Innovative city-scale modeling framework can help guide design of storm-resilient urban drainage systems

An innovative framework makes modeling integrated urban drainage systems at the city level simpler and more flexible, allowing easier identification of problematic areas as well as predicting the effects that infrastructure improvements and expansion may have on a city’s overall resilience to severe storms-induced vulnerabilities. The framework, called MetroFlow, was developed in a multi-year study led by civil and environmental engineering professor Marcelo H. Garcia at the University of Illinois Urbana-Champaign (UIUC).  Postdoctoral fellow Hao Luo is lead author on a new paper recently published in the Journal of Hydrology that presents the first comprehensive description and application of MetroFlow.

Many drainage systems in older metropolises are of the type known as Combined Sewer Systems, in which wastewater and stormwater flow through the same pipes. Expanding urbanization and an increase in extreme storm events can tax a city's drainage system. When the capacity of these systems is exceeded during heavy storms, the mixture of stormwater and untreated wastewater may overflow, flooding streets and residential basements, and discharge to nearby waterways. In addition to being a threat to personal safety – in 2021, for example, urban flooding led to the deaths of dozens of people when the remnants of Hurricane Ida hit the Northeastern U.S. – this type of flooding can also lead to water quality issues and public health concerns.

To identify when and where these overflows are likely to happen, MetroFlow views a city’s overall urban drainage system as a series of layers: surface and near-surface sewers, interceptors (that is, the pipes that divert sewage to water reclamation plants), connecting structures and deep tunnel systems. Chicago, which has 375 square miles of area served by a combined sewer system and a deep tunnel and storage reservoir system that collects and holds overflow until water reclamation plants have capacity to treat it, was used as a testbed.

Using a wide spectrum of data, including historical data from the four most intense storm events (2007-2016) in Illinois, the team modeled each layer of Chicago’s system independently using numerical or probabilistic models – whichever was more suitable based on data availability and the physical processes to be solved – then linked them together using MetroFlow’s framework.

“One of the main advantages of MetroFlow lays in its modular structure, which makes it possible to apply the most appropriate hydrologic and hydraulic model to an urban watershed depending on the available physical data,” Garcia said. “Often times there is limited information regarding the drainage system but the Illinois Urban Hydrology Model [one of the models built into MetroFlow] with its probabilistic approach, makes it possible to assess storm runoff and sewer flows in urban areas where deterministic models cannot be applied due to lack of data.”

The team then analyzed the way different parts of the layers interacted and responded (together and separately) to changing variables such as storm characteristics, conveyance and storage capacity of storm infrastructure. Their study showed that MetroFlow was able to predict the storm-driven combined sewer overflows, the performance of the drainage infrastructure and the effectiveness of the deep tunnel and reservoir system. In turn, they were able to identify hot spots likely to experience overflows during and after heavy rainfall.

Given how costly it is to repair or build new infrastructure, being able to identify problem areas and test possible solutions (for example, green infrastructure) before taking action will help city planners make informed decisions in the face of expanding urbanization and extreme weather events related to climate change. MetroFlow makes this process easier.

“With the layered approach we can incorporate newly developed structures in either an existing layer or a different layer depending on the nature and scale of the structure,” Luo said. “That allows the use of existing resources and reduces the computational power you need, and that increases the stability of the models.”

There are large-scale efforts underway in many cities to upgrade stormwater infrastructure to better handle the composite flooding and water pollution issues. Designing infrastructure in the face of escalating climate change requires thinking more comprehensively and accounting for the existing infrastructure and potential climate risks, Luo said. 

“Our purpose of this framework is to provide an approach to connect the cityscapes altogether and consider them as a system from the very beginning and from top to bottom,” she said.

The National Science Foundation and Metropolitan Water Reclamation District of Greater Chicago (MWRDGC) supported this work. Nils Oberg from the Carl R. Woese Institute of Genomic Biology at UIUC and CEE alumnus Dr. Blake J. Landry of the U.S. Naval Research Laboratory are co-authors on this study.  Garcia gratefully acknowledged the many different researchers and students (graduate and undergraduate) who contributed to the development of MetroFlow over a period of several years, including personnel from the MWRDGC. 

To reach Marcelo Garcia, call (217) 244-4484; email mhgarcia@illinois.edu

To reach Hao Luo, email haoluo2@illinois.edu

The paper, 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, is available here.