Why are we updating flood mapping for the Lower Chemainus River floodplain?

    Communities in the Lower Chemainus River area have a history of flooding, property damage, and transportation impacts from winter storms. Updated flood mapping is a critical step for mitigation and adaptation planning to reduce flood impacts. The previous floodplain maps for this area were prepared in the early 1990s and did not account for current issues like sea level rise or climate change. An update is needed to recognize changes in the floodplain area, development, new knowledge, and improved technology.

    The CVRD has partnered with the Municipality of North Cowichan, Halalt First Nation, and Penelakut First Nation to update floodplain mapping and develop a flood management plan.

    How was floodplain mapping developed?

    The process for creating floodplain mapping involved the following steps:

    • Surveying the rivers, surrounding land areas, and structures that affect how the rivers flow
    • Assessing how river channels have changed over time and how they might change in the future
    • Assessing how rivers flow under different conditions such as extreme storm events
    • Assessing how coastal impacts, such as winds and tides, affect flooding
    • Developing a hydraulic model of the study area to predict how flooding could occur in a defined weather event (i.e., 200-year flood)
    • Using the model to create floodplain maps

    How can a flood management plan help our community?

    A Flood Management Plan can help:

    • Identify areas at risk of flooding and erosion
    • Manage land development and infrastructure to help reduce future flood impacts
    • Enhance the river and riparian environment
    • Increase public safety
    • Inform emergency response planning
    • Identify mitigation options to reduce flood and erosion risk
    • Inform policies for future land use and development

    Why do areas in the Lower Chemainus River watershed flood?

    Rivers are part of our urban and rural areas that have been developed over time. When we build human uses near natural features, impacts can occur. In recent years, flood risk has been increasing due to:

    • Ongoing development in low-lying areas
    • Increased precipitation due to climate change impacts
    • Hot, dry summer periods affecting soil conditions and making land more susceptible to flooding and erosion
    • Loss of vegetation which helps slow runoff
    • More impermeable areas which intensify stormwater runoff

    Flooding in the winters of 2020 and 2021 have affected homes, businesses, roads, and major infrastructure, underlining the importance of creating a strategic flood management plan.

    What are some potential options to reduce flood risk?

    Flood mitigation options must be sustainable, cost effective, and technically feasible, while bringing community benefit and ensuring impacts are not shifted to other areas of the floodplain. Some examples of options that may be explored include: 

    • changes to physical infrastructure (e.g., roads, railway, bridges)
    • river restoration and enhancement
    • land use and planning policy to reduce development risks
    • alternate construction methods for buildings in flood-prone areas
    • managed retreat to reduce key infrastructure in flood-prone areas
    • emergency preparedness planning

    In the next steps of the study, potential mitigation options will be identified. Please share your suggestions of mitigation options that may warrant consideration by completing a comment form.