TRACK 2, DAY 2
“How Much Can This Stream Handle?”: The Evolution of Erosion Thresholds and Field-Based Assessment in Ontario’s Stormwater Practice
Paul Villard, GEO Morphix Ltd.
Wednesday, April 1, 2026 | 3:15 p.m. to 3:45 p.m. | Hall F
ABSTRACT
Ontario’s approach to erosion mitigation has evolved steadily over the past three decades: transitioning from peak flow matching to a geomorphically informed practice grounded in site-specific and receiver-appropriate erosion thresholds.
The 1994 MOE Stormwater Management Planning and Design Manual formally introduced the concept of channel-specific erosion thresholds, marking a shift from volume and rate control toward assessments of morphological resilience. Early analysis tools such as Time of Exceedance and Number of Exceedances were applied as coarse but practical screening metrics, helping to identify potential increases in erosive stress when post-development hydrographs exceeded critical flow thresholds.
Throughout the late 1990s and early 2000s, Conservation Authorities and consultants conducted extensive fieldwork to characterize channel response across southern Ontario better. Villard & Ness (2006) reviewed erosion threshold data across a wide range of TRCA watersheds, contributing to the growing body of evidence that many channels exhibit relatively low thresholds for adjustment. Their work reinforced the need to integrate reach-based sensitivity classification and channel-specific mitigation in stormwater planning.
Over time, more refined indices have been introduced, including the Cumulative Effective Work Index, Effective Stream Power, and Cumulative Effective Volume, which quantify not only the frequency and duration of threshold exceedance but also the magnitude of excess energy available for sediment transport. These indices rely on field-derived erosion thresholds as reference points for comparing pre- and post-development hydrology.
In recent years, the focus has shifted toward improving the methods used to define these thresholds, ensuring they reflect site-specific channel conditions and reach sensitivity. For example, where uncertainty exists or where channels are particularly vulnerable, field validation has been undertaken, or sensitivity analyses based on varying percentages of the erosion threshold have been used to bound potential risk and guide mitigation strategies.
Equally crucial to erosion mitigation is the preservation of landscape functions, including maintaining headwater drainage features or replicating their role in sustaining infiltration, regulating sediment inputs, and maintaining baseflow conditions. Recent years have also seen the broader application of Low Impact Development (LID) and green infrastructure strategies aimed at restoring or mimicking natural hydrology in urbanizing catchments.
In parallel, modelling approaches have improved, both in hydrological simulation and in geomorphic assessments, enabling a more accurate characterization of runoff timing, cumulative erosion potential, and stream sensitivity. Tools based on stream power and cumulative work are now used to evaluate reach-level vulnerability, potentially providing additional tools for prioritization and performance validation.
These advancements are helping to redefine how erosion thresholds are set, assessed, and addressed within Ontario’s evolving stormwater management framework.
ABOUT THE PRESENTER
Paul Villard, GEO Morphix Ltd.
For thirty years, Dr. Paul Villard has researched geomorphology in fluvial, coastal, and estuarine systems, with a focus on sediment transport and river evolution. Over the past twenty years, he has worked on hundreds of projects in Ontario, Manitoba, Alberta, California, and the northeastern U.S., including stream restoration and erosion assessments.