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QuakeImpact

The shaking

Dikes

A dike keeps the river and the sea out of land lying lower than they are. On the Fraser delta, the dike and the ground beneath it can both move during an earthquake.

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Delta dikes stand on liquefiable soil

A dike is an earth embankment that keeps the river and sea away from land lying below them. On the Fraser delta, dikes stand on loose, wet sand that can lose its strength during an earthquake. The embankment and its foundation can settle or spread sideways at the same time.

This does not mean every dike will fail. Performance depends on the ground, the shape of the riverbank and how the particular section was built. No assessment has established how the region’s dikes as a whole would perform.

Richmond’s results apply to the sections it tested

Richmond Fire-Rescue tells residents that computer models predict some earthquake damage but that the dikes will remain an intact barrier to flooding. The City’s engineering reports give a more limited result.

A 2016 analysis tested three sections of the Lulu Island dike in an earthquake expected about once in 2,475 years. It estimated 600 mm of vertical movement at No. 1 Road, 1,000 mm at Bath Slough and 500 mm at No. 4 Road. The provincial guideline allows 500 mm, so two sections exceeded it and one reached it. Sideways movement remained within the guideline at all three sections.

Three tested sections in the earthquake expected about once in 2,475 years. The findings do not apply to untested sections.

The estimates do not include about 0.3 m of additional settlement as liquefied soil packs down again. The report also warns that movement of several metres could occur where liquefaction is widespread and the riverbank is steep.

Later master plans reached the same conclusion for proposed dike designs: they would not meet provincial earthquake requirements without ground improvement or another approach. The estimated cost of densifying the ground was $9,000 to $18,000 per metre of dike.

These findings are not a survey of Richmond’s entire dike system. Eleven sections were analysed across four phases, and the reports say their results apply only where the tests were done.

Sea Island’s dike project addresses sea-level rise

Sea Island, where Vancouver International Airport sits, is ringed by a 15 km perimeter dike. The dike is being raised by about a metre to a total height of 4.7 m, along with the pump stations behind it.

That work responds to rising sea levels and flooding. Ground stability is part of the project, but it is not a seismic upgrade.

What you can do

  • Find out whether you live behind a dike. Municipal flood maps and emergency information show the protected areas.
  • Ask what has been assessed on the section near you. Richmond publishes engineering results for named sections. A resident of another municipality can ask for the same information. Results from one section should not be assumed to describe another.
  • Look up the ground beneath the dike and the area behind it. The official neighbourhood maps show liquefaction susceptibility across the western municipalities. Ground conditions explains how the ground changes the shaking and which municipalities are not mapped yet.
The full preparedness plan

Sources on this page

Numbered in the order they are cited. Each entry links back to where it is used.

  1. [1]ReportHolocene to modern Fraser River delta review, Canadian Journal of Earth SciencesCanadian Science Publishing · 2024Open the document ↗
  2. [2]ReportRichmondRichmond Fire-RescueThe liquefaction and dike-stability statements, and the Home Hazard HuntOpen the document ↗
  3. [3]ReportThurber Engineering Ltd., Seismic Deformation Analysis, Attachment 5 to the Lulu Island Dike Master Plan Phase 2Thurber Engineering for the City of Richmond · 2016PLAXIS 2D with the UBCSand model; vertical deformation exceeds the provincial 500 mm criterion at two of three sections at the 2,475-year event; flow slides "of several metres or more" where liquefaction is extensive. Results "only applicable at the sections analysed"Open the document ↗
  4. [4]ReportDike Master Plan Phase 3 (Kerr Wood Leidal)City of Richmond · 2019Proposed cross-sections "will not meet the performance requirements of the BC Seismic Design Guidelines for Dikes … without ground improvement"; densification costed at $9,000–$18,000 per lineal metreOpen the document ↗
  5. [5]ReportDike Master Plan Phase 4 (Kerr Wood Leidal)City of Richmond · 2021Same finding; liquefaction hazard "moderate and high for the 475 and 2,475-year return period events respectively. The resulting deformations would be large"Open the document ↗
  6. [6]ReportDike Master Plan Phase 5, Sea Island and Mitchell Island (Kerr Wood Leidal)City of Richmond · 2019Same finding; "liquefaction may result in a flowslide into the river for dike alignments along the river-bank due to lateral spreading"Open the document ↗
  7. [7]ReportSea Island Climate Resilience ProjectVancouver Airport AuthorityConfirms the 15 km perimeter dike, the raise of about a metre to 4.7 m, and pump-station upgrades. The pump-station count of eight is unconfirmed, and the ~$25M conflicts with a ~$12M figure published elsewhereOpen the document ↗
  8. [8]DatasetMetro Vancouver Seismic Microzonation Mapping ProjectWestern University / ICLR, with BC EMCR · 201729 Phase 1 maps published 2024; Phase II (Pitt Meadows, Maple Ridge, Langley) to late 2026Open the document ↗