Sanitary Sewer System Design

With population and employment on the rise, the City of Waterloo initiated an update to the previous City-wide sanitary sewer system. As part of this update, Stantec was retained to lead City staff and external software vendors and organizations through a Hydraulic Needs Assessment. The goal was to determine which hydraulic modeling software would best suit the City’s needs by reviewing collected information in the context of industry practices and model functionalities.

Questions surrounded the intended primary use of the hydraulic model, the preferred level of hydraulic analysis and infrastructure functionality, whether having integrated storm and sanitary was preferred, the desired output format and perceived use of the software for the City. Based on the City’s input and needs, weighted evaluation criteria were developed and used to compare and evaluate the model software development options.

The outcome of this assessment indicated the preferred approach was to develop a dynamic model that includes all pipes within the core area and sewers defined as trunks or sub-trunks for the remainder of the City. Long-term, the City identified the goal of transitioning to an all-pipes model throughout the municipality.

After thorough analysis, the selected software for developing the hydraulic model was PCSWMM. This decision resulted from PCSWMM exceeding the base needs, providing superior calibration and advanced topological tools, having a local software provider available for support, offering training and implementation, maintaining solid vendor availability and technical support and being the most cost-effective modeling software of all alternatives.

The resulting model was used to identify constraints for development considerations and assess existing and future conditions to refine recommended upgrades. The City is currently in the process of putting measures in place to move toward purchasing PCSWMM and implementing the Waterloo Sanitary Master Plan.

Continuous simulation of collection systems often requires long run times for complex computer models of large sewer system networks. Even still, this method of simulation can be useful for quantifying overflow frequency, pump runtime, storage utilization and dewatering characteristics – as well as other important indicators of wet-weather system performance.

In this project, the City of Columbus hired CDM Smith to size a sanitary sewer storage facility as part of the Livingston/James Sewer System Infiltration and Inflow Remediation Project. To avoid potential over- or under-sizing of storage facilities and to achieve a 10-year level of service, PCSWMM was selected for a continuous modeling approach, using precipitation data for 1949 through 2009 (61 years) from the Port Columbus International Airport gauge.

The dynamic wave model includes approximately 3,000 conduits serving an area of 4,700 acres. Using 2007 - 2008 flow monitoring data, seasonal (dormant and growth season) RTKs and initial abstraction parameters (IA) were calibrated to represent rainfall-derived infiltration and inflow (RDII). To reduce run times for the what-if scenarios, the 61-year inflow hydrographs were compressed (removing periods of low flow) and then routed through the sewer system.

The results showed that using long-term continuous simulation to size storage facilities can be practical, even for large complex sewer models. An advantage of this continuous simulation approach using PCSWMM and compressed inflow hydrographs is that it enables simulation of RDII without losing the influences from previous storm and moisture conditions, while maintaining reasonable runtimes to support generating and evaluating different design criteria for various levels of service.

Detroit’s sewer system spans approximately 900 square miles (2,300 km²) and its Combined Sewer Overflow (CSO) facility has a primary capacity of 1,700 million gallons per day. If the storage capacity is exceeded, the treated effluent is discharged to the river.

To avoid river discharge and address chronic basement flooding in the City of Detroit, PCSWMM was used to assist in the design of a new combined sewer system for the existing sewer district of Oakwood (~1,500 acres). The proposed new system design was configured to intercept a sufficient number of catch basins in the streets to prevent storm flows from surcharging the sewers collecting sanitary flows from the homes.

PCSWMM was used to lay out the eastern portion of the district which involved adding new relief sewers. The software helped to enable the placement of sewers graphically and generate associated input data required for the model.

A desired level of service criteria for the district was established by maintaining the peak HGL experienced during a 10-year 1-hour design storm event. Due to the revised design, approximately 50 to 75% of the storm flows have been effectively intercepted.

Performing the analysis and design using PCSWMM allowed for dynamic viewing and design modifications as the design proceeded. The revised design proved to be sufficient in preventing surcharging within 8 ft (2.4 m) of ground level and is equipped to meet the required level of service for the Oakwood District at a significant cost savings to the original proposed design.