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Water Service - Municipal Infrastructure

Development of Areal Rainfall Reduction Factors

Client:	Region of York
Location:	York, Ontario
Budget:	$87,000
Outcome:	Areal reduction factors used in the capacity analysis of large wastewater collection systems

XCG Consultants Ltd. was retained by the Region of York to complete a detailed analysis of spatial rainfall data in York Region with a view to developing appropriate areal rainfall reduction factors that could be applied in the analysis of the York Durham Sewerage System (YDSS). The YDSS is a long wastewater collection system that collects wastewater generated in Newmarket, Aurora, Richmond Hill, King City, Vaughan, and Markham and conveys the collected wastewater to the Duffin Creek WWTP in Durham Region.

The Region has completed significant hydraulic analysis of the system to establish the capacity of the current system and to establish the need and timing of upgrades. As the YDSS receives significant infiltration and inflows in certain communities, the hydraulic model considers both dry and wet weather inflows. In recent Master Plans, the Region has accepted a 25-year level of service for the YDSS. A peer review of model analyses completed in 2003 identified that a level of service in excess of 25-years was likely provided if a uniform rainfall assumption was made in the model analysis. As a result of the peer review, a detailed analysis of spatial rainfall was completed to establish appropriate areal reduction factors that could be used in the future.

The project was completed in two phases with the first phase dedicated to the analysis of rainfall data collected in 2002, 2003 and 2004. The second phase focused on the analysis of data collected in 2005 and 2000 and examined two storm events that had significant return periods.

The analysis utilized conventional statistical approaches to develop areal reduction factors for areas of varying size. The data analysis concluded that the areal reduction factor decreased as areas were increased. This is consistent with findings by others. Recommended reduction factors ranged from 1 for areas less than 900 ha to 0.68 for areas of greater than 25,000 ha. Since completion of this project, the Region has adopted the use of the areal reduction factor in its methodology for analyzing the capacity of the YDSS and assessing the timing of new projects.

The technical work completed for this project has been presented at a number of Technical Conferences including the ASCE Conference in 2006.