I live a few hundred yards from that dam (and fortunately a couple of hundred feet higher up the hillside), so over the past couple of days have been treated to the sight and sound of Chinooks in an almost continuous shuttle during daylight hours, bringing bags of stone and sand as reinforcement. Oh well, at least that huge fleet of Chinooks bought as a result of the public outcry over the lack of air transport in Afghanistan is being put to good use. I should add that the coordinated response of the emergency services seems to have been excellent - fast and appropriate - and the Chinook crews are doing a great job.
The reason the dam failed was a period of torrential rainfall, overfilling the reservoir and overflowing down the slipway (as designed) but with such force (the term "raging torrent" is not an exaggeration, if you've seen the video) that part of the concrete capping protecting the largely earth dam was torn off.
The dam was maintained and checked frequently, and has in the past been drained down for repairs, so it doesn't seem that maintenance was shirked. The dam seems to have been in OK condition, but it was an old design (it was built c.1830) and the ferocious weather evidently pushed it beyond its limits.
As well as the helos filling the hole in the dam, several powerful pumps are trying to lower the water level to below the lowest part of the damage, to allow people to return to their properties. Less than a quarter of the 6,500 residents have been evacuated (most of the houses are above the valley floor) but almost all of Whaley Bridge's facilities - shops, offices, workshops, pubs, the surgery and the library, the railway station and bus stops - are down in the valley floor and closed till further notice. So are most of the access roads, including the main road through the centre.
We are watching the weather forecast with anxiety, as more rain is forecast tomorrow.