Flood Protection System Prevents Flooding of Urban Core Area

During heavy rain events, we often hear from people who believe that the dikes and pump stations were a waste of money or that they are not working to prevent flooding. Over the last decade, existing dikes have been raised, new dikes constructed, new pump stations built, and gravel and log jams regularly removed from the Cowichan River.

The flood protection system has been designed to keep floodwaters from the Cowichan River and Somenos Basin from flooding into the urban core area; which, during last week’s rainfall event, they did. The flood pump stations located along the dikes are intended to pump accumulated rainfall runoff from within the urban core area over the dikes; which they did. The dikes and flood pump stations are working just how they were designed to work.

The flood protection system also contains some gaps that are required to accommodate existing roads, and there are a few creeks and streams outside of the flood protection works that are known to overflow during heavy rain events, causing some expected localized flooding.

During extreme rain events we expect, and prepare for, localized flooding in these known areas:

  • The system has three gaps to accommodate traffic and roads: at Canada Avenue, the Trans-Canada Highway, and Lakes Road. There is a procedure in place to systematically close these gaps as waters rise.
  • The area outside of the dike network is susceptible to flooding from streams or rivers that overflow. These locations can include but are not necessarily limited to the following locations: Mary Street/Philip Street, Rosewood Avenue, and Seine Road. 
  • Finally, culverts and drainage systems, including even those protected by the dike network, can become blocked preventing water from flowing into storm drains and towards the flood pump stations. Also, relatively flat areas protected by the dike network may still experience ponding if the water is not able to drain away to a storm drain or ditch.

 Some facts about last week’s flood and the 2009 flood:

  • Last weekend’s rain event was a major event, and was classified as a 1 in 50 year event, with the Cowichan River reaching a peak flow of 575m3/s
  • The significant flood in 2009, where about 300 homes were evacuated in the urban core area, was classified as a 1 in 7 year event, with river flows at 445m3/s

Water levels in the Lakes and Beverly area during last week’s flood were about 0.1 m higher than during the 2009 flood. Despite the higher flow in the Cowichan River and the higher flood levels within this area, there was no major flooding in the urban core area because the flood protection infrastructure protected the areas it was designed to protect.

In regards to the RCMP detachment, the flood protection works were never intended to protect the detachment or prevent localized flooding on Canada Avenue. It was too costly to extend the flood protection works up to the RCMP station, as it would require an extremely large pump station to essentially pump all of the rainfall runoff flowing down Holmes Creek.

Engineering a solution to the localized flooding in the Canada Avenue area is complex and very costly. The cost to raise the road to the 200 year flood construction level and replace the bridges is extremely high. Also the soils in that area are poor and the additional weight associated with raising the road could cause significant damage to the pipes buried within the road dedication. Raising the road could be avoided by extending the flood wall to the north but the poor soil conditions in the area make that option very costly as well. This is why the flood protection works in the Canada Avenue area not extended further north. However, the Municipality is looking at options to mitigation measures to reduce the frequency of flooding for parts of Canada Avenue north of the floodwall.

The Municipality does not seek to build diking where there is a low cost-benefit. The flood protection system has been built where there is significant improvements to protect which is why the focus was on protecting the urban core area. The flood protection works not only protects our urban core, but they also allow municipal crews to focus their efforts on mitigating flooding in other locations in the Municipality.

The new building off York Road is protected by the flood protection system. The Engineering department reviewed the proposed development to ensure that living spaces were constructed above the 200 year flood level, which is standard practice. The Municipality strongly discourages building in the floodplain outside of areas protected by the flood protection works.

There is extensive information on our website about the flood protection system, so that community members can learn more: Flood Protection. Should you have further questions about the dike network or pumping system, we encourage you to get in touch with our Engineering department at engineering@northcowichan.ca or 250-746-3103.

For more information:
Engineering Department