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eggsarascal

Whaley Bridge Evacuated

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Chalgravesteve keeps referring to the extra amount of water, due to excessive rainfall, causing extra erosion, I imagine this is a bit of a red-herring, since any amount of water percolating through seam/joints/fissures, due to any overflow, would cause the same erosion beneath the slab, the inflow being limited more by the width of the defective joint, than the volume/depth of water flowing over it.

O.K.

The greater the volume, the greater the depth and hydrostatic pressure ensuing, and therefore forcing "more" water through the joint.

BUT?

How much water was eroding the soil beneath the concrete before the excessive rainfall.

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28 minutes ago, Stubby said:

A quick look at the plans and specs should tell you if it was supposed to have rebar . Wonder if corners were cut ?

I would think they used the mesh sheets in the concrete ,and then these slabs sat on some kind of support at the edges. No bar used at all as it was just built as a cap not a structure? is my guess.

Its like a roof for the soil, but built in the cheapest way with the least metal and concrete possible.

Edited by Woodlover
Had a few beers and seen the light

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1 hour ago, Woodlover said:

I would think they used the mesh sheets in the concrete ,and then these slabs sat on some kind of support at the edges. No bar used at all as it was just built as a cap not a structure? is my guess.

Its like a roof for the soil, but built in the cheapest way with the least metal and concrete possible.

 

Does look very poor, I was surprised to see the concrete laid on top of what is basically fill. Something with a bit of aggregate would of held up better irrespective of the concrete letting water through.

 

Bob

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15 hours ago, difflock said:

Chalgravesteve keeps referring to the extra amount of water, due to excessive rainfall, causing extra erosion, I imagine this is a bit of a red-herring, since any amount of water percolating through seam/joints/fissures, due to any overflow, would cause the same erosion beneath the slab, the inflow being limited more by the width of the defective joint, than the volume/depth of water flowing over it.

O.K.

The greater the volume, the greater the depth and hydrostatic pressure ensuing, and therefore forcing "more" water through the joint.

BUT?

How much water was eroding the soil beneath the concrete before the excessive rainfall.

Once one of those slabs moves or cracks or drops, that’s it done. The water will just tear it apart. 

 

I worked out earlier that I thought the reservoir has a surface area around 100,000 m3

 

If the water cascading over the spillway way was 30cm deep and it looked at least that, then there is a volume of 30,000m3 that’s coming over that spillway in the hours that follow. That’s 30,000 tonnes of water travelling at speed.  Once it’s through any weak point it will tear it apart. 

 

I wouldn’t call that a red herring. 

 

Hindsight is a wonderful thing. But they could have cleaned and polished that spillway and brought it caked for its birthday but in that extreme circumstance it might still have failed. 

 

The only way that the whole thing could have been avoided would have been for the inflow to have been diverted before the reservoir overtopped the spillway. 

 

Im not disagreeing with anyone that maintenance could have and should have been better. The owners/operators should have done a better job. The inspection engineers should be more stringent, especially in the light of other dam spillway failures. 

 

But people are also expecting them to predict the impossible. In the past 50 years since they built that spillway, how many times previously had it been overtopped by anything? 

Probably never or only a few inch deep trickle. The normal full level of the reservoir will be some distance below The spillway

 

That allows additional emergency capacity. I’d be amazed if it’s normally closer than 1m from max water to the spillway. That’s 100,000 m3 of extra water  from the infill before it’s full to maximum. 

 

Thats the equivalent of 1000mm of rainfall in a few hours. If you double the catchment area, that’s 500mm of rain in the same period. Double it again and it’s 250mm. The area had 10/15mm of rain predicted. 

 

So it’s just my view that the combination of circumstances created a scenario that no one could have foreseen. The spillway then couldn’t stand the forces involved and failed. 

 

Its clearly not fit for purpose and a repair is not viable, needs a complete new modern engineering solution that enables the reservoir and dam to withstand the same or worse circumstances, because you can predict it now. It’s happened once so clearly it could happen again. 

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On 08/08/2019 at 09:46, Chalgravesteve said:

This is exactly their problem. If they dumped 1/3rd of the reservoir capacity every time they had heavy thunderstorms predicted, they would get so much criticism! They were between a rock and a hard place (although if the spillway was there then they wouldn't have had the problem!!) that they need the reservoir full to meet their high level demand, in a period of time not usually associated with torrential and sustained downpours. Its taken them a week with emergency pumps going full blast to lower the level, you simply cannot shift that volume of water in a short time frame, so the likelihood is that they probably needed to start shifting water OUT of the reservoir a week or more ahead, before the bad weather was even forecast!! 

 

And if you search on youtube for the USA spillway destruction, you will see that however good and solid it looks, the destructive power of flowing water is staggering once its found a way into the structure. 

 

Its OK if you are like the Hoover Dam, with huge solid rock formations acting as buttresses to the dam. They bored tunnels through that rock so that they could release water if they needed to. If they open those gates for 24 hours, the only thing that's going to happen to the tunnels is that the rock walls will get a little bit smoother than they were before!

 

I'm not defending in anyway a lack of maintenance (perceived or real - there was a quote from a reservoir panel engineer saying that she couldn't see anything that caused her concern in the pictures taken prior to the problem). 

 

 

If they could discharge the water using pumps "going full blast" why couldn't they have started draining earlier?, we've already established the Goyt had only just gone into flood, please don't try to tell me the runoff from the previous downpour had subsided before they started pumping.

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7 minutes ago, EdwardC said:

But that's not the catchment area, which maybe several square miles. Or in EU money, dozens of square kilometres.

 

Ok, a quick Google has revealed that the catchment area for Toddbrook Reservoir is 17 million square metres. Or in JRM money 4,200 acres.

Rather than trying to sound clever Google why the reservoir was allowed to overtop threatening thousands of homes.

 

Your infatuation with hating Tories out does mine, that really does  take some doing.

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56 minutes ago, eggsarascal said:

If they could discharge the water using pumps "going full blast" why couldn't they have started draining earlier?, we've already established the Goyt had only just gone into flood, please don't try to tell me the runoff from the previous downpour had subsided before they started pumping.

I can’t believe you don’t get this! I don’t know the reservoir or the way that it discharges to the canal network. I’m sure we are in agreement that you cannot pump the water out of the reservoir into the canal network. 

 

In order to empty the reservoir they had to bring in emergency pumps from the emergency services. That would appear to confirm in my mind that once the water is impounded into the reservoir there is no means of getting it back out. There is no drain/pump system to the goyt otherwise surely they would have used it! 

 

It seems entirely logical to me that there should have been a direct discharge pipe or pipes from the reservoir to the goyt. Open the gates and let it drain, just like pulling the plug on your bath. 

 

So to drain it earlier, you have to commandeer the local sports ground, mobilise the emergency services, and do all of the stuff they have had to do since the spillway failed, all on the basis that they might get 100mm of rain in August! 

 

Im happy to be corrected on this but everything I’ve seen and read so far gives a clear indication in my mind that once the water is in the reservoir, the only way it’s coming out when it’s below the spillway, is via the outflow to the canal system. But for 180 years it’s not needed a direct flow to the goyt. 

 

But because the spillway failed and potentially the bank below the spillway was eroded or compromised, they had to pump the water out (currently less than 15% full I believe now) because if the bank behind/below the spillway collapsed then the whole reservoir was going to discharge into Whaley Bridge

 

any solution they come up with for the future surely has to include a means of draining it to the goyt without having to bring in all those pumps 

 

 

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18 minutes ago, EdwardC said:

But that's not the catchment area, which may be several square miles. Or in EU money, dozens of square kilometres.

 

Ok, a quick Google has revealed that the catchment area for Toddbrook Reservoir is 17 million square metres. Or in JRM money 4,200 acres.

Absolutely right, and you have forgotten the underground streams and aquifers that also contributed to the local area being overwhelmed with water. 

 

The fact is, that 100mm of rain in August would not have come close to breaching it. 100mm in a week might have started to get harder. 100mm in a few hours, across the whole catchment, all being funnelled into one single location, a giant bath with no plug and only an overflow to protect it 

 

 

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