The Application of Radar Rainfall Data to Collection System Analysis

The design of major collection system elements such as trunk sewers and large pumping facilities must realistically account for sanitary inputs as well as possible wet weather flows. The wet weather component often originates in the older portions of existing communities that may be served by the new infrastructure. The designer's challenge is to predict peak flows for situations far in the future as well as embracing a wide variety of local collection systems located across a large regional sewershed. The Region of York owns and operates the major sanitary sewer system within its jurisdiction named the York Durham Sewage System or YDSS. Wastewater collected through the YDSS is conveyed to neighbouring Durham Region for treatment at the Duffin Creek WWTP. The YDSS system currently has a service area of 242 km² and a planned future service area of 448 km². Historically, the Region has evaluated the available capacity in the YDSS using a hydrologic-hydraulic capacity model. The model incorporates dry weather flow generation based on population and land use and wet weather modeling transforming design rainfall events into wet weather sewer flows. A peer review of the YDSS capacity model identified that previous capacity assessments were completed using the assumption of uniform rainfall across the entire service area of the Region. The use of this assumption generally resulted in over prediction of total peak flows. Accordingly, it was recommended that the Region undertake a detailed spatial rainfall analysis to identify appropriate areal rainfall reduction factors for the YDSS. The following paper presents the results of a study conducted on behalf of the Region of York to develop areal reduction factors that may be applied to wet weather design flow predictions.