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Coastal Flood

BC’s Lower Mainland is at the edge of the Salish Sea on the Pacific Ocean and is exposed to coastal floods, primarily in winter. Coastal flooding occurs when water levels in the ocean are higher than normal and ocean water covers adjacent land that is normally dry. In the region, such flood events could last from hours to days.

Higher than normal ocean water levels are generally a result of storm activities (storm surge, wave effects and wind) in combination with tides. Sea-level rise (SLR) will increase coastal flooding in the future, changing the current occasional flooding to more regular flooding.

Some components of ocean water levels are predictable (deterministic), such as tides. Other components are less predictable (probabilistic) and have varying likelihood of occurrence. Experts look at the combined effect on total water levels. Here’s a look at the components.

Components of a Coastal Flood

Total water level in a coastal flood = Tide + Surge + Waves + Set-up

Tides (Deterministic)

Tides are the periodic rise and fall of the ocean surface. Tide levels vary throughout the day, but are also subject to longer-term cycles caused primarily by the relative positions of the sun, moon and Earth. The maximum tidal elevation occurs once every 18.6 years in BC, but the level comes close to this for a few tides each year. These infrequent (annual) large tides are often referred to as king tides.

Storm Surge (Probabilistic)

A storm surge is a localized increase in water levels due to low-pressure systems in the atmosphere (storms). As these systems move from the Pacific into coastal waters, the localized low atmospheric pressure on the ocean causes the ocean water levels to rise.

Wave Effects (Probabilistic)

Large waves that break on the beach cause both a static increase in water level (wave set-up) and a dynamic, oscillating variation in water level (wave runup). <

Wind Set-Up (Probabilistic)

Wind set-up is associated with strong local winds blowing over shallow water towards the land. This wind results in a localized increase in the water level as the water is “piled up” against the shore.

Climate Change and Sea-Level Rise

Around the world, sea levels are rising due to melting ice caps and glaciers and the expansion of ocean water caused by warming. The impacts of sea‐level rise and coastal flooding pose threats to coastal communities, populations, infrastructure, economies and ecosystems.

Variations in local sea-level rise occur due to differences in topography, gravitational forces and ocean currents. The west coast of North America generally experiences lower-than-average global sea-level rise rates. In addition, geological processes can cause land to shift down (land subsidence) or up (land uplift) over time. When these considerations are combined with sea-level rise, this is referred to as relative sea-level rise (RSLR).

Relative sea level rise in a given area takes into account mean sea level rise (influenced by melting ice and the expansion of warming waters) and the vertical motion of the land, both subsidence and uplift.— based on information from the Union of Concerned Scientists (2015).

While sea levels are rising, there is high uncertainty in projections about the rate of change. For example, in 2014, the Intergovernmental Panel on Climate Change projected a sea-level rise of less than 1 m by 2100, but a more recent study projected a sea-level rise of several metres over 50 to 150 years. The more recent study accounted for the melting of the Greenland and Antarctic ice sheets, which has already begun.

The Government of BC at present uses 1 m of sea-level rise by 2100 and 2 m by 2200 to guide planning processes.