Urban Stormwater Runoff: A New Class of Environmental Flow Problem

Protecting the ecological integrity of freshwater ecosystems in addition to providing reliable and affordable water supplies to the growing human population is an ongoing challenge to water resource management and environment flow assessment. Urban stormwater runoff from conventional drainage systems degrades the ecosystems of streams and rivers by altering their volume and flow, resulting in a loss of biodiversity and ecological function. In order to assess and resolve this issue, Walsh et al. (2012) identified experimental causes by urban stormwater runoff and observed ways that have been implemented ways to protect them. They concluded that conventional stormwater management would needed to be completely revamped. Walsh et al. (2012) proposed to mimic the effects of informal drainage systems, where runoff is well-dispersed within surrounding vegetation (gardens or forests) or harvested in rainwater tanks. By providing filtration and large detention systems.—Genevieve Heger
Walsh CJ, Fletcher TD, Burns MJ. 2012. Urban Stormwater Runoff: A New Class of Environmental Flow Problem. PLoS ONE 7, 17

Walsh et. al. studied four small streams in eastern Melbourne; two of which almost completely lacked conventional stormwater drainage (Sassafras and Olinda), and two for which conventional stormwater management was utilized (Little Stringybark and  Brushy). The effect of conventional stormwater drainage was determined by assessing the flow regime and discharge in all four sites. Flow regime was assessed using 6-minute-time-step flow data from a permanent flow gauge, in addition to a limited record of water depth that was collected using OdysseyTM capacitance water level probes, logging 5-minute intervals.  Discharge for Sassafras and Stringybark was estimated manually using either a CMC 20 current meter counter or a Marsh-McBirney flow-mateTM velocity meter, 6–10 times at each site, and for Olinda, discharge was estimated using a quadratic relationship between the discharge recordings from a permanent flow station downstream (Melbourne Water Station) and corresponding depth logger readings. Lastly, the increase in runoff volume generated by impervious surfaces was assessed in order to quantify the volume of urban stormwater runoff. Streamflow coefficients (mean annual discharge depth divided by mean annual rainfall) of undeveloped, unregulated streams were compared to impervious runoff coefficients (mean annual runoff depth from an impervious area divided by mean annual rainfall) in order to calculate the difference, which represented the loss of evapotranspiration and equated to the excess runoff generated.
From the data collected, it was observed that the informal drainage management of Sassafras and Olinda permit better retention and infiltration, preventing severe ecosystem degradation, whereas the conventional stormwater drainage systems of Little Stringybark and Brushy pass runoff and all its associated pollutants directly downstream. Thus ecosystem degradation in areas of conventional stormwater management can be improved by using informal drainage systems.

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