Seton Portage Area Integrated Hydrogeomorphic Risk Assessment

For more information, contact:

Sarah Morgan
SLRD Emergency Program Manager
Email: smorgan@slrd.bc.ca
Telephone: (604) 698-6442

Apr 9, 2018
Electoral Area B

The Seton Portage Area Integrated Hydrogeomorphic Risk Assessment (“Assessment”) is now complete.

In 2017, BGC Engineering Inc. (BGC) was engaged to assess hydrogeomorphic hazards and risks in the Seton Portage area, including risk to life, buildings, critical facilities, business activities, culturally significant sites, and critical infrastructure like roads, bridges, railway, power, and communication lines. For the purposes of the Assessment, hydrogeomorphic hazards are defined as hazards stemming from Bear/Pete’s, Whitecap and Spider creeks and include floods, debris floods, debris flows and bank erosion on the Portage River (also known as the Seton River or Portage Creek). Additionally, hazards were assessed that could indirectly affect steep creek processes such as deep-seated landslides.

The completed Assessment has confirmed the potential for land movement events that put 59 buildings below the Bear and Pete’s Creek drainages above the tolerable individual safety risk threshold and at a high priority for risk mitigation measures. Of these 59 buildings, 44 are on land within SLRD Electoral Area B and 15 are on Reserve Lands administered by Tsal’alh. According to the BGC analysis, debris flow risks from Bear and Pete’s creeks are much higher than risks from other creeks at Seton Portage. Safety risk from Whitecap Creek was assessed as tolerable (note that there is one building at higher risk but there are plans to move it), and safety risk on Spider Creek was assessed as acceptable. Further study of Portage River was recommended.

The following table shows the calculated size and frequency of projected future debris flow events. Debris from an event in the 10 to 30 year return period is estimated to reach properties below the Bear and Pete’s Creek drainages, and debris from an event in the 30 to 100 year return period (and greater) is estimated to impact buildings, with increasing severity depending on the total volume of the debris flow event.

The report concludes that climate change will likely affect the frequency and magnitude of future debris flow events, with changing weather patterns increasing the potential for heavy downpours that push debris downslope. Additionally, permafrost is likely present above 2,100m in the watershed and rising temperatures would add to the debris load as rocks previously held together by ice are released as the ice melts. This would imply a higher frequency and possibly higher magnitude of land movement events in the future.

Engineered structural mitigation is possible and recommended by the Assessment, which identified mitigation on Bear and Pete’s creeks as being a high priority. However, at an estimated capital cost of $7 – $8M not including annual maintenance costs or the costs of clearing out debris after each debris flow event, securing the funding for structural mitigation will be a key area of focus moving forward.

The Assessment also recommended a number of non-structural mitigation options, including relocation, community education, emergency management planning, land use restrictions, and temporary evacuations during hazardous conditions such as heavy rainfall events. However, with the exception of relocation out of the area, none of these options would reduce the risks to a tolerable level without structural mitigation.

Download the complete Assessment

Community Information Meeting

Please join Officials and Staff from Tsal’alh and the SLRD for a presentation of the Assessment’s findings by BGC Engineering, followed by a Question and Answer session.

Meeting Details:

  • WHEN: Monday, April 16, 2018 (6:00 p.m. – 8:00 p.m.)
  • WHERE: Sk’il Mountain Community School (Sk'il Mountain Drive, Site 4)

To ensure that as many questions as possible can be answered at this meeting, community members are encouraged to review the Assessment and submit their questions prior to the meeting to one of the community contacts noted below:

All of the questions and answers brought forward at the meeting will be incorporated into the Frequently Asked Questions document linked above. 

A copy of BGC Engineering’s presentation from the Community Information Meeting will be available for viewing on this web page, as soon as it becomes available. 

Next Steps

Officials from Tsal’alh and the SLRD are reviewing the Assessment’s recommendations and considering next steps. Both organizations will continue joint advocacy efforts to secure funding for risk mitigation.

On a staff level, Emergency Program Managers will integrate this new information into existing emergency management plans. 

The SLRD will update this page as more information becomes available. 

Project Background

Seton Portage is situated on an alluvial fan and floodplain that separates Anderson and Seton Lakes, about 25 km west of Lillooet, BC. The community is found within the Traditional Territory of the Tsal’alhmec People, and also within the jurisdiction of Electoral Area B of the Squamish-Lillooet Regional District (SLRD). Built development is concentrated on the valley bottom, which is bisected by the Portage River (also known as Seton River or Portage Creek). Land tenure is a mix of Tsal’alh Reserve Lands and freehold property and private property. CN Rail, the BC Ministry of Transportation and Infrastructure, and BC Hydro own critical infrastructure in the Seton Portage area.

During the spring, summer, and fall months, the area is exposed to landslide risk and consequential flood risk from debris torrents originating in Whitecap Creek, and debris flows from Bear and Pete’s Creek on the northern slopes of Goat Mountain. Whitecap Creek flows into the Portage River from the northwest, and Bear and Pete’s Creeks are located across the valley to the south.

In 2015 and 2016, area was impacted by relatively large events originating from both drainages, with debris reaching properties on the Bear Creek fan at Seton Portage and the Whitecap Development property. Extreme runoff also occurred from Whitecap Creek in 2015 and 2016 that impacted Anderson Lake Road and the CN Rail line. Further, slides into the Portage River appear to have decreased outflow volume from Anderson Lake and future slides present a flood risk. A 2016 geotechnical report prepared by MFLNRORD identified an increased potential hazard and risk from terrain instability, including the potential for larger land movement events, and recommended a detailed risk assessment be undertaken to determine the risk to the public present in the Seton Portage area.

Prompted by these events and the MFLNRORD report, officials from the Squamish-Lillooet Regional District (SLRD) and Tsal’alh requested funding from the Province and Indigenous Services Canada (ISC) to move forward immediately on the report's recommendation to secure a detailed hydrogeotechnical analysis to quantify the hazards, evaluate the risks, and recommend potential risk reduction options. 

Funding for the study was confirmed in July 2017, and a multi-agency Steering Committee with representatives from the SLRD, Tsal’alh, EMBC, ISC and MFLNRORD was convened shortly thereafter. On July 24, 2017 the SLRD, acting as an agent for and under the authority of the Province of B.C., issued a Request for Proposals regarding the work, and BGC Engineering was selected by the Steering Committee to conduct the work.

The key outcome of the Project was to provide a comprehensive hydrological and geotechnical assessment, and risk assessment integrating all the main contributors to landslide, debris flood, and consequential flood risk including interactions between events originating from Whitecap drainage, Goat Mountain northern slopes, and Portage River outflow from Anderson Lake.

The aim was to provide Tsal’alh, the SLRD, local residents, and provincial and federal officials a clear picture of the landslide and consequential flood risk in the Seton Portage area, arrived at by using current best practices in landslide risk assessment. Additionally, the project was to present a wide range of possible mitigation options, including structural and non-structural approaches.

Background information and resources

 

 

Inside the SLRD > Current Projects & Initiatives > Seton Portage Area Integrated Hydrogeomorphic Risk Assessment