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Waterways Slope Stability Assessment

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Waterways Slope Stability Assessment

Map: Stability Zones

Slope Stability Map

[Click image above for larger view]

Watch

The March 7 "Here For You" presentation on slope stability in Waterways.

Background

The Regional Municipality of Wood Buffalo (RMWB) has received a report following the completion of a slope stability assessment for the community of Waterways.

On September 27, 2016, the Municipality directed Administration to conduct this assessment following requests from residents. The RMWB has applied to the Government of Alberta to have the costs related to the overall geotechnical assessment for Waterways covered through Disaster Recovery Program (DRP) funding.

The Recovery Task Force awarded the assessment work to Thurber Engineering, which monitored the slope between November 2016 and January 2017.

The Report

The assessment looks at general slope stability in Waterways and provides a general guideline for development in Waterways from a geotechnical standpoint. It also identifies areas in Waterways where there are risks and/or may be risks to development by property owners and residents looking to rebuild following the May 2016 wildfire.

Overall, the report found the valley slope is only marginally stable, meaning there is a potential for movement. The loss of tree cover in the area because of the May 2016 wildfire will only make the slope more prone to movement.

This will have an impact for several residents who own property within the study area at both the planning and redevelopment stages of recovery.

On Wednesday, March 8, the Wood Buffalo Recovery Committee (WBRC) directed the Recovery Task Force (RTF) to compile a report for Committee’s consideration that will include: an analysis of the community feedback received, a review of the technical aspects of the geo-technical reports, and identification of any financial issues associated with each of the options identified in the report.

The RTF will present this report at a meeting of the WBRC and residents will be encouraged to register as delegates.

Zones

The report divided the study area into zones based on the geological conditions and development risks of each area. See the report for the locations of each zone. The decisions made for next steps in Zone 3A will have an impact on development considerations for Zone 3.

  • Zone 1 - Development can proceed in this area, provided residents follow the standard Municipal requirements for permits and development.
  • Zone 2 - Development can proceed in this area. (A site-specific geotechnical investigation must be submitted at the time of the development permit application for each lot within this zone.)
  • Zone 2A – There is no existing development within this area. Development will not be allowed in the future in this area.
  • Zone 3 – Development can proceed in this area, however, the decisions for next steps in Zone 3A must be taken into account. There are risks in this are due to slope movements if careful planning, construction and maintenance are not followed. The Municipality has a duty to inform residents of the risks and restrictions on development in this area. (A site-specific geotechnical investigation must be submitted at the time of development permit application for each lot within this zone.)
  • Zone 3A – Mitigation options must be explored and a policy decision must be made by RMWB Council.
    This zone is one of the most sensitive areas of the valley slope within the former development area. According to the report, there is a high risk of ground movement that could affect not only the zone itself, but also the adjacent properties if adequate measures are not taken.
  • Zone 4 – Development will not be allowed within this area. The area will be placed in priority for re-vegetation.

Options for Mitigation in Zone 3A

The report outlines three options for the Municipality for this zone, which are presented below.

Option 1: Warn and Restrict

The Municipality could allow development following the same guidelines presented for Zone 3 above along with additional restrictions. Pile foundation walls may be required to provide sufficient resistance to handle the forces exerted by the existing soil structure creep movements.

Regardless of the type of insurance residents have, the cost of implementing changes to meet these restrictions may not be covered, as they may be considered new requirements and coverage is typically based on date of loss rather than date of rebuild.

Option 2: Municipality Constructs Community-level Structural Mitigation

A comprehensive geotechnical investigation to consider community-level structural mitigation options, as opposed to lot-level mitigation options, would be required to determine the best solution and cost to mitigate risk in this zone.

Structural mitigation would then have to be constructed before these property owners could proceed with development.

The cost of the study for the Municipality is estimated to be $30,000 to $40,000 for pre-design and $40,000 to $100,000 for detailed design work.

One option might be to expand and improve the existing retaining wall along Cliff Avenue, extending it from Tomlinson Street to Huggard Street. This may not protect the area between Cliff Avenue and Lower Cliff Avenue and a series of retaining walls may be required to protect the entire 3A zone.

A series of retaining walls could cost the Municipality in the order of $8 million to $16 million plus ongoing monitoring and maintenance costs.

The required geotechnical investigation and subsequent construction would likely delay the rebuild period more than one year.

Should the Municipality proceed with this option, a formal request would be submitted by the Municipality to insurance companies requesting a waiver on the two-year rebuild requirement for the property owners.

Option 3: Acquire and Mitigate

Zone 3A consists of 18 lots: eight were destroyed during the fire; one contains a standing home; six are vacant, yet privately held, and three are municipally owned. The Municipality could choose to acquire the 15 remaining lots.

Some regrading and embankment construction within the lots would be required to maintain the long-term stability of the slope. This would likely decrease the overall risk of the entire study area.

Some additional geotechnical work would be required to determine the regrading requirements. It is estimated that this study could cost the Municipality between $10,000 and $20,000.

While the Municipality could choose not to acquire the privately held vacant lots, acquiring the 15 lots would ensure that the area of highest risk was entirely under the care and control of the Municipality.

A total cost for acquiring the 15 remaining lots in the area cannot be calculated until appraisals are completed for each property. There would also be additional costs for regrading. Planning & Development can ensure that the area is zoned accordingly.

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