Flood Modelling and Mapping in BC’s Lower Mainland

Due to observed and expected future changes in snowmelt, precipitation patterns and sea level rise, in 2019, the Fraser Basin Council (FBC) undertook the Lower Fraser River 2D Flood Modelling and Mapping Project, a major component of the Lower Mainland Flood Management Strategy (LMFMS). The LMFMS is a collaborative initiative with the participation of over 60 governmental and non-governmental agencies that aim to reduce risk and strengthen resilience to Fraser River and coastal flooding in the Lower Mainland region. The multi-year undertaking is divided into three phases:

  • Phase 1: Understanding Lower Mainland Flood Risks
  • Phase 2: Building a Region-Wide Strategy, including the Lower Fraser River 2D Flood Modelling and Mapping Project
  • Phase 3: Taking Action

The Lower Fraser River 2D Flood Modelling and Mapping Project identifies the geographical boundaries and flood depths of a flooding event based on type and likelihood of occurrence. In addition to 14 base run scenarios (which included several climate change scenarios), 8 dike breach scenarios and 5 mitigation options were explored (four were modelled): dike raising, dike setbacks, sediment removal, land raising and upstream flow storage. The resulting maps can be used to help identify the structures, people and assets that are within flood zones, informing proactive flood risk reduction as well as emergency planning. The Lower Fraser River 2D Flood Model can be used by authorities to conduct additional runs and produce additional mapping suited to their needs. The project will help those with responsibilities for flood management to better understand current and future flood hazards, advance work on risk assessment to help prioritize high-risk areas, evaluate flood risk reduction options, and enhance emergency preparedness and response planning.


Identifying Actions

The Lower Fraser River 2D Flood Model projects the extent, depth, and velocity of water in the Lower Fraser River channel and on the floodplain (which is normally dry land) for floods of different sizes. The model covers a 170 km stretch of the Fraser River from Hope to the Salish Sea (Strait of Georgia). The model can also be used to show dike breach scenarios and the impact of possible flood mitigation options.
For this project in Phase 2 of the LMFMS, 27 flood scenarios were produced. Three simplified maps were developed, and 24 scenarios were simulated by the flood model: 14 base run scenarios (2 with simplified maps only); 8 dike breach scenarios; and 5 scenarios on the effects of different flood mitigation options (four of the five options were modelled).

Some of the model findings include:

  • Many areas not currently protected by dikes will be flooded by even a relatively small flood event, and the potential extent and depth of flooding in these areas increases with climate change.
  • The total area of floodplain flooded, and number of dikes overtopped increases significantly as the freshet flood scenarios become more severe.
  • Setting back new dikes from their current location to create more “room for the river” in flood events may also reduce flood levels in some locations.
  • The river channel will undergo substantial change over time as it adjusts to the new, climate change-driven flow regime. There is uncertainty in projecting specific flood water levels, but flood water levels are expected to increase substantially in the future.
  • Due to sea level rise, the “transition point” between coastal- and river-dominated flooding will likely shift upstream.

One of the primary objectives of the LMFMS flood modelling and mapping project was to support planning and decision-making related to various flood mitigation options. By simulating mitigation options in the model, their effectiveness can be assessed, and promising options selected for further investigation. Flood mapping information and data will be used to determine a community’s (and region’s) vulnerabilities to multiple flooding scenarios, which can in turn, inform a roadmap for action, including specific mitigation projects. The flood model and maps provide evidence on the nature, extent and distribution of flood hazards and risks across the region, as well as a compelling rationale to take action.

Implementation

The model results were used to support a Lower Mainland Flood Risk Assessment, which can help identify priorities for flood risk reduction at a regional scale. The Fraser Basin Council, as facilitator of the strategy development process, is working with partner organizations to develop the strategy as part of Phase 2. FBC will continue to engage with representatives from partner organizations to further explore the results of modelling and options going forward. To date, multiple jurisdictions in this region have accessed the model and the mapping and other outputs to support their planning and other flood management activities.

Outcomes and Monitoring Progress

The Fraser Basin Council has received preliminary input on a first draft of the LMFMS from federal, provincial, local and First Nations governments as well as several infrastructure and other regional entities. Many have been active participants throughout Phase 2, by providing financial support and/or expertise and other in-kind support. Discussions and project work will continue to support a second draft.

Next Steps

An update of the flood model was undertaken in 2022 to ensure ongoing accuracy and utility. Additional model runs are being conducted to account for updated climate change information and other scenarios of interest. Additional model runs can help decision makers explore how flood risk reduction options – such as different types of flood infrastructure or land use policies – perform when implemented at a regional scale, separately or in combination. This and other model applications will be explored and implemented over the coming years.

Resources