Good vegetation cover slows down the rate at which rainwater filters through the ground. But even this natural protection can be overwhelmed by intense rain, causing dramatic landslips like the recent example from Corrie Fee national nature reserve. Ewen Cameron and Ness Kirkbride reveal why in today’s blogpost.
The dramatic Corrie Fee landslips aren’t as rare as we might wish. Elsewhere in Scotland, landslips have blocked roads, watercourses and damaged property. With even more intense rain predicted as part of climate change, we will need to adapt and re-think some things we have long taken for granted.
The landslips shown above and they have been used by Quaternary researchers (who study long term climate change) as environmental archives, which can be used to interpret periods of past intense rainfall.
Debris cones are built up over time by repeated debris flows, rockfall. and occasionally snow avalanches. They occur all over Scotland, even our Sites of Scientific Interest in Glen Coe and Glen Feshie are not immune.
Individual debris flows are caused by intense rain saturating the ground, and in Corrie Fee they occurred within the confines of the rocky gullies. They need two things:
- sufficient silt, clay, and rock (not just rockfall) , and
- enough water delivered in one go, to fail and flow like a slurry of wet concrete.
Over time debris accumulates in the rocky gullies, and periodically heavy rain can result in a wet slurry type of slope failure we call debris flows. These flows can move fast, and have sufficient shear strength to transport very large boulders. The Corrie Fee debris cones will have built up over many thousands of years, from debris flow events like these.
Individual debris flow events bury soils and vegetation, and these organic layers can be radiocarbon dated. Debris cones are like peat bogs, as they are environmental archives. During the last 10,000 years there have been periods of frequent debris flow activity on cones, during wetter periods of climate history.
Debris flows originating from gully systems are not usually associated with grazing pressure, (unlike debris flows on open slopes). Rock fall, and more importantly smaller scale granular weathering of the bedrock and soil forming processes accumulate a mix of debris in the gully, When enough debris has collected all it takes is one sufficiently intense rainfall or snowmelt event to saturate the weathered debris in the gully floor. This can then start to move very quickly down the slope, initially collecting more debris by eroding a path and pushing some of the larger debris out at the sides leaving a track of abandoned debris on either side of its path down the hill, before slowing down and depositing a lobe of debris when the slope angle reduces and the water escapes from the rubbly mix. So, debris cones like the ones at Corrie Fee are good places to look at long term histories of extreme rainfall related landslides.
The photo shows there were several stages to these recent debris flows. The initial slurry like failures are thick and move like poured wet concrete, and then in the case of the left cone, water has eroded and reworked the lower part of the new debris flow lobe (its lobe front has been washed out).
This suggests that as the storm progressed there was not enough debris flow susceptible material in the left gully to continue to supply debris flows down the gully onto the cone. So the later part of the storm water eroded a section of the cone and created two nested mini alluvial fans of debris at the lower end of the left debris cone.
The other thing to note is that the largest run-out and sandy wash out has run over the area where pollen records were taken for the Quaternary of Scotland GCR site. Elsewhere where this type of wash out has occured in the past, you can find interbedded layers of peat and water deposited grit and gravel. These too can be used to look at past geomorphological events, and make inferences about past weather events like floods and debris flows.
Find out more about Scotland’s landslides here:
There is more to these landslip events than meets the eye !