Short duration intense rainfall causes an increase in rainfall derived infiltration and inflow (RDII) into aging sewer networks, which leads to Sanitary Sewer Overflows (SSOs). This study presents a generalised framework for assessing and mitigating the impacts of intense rainfall on sanitary sewer networks. The first part of the proposed framework involves a detailed hydraulic modelling to evaluate the performance of the sewer network. The second part deals with the development of SSO mitigation strategies based on Water Sensitive Urban Design (WSUD) approaches. This paper also demonstrates the application of the first part of the proposed framework for a case study catchment in Melbourne, Australia. The hydraulic performance of the case study sewer network during a wet and a dry year is presented. The analysis found that for the wet year, 11 manholes had sewer overflows, whereas 53 of 57 manholes in the network of 3.2 km had surcharges. Such a study will benefit the water authorities to develop mitigation strategies for controlling SSOs in their sewer systems.
Extreme rainfall events cause an increase in the flow into aging sewer networks, which can lead to Sanitary Sewer Overflows (SSOs). This literature review presents a complete assessment of the application of Water Sensitive Urban Design (WSUD) approaches as mitigation strategies for reducing rainfall-induced SSOs. The review highlights the various WSUD techniques identified in past studies for reducing sewer overflows. In these studies, it was identified that permeable pavements, green roofs, raingardens/bio-retention cells and rainwater tanks were the most popular WSUD strategies that have been extensively used in the past for the mitigation of sewer overflows. WSUD or “green” approaches also have enormous environmental, social and economic benefits when compared to the conventional “gray” approaches for sewer overflow mitigation. However, there have been limited studies conducted in the past that highlight and quantify the benefits of WSUD approaches for sewer overflow mitigation, particularly when such strategies are applied at a large scale (e.g., city scale). This review has identified the modelling software, SWMM, to be the most widely applied tool that has been used in the literature for WSUD modelling. It was also identified that with climate change-induced extreme rainfall events on the increase, WSUD-based “green” strategies alone may not be enough for the mitigation of sewer overflows. A suitable sewer overflow mitigation strategy could be green or a hybrid green-gray strategy, which would need to be identified based on a detailed context specific analysis.
Extreme climate events are becoming more intense in Melbourne in recent years. This increasing intensity of excessive rainfall has an adverse effect on the sewer network by causing sewage overflow hazards. Extreme rainfall events increase flow into the sewer system, both directly (inflow) and through infiltration into sewer. As a result of this Rainfall Derived Infiltration and Inflow (RDII), Sanitary Sewer Overflows (SSOs) may occur. These SSOs carry inherent risks to human health as well as lead to environmental pollution. This paper demonstrates a rigorous and efficient procedure of predicting RDII in a sewer system in Melbourne, Australia by using the Sanitary Sewer Overflow Analysis and Planning (SSOAP) toolbox. The SSOAP toolbox is a new freely available tool designed by the U.S. Environmental Protection Agency (EPA) for modeling of RDII. In the SSOAP toolbox, the U.S. EPA Storm Water Management Model (SWMM5) is incorporated for the hydraulic routing of the sanitary sewer system. For RDII flow estimation, SSOAP implements the synthetic unit hydrograph (SUH) method. In the literature, this procedure is recommended as the most accurate and industry standard methodology of determining RDII. The simplest SUH contains three triangular hydrographs to describe the fast, medium, and slow RDII responses. Each hydrograph has total of three parameters named R,T,K. R is the fraction of rainfall volume entering the sewer system as RDII during and immediately after the rainfall event, T is the time to peak, and K is the ratio of the time of recession to T. This method is known as the RTK unit hydrograph curve-fitting analysis.Nasrin et al., Modelling impact of extreme rainfall on sanitary sewer system by predicting rainfall derived infiltration/inflow 1.
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