The probability of a flood is typically expressed in one of two ways. A 100-year flood is a flood that has an estimated probability of occurring once in one hundred years. This can also be expressed as a one percent chance of occurring in any given year – or 1% Annual Exceedance Probability.
In British Columbia’s Lower Mainland, the large-scale floods most likely to impact multiple communities at once are:
- Fraser River flood, during spring freshet (snowmelt) and
- Coastal flood, during winter when storm surges combine with high tides
Floods can be caused by a number of processes. For example, local flooding can arise from heavy rainfall events that exceed the capacity of drainage systems or that cause nearby creeks to fill and overtop their banks. As reflected in the November 2021 flooding in the Fraser Valley and other locations, extremely heavy rainfall leads to rivers or other large waterways overflowing or flood infrastructure failures. See:
- a large snowpack on mountains of the Fraser Basin
- a cool spring followed by high temperatures that accelerate snowmelt
- episodes of heavy rainfall.
Climate change is expected to increase the risk of high spring temperatures and the potential of faster snowmelt as well as rain during the spring freshet.
1894 – Largest Fraser River flood of written record
1948 – Second largest Fraser River flood of written record. Caused loss of 2,300 homes and forced 16,000 people to evacuate. 10 residents drowned and 22,000 hectares of land were engulfed in water. As a result of the flood, Vancouver didn’t have a rail connection to the rest of Canada for three weeks.
Learn more about Fraser River Flood History.
1972 – Third largest recorded Fraser River flow at Hope, raising concerns of a large flood
2021 – Fraser Valley (Sumas Prairie) flood from flooding of the Nooksack River in Washington State
The BC River Forecast Centre analyses snowpack, assesses seasonal water supply and flood risk, and monitors and forecasts flows in British Columbia’s rivers and streams through modelling. Based on available data and modelling, the Centre issues high streamflow advisories, flood watch advisories and flood warnings. Learn more on the River Forecast Centre website.
The BC Storm Surge Forecasting Program has also been developed to provide forecasts for coastal storm surge events along the south coast of BC. This has operated between October and March in recent years.
Flooding is a natural process. With the exception of dams and storage reservoirs, there is very little we can do to prevent high water events on streams, rivers, lakes or the ocean. Therefore we need to focus on reducing the impacts, consequences and risks associated with flooding.
Flood risk consists of both the chance of an event happening and its potential impacts. Land use planning informed by floodplain management plans can reduce flood risk for new development areas. Flood risk is more difficult to manage in existing developed zones; however structural flood protection measures can reduce flooding of these areas. Similarly, some modifications can be made to individual buildings or properties. Education, preparedness, response and insurance measures can all help communities to reduce risk and be more flood resilient. Learn more under Reduce Flood Risk.