Pemberton Valley Flood Hazard

The Pemberton Valley is located on a floodplain, which means that it is prone to periodic flooding from nearby rivers, lakes and streams.

In 2017, the Pemberton Valley Dyking District (PVDD) was awarded funding from the Province of BC through Emergency Management BC (EMBC) to commission an update of the Lillooet River Floodplain Mapping (the last substantial update to the mapping was conducted in 1990, and since that time, modelling technology and methodology have evolved significantly).

Conducted by Northwest Hydraulic Consultants Ltd., the Lillooet River Floodplain Mapping Report ('the Report') was finalized in November 2018.

The updated flood modelling shows that peak discharge flows correlating to 50-, 100-, and 200-year floods are higher than previously understood and, as a result, current dike infrastructure does not offer the level of protection as was previously understood.

Sedimentation from the 2010 Mount Meager landslide and changes in the timing of flood events (floods are more likely to occur after rain-on-snow events in the fall, rather than at spring freshet) all contribute to the increased flood risk in the Pemberton Valley. Climate change is also expected to increase the peak flow rate over the next 80 years.

The likelihood of a 200-year flood occurring is once in every 200 years or a 0.5% chance in any given year; the likelihood of a 100-year flood event occurring is once in every 100 years or a 1% chance in any given year and the likelihood of a 50-year flood event is once in every 50 years or a 5% chance in any given year.

However, the magnitude of these flood events has increased. The report findings indicate that what was once thought to be a 1-in-200-year flood event, is now more likely to be a 1-in-50-year flood event.

In addition to providing insights into the increased flood hazard, the Report offers several recommendations to support flood management in the Pemberton Valley, some of which have been completed, or are already in progress.

Recommendations include:

  • Informing local residents of the increased flood risks (in progress; ongoing);
  • Updating emergency response plans (in progress);
  • Designating the new floodplain maps for the Lillooet River;
  • Notifying the provincial River Forecast Centre of the flood hazards in the Pemberton Valley (complete; the trigger points for high streamflow advisories, flood watch and flood warnings have been adjusted to reflect the new data);
  • Increasing sediment removal and conducting critical dike upgrades (in progress; PVDD);
  • Conducting additional modelling and mapping of the tributary flows, including the Birkenhead River;
  • Increasing monitoring with additional flow gauges on the Birkenhead River as well as tributaries.

These recommendations have implications for all levels of government and will influence land use planning as well as emergency flood response and recovery planning. Some of these measures will prove to be costly, but the jurisdictions involved (Líl'wat Nation, PVDD, SLRD, Village of Pemberton) will work together in an effort to secure funding for long-term mitigation measures. 

A working group consisting of emergency management personnel from all four agencies has been established and is developing an interim Integrated Flood Response Plan (IFRP). A formal, public IFRP is expected to be complete before October 2019. 

As well, the PVDD has begun the process of identifying flood protection improvement projects and options. Following this, the four jurisdictions will discuss and prioritize the projects, and will work together to pursue funding support from the provincial and federal governments to help advance further flood mitigation in the Pemberton Valley. 

All four jurisdictions will continue to share information and provide updates to the community through their respective communications channels (websites, social media, etc.)

Flood Scenario Animations

The following animations demonstrate the 200-year design flood for the Lillooet River. The animations represent worst case estimates of water inundation areas and water depths during a 200-year flood event.
Please read the Lillooet River Floodplain Mapping Report for more details on the hydraulic modelling.

Upper Lillooet River

Lower Lillooet River

 

DISCLAIMER
These videos have been prepared by Northwest Hydraulic Consultants Ltd. for the benefit of PEMBERTON VALLEY DYKING DISTRICT for specific application to the LILLOOET RIVER AND TRIBUTARIES FLOODPLAIN MAPPING. The information and data contained herein represent Northwest Hydraulic Consultants Ltd. best professional judgment in light of the knowledge and information available to Northwest Hydraulic Consultants Ltd. at the time of preparation, and was prepared in accordance with generally accepted engineering practices.
Except as required by law, these videos and the information and data contained herein are to be treated as confidential and may be used and relied upon only by PEMBERTON VALLEY DYKING DISTRICT, its officers and employees. Northwest Hydraulic Consultants Ltd. denies any liability whatsoever to other parties who may obtain access to this report for any injury, loss or damage suffered by such parties arising from their use of, or reliance upon, these videos or any of its contents.

 

 

 

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