WATER, WATER EVERYWHERE
With lowish levels being a concern this year on some canals the reservoirs have come under scrutiny – but it’s not just too little water the engineers worry about
Just how would the CRT manage an emergency in the event of a breach
The cold Pennine winter of 1830 saw a thick layer of ice cover the surface of the Lancaster Canal’s Killington Reservoir, high up in the hills west of Sedburgh. But then the weather turned milder, the ice began to break up and thawing snow on the surrounding hills swelled the waters. The wind veered around to the north, blowing the broken ice down towards the dam, where it blocked the overflow spillway. With nowhere else to go, the rising waters overtopped the dam and began cascading into the valley below…
The reservoir-keeper saw what was happening and, showing quick-thinking and presence of mind, grabbed a spade and set about creating a deliberate breach alongside the dam to get rid of the excess water. His prompt action prevented what would have been a major disaster if the dam had collapsed.
Almost two centuries later, we can still see what is probably the remains of the channel washed out by the water as the reservoir emptied itself through his makeshift cut. I’m visiting the reservoir with David Brown, a senior reservoir engineer with the Canal & River Trust, who is supervising work taking place at this site as part of a regime of inspection, maintenance and improvement work aimed at ensuring that the events of 1830 will never be repeated at any of the Trust’s reservoirs.
This isn’t just a CRT matter: all of Britain’s reservoirs are subject to statutory legal requirements – and with good reason. Disaster may have been averted at Killington, but the people of Cwm Carn on the Monmouthshire Canal weren’t so lucky in 1875, when heavy rain caused the dam to overflow and fail, killing 12 people.
Failures of two drinking water supply reservoirs in the 1920s resulting in 31 deaths led to the 1930 Reservoirs Act, and to today’s ten-yearly independent inspections and reports by members of a panel of qualified civil engineers – backed out by more recent legislation, with failure to comply being a criminal matter. And, to date, there haven’t been any more deaths from reservoir failures since then.
Often tucked away in valleys some distance from the canals they serve, CRT’s reservoirs form a part of its system that many boaters probably rarely think about – other than when their water stocks start to run low in a dry summer, as looked like being the case this year.
But it’s the opposite issue that most
concerns the Trust, which is spending £4.3m this year to ensure adequate protection against the risk of too much water – and it’s largely driven by those ten-year reports.
So what’s in the reports? Firstly, general information such as details of the reservoir and its catchment area; information about the ‘draw-down plan’ (for emptying it in an emergency); construction details and the local geology. Secondly, the findings of the inspection (with lots of detail about leakage and seepage).
Then the conclusions: is the draw-down plan adequate? Is supervision satisfactory? Will it survive a ‘once in 10,000 years’ flood event completely unscathed? Will it deal with the ‘worst meteorologically possible’ event with only minor damage? And, finally, there are recommendations on works needed to bring it up to scratch.
Killington is a relatively early reservoir dating from 1819 – and typically for this era, there are few records of how the dam was constructed. It’s believed to have a puddled clay core, but nobody can be sure without digging down. It’s quite normal for builders to have simply used what soil was available (such as sand or even peat) and not to make any attempt to dig down to a solid base before building it – often the original topsoil is still there under the dam.
It’s also been altered several times: the dam was raised to increase capacity (early engineers often underestimated water requirements) and we can still see where a large block of stonework was added to raise the height of the overflow
‘The report concludes that the dam will cope with all the weather can throw at it, the overflow is adequate, the level can be lowered in a hurry if necessary and, in general, it’s adequately maintained – but with a few provisos’
channel – and then a second overflow channel was added after the 1830 incident. The draw-off system has been changed more than once, as a siphon pipe set in the dam replaced the original culvert, and was then supplemented by a second siphon.
Unusually, rather than supplying a canal feeder channel, the water from the reservoir is fed back into the stream below the dam. It is then taken out several miles downstream (metered, to make sure the same amount is taken out as is put in), where a feeder carries it to the unnavigable northern reaches of the canal, and it eventually runs down the disused lock flight at Tewitfield to reach the navigable length, a full 15 miles from where it left the reservoir.
But the report concludes that the dam will cope with all that the weather can throw at it, the overflow is adequate, the level can be lowered in a hurry if necessary and, in general, it’s adequately maintained – but with a few provisos.
There are various recommendations which are mainly maintenance issues – so we can see one team at work replacing sections of the overflow channel’s base in concrete to deal with erosion, while another group are using a dye to investigate leakage in a further section of the overflow, and in the block of stonework mentioned above, prior to grouting work to seal the leaks.
Interestingly, two unrelated projects are taking advantage of the lowering of the water levels for this work. Firstly, a local community hydroelectric scheme is being set up, taking its feed from the cast iron draw-off pipe (which has involved some clever tapping into the pipe while still under some pressure).
Its turbine is currently working in parallel with the 1930-built valve house to control the take-off of water from the reservoir; in future the entire main flow to the Lancaster Canal will pass through the hydro system and generate 30kW for the local area, with the profits going to the community. All the work has been done by local companies, and it’s being championed as a demonstration of the capabilities and possibilities of
partnerships with communities, energy groups and CRT. The other project is to install a floating landing stage for a sailing club based on the reservoir and used a great deal by groups supporting disadvantaged children.
Our second site visit is to Foulridge Upper Reservoir, built to supplement Foulridge Lower Reservoir, which in turn feeds into the Leeds & Liverpool Canal just west of Foulridge Tunnel – and it’s quite a contrast. A much later reservoir dating from 1866, it’s hardly been altered since it was built, other than adding a low concrete wall along the top of the dam in the 1950s to deflect waves. But here, the issues aren’t about maintenance: they’re about coping with eventualities.
The inspection report indicates that an extreme flood would overtop the dam, washing away the path behind the concrete wall and ultimately threatening the dam itself; and that in addition, water passing down the overflow spillway could overflow the sides, wash out the adjacent ground, and again damage the dam.
The work in progress involves raising the dam crest wall by adding another half a metre of reinforced concrete – as we watch, a team are at work dismantling the wooden shuttering around a section that has recently been cast. Meanwhile the soil alongside the overflow spillway chute is being dug out and replaced in a way that will be better able to withstand a heavy flow of water. This involves putting concrete behind the channel side walls, installing a geotextile membrane and gabions (wire cages filled with stones) and reinstating the topsoil on top.
We’ve looked at just two of CRT’s 72 reservoirs requiring regular inspection. Work in progress elsewhere includes grouting spillways at Slipper Hill Reservoir, investigating leakage at Barrowford, and investigating the state of the crest wall at Winterburn (all on the Leeds & Liverpool).
Then there’s raising the dam at Tardebigge and lining the cast iron draw-off pipe at Upper Bittell (both on the Worcester & Birmingham), adding new siphons to increase the draw-off capacity at Bosley on the Macclesfield and dealing with mining subsidence at Elton on the Manchester, Bolton & Bury – the fact that the canal has been abandoned doesn’t free CRT of its responsibilities.
But if, despite all this work, the worst came to the worst and an emergency arose, how would CRT cope? There are inundation maps in place for every reservoir, showing the maximum extent of flooding in the event of a breach, and for those with the more serious consequences there are emergency plans detailing which buildings would need evacuating, and which roads would be unusable.
To find out if the plans work, a recent exercise was carried out by the Trust in conjunction with the British Dam Society (yes, there really is such an organisation). At the start, none of the staff knew that it was just an exercise – it began with a phone call in the middle of the night to say there was an emergency at Foulridge Lower Reservoir, and it ended with them demonstrating that it actually was possible to get one cubic metre per second of emergency pumping capacity set up to deal with it by the end of the same day.
So there are grounds to be reassured that even if, despite the inspections, reports, and recommendations, the worst were to occur, the situation could be handled with a rather less desperate response than one man and his spade.
And let’s hope that with wavering water levels on some canals this summer there will be plenty in store for next year.
Rebuilding sections of Killington’s overflow spillway base
Pressure grouting the spillway base
The Foulridge dam, with concrete crest wall
Good for a ‘once in 10,000 years’ flood? The 1819-built Killington dam
Repairs to Killington’s overflow spillway
Foulridge Upper Reservoir: rebuilding the spillway channel sides
Raising the Foulridge dam crest wall