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Linking stormwater control performance to stream ecosystem outcomes: Incorporating a performance metric into effective imperviousness

['Christopher J. Walsh', 'Waterway Ecosystem Research Group', 'School Of Ecosystem', 'Forest Sciences', 'The University Of Melbourne', 'Burnley', 'Victoria', 'Matthew J. Burns', 'Tim D. Fletcher', 'Darren G. Bos']

Date: 2022-02 

Stormwater control measures, such as raingardens, tanks, or wetlands, are often employed to mitigate the deleterious effects of urban stormwater drainage on stream ecosystems. However, performance metrics for control measures, most commonly pollutant-load reduction, have not permitted prediction of how they will change stream ecosystems downstream. Stream ecosystem responses have more commonly been predicted by catchment-scale measures such as effective imperviousness (percentage of catchment with impervious cover draining to sealed drains). We adapt effective imperviousness, weighting it by a performance metric for stormwater control measures aimed at stream protection, the stream stormwater impact metric. Weighted effective imperviousness can serve as a predictor of stream response to stormwater control. We demonstrate its application in a before-after-control-reference-impact experiment aiming to test if stream health is improved by dispersed stormwater control measures. Trends in weighted effective imperviousness showed wide variation in degree of stormwater control achieved in the six experimental sub-catchments, despite similar effort in implementing control measures across the sub-catchments. Greater reductions in weighted effective imperviousness (on a log-scale, on which stream response is predicted) per unit effort were observed in smaller catchments with lower starting effective imperviousness. While implementation of control measures was sufficient to expect a stream response in at least two of the experimental sub-catchments, we did not achieve the reduction in effective imperviousness that we were aiming for. Primary limitations to success were the lack of available space in these established suburbs, particularly for final control measures near pipe outlets into streams, and a lack of demand for harvested stormwater. The use of the continuous variable, weighted effective imperviousness, to measure impact on streams, and the protracted period of SCM implementation that varied among catchments, required a new approach to modelling “before-after-control-impact” experiments, which has potentially broader application.

Competing interests: I have read the journal’s policy and the authors of this manuscript have the following competing interests. The research and the University of Melbourne positions held by CJW, MJB, DGB, PP, JK, and SJI are all funded by the Melbourne Waterway Research-Practice Partnership, between the University of Melbourne and Melbourne Water (MW). MW is a corporation wholly owned by the Government of Victoria, with statutory responsibilities for the management of rivers and other waterways of the Melbourne region. They fund public-good research to further the environmental sustainability of their operations. In a previous role, TDF received payment as for his contribution to the engineering design of the Wicks Reserve Infiltration System, subsequently constructed by Knox City Council (KCC) and MW in the D8 catchment. Neither the authors’ partnership with and funding by MW, nor past work with KCC, alter their adherence to PLOS Water policies. The authors have no other competing interests.

Funding: The research activities reported in this paper span 19 years, and were supported by many funding sources. The primary research activities of this project were funded by: the Australian Research Council (ARC, https://www.arc.gov.au/ ) Linkage Program through grants awarded to CJW and TDF (LP0883610 and LP130100295) with partner organizations Melbourne Water (MW, https://www.melbournewater.com.au/ ) and Yarra Ranges Council (YRC, https://www.yarraranges.vic.gov.au/ ); and the ARC Future Fellowship Program through a grant awarded to TDF (FT10010044). Melbourne Water additionally funded research activities, at first through the Cooperative Research Centres for Freshwater Ecology and Catchment Hydrology, and later through the Melbourne Waterway Research-Practice Partnership ( https://mwrpp.org/ ). The Smart Water Fund of the Victorian Water Trust (SWF) also funded research activities. Programs for planning, allocating, building and maintaining stormwater control measures (SCMs) in the L4 catchment were funded by: the Victorian Government through the SWF, the Victorian Urban Stormwater and Recycling Fund, and through Melbourne Water as part of the Port Phillip and Westernport Threatened Catchments program and Living Rivers program; Melbourne Water; the Australian Government through the Caring for Our Country Investment Fund, administered by the Port Phillip and Westernport Catchment Management Authority; Yarra Ranges Counci; Yarra Valley Water (YVW, https://www.yvw.com.au/ ), who administered SCM incentive payments. Similar programs of SCM implementation in the D8 catchment were funded and managed by Melbourne Water in collaboration with Knox City Council (KCC) and South East Water. The authors collected performance data for D8 SCMs, and provided advice on some aspects of SCM design, but were not involved in program administration or implementation and received no project funding for works in the D8 catchment. None of the ARC, the Victorian or Australian Government funding bodies, or YVW played a role in the research. The authors worked with MW, YRC and KCC on the SCM implementation programs.

Introduction

Conventional urban stormwater drainage, delivering runoff from impervious surfaces to streams and rivers through hydraulically efficient pipes and sealed drains, severely degrades receiving stream ecosystems. Globally, urban streams are characterized by loss of sensitive species and shifts in ecological function [1], which result from alterations to flow regimes, channel form [2, 3], and water quality [4, 5]. These in-stream changes arise because conventional stormwater drainage increases the frequency and magnitude of disturbance. The disturbances experienced by channels and their in-stream biota are both hydraulic, arising from larger, more frequent high-flow events, and chemical, arising from the complex cocktails of pollutants associated with impervious runoff [6]. Hydraulic changes also simplify in-stream habitat, by increasing the capacity of streams to transport sediments [7], causing the loss of habitat features such as bars, benches, and woody debris [2].

In part as a response to increased awareness of the deleterious effects of conventional stormwater drainage to receiving aquatic ecosystems, alternative drainage approaches have been advocated across many jurisdictions over the last three decades [8]. While the protection of receiving waters has been a primary objective behind such approaches from the beginning [e.g. 9, 10], the connection between performance objectives of stormwater control measures (SCMs) and receiving water response has largely been opaque. Objectives have most commonly been set as performance metrics of (annual) pollutant load reduction [from the loads produced by conventional drainage: e.g. 11–13]. Such metrics fail to address the complexity, or match the time scales, of the hydrologic and geomorphic stressors to stream ecosystems created by urban stormwater runoff [14]. However, even SCM performance metrics that are explicitly linked to mechanisms of stream degradation [e.g. 15] cannot be easily used to predict the effects of the SCMs on their receiving stream ecosystem. Such prediction requires integration of SCM performance metrics with catchment-scale predictors of stream response.

Studies of stream ecosystem response to catchment urbanization have shown a predictable decline in ecological indicators with increased catchment urban density [16]. Progress in understanding of mechanisms for this decline has been hampered by a lack of consistency in catchment metrics used as predictor variables [17]. Total catchment imperviousness (TI, proportion of catchment covered by impervious surfaces) has most commonly been used, but several studies have shown that stream degradation is better explained when drainage connection is accounted for in addition to impervious coverage [16, 18–20]. If we are to understand differences or similarities in response to degradation of urban streams across regions, let alone the potential for reversing that degradation, we need consistent metrics of elements of urban infrastructure that are likely to drive stream ecosystem change. The most important such element is likely the stormwater drainage system [17].

Effective imperviousness (EI) is the proportion of catchment area covered by effective impervious surfaces, where ’effective’ connotes those surfaces that have an effect on the receiving stream. This measure arose from Leopold’s [21] demonstration that hydrologic changes in urban catchments are a function of both imperviousness and the extent of sealed stormwater drainage. In its simplest formulation, only impervious surfaces with direct connection to stormwater drainage are considered effective, while those that drain informally to pervious land are considered to have no effect. Such a formulation of EI was a better predictor of ecological response than TI in streams of peri-urban eastern Melbourne, Australia, where the extent and nature of stormwater infrastructure varies within and between catchments, ranging from informal drainage to surrounding pervious land, to formal pipe networks receiving runoff from curbed roads and private properties [16]. That EI was a better predictor than TI in such a setting provided evidence that conventional drainage of impervious areas has a greater impact on stream ecosystems than informal drainage. Impervious surfaces connected to the drainage system are undoubtedly important contributors to stream degradation given the substantial changes to catchment hydrologic processes that they cause [17].

However, it does not follow from EI being a better predictor that informally drained impervious surfaces have no effect. While the original formulation of EI made this assumption, a more parsimonious interpretation of EI being a better predictor of stream degradation than TI [16], would be that the effect of informally drained surfaces is substantially smaller than that of conventionally drained surfaces. While small, the effect of an informally drained surface is likely to depend on the length and nature (e.g. roughness and permeability) of the flow path between it and the stream. Walsh and Kunapo [18] attempted to account for such variability by weighting impervious surface areas by their distance to the nearest downstream stormwater pipe, drain or stream. However, their formulation conflated the probability that an impervious surface in an area with formal stormwater drainage systems was connected to a stormwater drain, with the variable effect of informally drained surfaces. These two effects could be separated by first identifying those impervious surfaces that are directly connected to the stormwater drainage network, and then modeling the degree of stormwater retention between informally drained surfaces and the stream, and weighting their area accordingly (between 1, if directly connected to conventional drainage, and 0, if draining to surrounding impervious land without significantly altering hydrologic processes).

A similar logic could be applied to the effects of SCMs. The performance of an SCM could be used to determine an appropriate weighting of the impervious area upstream of it. Water fluxes can be modeled through SCMs of known specification, allowing calculation of EI like that proposed by Walsh, Fletcher [22]. They proposed weighting impervious areas that drain to SCMs by a metric of overflow frequency, but it is also possible to weight by other aspects of SCM performance, such as restoration of filtered base flow or reduction in total flow volume [15, 23]. In this paper we advance the SCM performance metric proposed by Fletcher, Walsh [23] and Walsh, Fletcher [15] by using it and its subindices to formulate several alternative variants of EI. We advance the proposed approach of Walsh, Fletcher [22], by devising and demonstrating a method for combining performance metrics of complex arrays of SCMs to derive the catchment EI variants. In considering the effects of SCMs on EI, we advance the approach of Roy, Rhea [24], who removed impervious area draining to SCMs from EI, regardless of SCM performance.

We use data from a 19-year experiment of urban stormwater impacts in small-stream catchments of eastern Melbourne, Australia. The primary aim of the experiment was to test if intensive application of dispersed SCMs can restore stream hydrology, water quality and ultimately ecological state in receiving streams [15]. The restoration experiment was predicated on the hypothesis that impervious runoff delivered through conventional drainage systems was the primary degrading process requiring remediation to permit restoration of the receiving stream ecosystem. Our study catchments differed in their starting degrees of urban density and stormwater drainage infrastructure and, over 4 years, we applied different intensities, extents, and types of stormwater control measures in two experimental catchments. We are thus able to compare variation of EI variants as catchment-scale indicators of SCM performance over time and between catchments.

The study was conceived as a Before-After-Control-Reference-Impact (BACRI) experiment [15]. The classical approach to Before-After-Control-Impact experiments (and their extension to include reference sites in addition to control and impact sites) has been to divide samples into before and after periods to form a binary before-after effect, and to classify reference, control and experimental/impact sites categorically [25]: the presence of an experimental impact or intervention is tested by the interaction between the before-after effect and the control-impact (-reference) effect. However, such a categorical approach to testing the effect of SCMs in this experiment is not optimal because of the cumulative nature of SCM implementation; the continuous nature of EI, which was used to define reference and control catchments; and the variable degree of intervention. The protracted period of implementing the many dispersed SCMs in our study resulted from the logistical complexities of engagement with communities and authorities, and designers, builders and maintainers of the SCMs [15, 26–28]. Long periods of experimental manipulation are likely to be typical of similar endeavours [e.g. 24]. An alternative, but analogous, analytical approach is required to assess the effects of such continuous, variable interventions.

In this paper, we propose such a modelling approach and use the experiment’s implementation data and two contrasting simulated response variables to test and validate the model, and to also assess if the experimental manipulation (SCM implementation) was of a sufficient magnitude to expect a detectable response in the receiving streams. The primary focus of the paper is reporting and interpreting the implementation data of the experiment, and to propose and demonstrate new methods for assessing a) the degree of dispersed catchment stormwater runoff achieved by the SCMs implemented in the experiment, and b) their effect on downstream waters. Further investigations of stream response will use these data and approaches to assess the effects of the experiment on hydrologic, water quality and ecological variables measured in the receiving streams over the period of study. Before reporting and interpreting the SCM implementation of the experiment, we propose in the following section, a new variant of effective imperviousness, weighted by a performance metric for stormwater control measures. This measure will not only aid interpretation of the experiment, but more broadly allow prediction of how streams respond to stormwater control across catchments.

[END]

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