Ladybower Reservoir - Who pulled the plug and why?

[matty40s]

Yesterday saw a major flooding event in the Yorkshire side of the Pennines, it had been under Met Office warnings for days, and this certainly delivered. Meadowhall Shopping Centre should have cancelled their Christmas opening night far earlier - then nobody would have had to buy Primark pyjamas.

However, in the Derwent Valley below Ladybower Reservoir, yesterday and into today, have seen record levels on the Derwent, causing loss of life, major flooding and loss of business in places like Matlock and Matlock Bath...the A6 is shut in several places, continuing down into Derby which is now experiencing its worst floods in 20 years and it hasn't peaked yet.

Further down are marinas on the canals and in Shardlow itself, which can be inundated and many boats sunk as they do not normally experience rising waters.

For some reason yesterday morning/lunchtime, the Ladybower sluices let over 1/2 a metre depth out of the reservoir whilst the levels were fairly stable. The levels from the hills even now after the rain front has gone havn't gone halfway back to the level the release started at yesterday

1 day gauge map

maybe this was in anticipation of the forecast rain but Ladybower didnt get the worst of the rain, and the capacity is far higher than it reached yesterday in fact, it has been very well controlled in the last month

1 month gauge map

 

and is obviously able to deal with far larger volumes on a long term basis when needed.

3 month gauge map

So why was so much dumped yesterday morning/afternoon when the Derwent would have struggled to cope with the normal catchment run off as well as the lake ejection?

Has the Todbrook reservoir failure at Whaley Bridge at the start of August highlighted structural issues if concrete wash wall overflows are not maintained, and is this the case at Ladybower? 

This wouldn't have gained as much interest if someone from the guilty management at Severn Trent hadn't tried a disinformation headline late yesterday regarding a bloke taking photographs on the plug at the height of the drought in July......why was it paramount to release it this morning.....??

Then this afternoon arranging lovely pictures on the BBC of the plug draining exactly as it should..."being well used"

 

So who pulled the plug and then went shopping??

 

 

Edited November 8, 2019 by matty40s

[The PIT]

Good question. There must be a reason for it I can't see them making a big mistake like that was gonna be a good reason why the water was released. I doubt we'll ever know though.

[ChezWeather]

Witnessed the Derwent first hand in Matlock yesterday at its peak level, it was a truly incredible and also harrowing sight, but couldn't help wonder why so much water was released from Ladybower and what the level would have been like otherwise. 

Questions need to be answered by Severn Trent, im sure they won't however as people will just put this down to climate change, not that i am denier of such, but its pretty clear that Ladybower had an effect on the flooding in the Derwent Valley.

Few of my pics from Matlock below. 

 

 

Final one a few hours after peak level. 

 

 

 

[cuckoo]

Very interesting post.  Not sure, but I think something similar happened on the Derwent for the 2007 event. 

As many people on this site posted the 24-hr rainfall totals for Thursday were typically below 50mm for a lot of the catchment areas surrounding Matlock.  A wet day during a very wet autumn but still well below 2007 totals and well below the 100mm amounts that fell around the Don catchment area and onto the Derwent moors further North.

The response of nearby rivers like the Wye were nothing exceptional with levels presenting a flood risk to low lying areas only.  Yet the Derwent exceeded all time records at Chatsworth and in Derby.

Looking at your graph I believe mBDAT stands for metres below datum.  According to Google "the normal level of the Ladybower Reservoir at Ladybower Reservoir LVL in average weather conditions is between -0.21m and 13.03m".

Yet it is clear looking at your medium-term data that the reservoir level is being managed to keep the reservoir at or below the datum.  When the level starts to exceed the 0.0 datum there is a discharge event to move it back.  As anyone who goes there regularly will know Howden and Derwent dams often fill to full capacity and brim over the dam walls but Ladybower appears to be managed differently.

Maybe, as you say, there are safety concerns letting about letting Ladybower fill up and given the rainfall totals on the moors they visualised the mBDAT going strongly positive so preemptively dumped the water.  Looking at the graph it looks like a big overreaction as even now the levels are still well down compared to the datum.

The timing and quantity of the water dump for some of those downstream was disastrous, turning a minor flooding scenario into an all-time record breaking event.  Someone should be asking Severn Trent serious questions about what happened, but as you say, a few smoke-screen stories about the plugs, Boris Johnson in town with a mop and it should soon blow over.

[matty40s]

That's what concerns me, why was Boris drafted in to Matlock, not Sheffield, Rotherham or Doncaster. Why was it deemed to be a good time to berate some daft picture taking bloke, and why was it necessary to show Ladybower plughole working as almost normal as the valley below drowned.

[cuckoo]

severn trent info

above link about dam safety.  The section that caught my attention about embankment dams like Ladybower 

"If the downstream face of the reservoir is saturated with water, for example after very heavy rain or due to a leak from the reservoir, it will be weaker and more prone to failure by slipping along a plane of weakness within the embankment. The strength of the embankment can also be reduced over time. This reduction in strength can be caused by: • natural weathering • tree roots • burrowing animals"

Maybe there are some concerns about slope instability at Ladybower which triggered the water dump?   Doubt I'll find out without working there.

[The PIT]

I noticed earlier this year that despite plenty of rain Ladybower was lower than I expected so perhaps they have been doing some work which hasn't been completed yet.

[cuckoo]

Looking at the historical data, I've been reading your graph the wrong way round.  At the time Ashopton was visible, late summer/autumn 2018, the level is given as +13.01 mBDAT, which shows the positive numbers correspond to low reservoir levels and the negative numbers are when the reservoir is full. 

Reinterpreting the recent graph that you posted it now looks very different.  With all the rain on 07th the reservoir begins to fill and goes from +0.0 to -0.5.  The normal operating level at -0.21 has been exceeded.  Allow an embankment dam to overtop would be dangerous so water starts to be released into the river moving the level back towards -0.3 and the normal level.

Maybe there's no conspiracy after all! 

If they were prepared to let it fill to -0.5 though couldn't they have held it for longer at that level before gradually releasing back to -0.2 rather than the sudden release at the worst time?

  

[SevernTrent]

Hi

I'm from Severn Trent and I'm afraid that you're misreading our figures. A move from -1 (roughly) to -3 is actually an increase in water levels at the reservoir (as you'd expect when so much rain falls). A plus figure means the level is below the overflow and a negative is above overflow. A higher negative figure means the level is higher above the overflow (i.e. an increase in the amount of water in Ladybower). Had we released water, it would have likely gone from a negative to a positive. It clearly doesn't do this.

Just to confirm that we only discharge when asked to by the Environment Agency and we haven't had such a request since early October. We didn't release any water before or during the weekend floods and, in fact, we haven't operated the valves for well over a month.

In fact, our teams have been out and about trying to protect our assets from the flooding and helping with the relief efforts where we can.

Hopefully that clears things up for you.

[SevernTrent]

Hi

I'm from Severn Trent and we thought it was important to give you the correct information.

The initial premise - that we must have opened Ladybower on Friday is down to a misreading of our figures. A change from -1 (roughly) to -3 is actually an increase in water in Ladybower (as you'd expect with so much rain on the day). Basically,  a plus figure means the level is below the overflow (i.e. lower) and a negative one is above overflow (i.e. higher). A higher negative figure means the level is higher above the overflow which, obviously, wouldn't be the case if we'd opened up the dam and released into the Derwent.

As you're undoubtedly aware, we only release when asked by the Environment Agency (as they look after the Derwent) and we haven't been asked to do this since early October (i.e. over a month ago). There was no release on Friday.

In fact, our teams were out and about over the weekend protecting our own assets (treatment works and pumping stations) from the floodwater, as well as helping emergency services and local councils in the flood relief efforts.

I hope this clears things up.

[matty40s]

Thankyou very much for enlightening us, I do see one or two others worked out the upside down graphs, and it does make sense.....I still think doing the press release about the daft chap taking photographs was remarkably badly timed.

 

[SevernTrent]

The BBC story was nothing to do with us - they approached us and we tried to talk them out of it as we didn’t want to encourage others to do the same thing.

[The PIT]

Thanks for clearing that up.

[cuckoo]

8 hours ago, SevernTrent said:

Just to confirm that we only discharge when asked to by the Environment Agency and we haven't had such a request since early October. We didn't release any water before or during the weekend floods and, in fact, we haven't operated the valves for well over a month.

Thank you for coming on to explain the upside down graphs and to clarify that no valves were opened to release extra water before or during the weekend floods.

However a lot of water was still released into the Derwent from Ladybower through the plugholes (as you confirmed this occurs when the level is at a negative number).  The response of the Derwent (rising to all time records in a number of locations) was out of step with other nearby rivers (e.g. Wye, which stayed below levels where property was at risk) suggesting that the water coming through the Ladybower plugholes was making a big difference.on the Derwent. 

You said you didn't release any water before the floods but wouldn't it have made sense to do so?

Releasing water before the well forecast rain event and taking the reservoir level to a positive number would have left Ladybower in a position to absorb a lot more water prior to the level exceeding the plughole height and starting to overflow.  I don't have access to the Environment Agencys river level prediction models but prior to Thursday I would have thought the Derwent was at a level where it could cope with some extra flow and would only flood low lying land.  

I understand that it might have been the Environment Agency's call but it looks like the level is not being very actively managed and doing nothing prior to the weather warning may have been a missed opportunity to mitigate the level of impact of the floods.

[SevernTrent]

Sorry for the delay - yes, you're correct water was going down the spillway but that's obviously a controlled release that's pretty much BAU at this time of year. It would have contributed to river levels but the month's worth of rain in a day would be the factor that made the difference.

So a release from Ladybower only comes about if the Environment Agency requests it as it affects the Derwent, which is their responsibility. You'd have to ask the EA what the river was doing and whether it was a viable proposition before Friday. Our role only relates to the res I'm afraid and we wouldn't release withou the EA telling us to do so.