It's a major job that could take weeks if not months. In the short term to get it fixed as an emergency I don't know. Its a major job.
I think I saw on the BBC someone saying it was in essence a mud bank structure which is no longer used. If you look at the pictures you can see what looks to be earth under the blocks that have moved. And it looks like what has happened is the earth has been washed away in the over flow there was and now those banks are unstable. If you look at pictures from earlier this afternoon and this evening there is a marked worsening. If the top blocks go, the whole thing could go.
The trouble is with that earth exposed to the elements and a yellow warning of rain for the area over night that could lead to it simply being washed away. And the whole thing being undermined from the bottom in a mud slide rather than the problem merely being the reservoir being too full and at risk of over flow.
If there is too much water and it does go over the top, the whole thing really would be fucked though and that's the big danger, but it looks ropey even if it doesn't.
I think it's less about 'fixing' it and more about simply emptying the reservoir in the short term tbh.