UKH

Whaley Bridge - on the edge of the PDNP

Toddbrook Reservoir , above the town is severely damaged and more rain is predicted.

Fingers crossed for all in the area in which I grew up.

https://www.facebook.com/bbcnews/videos/901717940165105/

https://www.bbc.co.uk/news/uk-england-derbyshire-49189955

This shows some of the damage to the dam.

DAMage

Just hearing a 50/50 chance of failure within the next couple of hours.

Obviously they are trying to release as much water as possible to minimise the risk.

Terrifying.  

Its a CRT reservoir- a canal feeder. They are much more resource strapped than water companies and have had quite a few infrastructure failures in recent years. I'm not covinced their inspection regime is all it could be, hopefully this wasn't avoidable...

climate change is avoidable, unfortunately it was too late to mitigate this extreme weather event.

Complicated by mining works in the area too.

I've just got home (Kettleshulme) its raining pretty heavy still.....may go for a walk in a bit and have a look!

I was planning to do my first massive bike ride off the year on Sunday, up the Brickworks, down into Whaley, then up towards Buxton and the cat and fiddle. Might need a rethink. 

Love that area, looks like a lot of damage is on the cards. 

I wonder what it will take to wake the Conservatives up.

A dodgy dam above their homes. 

You'll be fine if the rain stops I reckon....its forecast to although Bollington is quite flooded in parts. Not sure you'll get near Whaley though but you could cut up past Windgather and down the road through Goyt! If you go on the Bollington page on facebook there's a video of a 4x4 going up past the units at the start of the brickworks road and its very flooded!' Parts of Macc are so waterlogged any rain at all is resulting in floods but its not too bad!

Yeah was planning to go via windgather, and avoid the dip into Whaley. I think I'll play it by ear if you say the Brickworks is knackered as well... 

BBC now saying that parts of New Mills now being evacuated.

. I think I'll play it by ear if you say the Brickworks is knackered as well... 

I'm going to Poynton in the morning so ill go that way and have a look.....I'll post back on this thread.....

Yea it’ll be the farms down by the river, most of the town is quite high up.

I would imagine their inspection regime is fine - it's set out in law.

in this instance it's not necessarily a lack of inspection or maintenance that's caused it. It could be very difficult to identify a weakness on the conc spillway by inspection - it could have voids from underneath and you wouldn't know. Or the design may not have been adequate in the first place.

it's not the first spillway to have been ripped up in a flood although others have tended to be older block masonry. either way this'll have knock on effects.

one of the main things is that you can react properly to this type of thing. you'll look a right proper charlie if your valves have seized up and you can't lower the water level.....

I have family on the Eastern side of the Peak, Darley Dale way - has the rain been as bad there?

Sorry, I don't know.

No, East is fine 

I don't know if its new legislation or not but there have been a few new abstraction systems without valves being installed on dams within Yorkshire. Vacuum (where I come in) is used to lift the water up a 2 foot pipe and over the damn wall then it syphons its self empty. 

It's designed to rapidly drain the res should something like this happen. 

Siphon are great but you need the lift pump - water or vacuum - to work, which in turn needs power or plant access.

I’ve always imagined the valves on a sluice having big old wheels that can be turned by hand when all else fails.

Could always have both.

One of my favourite lake related siphon type widgets are the vertical fountains driven by releasing dissolved CO2 from Lake Nyos - http://news.bbc.co.uk/1/hi/world/africa/1155057.stm

Yes they need power to work. They still have the sluice valves, the syphon system is a backup and apparently will drain faster too. 

forgive my ignorance, but is the spillway not designed specifically to take water if the overflows are overwhelmed?

just trying to understand if part of the structure ended up being "used" for a purpose for which it was never designed, or if it should have been able to cope but for some reason wasn't?

The spillway is damaged/undermined: https://www.bbc.com/news/uk-england-derbyshire-49199505

Given the available driving pressure from the atmosphere can't a siphon only be used to drop the level by ~10m below the top of the tube which presumably passes over the dam given you mention the need for a pumped start?

jk

Yes, the spillway should be able to safely handle water over topping the dam. It looks like water has got into one of the joints in the concrete slabs forming the spillway and or lifted it washing it away then continued undermined and breaking up the neighbouring slabs and wall, reducing the mass of the dam and its integrity.

jk

It is the sheer volume of water going over the spillway that has caused the problems:

https://twitter.com/Telegraph/status/1156976636320997377?s=20

Yup; real world pumps can only lift about 8 meters with vacuum given losses in the system, the hard limit is around 10 m depending on altitude and local air pressure etc.  Going off the reported area and volume on Wikipedia the average depth here is about 10 meters.  10 meters is also a lot of headroom against the next storm to prevent overtopping during stabilisation and to buy time to get serious external pumps in.

You can also start a siphon by submersing the whole pipe, closing valves at both ends, man handling it out and down the hill and opening the valves.  Kind of a desperate last resort though but a handy one as it can be done without any powered machinery - just some long fire hoses and some very wet people.

edit: well I never - a 15 meter rise in an atmospheric siphon by degassing to prevent cavitation nucleating the conversion to gas - https://www.nature.com/articles/srep16790

  Hopefully this situation ends well. Is it a one off in freak circumstances or are we about to embark on a mass panic that much of our infrastructure is approaching 200 years old and reaching its end of life?

The spillway dates from the 1970s according to Wiki.

funnily enough former conservative MP Edwina Currie was being interviewed on PM last night because she lives in Whaley Bridge, she said her cottage was not in danger, it is above the reservoir and is even older than the dam..  

Oh I can see the merit in a case like this of rapidly and without electrical power being able to dump even a meter of water let alone 10. I was really just wondering how/if siphons were used to drain reservoirs not equipped with sump valves.

I've done it myself to empty a pond: feed a hose in bit by bit allowing it to flood, kink it, drag it out down the hill and let go.

I'm not entirely surprised by that, I'd wondered if the inertia of the water might modify the result by resisting being torn apart in reality, seems I was right to be curious but not exactly about the physics.

jk

I think it’s a bountiful area to bait physicists in to arguing over interpretation of classical physics.

Even physicists forget that freezing and boiling are nucleated, statistical events and that bulk thermodynamics is a convenient high level simplification of statistical mechanics.  

It’s quite amazing when you can get a bottle of water from the freezer to flash-freeze at -20oC.  It’s utterly terrifying when it happens with superheated liquid water, which I imagine is the sort of state the water is in for the reported “super syphon”.  You’d presumably need very smooth boor pipes as well.

Worth noting that this doesn't work with normal lay-flat type hose that the fire service use. The pressure of water in a syphon is lower than local atmospheric to make it work, lay flat hose just squashes flat. Rigid 'suction' hose is what you need for this (or a normal garden hose for small scale stuff). 

thanks -so it looks like a maintenance issue then

is that right though? the spillway should be a smooth surface that the water runs over with no obstruction. if this was how it actually was, then why would the volume of water change things? if however it was not smooth e.g. there was an existing flaw, then (like a pothole in the road) it will grow and the more water the worse it will be. but there should ne no "pothole" there to begin with?

...or design or construction or operational or the flooding was outwith the design parameters, or some combination of those. I'd be guessing.

jk

Ah, so the story will be "we don't build things to Victorian standards anymore"

They often seem to be designed with steps and blocks (as in this case) to dissipate the energy of the falling water, presumably to protect the downstream part of the overflow system.

jk

It is also possible  that the erosion under the slabs was caused by seepage and piping resulting from higher reservoir levels and pressure.  Seepage water would be at much higher pressure than the water running down the spillway and would be much harder to spot under the spillway than it would be on the main structure..

yes, I have seen those, although the obstructions tend to be in channels in those that I have seen

anyhow, not that I am suggesting they are over reacting and of course it needs mending, but looking at the images, what has happened is that a few concrete blocks on the surface  on the opposite side of the water have come loose and several feet of soil etc underneath that has been scoured away. so if left will certainly get worse and lead to real structural weakness. but at present, I would be surprised if "surface" damage of this nature would in actual fact so reduce the structural strength of the dam as to make complete failure likely in its current state? it seems to be to be the protective layer that has been damaged - so if not repaired will lead to deeper problems, but at present I cannot see how the rest of the considerable thickness of the dam is so compromised?

Its thinnest at the top and the undercutting looks very deep to me.  If even just the top meter goes, the rest will wash away in minutes.  I don’t think there’s that much left at the top...

looking again at the pictures, I think you are right. it is deep. I was trying to find out what the core of the dam is made off but could not.

interestingly, the wikepedia page contains the following entry:-

"High rainfall levels resulted in damage to the dam’s spillway in December 1964.[1] The damage was repaired in 1965, but flood studies judged the spillway to be inadequate.[1] As a result in 1971 a new concrete spillway was added to the centre of the dam.[1][2]

In the 1980s, British Waterways carried out significant repair works to the dam to deal with leaks involving the mining shafts located around the dam. This resulted in a culvert being constructed under the current beach/launching area for the reservoir. A stone marker could be seen on the main beach showing the location of this for many years, but was relocated to the footpath opposite Toddbrook Lodge during access work for the 2009 draining.

It was known that the local coal mining industry had been a challenge for the dam's integrity for many years. The original builders were forced to purchase a block of coal below the dam in situ in order to ensure its mining did not cause structural issues.

The reservoir was also partially drained in 2009 for re-alignment works on the dam, and again in 2010."

so maybe not entirely unexpected...

That explains why they were putting sand bags by hand along the top on the upstream side. I was wondering why they bothered with puny little sand bags when there are meters missing on the other side.

do they not have monitors to check this though?

I've just been for a look.....what the pics on the internet don't illustrate well is that the area they are dropping bags of aggregate in to is actually a big hole and there is water seeping through. Great flying by the Chinook pilots.....they are back  and to very quickly and the logistics look well set up as the pi k up zone is on the road between Whaley and Buxton just near the outdoor centre.

 I was speaking with a guy who's house is just below and he said the dam itself is lined with clay and there have been a few repairs going back in time.

The weather is fine now but with the rain there's probably at least a couple of days where it'll continue to fill.

It looks worse in reality than the pics show and apparently is bowing hence the concern that it will burst!

Bloody impressive neat placement.

https://www.bbc.co.uk/news/uk-england-derbyshire-49201467

Yep.....they are doing an impressive job! I got some decent video on my phone but don't know how to put it on.here. At the pick up site its equally impressive....

FAO Thundercat....Bakestonedale road (brickworks) is open. A few new holes and debris but going up was ok earlier today...

whats the run out going be be like ? AKA everybody go surfing,, surfing Whaley Bridge .

it is but on the day one of our Finest has pass away who would expect  less from RAF https://www.bbc.co.uk/news/uk-england-cambridgeshire-49204090

thanks for the update.

at first glance i just thought that the conc panels had been ripped up by the uplift force and scoured underneath - so no flow over the spillway and it's not an imminent catastrophe.

I would guess the dam has a clay core, possibly only a couple of m thick, with the earth fill only realy there to make the thing stand up. All dams bow just like any beam sags under load. the question now is has it moved (they'll have survey records going back years) and is it moving?

if anyone is interested look up Teton Dam (in the US, some good vids online). Not a direct comparison, but evacuating people from downstream is definitely the right decision. 

Anyone know whether the road from Macclesfield through Rainow to Waley Bridge and on to Chapel-en-le-Frith is open and passable?   This is unclear on Google maps.

List of road closures from Derbyshire Constabulary.

https://www.derbyshire.police.uk/news/derbyshire/news/news/north/2019/july/...

I don't think so. The vacuum pump to start it syphoning can in this particular case only lift 6 meters (its a 200 mbar pump), so if the surface of the water is 6 meters or more below the top of the pipe it won't lift but if its closer to the top that 6 meters (this res normally sits 2 meters from the top) it will lift and start to syphon and as I understand it once it starts the weight of the water flowing down the discharge pipe will carry on drawing water up the intake pipe. I may be wrong in my understanding though.

What I do want to do is see it in action. A open ended 2 foot diameter pipe is going to move a lot of water. I'm looking forward to going to commission the vacuum pumps. 

Totally agree on wanting to see it in action.

If it were a chain I’d agree with you on the height thing - but it’s not a chain; falling water can’t pull rising water up, it can only cause a reduction in pressure behind it,  the atmosphere then pushes other water up in to that area of reduced pressure.  So you’re limited to the height rise corresponding to the difference between vacuum and local atmospheric pressure at the high water level.  Beyond that the pressure drops so much the water boils into gas breaking the siphon.

Or so I thought until the nature paper I linked above - I need to read that once I can get a copy...

Fun fact - you can’t properly hear big rockets like a Falcon 9 launching, as they’re so loud a vacuum is created in the low pressure part of the sound wave, attenuating it.

I missed the article you linked. That made very interesting reading. 

As the saying goes every day is a school day. 

.....and I see Boris Johnson now making promises about how he will help that he won't be able to keep.

There were plants growing in the concrete spillway (top photo here: https://www.straitstimes.com/world/europe/english-town-evacuated-after-rese... and at 0:50 here: youtube.com/watch?v=qQNh2lhYXBM& ) around the area where it collapsed.

Surely they are either a sign or a cause (or a combination both) of a weakness? Or are they insignificant?

I presume he will be offering to lie down in front of the floodwaters, in much the same way as he promised support against the expansion of Heathrow.

possibly, it's certainly a sing that the joints between the slabs should have been checked.

I'll take a wee bit of a punt and say it's probably a combination of two things; leakage through the core combined with defective slab joints. the downstream face would be more saturated and have higher internal water pressure nearer the surface. combine this with uplift pressure while it's spilling would likely suck material through any gaps in the joints. 

some leakage through the dam body is completely normal, and it's incredibly difficult to spot if it's becoming problem. you need years of motioning data which can sometime only hint at underlying problems.

this case shows why spillways on earthfill dams are best cut through natural ground on the abutments and not over the dam itself. but, given that this spillway is a retrofit it's not that easy. I can only guess that if this additional spillway hadn't been added the flood would have completely overwhelmed the original spill and ripped that out like at Ulley Dam 2007

Nice one, cheers! Ill be going up it and my typical speed is pretty much walking shows so I should spot them. 

I suspect that he or his team saw the grief that Theresa May got for failing to show up at Grenfell and thought it was an astute political move. I'm certainly no fan of Johnson but for many politicians it's "damned if you do, damned if you don't". What's Jeremy been up to this week?

Same thing he did immediately after Grenfell - jump on a (potential) tragedy to make political cheap-shots.  Only nobody cares enough to report it any longer...

Seem to be a few folk on here who have detailed knowledge of this topic, have you seen this recent report? (sounds like a bunch of experts reporting to EnvAgency post 2015) ? http://evidence.environment-agency.gov.uk/FCERM/Libraries/FCERM_Project_Doc...  

Key takeaways for me are in table 6.2 and 6.3 recommending that - category A dams (which this is) need to have inbuilt drawdown capable of lowering level by around 1meter/day (which clearly this dam did not) (table 6.2) and table 6.3 recommending inspection of high risk dams rather more frequently than yearly (like 2-3 times per week..). Of course this is recommendations not the law, but needs more attention than it is getting.

Well a minor Saturday afternoon beer turned into an epic binge, a lock in, a dreadful hangover and a cancelled bike ride. So I got nowhere near the Brickworks. 

He showed up which was good, but his 'promises' to rehouse everyone properly and rebuild the dam etc  are easy to make. Jeremy would have done a better job at listening properly.

Can you prove that? Is it normal to ask a few questions afterwards to establish some level of proof of learning? 

No, its my opinion. 

Personally I'd rather they didn't visit, their time would be better served being updated as appropriate by the ministers responsible and then acting on information. 

An mp or pm pitching up will just serve as a distraction for those on site trying to get on with their job. 

Better than a bike ride in my opinion.

Always good to get the priorities right!

I drove the bike down to Macc and rode the Cheshire lanes over towards Holmes Chapel, Sandbach etc. Living in Kettleshulme is good but any ride from home leaves a big climb to get back....and I'm no climber!

I'm not clear that the 1m per day is necessary or relevant in this case.  

This is based on MUCH larger drinking water reservoirs in areas of far greater population density.

Plans in place seem to working.  Part of those plans are to drain using external services as we have seen.  

I am not sure that the annual inspection isn't in addition to weekly or similar checks.

As of this afternoon the water is down 3m and continues at a rate of 10cm an hour.  

In reply to:

Residents who have been allowed to pop home have then refused to leave again. 

Putting emergency services at risk, or perhaps realising it won't actually collapse unless it rains again! 

Looking at this morning's Pennine damp, that appears a forlorn hope.

With a ridge to the south and the river to the north I think the attempts to cut the reservoir off from much of its supply should have worked pretty well.  The remaining catchment area looks about the same as the reservoir area so even the worst case 5 cm of rain shouldn’t have raised levels more than 10 cm.  

As it was, the rain didn’t fall there and it’s looking pretty empty now - https://www.bbc.co.uk/news/uk-england-derbyshire-49231384

no guarantee of that at all.

Wikipedia explains that the reservoir is filled from a feeder stream via a small weir, with unwanted water running alongside the reservoir and off down the valley. On Google Maps you can even see Todd Brook bypassing the reservoir along its northern shore. So blocking that would prevent most of this week's forecast rain from entering the reservoir.  The actual reservoir must be so silted up by now that it won't be long before it begins to appear to shrink very fast.

Geograph has a photo of the sluice on the feeder - I imagine there’s a lot of sandbags and mud now compared to this idyllic picture - https://www.geograph.org.uk/photo/1355597

Ever faster - more and more pumps are coming online and the cross section is getting ever smaller as it drains...

Rain or not is not really the issue right now.  The reservoir is half full.  The dam wall isn't stable.  The RAF have done a great job externally but until the water is lower the dam cannot be properly fixed. Half of a lot of water is still a lot of water, more than enough to do plenty of damage and destruction and death. Evacuation is the sensible thing to do right now.  

I can understand the professional caution of those responsible for its safety but I can also understand the pragmatic assessment of those who want to return. Given it survived as-is full to the brim and that it's the now unloaded upper reaches which were damaged and the lost mass has since been replaced it seems increasingly less likely to collapse while the water level is falling.

The logic of people choosing to live within the cordon in a ghost-town escapes me but people no doubt have their reasons, particularly those with animals or crops to tend.

jk

Bwahahaha... guess who's turned up in Whaley Bridge?

Wiki gives you a good idea about how "unknown" this (and other) dams might be. Possibly not built as specified. Possibly undocumented structures, weaknesses or historical repairs within.

I'll bet you £500 that the dam doesn't collapse! 

Would I bet my life on it by breaking the cordon - probably not!

What kind of crops might those be then? The kind that grow in the attic?

I understand your point. 

I also think it's an unknown quantity.  Built more than 100 years ago and not the first time it's had problems.  

The visible damage is shored up and hopefully you are right that having stayed up this long it's ok.  

But I wouldn't bet my life on there not being further problems lower down, or on the weight of all that ballast not moving anything unstable below it. 

The water is still higher than the damaged area.  

Who knows, people clearly have their reasons.

jk

Good drone footage of the works here - https://www.manchestereveningnews.co.uk/news/greater-manchester-news/drone-...

There was talk upthread about having more pumps at all reservoirs - I think it’s more sensible to move them when needed as here than to have them all sat idle everywhere - although the vacuum primed siphons Dax explained sound to me like a great way of adding capacity for relatively little capital cost.

The main step I think should be considered is pre-emptily lowering levels before forecast heavy rain, and having both a seasonal and daily week-ahead evaluation of forecast models feed in to level control.  There’s been such an improvement in numerical weather prediction in the last 15 years that I think this could be a largely automated process.  

For this specific reservoir it looks like half a meter of headroom and beefier separation of the brook and the water at the edge far from the dam could have averted this crisis.  Says the newly qualified armchair dam engineer...

Good drone footage from poly – only just had the time to take a quick look.

Let’s not mince about, this dam is collapsing; now, and ongoing. That’s why they have thrown the kitchen sink at it; just look at the number of pumps and that the outflow has overwhelmed the channel just beyond the toe of the dam. This is every effort to get the water level down asap.

You can see the track of the leakage flow form the hole has turned white from the cement/grout. I can’t see any water actively spilling into the channel from this, but there may still be a wee bit. This suggests that the leak is mostly plugged, and collapse is near to stabilising or possibly stopping.

Even one the water level is taken down below the leakage level it’s still very precarious and any raise in the water level will out it straight back to critical. There’s always the possibility of other damage at lower level and further movement (potentially caused by the rapid draindown itself).  

Would I be happy to kip in my bed immediately below the dam – would I f*ck!

I'm not sure how you can say the dam is actively collapsing from what can be seen in that video? The white streaks are leached from the ballast bags, they weren't present in earlier shots before it was deposited nor was there obviously any water leaking from the scoured section. It doesn't appear to have burst outward due to excessive internal hydraulic pressure, more the spillway surface has been lifted, broken up and removed by the overflow exposing the earth to the scouring action of the water which has locally significantly thinned and reduced the mass of the dam. It may have subsequently moved around where that mass was lost but the primary failure looks to have been erosion of the spillway surface.

jk

Exactly the same as happened at the Oroville dam in the USA in 2017. Once water gets underneath the concrete slabs and begins to undermine them it's only a matter if time. I remember once after a heavy storm seeing long strips of tarmac that had been rollered into a trench by a utility company but had been lifted out by water running down the road and carried bodily and deposited many yards  down the hill. 

Difference there seems to be the Oroville slipway was deliberately not part of the dam structure whereas with this one the damage to the spillway is damage to the dam itself

https://www.bbc.co.uk/news/uk-england-derbyshire-49199505

6th and 8th photo down you can see water escaping from the pit and further from the scour hole below the spilway wall - while the spillway is dry. this water is commign throught dam carrying material with it. in this situdation it's only a matter of time. and noone can tell with any certainty the amount of tiem before the progresive collapse becomes out of control.

the bags arn't filled with liquid.

In my experience, fresh bags of ballast contain considerable amounts of water.

Re 8th picture, I don't understand what you're saying.  I can't see any flowing water, your eyesight must be considerably better than mine!

I guess you didnt see the post from abr1966 above. I'm taking that at face value. It certainly goes to explain why they are still desperately lowering the water level while the spillway is dry. 

 And, you need a better supplier if you're paying for aggregate and being delivered water

Could that water be from the saturated ground - if it's bad as you say the residents wont be back for months, years?! 

if you look at the 6th and 8th photos (bbc link above); the brown streak down the grass outside of the pwillway wall. you can see the difference between flowing water and not (or at least much less in the 6th) that's a lot of water to be just from saturated ground.

the earth-fill will be effectivey saturated, and seepage happens, but if the water seepign out is carrying material that's a danger. not only are you losing structural intergiry, the danger is this becomes out of control. a small flow through a pinhole can very quickly scour out a larger hole and feedback loop to runaway collapse.

 the dam is (in civil engineergin parlance) f*cked and isn't goping to be refilled anytime soon. it'll take major repair and a lot of time before the reservoir is back. but for residents go get back the thing only needs to be made safe; this can be done in reasonable time.

I think you should get down to Whaley Bridge, pronto, and get your finger stuck in there

The rotor wash on the whirlybird was throwing a lot of water up over the bags as they approached over the reservoir...

quite agree. it's such a well known parable that you would have thought that the point could be made and understood so that people knew that this is a serious risk.

this isn't "elf and safety gown maaad" this is as close to a catastrophic collapse (and I mean catastrophic*) as has been for quite a while.

* for reference:

evidence.environment-agency.gov.uk/FCERM/Libraries/FCERM_Project_Documents/Lessons_from_Historical_Dam_Incidents.sflb.ashx

see tabel 1-1 and compare this reservoir; date built, height, capacity, failure by internal erosion and number of lives lost. (for general interest of the unusual; see Vaiont Dam page 111)

No, they're filled with mixed aggregate. Limestone seems likely given the geographical location but the colour makes me think chinaclay so I'd guess crushed granite. Either way it's the fines washed out, by groundwater as you contend, by rain as I suspect or because they were pre-soaked before being dropped.

Jk

I don't think the pictures you linked show what you think they do, in one of them water is still spilling in over the top, in the other I think you're just seeing the muddy streaks left after the overflow was sandbagged and stopped but it is hard to tell.

The brown streak outside the spillway on the grass is where the water in the spillway has undermined the wall and escaped, it isn't leaching out of the dam face, there's a photo where you can see the scoured hole under the spillway wall and in later pictures you can see it backfilled with bags, both sides of the remaining wall. 

Jk

I can't follow your argument. 

There was a leak through the core of the dam, this flow was washing out material: that's progressive collapse. Left unchecked the dam is a gonner. 

I have not seen anything in the press, or on the photos which clearly implies there was a leak through the core. 

And I've now read that report that you linked to (thanks) so I consider myself an expert! 

Post from our Johnny on the spot, arb1966, above.

And now in nat media

https://www.theguardian.com/environment/2019/aug/05/whaley-bridge-dam-very-...

Where are you getting this core-leak idea from, I haven't seen that reported anywhere? Are you saying it has somehow failed by saturation, coincident with the mechanical damage caused by overtopping? While I'm sure that's possible I'm not a big believer in coincidences.

So far as I can see the spillway has been damaged by the water flowing down it, partial or complete loss of a concrete slab has lead to rapid formation of a crater in the underlying earth leading to further collapse of the spillway surface up and downstream. Unchecked this would rapidly result in catastrophic failure either by erosion upstream to undermine the dam wall or mass loss, whichever occurs first. It has also allowed the overflowing water to escape under the spillway wall washing material away from the dam face outside the channel but this is surface water, it's not leached through the dam.

I don't disagree it would likely have collapsed had the damaged spillway been left exposed to overflowing water. I just don't think you need an internal leak to cause the damage visible, it's easily explained by erosion from above and easily dangerous enough in its own right to justify the intervention we've seen.

jk

As in my previous post 

Abr1966 " there is water seeping through"

Guardian article " there was a leak"

The only way it can leak is through the clay core, the waterproof element of the structure.

I'm sure there was water still coming down after the spillway was sandbagged, perhaps even after the level fell below the spillway top but it's not clear in any of the reports exactly where from. It's possibly coming past the sandbags (leakage visible in pictures), it's possibly draining the saturated soil within the crater, it's possibly coming from the clearly very thin section of remaining embankment at the top where the spillway has been undercut by overflowing water. Likely it was a combination and it obviously needed dealing with but it doesn't appear to be the primary cause of the incident. If it was seeping where the spillway meets the now eroded embankment at the top it will have been stopped/slowed initially by sandbagging and grouting then completely by lowering the water level.

The Guardian article is rather muddled and ambiguous, I wouldn't read much into that.

jk

You seem to have time on your hands 

The instigator was the flood damaging the spillway. You'll note that the slabs failed not at the point of highest velocity, the bottom of the chute. 

There's two likely factors that would contribute to weakness of the slab ...

Over to you

Everyone has been watching Chernobyl and got a bit excited!

From another Johnny on the spot (ie me), we'll start with the good news. As of this morning the level is down over 8m and the engineers should soon be in a position to start assessing whether they consider it safe for people to return.

I know it's nice to speculate on possible causes and how close we were to disaster, but I can tell you that the atmosphere in the village started off as widespread fear (I was on holiday last week but this was easy to pick up from the numerous FB posts), then went through helplessness to ones of a massive gratitude to those who have (hopefully) saved our village and a desire to help them however we can. This has mostly been in the form of feeding the 200 or so people from the army, police, fire, mountain rescue and other organisations who have put themselves out for our sakes. We are all massively indebted to them.

Yes, the dam in its current state is probably fkd. I'm not an engineer but I can't imagine it will be just fixed. I expect it will need rebuilding. That is if if gets replaced at all. The reservoir is a canal feeder and is no longer essential. Who knows if money will be found for it.

My understanding is that the slabs failed due to there being a big hole behind them where there should have been earth.  

This presumably has been happening slowly for a while and was then accelerated by the storm, which both exposed and worsened the problem.  

Have I understood correctly?  

Let’s have a picture of a bloke in a harness and helmet.  

 https://www.manchestereveningnews.co.uk/news/greater-manchester-news/worker...

Misalignment by settlement or biology exposing the substrate to erosion at the joints and the slabs to abnormal forces when exposed to flow. Undermining by flowing water or biology. Uplift by hydraulic pressure. Inadequate keying to the earth substrate or support against slipping from below. Reinforcement failure and cracking by water ingress, corrosion and freeze-thaw. I'm probably missing some.

jk

Presumably the Peak Forest and Macc canals.
I can't imagine there'd be too many people happy if they started running dry.

They are also fed by Combes reservoir.

When I said essential what I meant was in relation to when they were needed for the industries that relied on them

Walk or cycle near any Victorian reservoir or structure and you will see weeds (predominantly buddleia on bridges) growing between the bricks or stones and water penetrating from behind concrete, leaving calcite deposits, staining and plant growth. The Victorians left us a fantastic infrastructure, much of it heavily over-engineered because they had plenty of cheap labour and didn't do minimalist engineeering but through the last century we took that for granted and failed properly to maintain those lovely structures. The consequence of that became clear with the railways, which had not been maintained or upgraded since the war and began to break up under the strain of higher speeds and loads, causing a number of crashes. Railway re-privatisation may have been shambolic politically but at least it provided a reason for engineers to examine the infrastructure and begin to carry out maintenance. The same has not happened with the water infrastructure so events like this, where a spillway fails doing its normal job, will become more commonplace. Canals have been maintained because their importance as tourism and ecology assets have been recognised but the waterworks that supply and drain the canals have been neglected in the same way - a cursory inspection and fingers crossed that nothing extreme happens to test the integrity of the thing.

There's even a cheeky reference to Idwal Slabs in the comments!

Let's take the view that it was designed and built properly.

You've identified weakness of the joints. With ref to vid above, plant growth isn't a big deal unless it's woody; shrubs trees etc. So it's a possibility. 

And uplift. uplift shouldn't be a problem, part of normal design condition. If there were voids already underneath the slab, then they would lift and move more easily........ 

https://www.manchestereveningnews.co.uk/news/greater-manchester-news/watch-...

If the repairs are moving then I definitely think the evacuation is a good idea.  

For a pre-existing void to have been present under the spillway the earth fill must have washed out at some point, presumably recently. Where did it go, out the joints and down the spillway? Look at the highly contrasting colour of what washes out from the crater before the aggregate is dropped, is it credible that any resulting seepage streaks and staining were missed or ignored by previous inspections?

jk

I'm introducing the possibility that internal erosion may already have been present. This material could have been lost over the 50 odd years since the auxiliary spill was built.

The dam is a "pennine" embankment type, common of its era. Very narrow clay core supported by earth fill on either side. The scour of material reduced it's support. Quite possible that the core could move and crack...

The most interesting photo is the one of the spill shortly after the flood. No 7 on BBC I think. After the flood peak and before the spillway wall is undercut... What do you reckon?

The one entitled 'Part of the reservoir's spillway broke away on Thursday' I hadn't spotted it is out of sequence, the collapse incomplete and the side wall not yet undermined.

It's hard to be sure but it does look like more water is coming out of the hole than falling into it in that particular shot, I presume that's leakage resulting from the scoured undercut area at the top of the spillway. Very dangerous then but now thankfully several meters above the water level. Equally it could just be an illusion, the muddy exit water easier to see and the constricting sidewall deepening the flow further down the slope.

That first limited chunk of damage appears to perhaps have occurred during this footage  youtube.com/watch?v=GkMZPM4l1eQ& the water flowing out of that side of the dam toward the end of the sequence appears to be significantly redder/muddier than the rest. That appears to be the worst overflow event captured on camera.

So did the spillway continue crumbling after the overflow ceased or were there multiple waves of overflow?

several of the post-damage dry ish spillway pics appear to be from before the sandbags were added to protect the damaged section but in those the enlarged hole appears not to be leaking significantly. There is also somewhere I've seen a picture of the reservoir overflowing down 2/3rds of the spillway around the sandbags protecting the damaged section. I presume the overflow and damage occurred in waves rather than the spillway continuing its collapse after the overflow stopped.

jk

I don't know; date and time of each photo would help. 

From the downstream river gauge level the flood peaked only once. I believe once the WL dropped below the level of the auxiliary spill it didn't rise back up again. There was a risk of more rain, which would explain the sandbags accross the weir crest.

The undermining of the spillway wall and further collapse of slabs could have been caused by the spillway flow only. A bit duration dependant. What makes it interesting is you can see the flow is a lot less than the peak and doesn't look like much. 

If we look at the 1st photo of the BBC pics. There's water coming through a gap between the spillway slab and the weir crest (bagged off with a single row of sandbags). But on the left of shot there's 2 distinct wet streaks down the collapsed slab. This is a warning sign that would need to be looked at very closely.

The actual moment of onset of piping failure is maybe a bit moot. But if seepage though the soil turns to an actual hole with a visible jet of water comming out then you've probably got minutes/hours before it's breached.

Well, there are a few silly comments on the BBC news pages.  

This one is my particular favourite 

"Why on earth would you build a dam so high up anyway? Surely common sense suggests all dams should be built at the lowest point of a river."

You do know that most people are morons? 

Still, that's special.  

final post (with bugle) - not to you specifically but everyone. I'm looking at the same pics and videos, I just thought I would share an explanation of my interpretation of what I'm seeing and the mechanisms of dam failure but wanted to avoid a long rambling post.....

given the damage it's at least as likely as not the dam would be leaking.

flow of water through an earth dam is normal, and the water pressure should dissipate through the dam body and the flow go to ground beneath the downstream shoulder. (quick note on filters; the earth-fill material is held in place (in theory) by filtering, the gaps between the downstream particles are smaller than the upstream particle under hydraulic pressure. this goes for earth-fill, rock-fill etc.) taking potential internal erosion as, a function of, the pressure difference either side of a soil particle; if the seepage flow through earth-fill reaches the surface of the dam body and escapes to atmosphere it'll take material with it. you'll have the full (local) pressure differential and no filter. 

seepage escaping from the downstream face of a dam is bad, seepage escaping from the exposed face of a partially collapsed section is even more bad.

when I say it's collapsing; it is eroding, and this erosion won't stop.

they must have thought the dam was leaking or was at such risk of leaking that they pumped a load or cement grout into the repair; if the waterproof part (clay core) was intact just the reduction in pressure and ballast would have been ok. 

at first glance it doesn't look that bad, but I quite believe the person who gave it 50/50 chance of collapse.

I'v just noticed that the description given on the bbc mentioned "structural foam", looking back over the pics on the bbc website you can see it. you can rest easy knowing that they've done a proper repair..... with builders-foam :-D

Trains are running again as of this am.

I see the military have been awarded medals: https://twitter.com/Barney065/status/1158810639311261696?ref_src=twsrc%5Etf...

Very good. Now we need somebody to pen "The Ballad of Whaley Bridge" which people can perform for years to come in folksy pubs around the land, one finger stuck in their ear, singing very slightly flat in a vaguely north midlands accent to add authenticity.

Does such a song involve a man dressed as a maid buffing the singers with a feather duster, or is that a strictly County Durham thing?