Assessing the system performance of an evolving and integrated urban drainage system to control combined sewer overflows using a multiple-layer based coupled modeling approach

Many earlier developed and densely populated metropolises served by combined sewer systems have been implementing additional stormwater control measures to reduce the impacts of combined sewer overflows (CSO) driven by severe rainstorms. These mitigation measures call for robust modeling strategies to assess the overall performance before development and to predict vulnerabilities, and examine the underlying mechanisms causing them during or after development in a systematic manner. This paper first introduces a hydrologic and hydraulic coupled modeling program-MetroFlow that interacts and integrates multiple models built via diverse methods and tools. A typical integrated urban drainage system (IUDS) in the greater land area of Chicago in the US was investigated as a testbed. In addition, a large-scale Tunnel and Reservoir Plan in construction was evaluated as a mitigation measure to alleviate CSO risks. In this effort, three models were coupled into MetroFlow to account for the best available data of the multiple structural layers involved in this evolving system. Hydrological responses were analyzed under existing and projected system conveyance and storage capacities considering the full spectrum of rainfall conditions employing synthetic storms and real-time simulations of four historical extreme events to account for the anomalies exemplified by individual natural events. The overall effectiveness and resilience of the system were further assessed by the evaluation of normalized performance indicators (PIs) quantifying the reduction of total volume, duration and spread of CSOs. The cluster of simulation results collectively suggests that: (a) the proposed structural layer-based modeling approach is applicable in reproducing distributed rainstorm-driven CSOs for an evolving IUDS based on emerging digital-physical data; (b) CSO characteristics are rather multivariate functions of the rainfall tempo-spatial characteristics as well as the hydraulic performance of the collection system and (c) consistent improvement of CSO abatement was predicted with increasing conveyance and storage provided by the deep tunnel system. The projected system is predicted to capture 24-h storms of return period up to 10 years. MetroFlow's framework may also promote collaborative modeling, in which community-scale models can be utilized and merged to support extended-scale water and wastewater infrastructure planning, design and management.