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Chalgravesteve

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  1. 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.
  2. 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.
  3. 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
  4. 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
  5. 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.
  6. I don't disagree with any of this - except that if the engineering inspections say its all OK, why would they query that? The experts whose job it is to inspect and ensure the safety say its all ok! You would expect the engineering inspections teams to be absolutely aware of the California spillway failure and to be more stringent on inspections on anything constructed in a similar manner or a similar age etc. All we are doing of course, is speculating on what might have happened and what information people might have had. The one thing I really don't think anyone could have done, is accurately predict the volume of water that cascaded into the reservoir in the first place. If the wind had blown that storm 50 miles to the east or west, it would have saved 10 pages on an arbtalk forum as well as the reservoir!
  7. That's the whole point though. Unless you are suggesting deliberate negligence, no one knew or expected that the slip way would fail. Why would they? What crystal ball do you have that they dont have, so you know it was going to fail before it did? Their expectation is that it will do the job that it was built and designed for, 50 years earlier. Even if they allowed the reservoir to continue to fill once every year or so, until the water overtopped the spillway and flowed down it, the volume of water passing down the spillway in that "test" would be nowhere near the volume that came over in the storm. You cannot simulate that storm because the normal inflowing river only carries are certain volume of water in and the same volume would flow out over the spillway in the same time. As someone said earlier, if Cart had emptied the reservoir because a thunderstorm was predicted and then it rained for a bit but there was now not enough water for the canal network, they would have been ridiculed. I would have thought that if they tried to use the canal network to empty the reservoir into (as they must have some mechanism for getting water out of the reservoir and into the upper reaches of the canal to top it up all the time - is that gravity or pumped?) there is no way that the canal network could absorb that volume of water because of lock gates and sluices etc. Its a system that needs a controlled level of water. The river Goyt on the other hand, is more capable of handling a higher level of waterflow, Clearly though, given the fact that they have been pumping water out of the reservoir for a week, there is no direct controlled gravity feed discharge from the reservoir to the Goyt. They might be able to divert the inlet source but once its in the reservoir they appear to be stuck. You would hope that whatever solution they come up with to make the bank above Whaley Bridge safe, and the spillway safe, that this would incorporate a means of being able to discharge volumes from the reservoir safely.
  8. One thing with concrete, once its in place and surrounded by more concrete, is the friction between pieces is absolutely immense. You try breaking up a concrete driveway. You might crack it but you cant lift pieces out. Once you do make a hole though, it starts to work the other way, the weight of the concrete and the void underneath means that a piece can fall and what was previously seemingly impossible to break now snaps because its brittle. Those photos that I've seen of the damaged spillway, don't appear to show reinforcing bars or mesh in each slab, so its "just" concrete. Once the spillway surface is breached for whatever reason, with that level of water passing over it, there is only going to be one outcome, unless the water flow stops.
  9. I'm only giving you my opinion on what I've read and what I know about reservoir design (having actually had a reservoir designed and built for me). I do know that there is a mandatory requirement for inspection on an annual basis and I think I'm correct in saying that an independent reservoir panel engineer has to inspect at least every 10 years. I'm also reasonably sure that I've read somewhere that an independent panel engineer said there was nothing in photos taken prior to the failure that caused them concern and I also think there was a report saying that an IRP engineer inspected and passed the reservoir (which would include the spillway and and any vegetation growth on it) in April of this year. I'm happy to take on board other peoples views, I was interested in the thread in the first place because of the design issues that came up in my own project and I felt I had something to contribute. I hadn't considered that the lack of maintenance might apply to the lower network of canals resulting in a reservoir being kept over full in order to ensure that there was sufficient water for the canals, rather than the reservoir itself. Clearly if you are lost for words about my comments, you have personal knowledge of the site, the extensiveness of the vegetation and the fact that trees have grown through the spillway slab and rendered it ineffective. If you do have that level of actual factual knowledge that the spillway was fatally damaged before the storm and the overtopping, that might amount to negligence for individuals and the CaRT and that is an extremely serious matter.
  10. I don't see how they have "let the slipway fail"? I would struggle to believe that the slipway was originally constructed with a void underneath it, if there was a void there all the time, then surely there would be a means of access to that void to ensure that nothing was happening in the void out of sight? So, logically, there shouldn't be a void and the concrete slabs and edges are a reinforced area, on top of the reservoir banking, designed to handle the water from the emergency situation where the reservoir overfills and overtops, so that catastrophic erosion and collapse doesn't occur. I think that with the events of the past few weeks, there is every chance that the reservoir bank itself, made from soft soils and loose aggregate, under the slipway has become damaged through erosion. Clearly they cannot take the risk that a repair to something that has clearly failed once, and a repair onto something that might be unstable as well, so they need to do a rebuild job. If I was them, I would be removing a section of that banking and rebuilding the whole thing in concrete where the new water spillway will go. I would be putting hydraulic sluice gates into it at the same time, so that if you ever had to reduce the water level again, quickly in an emergency, you can do so by opening the gates and controlling the outflow through and an area that is designed to withstand the forces involved, instead of having to lay countless pipes around and over the edge and pumping it out. Open the gates and let gravity do the work for you. I would be putting a double gate on all sluices, and not relying on one gate. In the meantime, the inflow is blocked and what would be entering the reservoir is directly flowing into the river/canal system? As we move into the winter/high rainfall periods, this might put low lying areas close to those water courses more at risk than they were previously, because the reservoir is not there to absorb short term high volumes of rainfall as it has done for the past 180 years, until they have rebuilt the overflow spillway.
  11. Ah I see, the network is pissing water out everywhere so they need lots of water at the top! They do need to keep the reservoir reasonably full, it would actually probably cause more maintenance issues of the reservoir itself if you allow the upper sections to dry out and become overgrown. There will always be two sides to a story, its probably far easier for them to keep pushing water into the system than to shut and drain sections (which causes disruptions to others - who will complain vociferously when it directly affects them, but will otherwise just let it pass when it doesn't!), but that becomes necessary when safety is compromised. Modern policy on a lot of infrastructure these days seems to be patch and repair until its so bad it can't be!
  12. 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).
  13. The reservoir has been there 180 years and this is the first time it’s been breached? That’s once in180 years then you could argue that without the spillway being constructed 50 years ago, the dam would have overtopped and it would have collapsed. The spillway, despite sustaining damage did its job
  14. I saw a YouTube video showing the overflow channel that comes around the side of the reservoir with deep fast flowing water. It was flowing well before the slipway started to discharge (into the same channel). The channel couldn’t handle the volume of water being thrown into it and areas like a children’s playground were getting flooded. So there were efforts to divert the inflow to the reservoir before it overtopped the spillway.

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