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eggsarascal

Whaley Bridge Evacuated

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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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