There has been another big rockfall on the cliffs at the northern end of Newport (Traeth Mawr) beach. It's not as big as the last one I reported, but it has left a distinct cone about 5m high against the foot of the cliff face, where it now rests on part of the wave-cut platform. The debris consists entirely of slabs and smaller fragments of local shale and mudstone, from high on the cliff face.
Not sure of the date of the fall, but it was probably in the first half of February 2021 during a spell of stormy weather.
So what's going on here? there seem to be two main factors in th apparent acceleration of cliff face "events", both associated with global warming and climate change.
1. There is now much more variability in wind directions than there was fifty years ago. That means that northerly or north-westerly gales are much more common, with storm waves battering the cliff base and triggering collapse events. Previously, these cliffs were rather well protected, and there was a barrier of broken debris at the head of the beach at around HWM. This protective barrier has now been dissipated.
2. Because of increased rainfall totals, and increased wetness in the winter months, the water table in the rock behind the cliff face is higher. This means there is increased pore water pressure and also greater lubrication in fissures and fractures -- so that when storm waves are sending vibrations into the cliff, and when air under pressure is forced into caves, cliff collapse becomes more likely.
In addition to this latest large-scale collapse, there are a number of smaller ones along the cliffline in the vicinity.
8 comments:
Is there any news on the rock debris cone from the previous rockfall just over a year ago on this part of the beach?
Dave
It's virtually all been shifted away by the waves. WhenI get a bit more time I'll take a closer look, to see how much modification there has been on the rocks that came down on that occasion.
The previous (bigger) rockfall in Nov 2019 was almost entirely gone within 7 months. See this post:
https://brian-mountainman.blogspot.com/2020/07/newport-rockfall-removal.html
Are you interested in the modification of the actual rocks themselves? By the action of sea waves on them?
I'm intrigued by this as I recently watched the old programme about a tsunami in 1607 along the Bristol Channel coast. Not sure I fully agree with the thesis. One of the presenters was very into the impact of the waves on the rock formations on Sully and along the Glamorgan coast. I'd want quite a bit more evidence of that.
Dave
Is this latest rockfall slightly to the east, but overlapping with the previous one? This is from comparing photographs with a cave visible on either side of the spread of rockfall in both images (unless there are multiple caves along this bit of cliff).
Dave
Yes Dave -- I have always been interested in stone shapes, and measures of angularity / roundness. I'd like to check how long it takes for wave action on a beach like this, around HWM, to start to round off the edges of sharp fragments following a rockfall. These shales and mudstones are of course very soft, and so the rounding process might be quite fast..... watch this space! But I doubt that a single tsunami could have rounded pebbles and stones to any great extent, even if there were some huge waves with a vast amount of turbulence. But an "event" like a tsunami, lasting a few hours at most, would leave a lot of mess -- and of course it could pick up cobbles from existing storm beaches and redistribute them.
See this one on shapes:
https://brian-mountainman.blogspot.com/2015/04/stone-shapes.html
There are multiple small caves here -- I think the two falls are in the same embayment, but not immediately next to one another.
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