An educated modeling approach for RDII mitigation plan

Problem Statement

Rainfall-derived inflow and infiltration (RDII) has been identified as the main cause of sanitary sewer overflows (SSOs) and combined sewer overflows (CSOs). The region of York is implementing a mitigation plan to reduce high RDII and reserve sewer capacity for the City to add new development. Understanding the sources of RDII is essential for determining the system limitation and developing an effective mitigation plan.

Objective

The objective is to identify the source of high Inflow/Infiltration (I/I) over 37-ha residential area in northeast Newmarket, Ontario. In 2016, the lateral lining rehabilitation were conducted, with flow monitoring data covering both pre and post lining condition. Through controlled modeling experiments, this study quantifies the I/I reduction from lateral lining, and provide alternative solutions to mitigate the remaining I/I.

Methodology

A large amount of the I/I is generated from perched water, collected in manmade trenches around the collection system network. Those trenches include house trenches (buffer area around the house), lateral house connection trenches and mains trenches. Groundwater flow underneath the non-disturbed lawn areas also contribute to I/I at a lower and slower rate. To represent all the RDII sources, a detailed model is constructed for the study area in Newmarket, combining various GIS information and windshield survey field data. Hydrologic and hydraulic components are accurately represented for each sewer and manhole. The model is calibrated for both pre and post lateral lining condition. Impact of the lateral lining on I/I is quantified, with peak flow reduction of 33% and total volume reduction of 34%. Finally, mitigation alternatives with roof drainage redirection away from the house buffer and storm sump pump installation are evaluated.

This educated modeling approach can be applied to other areas to identify and quantify I/I sources. When combined with cost-benefit analysis, I/I mitigation programs can be defined for design implementation.