6.3: Mitigating the Effects of Mass Wasting

We cannot prevent mass wasting, but in many situations, there are actions that we can take to delay it from happening. Where we cannot delay it, there are things we can do to mitigate its damaging effects on people and infrastructure. Where we can neither delay nor mitigate mass wasting, we should have the sense to stay out of the way.

Preventing and Delaying Mass Wasting

It is comforting to think that we can prevent mass wasting by mechanical means, such as the bolts in a road cut or by draining water out of a slope, or by building physical barriers. What we must remember is that the works of man are weak and pathetic things compared to the works of nature. The rock bolts in road cuts will start to corrode within a few years, and within a few decades many of them will lose strength. Unless they are replaced, they will no longer be supporting that slope. Drainage holes eventually get plugged up with sediment and chemical precipitates, and, unless they are periodically unplugged (which is difficult) their effectiveness will be reduced. Eventually, unless new holes are drilled, the drainage will be so compromised that the slide will start to move again.

The important point is that our efforts to “prevent” mass wasting are only as good as our resolve to maintain those preventative measures. We may have created systems to ensure that others will carry out that maintenance in future decades or even future centuries, but there is nothing to ensure that it will be done.

We must be careful to avoid doing things that could make mass wasting more likely. The most common anthropogenic cause of mass wasting is road construction, and this applies both to remote gravel roads built for resource extraction and to large highways. Road construction is a potential problem for two reasons. First, to create a flat road surface on a slope inevitably involves creating a cut bank that is steeper than the original slope and might also involve creating a filled bank that is both steeper and weaker than the original slope (Figure \(\PageIndex{1}\)). Second, roadways typically cut across natural drainage features, and unless great care is taken to reroute the runoff water, and to prevent it from pooling, oversaturation of materials can result.

Figure \(\PageIndex{1}\): An Example of a Road Constructed by Cutting into a Steep Slope, and the Use of the Cut Material as Fill

Apart from water issues, builders of roads and other infrastructure on bedrock slopes have to be acutely aware of the geology, and especially of any weaknesses in the rock related to bedding, fracturing or foliation. If possible, we should aim to avoid certain areas and build somewhere else—rather than try to stitch the slope back together with rock bolts.

It is commonly believed that construction of buildings on the tops of steep slopes can contribute to the instability of the slope. This may be true, but probably not because of the weight of the building. A typical house isn’t likely to be heavier than the hole in the ground that was made to build it. A more likely contributor to instability of the slope around a building is the effect that it has on drainage.

Monitoring Mass Wasting

In some areas it is necessary to establish warning systems so that we know if conditions have changed at a known slide area, or if a rapid failure, such as a debris flow, is on its way. Some areas are monitored 24-7 with optical devices. A simple mechanical device for monitoring a slide is shown on Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): Part of a motion-monitoring device at the Checkerboard Slide near to Revelstoke, BC. The other end of the cable is attached to a block of rock that is unstable. Any incremental motion of that block will move the cable, and this will be detectable on this device. (Author photo, CC BY 4.0)

Figure \(\PageIndex{3}\): Mount Rainier, from Auburn, Washington

Mt. Rainier in Washington State has the potential to produce massive mud flows or debris flows (technically lahars) with or without a volcanic eruption, and over 100,000 people in the Tacoma, Puyallup and Sumner areas are in harm’s way because they currently reside on deposits from past lahars (Figure \(\PageIndex{3}\)). In 1998 a network of acoustic monitors was established around Mt. Rainier. The monitors are embedded in the ground adjacent to expected lahar paths. They provide warnings to emergency officials, and when a lahar is detected the residents of the area will have anywhere from 40 minutes to 3 hours to get to safe ground.^[1]

It is critical to monitor the weather conditions and stream flows to assess changes to the risk of slope failure. Changing weather conditions can be failure triggers, especially in the following ways:

• extreme rain events saturate and weaken surficial materials (and even bedrock),
• extreme rain events lead to stream flooding and so to eroded banks, contributing to failures,
• sudden warming events can trigger rapid snow melt, increasing the amount of water in surface materials and streams, and
• persistent drought will dry out sandy materials making them more prone to failure.

Mitigating the Impacts of Mass Wasting

In situations where we can’t predict, prevent, or delay mass wasting hazards, there are some effective measures that can be taken to minimize the associated risk. For example, many highways in temperate mountainous regions have avalanche shelters like that shown on Figure \(\PageIndex{4}\). In some parts of the world similar features have been built to shelter infrastructure from other types of mass wasting.

Figure \(\PageIndex{4}\): A Snow Avalanche Shelter on the Coquihalla Highway. The expected path of the avalanche is the steep un-treed slope above.

Finally, in situations where we can’t do any of these things, we simply have to have the common sense to stay away.

Media Attributions

• Figure \(\PageIndex{1}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{2}\): Photo by Steven Earle, CC BY 4.0
• Figure \(\PageIndex{3}\): Mount Ranier from Centennial Viewpoint by Ron Clausen, 2017, CC BY SA 4.0, via Wikimedia Commons, https://commons.wikimedia.org/wiki/F...Washington.jpg
• Figure \(\PageIndex{4}\): Photo by Steven Earle, CC BY 4.0

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1. Monitoring Lahars at Mt. Ranier, US Geological Survey, https://www.usgs.gov/volcanoes/mount...-mount-rainier ↵
2. Moore, D. P. (1976). The Rubble Creek landslide Garibaldi, British Columbia (Thesis, University of British Columbia). https://open.library.ubc.ca/collecti...tems/1.0052710 ↵