Whaley Bridge Evacuated

[eggsarascal]

48 minutes ago, openspaceman said:

Here's a quick sketch of my take on this from looking at the map.

 

The Combs  and Toddbrook reservoirs (dark green) feeders (yellow) fill the High Peak Canal (canals use a lot of water when boats go through the locks, about 30 tonnes in our local canal as the locks hold two 70ft narrow boats).

 

Because the canals follow the contour and the Goyt is lower than the canal after Whalley bridge it has to be filled from Toddbrook and Combs reservoirs. Eggs has said it is also filled via the Macclesfield canal and the Bosley reservoir which is 25km from the junction with the high peak canal which starts in Whalley bridge  about 9km south of the junction.

 

The Fernlee and Errwood reservoirs do not seem to feed the canal but presumably also have the function of controlling flow on the Goyt. The trouble is the maps only show open water so I do not know what any of the reservoirs also serve by underground pipes.

 

So the only ways out of the Toddbrook reservoir are via the feeder to the High Peak canal, the weir at the northern tip  and then the overflow that runs  curving clockwise   in front of the dam and thence into the Goyt and the spillway over the top of the dam which meets the above overflow.

 

Any excess that ends up in the High peak canal ( shown red) flows over a weir and into the Goyt (shown light green) near the start of the canal at Whalley bridge.

 

As the Toddbrook reservoir was full, and following very heavy rain, all the excess ended up in the Goyt, which seems to have coped with it, as I said in an earlier post there was the possibility of holding back water at the Fernlee and Errwood reservoirs plus the Combs reservoir if the Goyt could not handle the excess from the Toddbrook system.

 

So it looks like everything worked as planned except the spillway  was damaged when it was over-topped with an unprecedented flow.  I can see the spillway was poorly constructed or maintained for the flow that came over it, Eggs asserts this was predictable and attempt should have been made earlier to lower the level of the Toddbrook reservoir via the overflow into the Goyt if I infer his post correctly.

 

 

My personal guess is that there wasn't much of a void below the spillway slabs prior to it being over-topped and the damage was all done during the surge because the joints between the slabs had  deteriorated.

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.

[openspaceman]

15 minutes ago, eggsarascal said:

 either way the slipway failed due to lack of maintenance.

I think you mean spillway?

 

The reason is there was ambiguity early on with the  word slipway. It was used to refer to a point at the North of the reservoir where there seems to be a boating club and its slipway (I marked it on my sketch as "SLIP"). At this point there is also a weir where water leaves the reservoir and falls into the normal outflow, it seems to fall into the part of the Toddbrook  that has skirted the northern edge of the reservoir and then enters the curved outflow channel that runs in front of the dam and on into the Goyt.

 

There are weirs also at the upstream entry to the Toddbook  reservoir and halfway to the dam on the northern edge which allow water to divert into this part of the diverted Toddbrook and go directly to the outflow, although trying to interpret the map is not easy.

[eggsarascal]

1 minute ago, openspaceman said:

I think you mean spillway?

 

The reason is there was ambiguity early on with the  word slipway. It was used to refer to a point at the North of the reservoir where there seems to be a boating club and its slipway (I marked it on my sketch as "SLIP"). At this point there is also a weir where water leaves the reservoir and falls into the normal outflow, it seems to fall into the part of the Toddbrook  that has skirted the northern edge of the reservoir and then enters the curved outflow channel that runs in front of the dam and on into the Goyt.

 

There are weirs also at the upstream entry to the Toddbook  reservoir and halfway to the dam on the northern edge which allow water to divert into this part of the diverted Toddbrook and go directly to the outflow, although trying to interpret the map is not easy.

Yes, spillway. How many times have I got that wrong in this thread...?

[eggsarascal]

1 hour ago, openspaceman said:

Here's a quick sketch of my take on this from looking at the map.

 

The Combs  and Toddbrook reservoirs (dark green) feeders (yellow) fill the High Peak Canal (canals use a lot of water when boats go through the locks, about 30 tonnes in our local canal as the locks hold two 70ft narrow boats).

 

Because the canals follow the contour and the Goyt is lower than the canal after Whalley bridge it has to be filled from Toddbrook and Combs reservoirs. Eggs has said it is also filled via the Macclesfield canal and the Bosley reservoir which is 25km from the junction with the high peak canal which starts in Whalley bridge  about 9km south of the junction.

 

The Fernlee and Errwood reservoirs do not seem to feed the canal but presumably also have the function of controlling flow on the Goyt. The trouble is the maps only show open water so I do not know what any of the reservoirs also serve by underground pipes.

 

So the only ways out of the Toddbrook reservoir are via the feeder to the High Peak canal, the weir at the northern tip  and then the overflow that runs  curving clockwise   in front of the dam and thence into the Goyt and the spillway over the top of the dam which meets the above overflow.

 

Any excess that ends up in the High peak canal ( shown red) flows over a weir and into the Goyt (shown light green) near the start of the canal at Whalley bridge.

 

As the Toddbrook reservoir was full, and following very heavy rain, all the excess ended up in the Goyt, which seems to have coped with it, as I said in an earlier post there was the possibility of holding back water at the Fernlee and Errwood reservoirs plus the Combs reservoir if the Goyt could not handle the excess from the Toddbrook system.

 

So it looks like everything worked as planned except the spillway  was damaged when it was over-topped with an unprecedented flow.  I can see the spillway was poorly constructed or maintained for the flow that came over it, Eggs asserts this was predictable and attempt should have been made earlier to lower the level of the Toddbrook reservoir via the overflow into the Goyt if I infer his post correctly.

 

 

My personal guess is that there wasn't much of a void below the spillway slabs prior to it being over-topped and the damage was all done during the surge because the joints between the slabs had  deteriorated.

70'10" goes into all the looks on the K@A, sling the boat in on her own and 72' will go in. The difference with the K@A to most other cuts is the back pumps at Foxhangers, so the same storage isn't needed. Is it something like 5 million gallons pump back up on a busy day?

[openspaceman]

34 minutes ago, eggsarascal said:

Yes, spillway. How many times have I got that wrong in this thread...?

I only noticed it then.

 

Incidentally googlearth shows it empty in 1945 (and no sign of water in the canal but it is B&W so hard to tell)and no spillway visible, all the later photos show it at well below capacity  with most of the boat slipway showing.

16 minutes ago, eggsarascal said:

70'10" goes into all the looks on the K@A, sling the boat in on her own and 72' will go in. The difference with the K@A to most other cuts is the back pumps at Foxhangers, so the same storage isn't needed. Is it something like 5 million gallons pump back up on a busy day?

Yes they back pump for the Caen Hill flight because there is no water at the top of the flight to make up and I think they had to do this from the beginning with steam pumps. Even with limited pumping the one by me tends to be shut for navigation from June till the Autumn, it was only re opened in 1991 after being closed for 50 years.

[eggsarascal]

41 minutes ago, openspaceman said:

I only noticed it then.

 

Incidentally googlearth shows it empty in 1945 (and no sign of water in the canal but it is B&W so hard to tell)and no spillway visible, all the later photos show it at well below capacity  with most of the boat slipway showing.

Yes they back pump for the Caen Hill flight because there is no water at the top of the flight to make up and I think they had to do this from the beginning with steam pumps. Even with limited pumping the one by me tends to be shut for navigation from June till the Autumn, it was only re opened in 1991 after being closed for 50 years.

I was going to get above myself and say Foxton used to be back pumped, the more I think about it the boiler house was there to power the inclined plane, probably the prettiest place 

 on the cut IMO.

Edited August 12, 2019 by eggsarascal

[Chalgravesteve]

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.  

 

 

[difflock]

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

[monkeybusiness]

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? 

[Chalgravesteve]

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.