Reducing the risks of flooding through science, sound planning, and strategic investment

By District Staff on Thursday, Oct 19, 2017

WATCH | View this video to get an overview of the work we're doing to reduce the risk of flooding caused by debris flows 

The DNV is one of the world's best places to live, work, and play, due in no small part to our beautiful natural setting — mountain vistas, towering trees, ocean playgrounds, and streams and creeks brimming with life.

But all of this beauty comes with a certain level of risk.

After all, mother nature is quite determined to slowly but surely wear down the North Shore mountains and wash them into the sea, using our abundant winter rain as her primary tool. And that means an ever-present chance of flooding.

So how are we managing the risks that come with living in a rain forest, and protecting everyone against the impacts of natural hazards, such as floods?

With science, sound planning, strategic investment, and a lot of hard work.

Managing the effects of debris in streams

In 2014, following unusually high rainfall, several creeks across the District — including Kilmer, Thames, and Gallant — overflowed, damaging over 100 homes, businesses, and a school.

While drainage infrastructure, such as culverts, existed in all of these areas, those systems were built to handle clear water floods, not flood waters filled with debris, such as fallen tree limbs, rocks, or mud.

It's the debris in the water that blocked culverts and other drainage systems, and caused creeks to overflow onto surrounding streets.

How the debris flow study worked

Given that floods caused by debris are becoming increasingly likely due to the impacts of climate change, we quickly moved to re-evaluate all of our creeks, assessing them for debris flow hazards.

We hired BGC Engineering Inc to complete a full Debris Hazard Risk Assessment for 35 debris flood-prone creeks across the District, and recommend ways to reduce the risks.

To conduct the study, BGC divided our creeks into two types: Urban creeks (those creeks accessible by road and serviced by our stormwater infrastructure) and Indian Arm creeks (all creeks on Indian Arm north of and including Sunshine Creek).

For the urban creeks, they:

  • assessed the potential frequency, size, and extent of debris flows
  • estimated the risk posed by these hazards to people, buildings, and infrastructure
  • prioritized locations for specific actions to reduce risk
  • identified potential risk reduction actions, along with cost estimates

With the study results and recommendations in hand, staff then developed a long-range implementation plan to address the 15 creeks most in need of risk mitigation.

Putting our implementation plan into action

Work on our three highest priority creeks — Kilmer, Thames, and Gallant — has already been completed or is underway.

For each of these three creeks, we turned to the same strategy: debris basins.

Simply put, a debris basin traps debris such as rocks and tree limbs, allowing the water to flow unimpeded through the storm drainage network.

All of the debris that is washed down the mountain side during heavy rain is caught by a straining structure, and held in the basin. Once the heavy rain event has ended, staff can easily use machines to remove and dispose of all of that debris, ready for the next heavy rain.

debris flow diagram


Looking to the future

We invested approximately $2.5 million in reducing the risk of flooding along our three highest priority creeks, with partial funding provided by Build Canada.

We estimate that the total cost for all mitigation measures for all high-priority creeks will be approximately $7 million.

The remainder of the work will be prioritized and considered for funding during our annual budget process, and within our available resources, and staff will continue to apply for senior government funding, wherever possible, to offset the direct costs.

Given our climate and natural surroundings, it's unrealistic to think we can eliminate the risk of flooding entirely, but we are hard at work increasing the safety, and reducing the risks as much as possible.

Want to learn more?

If you're interested in seeing a debris flow basin in action, take a trip to the trailhead at the end of Braemar Rd E., where we have recently installed one along Kilmer Creek.

Or visit our website to learn more about how we manage extreme weather.

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