The Landslide Blog is written by Dave Petley, who is widely recognized as a world leader in the study and management of landslides.
BBC News ran an item yesterday about the Snake Pass, the major road that links the two major northern English cities of Manchester and Sheffield. The Snake Pass is a beautiful highway, passing through the Peak District National Park on a route that is winds along the valley floor before crossing the hills and then descending into Glossop.
The article highlights the threats to the road posed by landslides, which have become increasingly common. A key section reads as follows:-
“But closures due to landslips are becoming so frequent that Derbyshire County Council has warned it cannot afford to keep repairing the road, and has asked for government help to keep it open.”
…
“Derbyshire County Council said it was increasingly having to resurface Snake Pass following landslips, but it does not have enough money for bigger repairs.”
I have written before about the challenges on this road, but high quality Google Earth imagery highlights the issues. This is one section of the road, located at [53.40238, -1.78598]:-
In the foreground you can see a section of the road that is closed off and controlled by traffic lights. This image was taken in 2022, but the situation remains to this day. Clearly this section of the road has slipped due to erosion by the river. However, it’s also evident that the road is crossing a much large landslide system, with a steep back scarp beyond the trees. On this image the landslide does not appear overly concerning (but see below).
Just around the corner there is another section that had similar issues in 2022:-
Note the slightly odd, hummocky topography, which to any engineering geomorphologist spells trouble. Elsewhere the problems are less obviously acute, but there are clear signs of running repairs to the road, which has had to be resurfaced in short sections:-
These images clearly show problems, but on first inspection it is not obvious that they represent the types of issues that could cause closure of the road. However, if we look at Lidar imagery of the area (available here) the the true picture becomes clear. This is the section of road and the surrounding terrain:-
Let’s take a detailed look at the site of the first Google Earth image above:-
Note the smooth topography to the north and south. The valley floor runs east-west across the images, and the road can be clearly seen on the north side, low on the slope. The massive area of highly disrupted terrain is the landslide complex, with the backscarp high on the hillside.
This area was probably not covered by ice in the most recent (Devensian) glaciation, but the conditions would have been periglacial. These landslides are thought to have formed during this period. As the first Lidar image shows, the entire section of the road is crossing these large, ancient landslides. The movement we are seeing that is causing such damage is the reactivation of these slopes.
Whilst these landslides have shown comparatively low activity since the road was built 200 years ago, they are now responding to a changing climate, and in particular to both large storms and longer periods of prolonged rainfall. Their rates of movement are increasing, and this is causing the issues that the local authority is struggling to manage. This is going to get worse.
And thus, the County Council is realistic about the prospects of being able to maintain the road unless there is a substantial injection of funding from central government. The road runs through a landscape that is heavily protected against development, and stabilising these slopes, whilst technically possible, would be a major undertaking. Finding a new alignment might be the best option, but it would be very expensive. All of the valleys in this area have these large landslides.
Finally, I note with some amusement the comments made by a cycling campaigner at the end of the BBC article that the road could be turned into a cycling and walking trail. Whilst this sounds superficially very appealing, in reality stopping maintenance as an active road would mean that the alignment would rapidly deteriorate, and the route would no longer be usable by bicycles. The campaigners should take a look at what happened on an adjacent valley at Mam Tor, which was also a major highway linking Sheffield and Manchester. The road closed in 1979 and the route rapidly deteriorated. Footpaths could be maintained, but the precarious nature of the alignment along the Snake Pass mans that the road would rapidly be lost.
The future of the Snake Pass undoubtedly needs a national debate. The road is a crucial strategic route that links two major cities. Its loss is hard to imagine and would have major implications, but it is not viable in the long term without a major intervention.