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Posted: Oct 7, 2021
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Flood Mapping

Somass Flood Study Header

Rivers, lakes, and the inlet form the landscape of our region. We have studied flood management for the Somass River System so we can plan effectively for its future.

Somass Watershed Flood Mapping Overview

The results from the Somass Watershed Flood Management Plan provide the first ever comprehensive flood risk information covering areas of Sproat Lake, Sproat River, Great Central Lake and the Ash River, and, additionally, provides updated flood risk information for the Alberni Inlet, Somass Estuary, Somass River, and the Stamp River where previous floodplain mapping was completed over 20 years ago.

Flood mapping is an important step in watershed management planning. It provides a more accurate baseline of anticipated flood risks if no actions are taken. The knowledge gained on this complex river, lake and tidal water system will help the community take a collaborative and informed approach to mitigation efforts because it provides greater certainty about potential flood events and associated impacts.

Prior to the completion of the Somass Watershed Flood Management Plan, the ACRD had minimal data and information to inform flood risk mitigation for new developments and the Zoning Bylaw provided only general guidance on flood construction levels not tailored to the local watershed and it's unique processes.

Project Timeline

The project commenced in 2018 and has since moved through the stages of data collection, hydraulic modelling, flood mapping, community review, and in 2020, the finalized report, maps, and models. A peer-review of the findings will take place in 2021.

During the community review stage, the ACRD engaged in a number of public information campaigns including presentation of draft findings on the website, social media postings, surveys, and a public information event hosted by Northwest Hydraulic Consultants (NHC) on March 10, 2020.

The final report was presented a first time to the Alberni Valley and Bamfield Directors Committee on November 4, 2020 and options for implementing floodplain mapping in the electoral areas were discussed at the January 27, 2021 Electoral Area Directors Committee meeting.

A peer-review process is underway Spring 2021 to ensure the methodology and research is sound and that the findings are accurate.

Next Steps

If purchasing or developing properties located within a designated flood area, flood impacts on development should be considered. The ACRD is considering options for adopting flood construction levels as part of the Zoning Bylaw or Floodplain Bylaw. In the meantime, any building permit for development within the flood risk area will likely be subject to an assessment from a geotechnical engineer to establish a flood construction level and to confirm whether the land is safe for the intended use as it relates to flood risk.

This site-specific assessment will take into consideration all available technical information and provide development recommendations regarding flood risk mitigation. A BC Land Surveyor may be required to confirm elevations and location of a building(s). The geotechnical assessment would be registered to the property title as a Section 219 restrictive covenant in accordance with the Community Charter.

Visit the ACRD Building Inspection page for more information on Building Permits.

Data Analysis

Through 2019, the project team captured data from water sensors, terrestrial and bathymetric surveys, hydrology analysis, and geomorphic assessments for use in the development of a hydraulic model of the rivers, lakes, and coastal areas within the Somass Watershed. The hydraulic model was used to develop the final floodplain maps and Flood Construction Levels. Key steps have included:

  • Surveying the rivers, surrounding land areas, and structures that affect how the rivers flow.
  • An assessment to chart how the river channels have changed over time and how that affects flows today and potentially in the future.
  • A hydrology assessment to confirm what happens to the rivers under different conditions, including extreme weather.
  • A coastal assessment to consider how local winds, tide water levels in Alberni Inlet, and lake levels may affect flooding now and in the future with consideration of climate change effects on precipitation and sea levels.
  • Creation of a hydraulic model of the study area that predicts how flooding could occur in a defined weather event. The model was tested and validated by qualified engineers to confirm it is able to reflect actual flood conditions.
Project Reports, Maps, and Videos

Visit the pages below to find:

Additional Background Resources
Contact
Please contact:
Alberni-Clayoquot Regional District
Planning and Development
250-720-2700
planning@acrd.bc.ca