San Francisco is home to approximately 880,000 residents and is one of two cities in California that uses a combined sewer system to collect and treat sanitary and stormwater flows. The San Francisco Public Utility Commission (SFPUC) owns and operates close to 1,000 miles of sewer mains, 3 treatment facilities, 200 million gallons of storage, 26 pump stations and 36 combined sewer discharge outfalls. The terrain of San Francisco is famous for its topography that includes steep hills, low valleys and, flat low-lying areas.
Developing the City of San Francisco’s combined sewer system numerical model came with several overarching challenges:
To help address these challenges, SPFUC uses Innovyze software, InfoWorks ICM.
There are many numerical models available in the engineering industry today. The City of San Francisco selected InfoWorks ICM for its combined sewer modeling due to its remarkable performance in three key areas:
The use of these features for San Francisco’s modeling work is described below:
The combined sewer system is designed to collect and convey flows for a design storm. In extreme storm events, excess stormwater flows may not enter the sewer system and combined sewer flows may exit the sewer system at some locations. In flat topographies, these overland flows pond in the area until system regains capacity. However, with San Francisco’s topography of steep hills, low valleys and low-lying flat areas, the overland flows often transport over the street surface and either enter back into the sewer system or pond at another low-lying location. The location of the origin of the overland flows and the eventual location of re-entry into system or ponding can be very different.
The two-dimensional (2D) module in InfoWorks ICM allows generating a surface mesh using ground surface elevation data. In extreme storm events, when there are overland flows on the ground, InfoWorks ICM enables the 2D module and routes the overland flows by solving the surface flow transport equations for each of the mesh element. Allowing the model to mimic the transport of overland flows is extremely helpful to characterize the performance of the sewer system. The figure below illustrates an example of transport of overland flows on the ground surface. The ability to visualize the fate and transport of overland flows with increasing accuracy has provided the planners and engineers a higher confidence in the model, and its use in sewer infrastructure projects.
Many of the treatment facilities, pump stations and combined sewer discharge outfalls convey and treat the flows differently during dry vs. wet weather. Additionally, during wet weather the operation of some facilities varies depending on the amount of rainfall and the combined sewer flows in the system. InfoWorks ICM has a robust module called “Real time control (RTC)” that allows programming different types of pump stations, gate and valve structures. It also allows programming the set-points for the operation of these facilities. The RTC logic is easy to program and understand and allows a much better representation for simulating the different treatment pathways (i.e. secondary vs. primary treatment facility vs. combined sewer discharge outfall) for any storm event.
City of San Francisco has several engineers from different locations that work on the model, and often working to solve the same problem concurrently. The workgroup-based databases and configuration management system has provided a seamless approach for several engineers to update the model, develop scenarios for analysis and generate results using the same network. The advantage of this feature has led to less time required for database management and conflict resolution, and higher team productivity.
“The model allows me to have confidence in knowing not only what the numbers are, but also what the numbers visually mean in terms of hydraulic performance.” Julio Maravilla, an Engineer at SFPUC said.
The features of InfoWorks ICM, as described above, support this confidence and as such empower the engineers at SFPUC, like Maravilla, to make sound decisions in managing San Francisco’s combined sewer system.