Protection of stream ecosystems from urban stormwater runoff

Abstract

There is now widespread recognition of the degrading influence of urban stormwater runoff on stream ecosystems and of the need to mitigate these impacts using stormwater control measures. Unfortunately, however, understanding of the flow regime requirements to protect urban stream ecosystems remains poor, with a focus typically on only limited aspects of the flow regime. We review recent literature discussing ecohydrological approaches to managing urban stormwater and, building on the natural flow paradigm, identify ecologically relevant flow metrics that can be used to design stormwater control measures to restore more natural magnitude, duration, timing, frequency and variability of both high and low flows. Such an approach requires a consideration of the appropriate flow and water quality required by the receiving water, and the application of techniques at or near source to meet appropriate flow regime and water quality targets. The ecohydrological approach provides multiple benefits beyond the health of urban streams, including flood mitigation, water supply augmentation, human thermal comfort, and social amenity. There are, however, uncertainties that need to be addressed. Foremost is the need to define ecologically and geomorphically appropriate flow regimes for channels which have already been modified by existing land use. Given the excess of water generated by impervious surfaces, there is also an urgent need to test the feasibility of the natural flow paradigm in urban streams, for example using catchment-scale trials.

Keywords

ecohydrology natural flow regime stormwater runoff urban hydrology urban streams

I Introduction

The importance of urbanization and particularly stormwater management as an agent of hydrologic change has long been recognized (e.g. Hammer, 1972; Leopold, 1968). In more recent times, substantial evidence of the impacts of these hydrological changes on stream geomorphology (Hawley et al., 2013; Vietz et al., 2014) and ecology (Paul and Meyer, 2001; Walsh et al., 2005b) has been presented in the literature. Addressing the impacts of stormwater runoff has been the subject of many studies (see, for example, Wenger et al., 2009), inspiring a range of new approaches to managing urban stormwater.

Perhaps surprisingly, some of the reviews that deal with the impacts of urban stormwater runoff and of their mitigation seem to ignore the protection of urban streams (the common receiving environment for urban runoff) or mention it almost as an afterthought. For example, in the recently published Handbook of Urban Ecology, Worrall and Little (2011) limit their consideration of ecological services to those provided at the local site scale by various stormwater control measures (e.g. green roofs, swales, wetlands), without consideration in their discussion of the links to receiving water ecosystems. Similarly, Douglas (2011) summarizes the changes in urban water balance and quality caused by urbanization, before describing briefly the dramatic changes to channel morphology that result, but with no explicit reference to the impacts on stream ecosystems.

In this paper, we describe ecohydrological approaches to stormwater management, with the explicit objective of protecting the health of urban streams, and consider the broader local- and landscape-scale benefits of this approach, along with impediments to its adoption. The term ecohydrology describes the ‘understanding of relationships between hydrological and biological processes at the catchment scale to achieve water quality improvement, biodiversity enhancement and sustainable development’ (Zalewski and Wagner, 2005: 265). Given the short length of this paper, we largely limit our discussion to hydrology, while acknowledging that water quality is also an important driver of aquatic ecosystem health.

Importantly, we conclude that an ecohydrological approach to managing urban stormwater is not only necessary to protect and restore urban stream ecosystems, but can also provide broader benefits to the human communities living in urban catchments.

II The rationale for an ecohydrological approach to managing urban stormwater

Until the last two decades, the primary focus of urban stormwater management has been on flood mitigation, resulting in the construction of hydraulically efficient drainage infrastructure, made up of pipes and lined channels (Booth and Jackson, 1997). Burns et al. (2012) termed this the ‘drainage efficiency’ approach. A wide recognition of the impacts of such an approach to stormwater drainage on water quality and flow regimes developed in the 1990s (Makepeace et al., 1995; Novotny and Olem, 1994; Novotny and Witte, 1997). This led to a more integrated approach to stormwater management, incorporating water quality treatment and mitigating hydrological changes (US Environmental Protection Agency, 1983, 2000). The concept of low impact development (LID), for example, aimed to maintain a natural site water balance through hydrologic landscapes which were ‘functionally equivalent’ to their pre-development state (e.g. Prince George’s County Department of Environmental Resources, 1993).

In practice, however, many attempts to mitigate hydrological impacts of urbanization have been characterized by a focus on a narrow range of flows, with the primary focus being on peak flows (Burns et al., 2012; Petrucci et al., 2013a). This has often led to large end-of-catchment attempts at attenuation, which are likely ineffective in mitigating effects on stream geomorphology (Elliott et al., 2010) and ecology (Wenger et al., 2009). By focusing on a single aspect of the flow regime (high flows), there is a risk of perverse outcomes (Petrucci et al., 2013b). For example, detention basins, designed to reduce peak flows downstream, may potentially increase erosion downstream, by extending the duration of flows above the flow threshold able to do work on the channel (Bledsoe, 2002; MacRae and Rowney, 1992; Tillinghast et al., 2011). An approach focused on high flows only will not be sufficient to deliver ecosystem health, given the importance of low flows in providing wetted habitat (Hamel et al., 2013; Poff et al., 2010; Price, 2011).

III Understanding the whole flow regime

1 Towards a natural flow regime for stream protection

Poff et al. (1997) proposed the ‘natural flow regime’ paradigm, arguing that flow is a master variable controlling the dynamic character of aquatic ecosystems, and thus a primary determinant of their ecological integrity. For example, flows determine the size and depth of benthic substrate, the extent and timing of wetted habitat, and the hydraulic and chemical conditions experienced by stream biota. There is general agreement that the five general components of flow regime which determine ecological processes in river ecosystems are the magnitude, duration, frequency and timing of flows and the rate of change between flow rates (Olden and Poff, 2003; Poff et al., 1997).

In 1997, Poff et al. lamented that in rural catchments consideration by researchers and practitioners of environmental flow requirements was often quite narrow, with a singular focus on minimum flows. The converse argument could be made about urban catchments, where peaks have been the primary focus. For example, studies of urbanization impacts on stream hydrology have most commonly reported on changes to flow metrics related only to high flows, such as lag time, flood magnitude and duration, and some consequences for channel morphology (see review by Jacobson, 2011). Studies of the impact of urbanization on baseflows are relatively few (Price, 2011). The poor understanding of baseflow responses limits our ability to mitigate or manage their alteration by urbanization (Hamel et al., 2013).

Within the last decade there has been an increasing focus on more holistic, ecohydrological approaches to managing flow regimes in catchments containing urban land use. Several authors have described elements of such approaches, although explicit consideration of the five flow-regime aspects described by Poff et al. (1997) has been rare. Booth et al. (2002) described the evolution from a peak flow-based approach to a volume-based approach in the USA, noting that both approaches leave substantial parts of the flow regime disturbed (see also Petrucci et al., 2013a, 2013b). Booth et al. (2002: 840) described a transition towards ‘hydrologic restoration through watershed planning’, as a central tenet of LID (Dietz, 2007; Toronto Region Conservation Authority, 2010; US Environmental Protection Agency, 2000).

Most recently, a number of authors have begun to spell out the requirements for delivering the ‘natural flow paradigm’ in urban catchments. For example, Burns et al. (2012) proposed the ‘flow regime approach’, the central premise of which is the restoration or protection of natural hydrologic processes at the individual land parcel scale, with the aim of restoring more natural low and high flow hydrology at the catchment scale. Their approach builds on the ideas developed under concepts such as LID and source control (Petrucci, 2012), but goes further in terms of specifying the elements of the flow regime which need to be addressed by stormwater management. Walsh et al. (2012) also identified specific aspects of the flow regime altered by traditional stormwater management, and identified broad approaches to protecting and restoring flow regimes. Importantly, the authors used empirical data to demonstrate the link between the degree of flow regime disturbance and the health of the receiving stream.

While there is agreement on the merits of protecting and restoring more natural flow regimes in urban and urbanizing catchments, stormwater managers need guidance on how to achieve such an objective. Such guidance requires the development of explicit links between components of the flow regime and ecological and geomorphic values, selection of appropriate flow metrics, and specification of performance targets.

2 Selecting ecologically relevant flow metrics to guide stormwater management

In the ecohydrological literature, many flow metrics have been proposed (Mathews and Richter, 2007; Price, 2011; Smakhtin, 2001; Wenger et al., 2010). Richter et al. (1996) proposed a suite of 32 metrics which they called ‘Indicators of Hydrologic Alteration’ (IHA). The relative ease of generating and calculating metrics – for example, Olden and Poff (2003) examined a suite of over 170 metrics – has resulted in difficulties in determining which metrics should be used to guide flow management, particularly in the urban context. Indeed, identifying which of the many proposed metrics explain ecological responses has proved challenging, in part because of interacting and confounding effects (Poff and Zimmerman, 2009; Poff et al., 2010). Poff and Zimmerman (2009) concluded from a review of 165 studies that establishment of generalizable quantitative relationships between flow alteration and ecological response was not possible. They identified the need to assess relationships between flow and ecosystems across a gradient of disturbance.

Konrad et al. (2008) showed that multiple streamflow metrics were associated with the richness and abundance of a range of sensitive aquatic taxa, with flow acting as a likely limit on biotic assemblages, while Roy et al. (2005) observed similar relationships for fish. Steuer et al. (2010) advanced the statistical relationship approach for 83 flow metrics and indicators of algal, invertebrate and fish communities. The metrics found to be important included several from each of the major flow characteristic groups (except seasonal timing); magnitude, frequency, duration and variability. Several authors have argued against single metrics, suggesting that integrated measures based on flow duration curves (FDCs) provide far more information (Petrucci et al., 2013b). Gao et al. (2009) propose the use of metrics based on the flow duration curve, such that deviations from the reference could be represented by integrated metrics. Their suggested use of metrics which capture cumulative deficit (‘eco-deficit) or surplus (‘eco-surplus’) from a reference FDC is based in part on the desire to reduce the number of metrics, given that many of the 170 IHA are known to be highly correlated (Olden and Poff, 2003).

Statistical approaches to metric selection have their place, particularly in demonstrating the link to ecological outcomes, but additional insights may be provided by studies that identify the mechanisms causing alteration of the flow regime. Walsh and colleagues have been pursuing this path, initially with a single-focus on runoff frequency (Walsh et al., 2005a), then developing a more integrated metric of retention capacity (Walsh et al., 2009) and most recently presenting an integrated suite of metrics (Walsh et al., 2012), drawing on the environmental flow framework ecological limits of hydrological alteration (ELOHA), developed by Poff et al. (2010). Walsh et al. (2012) proposed that urbanization creates an environmental flow problem, wherein the excess runoff volume (which expresses itself through increases in the frequency and magnitude of runoff events) leads to the degradation of receiving waters.

Expanding this theme, Burns et al. (2013) proposed a framework for determining flow regime performance objectives, setting targets for (1) reductions in runoff frequency and total annual runoff volume and (2) restoration of baseflows. They proposed that a target based on initial loss (the amount of rainfall needed to generate a hydrological response) could be used to return more natural runoff frequency. To do so they draw on studies from a range of undisturbed catchments. Such an approach seems consistent with the ‘natural flow regime’ paradigm (Poff et al., 1997), whereby hydrological behaviour of natural catchments is used as a template.

There is an increasing recognition that site-scale channel reconstruction approaches to urban stream restoration are failing to achieve ecological objectives (Laub et al., 2012), and restoration of channel form and function, integral to the ecological condition of a stream, requires catchment-scale activities (Bernhardt and Palmer, 2011). Vietz et al. (2014) demonstrated that ecologically relevant physical attributes of streams – such as the depth of benthic sediments, bank condition, and bars and benches – are affected by low levels of connected imperviousness (approximately 3%). They identified the stormwater drainage system as the primary driver. Importantly, it is not necessarily only the bankfull flow that affects geomorphic change; smaller, more frequent events often drive change (MacRae and Rowney, 1992). Nevertheless, specific flow components that drive physical change in urban streams are commonly estimated based on a single threshold of erosion potential (Elliott et al., 2010), even though there are a suite of physical attributes that may be important ecologically, and sensitive to changes in flow (a paradigm well understood in rural settings).

In summary, while the literature on ecohydrological metrics remains in flux, with many recent attempts to distill down the large number of candidate metrics (e.g. Gao et al., 2009; Poff et al., 2010), there is now useful guidance to aid the design of stormwater control measures. Stormwater design standards which aim to restore (1) runoff frequency, (2) baseflow and (3) total annual volume from a given site or impervious area to near their natural level appear likely to eliminate or mitigate any impacts to receiving waters. The remaining challenge is to translate such site-based targets (which are the appropriate scale to guide stormwater management) into broader catchment-scale metrics, which can be used for regional planning.

3 Understanding the mechanisms that drive disturbance

Alteration of catchment hydrology by urbanization is a result of two principal drivers: (1) the creation of impervious surfaces and (2) the connection of these surfaces to receiving waters through hydraulically efficient pathways such as pipes (Walsh and Kunapo, 2009). Changes to properties of urban soils may also have some effect (Shuster et al., 2005). Concomitantly, channelization, including channel straightening, of streams and disengagement from their floodplains (both through incision and excavation) have increased the efficiency with which flow is transferred from headwaters to catchment outlets.

Studies characterizing catchment-scale changes have historically (and in some cases currently) focused on total imperviousness (TI) (Brabec, 2002; King et al., 2010; Schueler, 1994), despite Leopold (1968) noting the importance of the nature of the connection of the impervious surfaces to receiving waters. The terms effective imperviousness (EI) and directly connected imperviousness (DCI) are thus used to describe the proportion of a catchment made up of impervious areas directly connected to receiving waters via a constructed drainage system. Such metrics provide a better prediction than TI not only of changes to the flow regime (Brabec, 2002) but also of water quality (Hatt et al., 2004), geomorphic condition (Vietz et al., 2014) and a range of ecological indicators (Catford et al., 2007; Newall and Walsh, 2005; Walsh, 2006).

The earliest predictions of EI were rather simplistic, however, with a uniform ratio between TI and EI typically used for a given land-use type (e.g. Booth and Jackson, 1997; Wang et al., 2001), thus ignoring inevitable variation between catchments in drainage connectivity (Lee and Heaney, 2003). With advances in Geographic Information Systems (GIS) and spatial modelling capability, more precise means for estimating effective imperviousness have developed. Han and Burian (2009) used GIS methods to classify each impervious area as connected or unconnected. Walsh and Kunapo (2009) took the concept further, by considering measures of imperviousness that provided the best prediction of ecological response. Using regression models, they found that attenuated imperviousness (AI, whereby impervious areas are inversely weighted by the length and nature of their flow-path to the receiving water) was a much stronger predictor of biotic assemblage composition than total imperviousness.

The focus on the nature of imperviousness (i.e. EI or AI) is an important step towards demonstrating the mechanistic links between urban catchments and their streams, identifying causes of disturbance, and identifying mitigation strategies (Roy and Shuster, 2009). These impacts can then be directly targeted by mitigation strategies, which aim to ‘disconnect’ (at least for typical rainfall events) the impervious areas from receiving waters (Shuster et al., 2007; Walsh et al., 2005a).

IV Stormwater management: from simple nuisance to complex threat and opportunity

1 Integrated management of flow regimes and water quality

As described previously, stormwater management has evolved from once largely being singularly focused on flood mitigation to now being complex, multi-objective and highly integrated. In the 1990s and early 2000s, environmental concerns drove major innovation in stormwater control measures, leading to techniques with a primary focus on pollutant load and peak reduction (Burns et al., 2012). While this represented a significant step forward, the approach was missing an explicit link to the needs of receiving waters (Booth et al., 2002; Dietz, 2007). Urban streams, for example, are typically much more sensitive to variations in pollutant concentrations than long-term pollutant loads (Taylor et al., 2005), meaning that they may not be effectively managed simply by setting load reduction targets, just as attenuation of peak flows only is unlikely to be sufficient to avoid loss of biodiversity.

In recent years, the development of stormwater strategies which aim to simultaneously return more natural water quality while holistically delivering more natural flow regimes is increasingly common. For example, stormwater bioretention systems, widely tested for their pollution reduction performance (Davis et al., 2009) are increasingly being assessed for their ability to restore infiltration and groundwater flows and to mimic features of the natural flow regime (e.g. Davis et al., 2012; DeBusk et al., 2011; Endreny and Collins, 2009). DeBusk et al. (2011) note that bioretention systems could be designed to mimic interflow patterns from natural streams and argued that such an approach should become a standard requirement of bioretention designs. The feasibility of delivering natural flow regimes is, however, questionable without a means of reducing the excess flow volume that results from impervious areas (Walsh et al., 2012), making stormwater harvesting a likely critical element of the ecohydrological approach. Ultimately, stormwater managers need to set objectives that are directly related to the needs of receiving waters; Burns et al. (2012) suggest that flow-regime management should be incorporated into every set of stormwater performance objectives, alongside water quality and flood mitigation.

2 Integration of scales and the use of centralized and decentralized solutions

Restoring more natural flow regimes at the catchment scale requires actions to be undertaken first and foremost at the land-parcel scale, where opportunities for restoring pre-development water fluxes (infiltration, evapotranspiration and runoff) are greatest (Burns et al., 2013; Hamel and Fletcher, 2013). It is this rationale that underpins existing concepts such as source control (Mikkelsen et al., 1996; Petrucci et al., 2013a) and LID (Freni et al., 2010). Without retention of runoff at or close to the source, volumes of water downstream will make restoration of natural flow regimes impractical, particularly where the large excess volume from impervious surfaces has not been reduced.

Space constraints at individual land parcels may not permit all aspects of the pre-development water balance to be restored, meaning that a combination of decentralized (at-source) and more centralized (downstream) techniques (e.g. McArdle et al., 2011) will be required. In recent times, some authors have proposed novel solutions to the challenge of integrating stormwater management into the urban landscape, such as Endreny’s (2004) socio-ecological framework.

3 Stormwater as a coupled threat and opportunity for urban communities

While stormwater represents a threat to urban streams, it also represents an important opportunity, as the volume of excess runoff is generally of a similar order to the total water demand for a city (Mitchell et al., 2003). Several studies have demonstrated the potential for stormwater harvesting to help in restoring flow regimes (Ashbolt et al., 2012; Fletcher et al., 2007; Grant et al., 2012; Hamel and Fletcher, 2014; Herrmann and Schmida, 2000). To be successful in mitigating stormwater impacts on streams, stormwater harvesting should be designed around the flow regime metrics described in section III.2; that is, they should be designed to restore the frequency of runoff and overall flow volume from a given impervious area to that which would have occurred in the natural state (Walsh et al., 2012).

Given the need to ensure that baseflows are also preserved or restored, harvesting alone will not be sufficient, and must be complemented with techniques to restore baseflows (Burns et al., 2012). Walsh et al. (2012) proposed stormwater as an environmental flow problem (sensu Poff et al., 2010) and argued that stormwater harvesting, when applied at the right scale and with the right operating rules, is essential to reduce the excess runoff volume resulting from impervious areas. They pointed out the importance of intercepting runoff before it enters the receiving waters, highlighting the further degradation caused by large-scale end-of-catchment harvesting schemes which divert water from the urban stream itself, thus likely depleting baseflows, rather than preventing the input of stormwater to the stream.

The benefits of the ecohydrological approach extend beyond the protection and restoration of urban streams. By retaining water in the urban landscape through a focus on stormwater harvesting, infiltration and irrigation of the urban landscape, there are significant and catchment-wide benefits in terms of landscape amenity (de Graaf and van der Brugge, 2010), mitigation of the urban heat island effect (Endreny, 2008), flood mitigation (Tourbier and White, 2007) and improvements to human well-being (Benedict and McMahon, 2006).

V Impediments and uncertainties of an ecohydrological approach

Despite increasing recognition of the need to restore flow regimes of urban streams as close as possible to their pre-development levels, several important questions remain unanswered. First, identifying the target flow regime can be difficult in the common situation where there are neither pre-development flow data available nor a nearby reference catchment. Modelling has been applied to this problem (Elliott et al., 2010), but is afflicted by high levels of uncertainty, particularly in the light of a changing climate (Fletcher et al., 2013). A related challenge is in predicting the effects of proposed stormwater management strategies on all aspects of the flow regime. Modelling the effect of techniques such as infiltration systems on baseflow processes is particularly problematic (Elliott et al., 2010) and requires detailed understanding of the hydrologic behaviour of the catchment prior to urbanization, as well as how it may be affected by the presence of urban infrastructure (Hamel and Fletcher, 2013).

Changes in flow volume and peak flows following urbanization result in channel enlargement, such that urban streams are significantly deeper and wider than their pre-development state (Tillinghast et al., 2011). Given this context, identifying a ‘target’ flow regime becomes more difficult, because the pre-development flow regime will have different hydraulic and wetted habitat consequences in the new channel than it would have in the pre-development state (Vietz et al., 2014). An enlarged channel may reduce velocities at lower flows, but will contain greater magnitude flows (due to less floodplain engagement) and exert greater disturbance on the channel bed, banks and biota. How to address altered channel morphology in conjunction with stormwater control measures to improve aquatic habitat is thus an important research question, and one where the role of channel design in restoration efforts (Larson et al., 2013), or assisted recovery (Vietz et al., 2014), will be paramount.

By definition, an ecohydrological approach will also require allowing streams more room to move, such that floods can spread out onto floodplains without causing major cost or inconvenience to communities. Vietz et al. (2012) suggested that providing hydrologic engagement of the floodplain, as well as space for lateral, stream-driven migration, could ultimately reduce channel enlargement. While historically development often started in the floodplain, to take advantage of the various services (water supply, waste disposal, food production, etc.) offered by floodplains, an ecohydrological approach will require floodplains to be valued primarily for their ability to support and regulate natural processes in urban streams (Bernhardt and Palmer, 2007).

A further consideration is that stream geomorphology is driven not just by hydrology but also sediment inputs, particularly coarse-grained sediment that plays an important role in the provision of stream habitats and channel recovery (Vietz et al., 2014). How urbanization alters the coarse-grained sediment regime is poorly understood, with fine-grained sediment (silt and clay) often the focus (Chin, 2006). The role of coarse-grained sediments in stream recovery will be particularly important as flow regime is increasingly addressed. Unfortunately, the techniques used to mitigate hydrology at the catchment scale (infiltration, harvesting, etc.) have the potential to further deplete the supply of coarse sediment inputs to urban streams (Vietz et al., 2014). A future challenge will thus be the design of systems which retain the fine-grained sediments, known to be a central driver of stream pollution (Pitt et al., 1995), while allowing coarse-grained sediments to pass.

To date, tests of the feasibility of restoring pre-development flow regimes have been essentially limited to modelling studies (Burns et al., 2012; Fletcher et al., 2007; Hamel and Fletcher, 2014; Tillinghast et al., 2011) or to empirical studies of individual systems (Davis et al., 2012; DeBusk et al., 2011), with empirical studies at the whole-of-catchment scale only at a very preliminary state of implementation (Fletcher et al., 2011; Shuster et al., 2010; Thurston et al., 2008). There is an urgent need to pursue such trial catchment retrofits in a range of contexts which cover variability in climate, physiography and impacts of other catchment infrastructure (Kaushal and Belt, 2012). Doing so will help determine whether the natural flow regime is an appropriate and feasible target for urban stormwater managers and, if so, how it can be best delivered.

Footnotes

Acknowledgements

We thank Tim Warner and two anonymous reviewers for their comments on an earlier version of this manuscript.

Funding

Fletcher is supported by an ARC Future Fellowship (FT10010044). All authors are also supported by the Melbourne Waterway Research-Practice Partnership (funded by Melbourne Water). This work derives in part from research funded by the Australian Research Council’s Linkage Program (LP0883610 and LP130100295) and the Cooperative Research Centre for Water Sensitive Cities, which also supports Vietz.

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