Scenarios for redesigning an urban drainage system to reduce runoff frequency and restore stream ecological condition A. R. Ladson*, S. Lloyd**, C. J. Walsh**, T. D. Fletcher*, P. Horton* * Institute for Sustainable Water Resources, Department of Civil Engineering, Monash University, Vic 3800, Australia.
(E-mail:
[email protected];
[email protected]) ** Water Studies Centre, School of Biological Sciences, Monash University, 3800, Australia (E-mail:
[email protected];
[email protected])
Abstract In urban areas, efficient drainage of impervious surfaces means that polluted stormwater is frequently delivered to streams. Commonly, catchment urbanization can increase runoff frequency by a factor of 10 or more, as the effective imperviousness - the proportion of the catchment that consists of impervious surfaces drained to streams - is increased. This causes a decline in stream health. To decrease runoff frequency, effective imperviousness must be reduced. This requires urban drainage systems to be redesigned, using techniques such as infiltration and rainwater harvesting, so that stormwater from small rain events is not piped directly to streams but instead is infiltrated, reused or retained. We have developed scenarios that explore alternative urban drainage systems appropriate for a small partly urbanised catchment in Melbourne’s east. These scenarios incorporate, biofiltration basins, swales and dual purpose rainwater tanks that supply water for householders. Our results suggested that sufficient reductions in effective imperviousness and runoff frequency are possible to achieve improvements in stream health. Keywords Stream restoration, runoff frequency, urban drainage
INTRODUCTION Urbanization has a profound effect on streams. With the spread of efficiently drained roads, roofs, car parks and other impervious surfaces over catchments, runoff frequency, magnitude and quality are changed such that streams degrade morphologically and ecologically. Symptoms of this “urban stream syndrome” (Walsh et al. 2005a) include loss of aquatic species from streams, degradation of instream and riparian habitat through channel instability and enlargement, and disruption of stream ecosystem processes such as the retention and processing of nutrients. Protection of receiving stream ecosystems from the impacts of urban stormwater runoff requires comprehensive stormwater management that directly addresses the causes of ecosystem impairment (Jones et al., 2005). For most regions where stormwater management aims to protect stream ecosystems, performance standards apply to pollutant loadings (Victorian Stormwater Committee, 1999), or attenuation of runoff volume (Watershed Management Institute, 1997). In Victoria, environmental targets for urban stormwater require a 70% reduction in average annual litter load, an 80% reduction in average annual suspended sediment (TSS) load, and a 45% reduction in average annual total phosphorus (TP) and total nitrogen (TN) load (Victorian Stormwater Committee, 1999). Managers who aim to meet such standards naturally tend to concentrate on controlling runoff from large rain events, as these deliver the largest loads. In doing so, reducing runoff volume and frequency of flow is largely ignored. However, control and retention of the relatively small, and most frequent rain events may be a more ecologically relevant objective (Walsh et al. 2005b).
Water Practice & Technology Vol 2 No 2 © IWA Publishing 2007 doi: 10.2166/WPT.2007053
Scenarios for redesigning an urban drainage system to reduce runoff frequency and restore stream ecological condition
Consider the difference between runoff production in rural and urban areas. Not all rain contributes to runoff. At the start of a storm, some rain is intercepted by vegetation, retained on the surface of a catchment, or infiltrated into soil. In a study of 1059 bursts of rain on natural catchments in Victoria and the Australian Capital Territory, Hill et al. (1998) found that initial losses - the quantity of rainfall required before runoff commenced - was always greater than 10 mm for 21 of the 22 catchments and was commonly between 20 mm and 40 mm. Urban catchments have less vegetation, so interception losses are reduced, less water can infiltrate into the soil when it is covered by impermeable surfaces and there is also less opportunity for water to pond because of hydraulically efficient drainage works designed to rapidly convey water from runoff events. This means that initial losses are greatly reduced. Richardson et al. 2004 suggested that common materials used in urban areas, such as concrete and asphalt, require a threshold rainfall of between 0 and 2 mm before runoff is produced. The roads, roofs and car park studied by Hollis and Ovenden (1988) had initial losses that were usually less than 2 mm. Similar values of initial loss are found in urban catchments in cities throughout the world. For example, of the 26 urban catchments studied by Boyd et al. (1993), 19 (73%) had an initial loss less than 1 mm, while 23 (88%) had an initial loss less than 2 mm. Clearly, urbanization can reduce the catchment initial loss by a factor of 10 or more. Changing the initial loss has a large effect on runoff frequency because small rainfall events occur much more frequently than large rainfall events. A few millimetres of rain falling on a car park, for example, will be sufficient to cause polluted surface runoff that will flow into entry pits and then to streams via the urban drainage network. In a climate such as Melbourne’s, where on average it rains every few days, streams in urban areas will be frequently disturbed. In rural catchments the higher initial loss means that a few millimetres of rain will not produce runoff that reaches a stream. Recent studies in small streams of eastern Melbourne demonstrated that impervious surfaces with direct piped connection to streams had a much greater effect on water quality, and the species of algae and invertebrates in the streams, than did impervious surfaces with informal drainage to streams (Hatt et al., 2004; Taylor et al., 2004; Walsh, 2004; Walsh et al., 2004; Newall and Walsh, 2005). The informal drainage systems of those studies would only have intercepted small rain events, and yet they seemed to have a strong protective effect on stream ecosystems (Walsh et al., 2005b). Walsh et al (2005b) proposed a design standard to minimize the impact of an impervious surface to a receiving stream: retention of all runoff on site (for re-use, infiltration or evapotranspiration) up to the size of a storm that would have generated overland flow from the site in its pre-urban state. Such a site could be considered to have an effective imperviousness (EI) of zero. Directly connected impervious surfaces have EI = 1. Ladson et al. (in press) proposed an index to calculate the EI of impervious surfaces that retained less runoff than required to achieve EI = 0 (Figure 1). For catchments of eastern Melbourne streams, to reduce EI of an impervious surface to zero requires retention of all runoff from rain events up to about 15mm (Walsh et al., 2005b; Ladson et al. in press).
Scenarios for redesigning an urban drainage system to reduce runoff frequency and restore stream ecological condition
Figure 1 Drainage connection index, CI for the Dandenong Range, Victoria, Australia, where the first (approx.) 15 mm of rainfall is retained under natural conditions. RH axis shows cumulative frequency of days with rainfall less than value indicated on bottom axis. Based on frequency of daily rainfall at Croydon, 1900 - 1994 (adapted from Ladson et al, in press) Models developed for small streams of eastern Melbourne predicted that for any improvement in instream ecological condition, catchment EI needs to be reduced to
