Rehabilitation of the Pickering and Ajax Flood Control Dykes Class Environmental Assessment

Toronto and Region Conservation Authority (TRCA) is undertaking a Conservation Ontario Class Environment Assessment (EA) for Remedial Flood and Erosion Control Projects for the study of rehabilitation options for two (2) existing flood control dykes referred to as the Pickering and Ajax dykes.

The dykes were constructed in the 1980s to provide flood protection for the Village East community in Pickering and the Notion Road/Pickering Village community in Ajax, which are both designated Special Policy Areas (SPA).

Recent studies completed by TRCA have identified various deficiencies in their construction which prevent them from meeting current engineering design standards and factors of safety (FOS). It is desired to undertake rehabilitation of the dykes to meet current engineering standards and FOS while maintaining, or improving, the existing level of flood protection to the surrounding communities.

Background

The Village East and the Notion Road Pickering Village communities in the City of Pickering (Ward 3) and Town of Ajax (Ward 1) are located within the regulatory floodplain of the Duffins Creek watershed. This area has a long history of flooding, with 634 buildings susceptible to flooding during a Regional Storm (Hurricane Hazel) event.

Due to the flood vulnerability of the community, the area was designated as a Special Policy Area (SPA) to provide for the continued viability of existing land uses. In addition, flood protection measures were constructed in the 1980s to provide flood protection up to and including the 500-year storm flow.

The flood protection measures consisted of two flood protection dykes:

  • The Pickering Dyke, constructed in 1985, extends for approximately 1,150 metres, and is located north of Kingston Road extending from Brock Road eastward to east of Notion road.
  • The Ajax Dyke, constructed in 1984, extends for approximately 325 metres, and is located west of Church Street South extending north from near Mill Street.
Study Area for the Pickering Ajax Dyke Rehabilitation EA
Study Area for the Pickering Ajax Dyke Rehabilitation EA

As part of TRCA’s flood control program, these dykes have been inspected annually and after every significant rain event. In 2007, a major erosion scar was identified and repaired in 2008. In 2009, TRCA undertook a detailed assessment and “level of service” study of the dykes and adjacent creeks in order to develop a permanent long-term solution to address erosion issues.

In response to the recommendations from the 2009 study and in order to update flood modelling for the area, TRCA completed a multi-faceted study of the Pickering and Ajax SPAs and dykes in 2018. The objectives of the study were to:

  • Develop a more advanced hydraulic model of the area to characterize flood risks within the SPA
  • Assess the level of service and structural competency of the Pickering and Ajax flood control dykes
  • Develop a preliminary restoration strategy for the dykes

Based on the results from the 2018 hydraulic modelling study, it was determined that the targeted level of flood protection to the 500-year event is not provided by the existing flood control dykes. The Pickering Dyke provides flood protection for the 100-year storm and the Ajax Dyke provides flood protection for the 50-year storm.

Factors contributing to the reduced level of flood protection afforded by the Pickering and Ajax Dykes include: reduced dyke elevations due to settlement, and less sophisticated hydraulic modeling methods used as part of the original design process.

The Dyke Level of Service and Rehabilitation Report (2018) identified a number of deficiencies for both the Pickering and Ajax dykes based on field and geotechnical investigations. The deficiencies included:

  • Excessive vegetation
  • Erosion
  • Deteriorating dyke toe protection
  • Improper dyke construction materials

  • Blocked flap gates
  • Sediment and debris in culverts
  • Slope stability issue
  • Settlement/low areas on top of the dykes

vegetation and erosion on the Ajax and Pickering dykes
Left: Excessive vegetation on the Ajax dyke; Right: Shoreline erosion opposite the Pickering dyke
excessive vegetation and grass and gravel on Pickering and Ajax dykes
Left: Grass-covered dyke with some areas of excessive vegetation growth on the side slopes; Middle: Excessive vegetation growth on the top and side slopes of the Pickering dyke; Right: Gravel trail along top of Pickering dyke with excessive vegetation growth on the side slopes

The results of the geotechnical stability analysis indicate that the current dykes do not meet current engineering design standards. As such there is a high potential of dyke failure under an extreme storm event.

Nine (9) options to address the structural/slope stability issues and rehabilitate the dykes were developed in the 2018 study. Due to the scale of the required rehabilitation work, in order to move forward on the rehabilitation plans, the 2018 report recommended that a further study be undertaken by TRCA in accordance with the Conservation Ontario Class EA for Remedial Flood and Erosion Control Projects to assess all potential impacts and refine options for the rehabilitation of the dykes.

The report also recommended that TRCA undertake general repairs and maintenance immediately, prior to initiation of the EA Study, including erosion protection for priority sites and flap gate maintenance. TRCA initiated maintenance and repair work on the dykes in 2018.

At this time TRCA is undertaking a Conservation Ontario Class EA focused on determining the preferred solution for rehabilitation of the existing dykes.

The dyke rehabilitation objective is to provide the same, or greater, level of flood protection as currently provided (the Pickering Dyke provides flood protection for the 100-year storm and the Ajax Dyke provides flood protection for the 50-year storm) and enhance the dykes to meet current engineering design standards.


Questions?

Please contact:
PADR Coordinator
Toronto and Region Conservation Authority (TRCA)
101 Exchange Avenue, Vaughan ON L4K 5R6
Email: PADR@trca.on.ca
Website: www.trca.ca/PADR