Three Sliding Houses EarthCache
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When you arrive at this location, you will notice three driveways leading off the edge of the cliff.
Before October 1999, three homes with a spectacular view of the North Saskatchewan River existed here. On October 23 of that year, this changed, when a massive landslide caused these homes to slide down the bank into the valley below. How did this happen? Hopefully this Earthcache will shed some light on this unfortunate saga.
The bluffs beneath Whitemud Road lie at the outside of a meander on the river. This river bend is the feature from which the “Riverbend” neighborhood takes its name. It is also the feature that continues to threaten homes along the bluffs. At the outside of a meander, a river flows at its fastest. The fast moving river has more energy, and the erosive power of the flowing water increases. This can lead to Bank Scour, the removal of a riverbank by water and the materials it carries. Where bank scour occurs, Mass Failure is a potential outcome. In the case of this location, the Mass Failure brought down three homes with it. A Mass Failure is an event where sections of an eroded bank slide, slump or topple into a river or stream. Where a bank has no vegetation, consists of sandy material, or is high and steep, the likelihood of mass failure is even greater.
When the landslide took place, it was not a uniform event. Firstly, a block of land fell into an 18-meter deep graben (a graben is a depressed block of land). This took place almost directly behind the home of Mr. and Mrs. Skinner, which was the middle of the three destroyed homes. This left two margins of land on either side of the “hole”. These points were very unstable. The first one to collapse was the North side, followed shortly thereafter by the South side. While the North side fell because the original slide had undercut this margin, the South Side of the graben collapsed because of immense groundwater flow. This was followed by several smaller slides that left the landscape much like it appears today.
A second factor was also found to affect the bank stability. Large amounts of groundwater flowing towards the river further weakened the stability of the bank. This continues to be a problem for nearby residents. An underground stream about halfway down the bank continues to weaken the slope, and neighbours have installed wells to limit the flow of groundwater. The specialized equipment diverts 250,000 liters of water a day to storm sewers and into the river. There are currently twenty-five homes on this street, and nearby 154 Street (to the North) that remain at risk. Currently four of these homes are uninhabitable because of the imminent danger of another mass failure event. The city engineering department monitors the bank stability with the red markers (see right) that are very present in this area. These sense ground movement, and provide signs if another event may be forthcoming. In the 1970’s, a smaller slide took place along 154 Street, one block North of this site. No property was damaged and no one was injured. The slide is still visible if you drive along this street. Perhaps this should have served as a warning that this was not a suitable building location?
Logging Requirements:
You will need to show that you learned something at this site. I wouldn’t want you not learning anything, and then going and building a house at a dangerous location! This is also a requirement of the Earthcache program.
Please answer two of the following questions:
1) Based on the neighbours’ current bank, estimate how much land was lost in the 1999 slide? (How far from the current bank to the former bank location?)
2) Based on your knowledge from this Earthcache, provide another location in the City of Edmonton that might be prone to bank erosion. You may provide a Lat/Long, or a description (ie. end of X Street, in X Neighborhood)
3) How wide do you estimate that the current graben is? Remember, a graben is the block of land that is depressed from the surrounding land.
You must also post a picture with your log. Don’t get too close to the edge though!
Update: Earthcache guidelines no longer allow a photo requirement in order to log an earthcache. However, I enjoy looking at all the interesting photos that get uploaded on this cache, so I really encourage you to do so!
References:
Edmonton Journal, April 9, 2008 “Owner defiant after order to leave endangered home”
Peter Barlow, et al, “Setbacks in Edmonton, Alberta, Canada”, Instability Planning and Management: Seeking Sustainable Solutions to Ground Movement Problems. (Telford: 2002) at 75.
Bowes v. Edmonton [2007] C.C.S. No. 9138- Lawsuit vs. city with many geotechnical details.
Additional Hints
(Decrypt)
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