How One Community Is Using Radar Rainfall to Reduce Overflows and Basement Flooding While Enabling Real-Time Control of Its Wastewater System

The purpose of this presentation is to describe how a Great Lakes community is addressing its goals of reducing overflows and basement flooding while enabling real time control of its wastewater system. Peel Region believes it will require best in class measured and predictive rainfall data, to create efficiencies for conveyance and treatment, that will allow the Region to face the unique challenges of climate change, population growth, and aging infrastructure. Peel has implemented the program described in this abstract, in its 2020 Water and Wastewater Master Plan for the Lake-Based Systems. The Region of Peel, Ontario (Peel or Region) is responsible for the operation and maintenance of the sanitary sewer network, pumping stations, and wastewater treatment plants within its collection system. Peel is composed of the Cities of Brampton and Mississauga and the Town of Caledon that drain a combined surface area of 1,246 km2 to Lake Ontario. Wastewater services are also provided by Peel to neighboring Toronto and York through inter-regional servicing strategies, thus making accurate rainfall over Peel and for neighboring jurisdictions of increasing importance for managing wastewater flows. Wastewater is conveyed by gravity, generally from north to south through the collection system to the wastewater treatment facilities at Lake Ontario. Peel provides wastewater system services to Mississauga, Brampton and parts of Caledon. Demands are placed on the wastewater system because Peel is one of the fastest growing municipalities in Ontario, in excess of 1.4 million people. The Region's wastewater system comprises two principal trunk systems: the west trunk system that conveys flows along and near the Credit Valley to the Clarkson Wastewater Treatment Plant, and the east trunk system that conveys flows along and near the Etobicoke Creek Valley to the GE Booth Wastewater Treatment Plant. Planned construction of a 11-km of the East to West Diversion Trunk Sewer will require RTC and forecast precipitation. There are over 3,600 km of wastewater pipes in Peel, with 300 km of trunk sewers consisting of pipes ranging from 750 mm to 3,150 mm in diameter, servicing 294,000 customer accounts. Climate change and population growth continue to put pressure on the wastewater system. Recently, a climate change mitigation strategy was endorsed by the Region of Peel Council to reduce basement flooding due to sewer backups. Understanding system response to rainfall and taking proactive measures to reduce I/I are strategic actions aimed at reducing flooding issues region wide, and to maintain wastewater treatment capacity in the long-term. The flash flood that occurred on July 8, 2013, had a larger impact on the Region of Peel and City of Toronto than any storm since Hurricane Hazel (1954), and reinforced the need for better preparation and operation of the wastewater system. The Region is seeking to improve the wastewater system for future growth, achieve cost efficiencies, derive environmental benefits, and to lower the risks to human health. Improved rainfall monitoring, both current and forecast, are key to achieving these goals: increasing volume of wastewater treated; reducing incidents of overflows/basement flooding; and achieving effective operation of real-time controls. These goals can be more efficiently achieved through accurate and representative rainfall monitoring. This is achieved through the combination of weather radar and rain gauges. Radar-based precipitation measurement that relies on both gauges and radar is called GARR, or gauge adjusted radar rainfall. The target region and neighboring weather radars (both Canada and US) are shown in Figure 1 with range rings indicating coverage distance. Current and forecast rainfall is needed to meet the Region's objectives. Technical details on forecast rainfall is not discussed here but will be presented along with GARR during the presentation. The GARR system embodies both near-real-time (NRT) and End-of-Month (EOM). During EOM, Quality Control (QC) of rain gauges and radar removes or mitigates radar anomalies and erroneous or inconsistent gauge measurements. Gauges identified as consistent are used to bias -correct the radar precipitation rates. Thus, GARR is the combination of two sensor systems that results in better maps of rainfall than either sensor system could produce alone. GARR provides coverage of targeted areas at spatial and temporal resolutions commensurate with the sewer catchments. GARR is needed for better understanding and modeling of:

-- Basement flooding from both Sanitary Sewer Overflows (SSO) and Combined Sewer Overflow (CSO), and Facility By-Pass
-- Inflow and Infiltration Calculations
-- Hydraulic model calibration
-- Stormwater flood modelling

To illustrate the properties of GARR required by the Region, we summarize the characteristics of the extreme event, July 8, 2013. GARR was produced with weather radar and rain gauges from multiple networks from Region of Peel (29), City of Mississauga (14), and Credit Valley Conservation (13). Figure 2 depicts the spatial distribution of rainfall depth depicted by the color scale ranging 11.9 - 138.3 mm with a mean of 65.3 mm over the target area. Rain gauges are shown by black triangles with depths in mm. The scatter plot in Figure 3 shows bias-corrected radar (note 1:1 line) versus rain gauge depths for a selected period. Gauges excluded are shown in red, whereas rain gauges that pass QC (included) are shown in blue. Quality control removes bias in the GARR rainfall estimates, and generally improves accuracy from more than ± 20% to less than ± 5%.

Benefits of Presentation
This presentation should be selected because it shares the experience and challenges faced in characterizing rainfall. While GARR is an accepted technology in wastewater, the needs in the Great Lakes Region are unique and would benefit other communities with similar climate and population/development pressures.
Status of Completion
At the time of submittal this project is complete in terms of setup and operations. Historic data analysis was completed for requested storm events in March 2019. End-of Month quality-controlled GARR for all storms in May to November 2019, was completed in April 2020. Continuous production of Forecast and NRT GARR is operational beginning April 2020, and EOM GARR production continues.

Conclusion
Besides climate change, growth will continue to place pressure on water and wastewater infrastructure in Ontario. GARR is fit-for-purpose and helps address objectives of the 2020 Water and Wastewater Master Plan for the Lake-Based Systems.