Dikes around Duncan did indeed protect against catastrophic flooding
In response to Peter Rusland’s letter published last week online entitled “Past diking projects didn’t prevent flooding,” I want to offer the following information.
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 this month’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; in other words, the dikes and flood pump stations are working exactly as they were designed to.
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:
• There are three gaps to accommodate traffic and roads: at Canada Avenue, the Trans-Canada Highway, and at the bottom of Lakes Road hill. 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 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.
It’s also important to note that this month’s rain event was classified as a one in 50 year event, with the Cowichan River reaching a peak flow of 575m3/s. During the significant flooding we experienced in 2009 (where about 300 homes were evacuated in the urban core), the flooding was classified as a one in seven year event, with river flows at 445m3/s. In other words, we had more water this time than in ’09, but we got through that with no major flooding in the urban core. Again, evidence that the flood protection system did its job.
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 would have been 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.
As to the localized flooding in the Canada Avenue area, the solution 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, in part because 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. Alternatively, we could extend the flood wall to the north, but again, the poor soil conditions in the area make that option very costly. As a result, we must be careful in how we attempt to address these issues with the 2020 Canada Avenue Flood Gate Project.
In summary, we don’t build dikes in areas where there is a low cost-benefit. The flood protection system has been built to protect homes and people, rather than to prevent occasional high water on our road network, especially where alternate routes are readily available.
As to the reference to the new building off York Road, that one is fully protected by the diking system. Our Engineering department reviewed the proposed development to ensure that living spaces were constructed above the 200 year flood level, which is standard practice. As a matter of OCP policy, we strongly discourage 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, just go to www.northcowichan.ca/floodprotection.
Further information is also available from our Engineering department at engineering@northcowichan.ca or 250-746-3103.
Al Siebring, mayor
North Cowichan
