Groundwater Sapping: An Under-appreciated Driver of Slope Instability with Important Implications for Erosion Control in the Valleys of Southern Ontario

Presentation Overview:

Groundwater sapping is responsible for slope instability and erosion over a range of spatial and temporal scales within the valleys of southern Ontario.

Groundwater sapping is a geomorphological process through which sediments are eroded from a slope by near-constant groundwater discharge at a particular location. It produces distinct slope hollows to entire valleys, with amphitheater-like heads, steep side walls, and a flat floor.

Newly available LiDAR-derived topographic data for portions of southern Ontario have catalyzed widespread recognition of slope instabilities attributable to groundwater sapping. An inventory of mass movements along Toronto’s Mud and Yellow Creek ravines revealed at least eight sites where groundwater sapping has formed retrogressive headscarps, some of which pose a risk to residences and infrastructure.

Distinguishing slope instability driven by groundwater sapping from that driven by landsliding is critical to slope stability assessment.

Standard geotechnical practices for predicting long-term stable slope morphology and erosion hazard limits fail to accurately represent conditions following continued groundwater sapping. An example is provided from the Patterson Creek valley, in Richmond Hill.

Solutions aimed at mitigating instability and erosion driven by groundwater sapping must prioritize redirection of groundwater away from the sapping site (e.g. French drains) instead of, or in addition to, stabilization of present topography.

Learning Objectives:

  • Learn about the contribution of groundwater sapping to slope instability and erosion within the valleys of southern Ontario.
  • Learn how to recognize groundwater sapping, based on field observation and interpretation of LiDAR-derived topographic data, by drawing attention to unique morphological features it produces.
  • Understand why long-term risks posed by groundwater sapping cannot be effectively assessed or mitigated based on standard geotechnical practices.

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