as well as displaying negligible in-channel habitat. Absence of some or all reinforcement, particularly where the channel is not enlarged or straightened, supports the development of in-channel and bank habitats and promotes connectivity. Thus, this analysis across three different European catchments shows important, deep-seated associations between the reinforcement of urban rivers, engineering modification of cross-profile and planform and the complexity and connectivity of habitats in the channel and riparian zone. These habitat characteristics are extremely important for river ecology and form a basis for river rehabilitation efforts, which are described in the next section.
Opportunities for Change
Given the geomorphological and ecological consequences of urban development and also the spectrum of river cross profile, planform and reinforcement types and their vegetation and physical habitat characteristics, it is apparent that rehabilitation or enhancement of urban rivers to a less engineered state is likely to lead to an increase in the diversity and connectivity of physical habitats and also a more diverse and complex vegetation structure. This, of course, assumes that water and sediment quality are not so degraded that ecological benefits cannot accrue from improvements in connectivity and physical habitat within the channel and riparian zone.
Three examples of the types of change that could be achieved by rehabilitation or enhancement are shown against the backdrop of the PCA biplot in Figure 4 (Boxes 1–3). Box 1 represents locations where there is space to change unreinforced, straightened river channels into a meandering course from which fluvial processes can induce further recovery. This was the approach adopted on a section of the River Cole, West Midlands, UK. This river has an almost entirely urban catchment, and an extremely successful scheme cut a new sinuous, trapezoidal channel to replace the river's historically straightened course. No artificial creation of bed or bank forms were imposed and there was no reseeding of the banks, but bank and bed adjustment proceeded rapidly (Gurnell et al. 2006a) and natural hydrochorous (water-transported) seeding of the banks resulted in the establishment of a complex, diverse riparian vegetation cover within 3 years (Gurnell et al. 2006b).
Three scenarios of change in the rehabilitation or enhancement of urban rivers plotted in relation to the pattern identified in the 143 surveyed urban river reaches (1) introduce meanders to a straightened reach; (2) remove (some) reinforcement and, where possible, introduce meanders to a straightened, reinforced reach; and (3) introduce artificial habitats to enhance reinforced reaches.
Box 2 represents locations where reinforced, straightened channels are transformed into an unreinforced state with the addition of a more sinuous planform, where possible. This type of transformation also depends on having space for the river to adjust laterally, although retention of reinforcement on one bank or along critical bank sections is often sufficient to dampen lateral movement. This type of rehabilitation has been applied to a part of the River Brent, London (River Brent Site 2007). Here, the previously enlarged, straight, reinforced channel has been replaced by a largely unreinforced sinuous course. In some short sections, reinforcement has been retained on one bank to protect infrastructure, but the opposite bank is free to adjust. Wetlands and backwaters have been incorporated into the design.
Box 3 represents constrained locations where full reinforcement or reinforcement of both banks must be retained to protect infrastructure. If the reinforced banks are set reasonably far apart, there is potential to introduce new features into the intervening floodway. In these situations, it is often possible to introduce reinforced, streamlined benches along a margin of the channel, where aquatic or wetland plant species can be planted to suit the level of the bench and thus its hydrological regime. Martin-Vide (2001) described the design of a sinuous low flow channel and constructed wetlands within a floodway through Barcelona, where both features are stabilised by a significant level of reinforcement. In the most confined situations, floating structures can be chained to the channel edges, so that they move with the water level, providing marginal habitat, shade and shelter without affecting channel conveyance (e.g. Thames21 2007).
A Broader Human Context
Environmentally sensitive urban river management is becoming more critical as the world's population concentrates in cities. According to the United Nations Population Fund (2006) report on the state of world population, almost half (49%) of people lived in urban areas in 2005 and the forecast urban population growth rate (2005–2010) is 2.0% in comparison with an overall growth rate of 1.1%. Therefore, it is not surprising that urban ecology, which attempts to integrate physical, ecological, cultural and socioeconomic properties of urban areas (Alberti et al. 2003; Grimm et al. 2000; Pickett et al. 2001), is a rapidly developing area for research. This field incorporates humans explicitly into all aspects of ecology, because humans change the expression of the rules that govern life across spatial and temporal scales (Alberti et al. 2003). The fate of rivers and the services they provide to cities depends on such an integrated understanding of their human-impacted functioning.
Bolund and Hunhammar (1999) recognise two groups of ecosystem services offered by rivers in cities: microclimate regulation and recreational and cultural values. If rivers are to deliver these services, they need to be incorporated into the planning process in an explicit, integrated and sustainable way. While historically, urban rivers have been managed as a resource that was exploited for human benefit, particularly for water supply, wastewater disposal and flood management, Findlay and Taylor (2006) argue that social, political and environmental factors together provide robust justification for the rehabilitation of urban streams. The increasing importance of water-front locations to property values undoubtedly has resulted in significant building encroachment along city river margins, but Pinch and Munt (2002) stress that the starting point for any planning proposals, designs and decisions should be the river, that brown-field sites bordering rivers should not automatically be seen as ‘ripe for development’ and that development proposals should be seen primarily from the river's perspective, enhancing rather than extracting value. This concurs with the views of Petts et al. (2002), who see rivers as a catalyst for city regeneration. They note the need for land use to be planned to make the most effective use of the river, and for the river to be planned and managed so that it is an asset to the local community. Placing the river at the centre of urban planning implicitly recognises that a healthy river is crucially important and that river health depends not only on improving water quality but also on providing space for channel-margin connectivity and habitat complexity. As has been demonstrated throughout this article, the existence of a connected riparian zone is crucial to river health and quality, and so a planning framework that emphasises the importance, wherever possible, of giving the river space for such connectivity is fundamental to the delivery of river ecosystem services. Such a planning framework needs to be supported by a legislative framework that recognises river networks and margins as morphologically complex and often dynamic entities, but this remains a challenge in many urban contexts (e.g. Lamaro et al. 2007).
Short Biographies
Angela Gurnell is Professor of Physical Geography, King's College London, UK. She has written widely on the hydrology, geomorphology and ecology of rivers, including over 100 refereed journal articles, 25 book chapters and 10 edited books and journal special issues. Her recent research has been concerned with two main themes: the role of vegetation in influencing the form and function of river systems; the characteristics and rehabilitation of urban rivers. She holds DSc, PhD and BSc degrees from the University of Exeter.
May Lee undertook research for a PhD at King's College London, UK. Her research focused on urban river characteristics. She is currently working as a teacher at St. Catherine's School, Bramley, Guildford, UK.
Catherine Souch is the Head of Research and Higher Education at the Royal Geographical Society (with IBG). She has published on the hydrology, geomorphology and biogeochemistry of lakes and wetlands and on the environmental impacts of urbanisation. She holds a BA degree from the University of Cambridge and MSc and PhD from the University of British Columbia.
Acknowledgements
The authors thank Dr Mark Taylor and a second, anonymous, referee for their considered and constructive comments on the original manuscript.
