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How do coastal defences (walls, groynes) cause erosion further down the coast?

I know that defences such as sea walls and groynes can cause more erosion down the coastline in another place (where there are no defences).

However, I don't get why this is. I had always believed it was (Option A), but I just saw some stuff online which suggests it is (Option B).

Option A = Defences such as walls prevent erosion in one place on the coastline, but make erosion even worse in other places along the coastline that don't have defences, as the defences mean the marine energy is focused down the coastline to where there are no defences, meaning the place with no defences erodes away even quicker than it did before defences were installed along the coastline.

Option B (from the textbook) = Sea walls and groynes defences protect part of the coast, "but they also interrupt the flow of material by longshore drift". "The beach downdrift where there are no defences is then starved of material, and so its cliffs are exposed to wave attack, increasing coastal erosion".

Which one is true? Or are both options true?

Also, in option B, where it says "they interrupt the flow of material by longshore drift", does that mean that the defences stop longshore drift happening further down the coastline by making all the sediment be deposited at the defence rather than being transported via longshore drift? Ie, does that mean the defences trap sediment and prevent it being deposited elsewhere (and so preventing it from protecting the coastline elsewhere as it isn't being deposited anywhere else)?


Thanks
(edited 7 years ago)
From what I remember of A2 Geology, option B sounds more likely. If you put in groynes for example, the sediment gets trapped by the groynes and is then unable to be transported further along the coastline by longshore drift. Therefore, areas further along the coastline aren't being replenished with transported material, so erode more quickly. So you are correct in saying that the sediment is trapped by the defences, and this prevents areas further along from being protected :smile:
Original post by blobbybill

Option A = Defences such as walls prevent erosion in one place on the coastline, but make erosion even worse in other places along the coastline that don't have defences, as the defences mean the marine energy is focused down the coastline to where there are no defences, meaning the place with no defences erodes away even quicker than it did before defences were installed along the coastline.


This account does not pass even a cursory sanity check. If a wave crashes against a sea wall in Skegness its energy is dissipated and it cannot possibly cause any damage in Lowestoft. There can be no transfer of damaging energy from one place to another.
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blobbybill
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Original post by Leviathan1741
From what I remember of A2 Geology, option B sounds more likely. If you put in groynes for example, the sediment gets trapped by the groynes and is then unable to be transported further along the coastline by longshore drift. Therefore, areas further along the coastline aren't being replenished with transported material, so erode more quickly. So you are correct in saying that the sediment is trapped by the defences, and this prevents areas further along from being protected :smile:


Thanks. Looking in other textbooks, they all seem to explain it in the sort of way of option B rather than option A.

Thank you for the help!
Original post by blobbybill
Thanks. Looking in other textbooks, they all seem to explain it in the sort of way of option B rather than option A.

Thank you for the help!


You're welcome! :smile:

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