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eggsarascal

Whaley Bridge Evacuated

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12 hours ago, eggsarascal said:

What you infer is correct, CaRT could, and should have started draining earlier. Do they have a telemetry system on their reservoirs to notify them that they are overtopping at an alarming rate?, either way the slipway failed due to lack of maintenance. Some years back some Herbert decided lowering the levels on the network was a good idea due to most of the water being lost was from the top six inches?, what he forgot was those six inches dry out and become incapable of retaining water when the capacity is needed. We still haven't worked out why the Goyt wasn't utilised when it had only just gone into flood. I might be a conspiracy theorist but I'd have a tenner with anyone that the reason is CaRT we're afraid of running out of water later in the year.

I don't have any axe to grind with either side of the argument, I'm trying to view it logically and see if one argument (for more/better maintenance and drain it earlier) stacks up against it being an entire unpredictable event. 

 

I can understand why they would hold the water level at close to maximum if they can in August (which must be predicted to be the time of most use and least rainfall), but maximum level on a reservoir is NOT just below where the spillway would start to flow, it would be at least 1m below that? One would assume that the lack of potential replenishment for the reservoir in the event of a drought period would be in the mid June to Mid Sept period. Therefore, by 1st August, the reality is that they need to be able to guarantee no more than 8 weeks of use from the volume retained, as the natural rainfall and replenishment should be active by late September again.  

 

My own small reservoir has an overflow pipe that starts to flow when water is within 50cm of the top of the bund, and we can open that overflow to max which takes it back down to 80cm within an hour. Given that the only way that it fills up is from rainfall actually landing on the surface, or our own pumps, that is easily manageable. The volume is also manageable. It holds 9,900 m3, but has a useable volume of around 8,500m3 max. Potentially I can need 60m3 per night to irrigate the golf course or more in drought conditions. So if i need to irrigate every night between 1st May and 30 September I need 9180M3 or more. But I can abstract 20m3 per day for 365 days a year, so in the same period I can add 3,000m3 to the reservoir (provided that its not filled to the overflow). So it is all about managing the inflow against the demand.

 

When you look at the maps, and see that Todd Brook is what flows in to fill the reservoir, and that there is a weir part way along the northern edge which is a man made channel which then flows around and below the spillway (the spillway discharges into the same channel) and that channel was flowing very fast and deep before the spillway overtopped. I can only conclude that the volume of water that was dumped from the sky, in a very small, very specific area, which then all flooded down into the Toddbrook reservoir, was an absolutely off the scale one off event. If the volume of water was more widespread, I didn't hear of any of the other reservoirs in the immediate location (and the very useful map shows 5 in total) coming under any threat at all. 

 

Eggs, you even say yourself in this post, that the Goyt had only just gone into flood. So, for one single reservoir to get hit by such a volume of water in such a short space of time is absolutely impossible to predict. Ultimately, the spillway had been inspected, and passed. Clearly when subjected to extreme forces, it failed and the consequences of that failure must now surely have ramifications across the whole network. I just cannot see how anyone can just take a simplistic view that it was a lack of maintenance and they should have drained it earlier. On the same basis they should have drained the other 4 reservoirs in the are as well, but they didn't fail or even overtop? It was a combination of way more factors than that, and a "perfect storm" that almost ended in a catastrophe.

 

The upside of this of course is that it has been a massive wake up call to CaRT as well as anyone else involved with reservoir storage, that the overflows and mechanisms for draining need reviewing and bringing up to standard.

 

Incidentally, on one of the overheads of the spillway, there appears to be an outlet/overflow pipe exit to the right of the spillway. There is certainly a man made channel there, but I don't recall seeing any water flowing from this either before the spillway failure or as a mechanism for draining the reservoir.  

 

 

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Unfortunately there will now be the usual Mega expensive over-reaction, with massively over-engineered concrete structures installed where they are not ever likely to be needed i.e. justified.

And no doubt another layer of highly overpaid managers to supervise and obviously, manage such structures.

Until the next unexpected occourance.

Bit like the Worlds Armies always preparing for the next war based on what happened in the last war.

mth

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They seemed to get the water level down below the spillway super-quickly - every picture I saw of the damaged section had no water running over any of it. How was this achieved - is the weir adjustable to set the ultimate reservoir level? 

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8 minutes ago, monkeybusiness said:

They seemed to get the water level down below the spillway super-quickly - every picture I saw of the damaged section had no water running over any of it. How was this achieved - is the weir adjustable to set the ultimate reservoir level? 

Gravity plays a huge part. The reason the reservoir overtopped was that the volume of the inflow exceeded the capacity of the outflow. If you leave the plug in your bath and the taps running, then the bath will continue to fill. When it reaches the overflow, if the volume coming into the bath exceeds the speed at which gravity takes it down the overflow, it will continue to fill, and once the water gets above the overflow then the volume of water going through the overflow will increase due to increased water pressure. Eventually, if the inflow continues, the water will reach the edge of the bath and the thing will overtop. to stop it, you turn off the tap and the overflow will quickly recover the position.

 

In the case of Toddbrook, it stopped raining at the volume that caused it to overflow. It can only continue to overflow at that rate if the volume of rainfall coming off the hills keeps pushing it into the reservoir faster than the gravity safety valves can handle it. Once the inflow slows down, it will stop overflowing reasonably quickly. The pumps and the rest of the gear all came after in my view, when they had decided quite correctly that the dam was possibly unstable and could collapse and so the bulk of the volume needed to be reduced as well.

 

  

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10 hours ago, Chalgravesteve said:

Incidentally, on one of the overheads of the spillway, there appears to be an outlet/overflow pipe exit to the right of the spillway. There is certainly a man made channel there, but I don't recall seeing any water flowing from this either before the spillway failure or as a mechanism for draining the reservoir.  

If you go to

 

https://osmaps.ordnancesurvey.co.uk/53.32697,-1.98893,18

 

You can swap between the map and aerial photo and see there is a drain to the overflow from the middle of the dam and the feeder outlet to the canal (the reservoir's main function where one would expect most of the water to leave the dam) the feature between the spill way and this drain is indeterminate.

 

Also note in the aerial photo the water level is above the boat slipway and water is flowing over the weir to the overflow, no other photos I have seen show the reservoir this full but there is no date on the imagery.

 

I just wonder if historically, prior to 1945, there was no spillway and the reservoir was filled more conservatively. Then recently with the demand for leisure boating the decision was made to increase the static level for dry periods and the spillway was added to allow for this.

 

The wikipedia article is interesting and points to the feeder running in by the transhipment building also, I wish I had read this before I tried to deduce it from the maps.

 

Interestingly the first flush of run off from the Toddbrook does not enter the  reservoir until the stream is running several inches  higher.

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On 13/08/2019 at 16:52, Chalgravesteve said:

Gravity plays a huge part. The reason the reservoir overtopped was that the volume of the inflow exceeded the capacity of the outflow. If you leave the plug in your bath and the taps running, then the bath will continue to fill. When it reaches the overflow, if the volume coming into the bath exceeds the speed at which gravity takes it down the overflow, it will continue to fill, and once the water gets above the overflow then the volume of water going through the overflow will increase due to increased water pressure. Eventually, if the inflow continues, the water will reach the edge of the bath and the thing will overtop. to stop it, you turn off the tap and the overflow will quickly recover the position.

 

In the case of Toddbrook, it stopped raining at the volume that caused it to overflow. It can only continue to overflow at that rate if the volume of rainfall coming off the hills keeps pushing it into the reservoir faster than the gravity safety valves can handle it. Once the inflow slows down, it will stop overflowing reasonably quickly. The pumps and the rest of the gear all came after in my view, when they had decided quite correctly that the dam was possibly unstable and could collapse and so the bulk of the volume needed to be reduced as well.

 

  

So there was no manual intervention to limit/reduce the volumes of incoming water once the failure became apparent then - it was pure luck that the input reduced to a volume that the normal outflow could handle before the spillway eroded further (which would have inevitably compromised the dam)? 

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CaRT may have to go back to the Government for support!, that's you and me, folks. Can anyone remind me what sort of money Parry is on while the trust is coining in millions from people who use the network?

 

 

Edited by eggsarascal

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