Holding back the tide
After one of the hottest summers on record, the deluge came down hard. It is this type of switching between extremes that climate change specialises in, and it causes engineers nightmares by bringing unprecedented challenges.
In Derbyshire, the Whaley Bridge dam came perilously close to collapse, while across the North the repair bill will runs to millions with bridges destroyed, roads severely damaged and hundreds of houses flooded.
While the evacuation of Whaley Bridge, just beneath Toddbrook Reservoir ended on Thursday (7 August), with the dam having been made secure at the time of writing, questions remain over the legacy issue the country is facing with many of its older civil infrastructure.
An RAF Chinook drops sandbags on the Toddbrook Reservoir dam, in a bid to stop the 19th century structure collapsing.
More than 2,000 homes are at risk and thousands of people have been evacuated from Whaley Bridge.https://www.itv.com/news/2019-08-01/evacuations-in-whaley-bridge-as-dam-wall-partially-collapses-in-derbyshire-town/ …
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Environment department Defra suggested it would not launch a national review of similar legacy dams and other structures for the time being, despite concerns raised by experts.
David Hindle, is a semi-retired chartered engineer with the Institution of Royal Engineers and the Institution of Civil Engineers. He is also a Territorial Army Colonel, and has worked in engineering for 40 years on a range of structures.
He told Transport Network there were 'enormous legacy issues', as Victorian structures across the country reach the end of their design life span. On top of natural degradation, many of these structures have been re-purposed or are now used for much larger, modern stresses than were originally intended - as in the case of Hammersmith Bridge, which recently closed to motor traffic.
The Canal and River Trust, which owns the Toddbrook Reservoir, said the damage to the spillway – a structure that allows floodwater to pass over, through or around a reservoir when it is full – occurred after a period of very heavy rainfall on 31 July.
It said that it was ‘still too early to say how this has happened and a full investigation will follow the emergency response’ but that the intense rainfall ‘is thought to be a significant contributory factor’. Engineers on site will commence surveying the dam once it is safe to do so.
It added that the last detailed expert inspection of the reservoir ‘took place by highly trained reservoir engineers in November 2018’. It said this was an independent 10-year inspection, with the reservoir also inspected annually and the dam visually inspected twice weekly by Canal and River Trust operatives. The reservoir was built in 1831.
The pressure from the rainwater may have caused the concrete construction joints to fail on the dam's spillway, Mr Hindle speculated, warning that if there has been damage to the dam’s clay core it will probably need to be fully replaced.
BBC Radio Derby
Whaley Bridge dam repair: How events unfoldedhttps://bbc.in/2yVxIyN
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It is not just water damage in land that is presenting a threat to the nation's infrastructure – the coast is in retreat too.
The Committee on Climate Change (CCC) has said that 520,000 properties in England, including 370,000 homes, 'are located in areas at risk of damage from coastal flooding and 8,900 properties are in areas at risk of being lost through coastal erosion'.
Damages from flooding and erosion are over £260m on average each year, the CCC said and predicted that by the 2080s up to 1.5million properties may be at risk.
'In addition, 1,600km of major roads, 650km of railway line, 92 railway stations and 55 historic landfill sites are at risk of coastal flooding or erosion by 2100.'
One new technique to be used in the fight against coastal erosion is called 'sandscaping' - a strategy pioneered in the Netherlands.
In what is thought to be a UK first, a £19m sandscaping project will see about 1.8 million cubic metres of sand pumped ashore to protect around 400 homes through the creation of a giant sand dune on the Norfolk coast. Two thirds of the cost of the Bacton and Walcott barrier is being paid by Shell and other companies operating the Bacton Gas Terminal, which processes a third of the UK’s gas. Another £5m is being funded by the Environment Agency and North Norfolk council is paying £500,000.
However this is not a permanent solution and the dune is only predicted to last 15-20 years at most before it is washed away.
Roads also face problems from shifting ground.
Mr Hindle pointed out that many roads, for instance in areas around Wales and Bath, are built on unstable land with whole hillsides subject to geological movement.
This makes them more likely to experience landslips in the event of heavy rain. While authorities often pave over the problem every few years with new asphalt layers, sometimes the entire foundations will need to be dug out and rebuilt with the earth above and below the road secured. Mr Hindle works as a technical adviser for ISS Ltd (Innovative Support Systems) which was called out on this type of a job to the A4233 in August 2017 on the Maerdy Mountain Road Improvements scheme.
The Ramwall being assembled on the A4233
ISS provides a unique Ramwall ground support system that can be used on various types of embankments including railways, motorways and roads and was originally built for rail projects to replace the use of gabion baskets.
It is a simple gravity wall with continuous steel reinforcement and can be anchored into the ground and the embankment behind.
Ramwall (reinforced aggregate matrix wall) uses a steel mesh approved to BS 4483:2005 and galvanised to BS EN ISO 1461:2009 to give a life expectancy of at least 100 years or more.
On the A4233, a retaining wall was need along the toe of a re-profiled slope, which had suffered a landslip after heavy rain. Piles were sunk on the downslope and the Ramwall used above the road.
Lead contractor on the £1.1m scheme, Alun Griffiths said: ‘Due to access constraints, a bespoke piled edge beam detail was designed which required only a small pile rig and mitigated vast temporary works. Piles were also designed with grout rather than concrete to increase reinforcement cage sizes without increasing pile diameter.
‘The design allowed for up to one metre of embankment erosion in the future. This pile system and edge beam increased the rigidity of the “RAMwall” retaining structure, which consists of mesh panels that were O-ringed together for quick onsite construction.’
The Ramwall was mostly one metre in height increasing to 1.5 metres over a 30-metre stretch of the slope. It is filled with graded granular rock fill and backed by a waterproof membrane and provided with a back of wall catch drain at the bottom of the slope constructed using concrete canvas.