Last year I helped compile a geotrail for the Churnet Valley in Staffordshire, England. Just as we were about to publish the trail I discovered that a key footbridge had been closed temporarily. This involved a hasty rewrite including an alternate route. I went back this week to see if it had reopened. It hasn’t, but it is now clearer why it is shut – the dam beneath the footbridge is failing.
To set the scene, the area is situated in the Upper Carboniferous, Bashkirian (Westphalian) Coal Measures which were extensively mined for both coal and ironstone. The Froghall Ironstone used to outcrop on the western side of the valley shown in the map below. The spoil, predominantly red siltstones, from ironstone mines on the western side of the valley were used to build an embankment across the valley to take a tramway from coal and ironstone mines further up on the eastern side of the valley. The valley was dammed and a lake formed up stream. It is this spoil dam that is now disintegrating.
In recent times, the embankment was replaced with a footbridge and a spillway from the lake forming a stream under the bridge as can be seen in this photograph taken last year. Just above the stream bed is intact siltstone bedrock but soon passes upwards into loose spoil.
A view from the other side of the bridge last summer shows siltstone spoil underneath the eastern bridge support.
Now contrast this with the scene yesterday.
There is now no water flowing through the spillway as the lake level above the dam is below the spillway lip. Instead, the water is now flowing through the dam spoil material itself and the spoli is being washed out in the process, and further undermining the bridge support.
Fortunately, there is another, intact, tramway embankment a few hundred metres downstream of this dam which should buffer the effects should the dam totally fail and the lake drain catastrophically.