Denison Road West Upper Slope Stabilization – Class Environmental Assessment

Toronto and Region Conservation Authority (TRCA) has formally commenced Conservation Ontario’s Class Environmental Assessment for the Denison Road West Upper Slope Stabilization Project, in the City of Toronto.

Project Location

The study area is located southwest of the intersection of Lawrence Avenue West and Jane Street, in the City of Toronto. Residential properties on Denison Road West back onto the crest of a steep valley slope along the Humber River. Four properties and a TRCA-owned public parkette which are situated near Raymore Park make up the study area.

Learn More About the Project

Project Background

TRCA has been monitoring this area since the early 1980s due to its known erosion risks. The Humber River is a large, fast-moving watercourse which has the potential to cause major changes to the surrounding landscape. At that time, TRCA constructed a 350 metre long revetment along the eastern valley wall below Denison Road West. The intent of the project was to stabilize the bank of the river and the slope above it.

Following a major storm event in August 2005, high water levels topped the revetment and the slope failed. In response, TRCA retained Golder Associates to carry out a thorough slope stability review. Completed in 2009, their review found that the slope was unstable and at risk of further failure. Left unchecked, instability can lead to loss of tableland, putting residences near the top of the slope at risk.

In 2015, TRCA subsequently retained Terraprobe to carry out subsurface investigations and develop detailed designs. Designs were to address the lower slope instability where erosion risk from the Humber River was highest. TRCA’s preferred design included replacing the failed revetment with an armourstone wall and regrading the slope directly above it.

From 2016 to 2017, TRCA constructed the wall designed by Terraprobe with minor alterations due to one property opting out of the project. TRCA also included plantings to further stabilize the ground above the wall.

The Current Class EA Study

The lower slope work performed in 2016 and 2017 was only the first phase in addressing the risk of future slope failure. Now, TRCA is commencing a second phase of work which will investigate possible measures to address upper slope instability. The current study focuses on four properties and a parkette located above the previous project area. By including several properties, a continuous treatment across the entire slope may be possible. This is the most effective way to provide a long-term, low maintenance solution for slope stabilization.

This project is being planned in accordance with Conservation Ontario’s Class Environmental Assessment for Remedial Flood and Erosion Control Projects (amended 2013), because it is the approved process for this type of undertaking. TRCA invites residents of the community to participate in this study, which includes public and stakeholder engagement. Public input is a fundamental part of the planning and design process for this project. To learn more about the public consultation elements of a Class EA project, see here. For a general overview of EAs, see here.

Scope of Work

As part of this study, the geotechnical investigation will determine the cause and extent of slope instability. These factors help TRCA’s consultant develop alternative solutions to address risk to public and private property. Secondly, TRCA evaluates each alternative and selects a design for the preferred alternative. That design is finalized following a 30 day public review period. Finally, TRCA then develops detailed designs for implementation which are based on the preferred alternative. The final design will provide long-term protection for essential structures at the top of the slope in the study area.

An armourstone wall and soil stabilization measures at the toe of a steep slope overlooking the Humber River
Previous lower slope work done by TRCA behind properties on Denison Road West, with evidence of erosion and slope failure still visible on the upper slope.

Currently, TRCA’s geotechnical consultant is in the process of analyzing the information collected during their investigation. The consultant will then develop a selection of alternative conceptual designs. TRCA will be coordinating the design review and public consultation stage of the study, beginning in 2020.

Construction Information

Construction planning will begin once a preferred alternative has been selected and detailed designs are underway. The construction phase of the work is tentatively planned for late 2020 or early 2021.

Public Consultation

The Conservation Ontario Class Environmental Assessment for Remedial Flood and Erosion Control Projects is being used to conduct this study. In previously developed areas, Conservation Authorities have approval to use this assessment process for projects that will protect human life and property from potential flood or erosion problems. The Class EA process outlines the steps required to determine the best remedial erosion control solution. Several alternative solutions are considered for a project under the Class EA. Each alternative undergoes detailed technical evaluation as well as public review and consultation. Finally, this review process identifies a preferred alternative. The preferred alternative offers the best means to protect against flooding and erosion and also mitigates other environmental effects which could result from the project.

The Conservation Authority Class EA process guarantees opportunities for public and stakeholder participation throughout the planning stages of a project. TRCA will coordinate these consultation opportunities, and then post updates and scheduled meetings to this web page.

Figure 2 shows the project planning process, which follows Class EA requirements.

Figure  2. Class Environmental Assessment process.

EA Documents

Notice of Intent

Project Updates

November 2019

TRCA’s geotechnical consultant performed investigative work on all properties participating in the project. They advanced boreholes in all participating properties over three days of drilling work. TRCA uses investigations like this because they determine the long term stability of the slope. They also help predict which structures may be at risk due to future erosion.

A borehole drill being operated by workers in jumpsuits and hard hats
Workers conducting a geotechnical investigation by advancing a deep borehole on one of the study properties.

October 2019

TRCA is currently in the process of scheduling slope stability assessments with the geotechnical engineering consultant for the project. Additionally, this consultant will also develop design concepts for slope stabilization works.

Contact Us

For more information about this project, please contact:

Nivedha Sundararajah, Project Manager, Erosion Risk Management
Toronto and Region Conservation Authority
Phone: (416) 661-6600