Diversion of stormwater first flush: an alternative method of managing pollution to urban waterways

The problem of stormwater runoff as a major contributor to urban waterway pollution is increasingly being recognised throughout the industry. Additionally, the stormwater first flush, either the initial period of runoff during a storm event or a seasonal first flush occurring after a long dry period, can contain higher pollutant loadings than the remainder of the stormwater runoff. In order to address these problems, a move towards sustainable development and effects based management is emerging, which leads to the need for efficient and effective methods of managing pollution within a catchment. This dissertation looks at the diversion of stormwater first flush into the existing wastewater network as an alternative option to that of the containment of wastewater overflows for managing pollution to urban waterways. A high level desktop assessment of the Norman Creek catchment in Brisbane was undertaken to investigate the concept of the stormwater first flush diversion into the existing wastewater network. It was found that a total of 23 extended dry periods occurred from January 2013 to July 2015, and all of these were found to be followed by a small storm event. This could be thought of as 23 small seasonal first flush events. Five of these storm events were simulated in a calibrated wastewater network model to assess areas within the network that may have spare capacity. A more detailed assessment was then undertaken to identify three potential diversion locations. The inflow expected from a stormwater diversion into the wastewater network was simulated with the addition of fixed 100 L/s, 200 L/s and 500 L/s inflows at each of the selected diversion locations in the model. The model results indicated there is existing capacity within the Norman Creek wastewater network for flows of up to 200 L/s to be diverted from the stormwater system into the existing wastewater network at two of the diversion locations, and up to 100 L/s may be acceptable at the third location