I was surprised that a house was built in the path of the mud slide in British Columbia early this month, July 4th.  Built right in a mud channel.  Particularly in view of the fact that the Ministry of Transport reported that more slides were possible – they knew. (Ref. 1) Why were houses approved for construction in such an unstable area?  Who is liable?

Assessing the potential for a mud slide in an area and quantifying the risk for residents is not rocket science.  Someone had done sufficient assessment to warrant the report of the Ministry.

You don’t see the messy stuff mentioned as such in engineering books but it’s there.  It’s classified as very soft clay and silt with a little sand and gravel and a few cobbles.  The analytical procedures are in the books too – reliable slope stability methods of analysis understood by experienced engineers.

So, what’s the big surprise about the mess in BC and why were people allowed to build there?  We’re not talking something small here.  We’re talking about a slide that engulfed a house to a depth of several feet according to pictures on line.  And moving fast too as seen in one video of a fellow running out of the way just in time.

You don’t need a lot of data to do such an analysis either and the data is readily available in the public domain:

• The history of mud slides in the area and the rain fall at the time
• Topographic maps to give you the shape and slope of the ground, and the location of mud channels
• The results of terrain analysis identifying features relevant to mud slides, and evidence of past events (screen grabs from video taken from low flying drones has had a big impact on the engineering analysis of terrain in an area)
• Soil maps (surficial geology maps) to tell you that a mixture of clay and silt underlies the area

Engineers have done this type of assessment often, and everywhere throughout North American and around the world.  And based on the news and Transportation’s report, likely for this area as well.

So why the surprise?

Why would a person even mistakenly build a house in a mud channel?  I can’t help but think there would be local knowledge that would kick in even if the government was silent.  Would you build a house in a river channel or on a flood plain between wet seasons?

Reference

1. Notes taken from a CBC News report online.  “Debris flood, a mix of mud, gravel and cobbles 200 km east of Prince George, BC in a flood prone area 1:30 Saturday morning July 4, 2020.  Waist deep.  Second one that day nearby.  Ministry of Transport said more likely in the area”