A look at Derby’s new flood defences
- Credit: Archant
As worries about the effects of rising river levels come to the fore, Geoff Ford finds out about Derby’s new flood defence system
Global warming is something we simply cannot escape and while there does seem to be progress with landmark agreements on the international stage, the effects are being felt with increasing regularity and intensity. Derbyshire has escaped the devastation seen in Cumbria in recent years but extreme weather events now seem to occur anywhere.
Marketing Derby chief John Forkin has highlighted that the city of Derby does not make good use of the River Derwent, one of its prime assets and also historically important as the place that saw the birth of the Industrial Revolution. New apartments have appeared along the water’s edge and it is hoped that the Riverlights development will be the catalyst for the city to turn and embrace its river once more.
It is reassuring, then, to see that work on Derby’s Our City, Our River flood defence programme is well underway to protect homes and businesses against the threat of flood damage in the future. The Environment Agency had been working on the scheme for a number of years before work on the ground began in early 2016. The scheme is to be delivered in three packages and should be completed in 2023. Work is in progress on Package 1 and Project Manager Dave Powner recently took time off from his busy schedule to give me a guided tour.
‘There are over 2,200 properties and businesses that we’ll protect,’ Dave told me. ‘Specifically they’re in the Little Chester area, which is prone to flooding, together with parts of the city centre. Package 1 will cater for the potential flooding around Little Chester. Packages 2 and 3 will protect the city centre and areas further downstream. The overall scheme has been assessed at a cost of £95 million.’ The first phase has been funded with £17.1 million awarded as a Flood Defence Grant with a further £13 million available from the Growth Fund.
There are some existing flood defences but, Dave says, these are in a poor state of repair. ‘They are not sufficient to cater for current forecasts. We are now building them to a much higher specification to take account of a one-in-a-hundred-year event. The Derwent has a big catchment area in the Peak and that is taken into account when predicting what effect a one-in-a-hundred-years event would have down here.’
Cumbria, and in particular Cockermouth, has seen these events all too frequently. The 2015 floods in the wake of Storm Desmond were the fourth time that Cockermouth had been severely hit since 2005. Record rainfall in November 2009 brought severe flooding to Cockermouth and a police officer lost his life as a bridge was swept away. By June 2013 a £4.4 million flood defence scheme had been completed in Cockermouth but this was overwhelmed in December 2015 as the rainfall record was broken again. Four hundred and sixty-six properties in Cockermouth were flooded, around 5,200 across the UK. The economic losses were estimated at £2bn and another life was lost in Cumbria.
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‘It’s so difficult to predict future events,’ says Dave. ‘We thought the flood defences in Cumbria would last for a hundred years but two years later the flooding was worse still.’
Contractor GBV Ltd (a partnership between Galliford Try and Black & Veatch) began work on the Our City, Our River project in January 2016 by removing trees that were within the flood defence construction work area. A comprehensive replanting programme will ensure that there is a net gain in the number of trees once the project has been completed.
An archaeological investigation by Trent & Peak Archaeology included the remains of the Roman encampment and the settlement of Derventio. ‘Little Chester Roman fort has been the subject of a number of surveys over the years,’ says team leader Gareth Davies. ‘This offered the opportunity to tie together earlier observations and ask new questions of this site.’ The survey also brought together the community who volunteered to help with the Big Dig last May and processing artefacts recovered from the survey.
The new flood defence project begins to the north of the city with the protection of commercial properties along Alfreton Road. The Derby Rugby Club fields and Darley Fields recreation ground are designated as flood plains with the commercial properties protected by a new flood defence wall. This is a physical barrier of large steel sections pushed into the ground to prevent underground seepage, with around 1.5 metres (5 feet) above ground to contain rising floodwater.
Galliford Try project leader Peter Richardson explained the piling process. ‘The piles are 4.7 metres (15.4ft) long and vibrated into the ground, a more efficient method than the old steam hammer. The vibration moves the ground, it’s like a knife cutting through butter. This is a weathered mudstone so we don’t need to go deep into stone. It is a steady push.’ Each pile on this half mile stretch takes just two minutes to drive in, but with setting up time, it is possible to install ten piles in an hour.
‘The old flood defence, a clay bank, has been topped up by around 900mm (3ft),’ Peter continued. ‘The pile hits a clay lens to stop the water from the football pitches coming underneath. That is a big problem if the flood lasts for a long time, the water getting underneath from infiltration. Along the back of the industrial estate the piles left are in a rusty state but are capped to make them more aesthetically pleasing.’ The piles are corrugated for stiffness, interlock with the adjacent piles and the joints are seal welded. Nearer the river a lower bank with its footpath along the top will remain to protect the field should the flooding be less severe.
In the Darley Abbey conservation area each property must be dealt with individually. ‘With a conservation area, we are limited to what form of protection we can offer,’ says Dave Powner. ‘We will be asking residents/owners of properties at risk of flooding if they want property protection which includes waterproof doors and flap valves on outlet pipes to prevent flood water from coming in. This is covered by the scheme at no cost to residents.
‘During recent flood events the Darley Abbey bridge has been struck by debris coming down the river which has caused some damage. It is likely that the current weight restriction will have to be reduced as a safety precaution. We believe that the ownership of the bridge was passed on to Crown Estates following a liquidation order. The City Council is contacting Crown Estates to find out if they will be carrying out any structural assessment.’
Along Darley Park, to the west, the slope of the ground is a natural defence while on the east bank the wall continues around the Darley Fields sports changing facilities and protects two of the oldest properties in Little Chester, Derwent House and Stone House Prebend, the home of Joan and John D’Arcy. Their house is possibly the oldest private dwelling in the city, dating back to 1593, and has been protected from the river by a 5ft-high earth bank erected by the Environment Agency in 1968.
‘There has been high water,’ says Joan, ‘but the existing defence worked in 2000 and 2001. The water came over the river bank but our defence wasn’t breached. It was around two feet from the top.’ Beyond the earth bank are the remains of a small wood, once a tree nursery. ‘It’s a short copse,’ Joan added, ‘but it came to the bottom of the flood bank and these trees had to be felled. That was the worst bit for us. There were an awful lot of trees felled.
‘The workmen have been as thoughtful as they could be and friendly but, of course, heavy machinery is noisy. The new wall should be three feet higher than the present bank.’
The existing Prebendary house was built during the Reformation with little in the way of foundations. Stone from Roman walls can be seen peeping through the lawned garden so the couple were pleased to learn that the new wall would be a little further from the house than first thought. ‘After the archaeological investigation it was decided to move the wall away from the house,’ John said. ‘That was a considerable relief to us, that they wouldn’t be putting in the piles so close to the house.’
Parker’s Piece sports ground, site of the Roman bath houses, remains as a flood plain. The route for the wall has been moved towards the river to avoid disturbing the Roman remains and the cricket pavilion will not be affected. Trees will be replaced on the dry side of the wall and a new, wider footpath will lead to the Handyside railway bridge, now used as a footbridge across the river.
The wall on the east bank continues behind the derelict A W Bliss factory towards Etruria Gardens and through the former G F Tomlinson building which is to be demolished. Passing the City Road Business Centre, the wall comes to an end next to the Waterside Inn (formerly The Bridge Inn), and ties in to St Mary’s Bridge. This long section of continuous wall is close to residential and business properties and Dave Powner says different piling techniques are used according to the location. ‘We are using a technique where the machine grips onto a pile that is already done and pulls the next one down. These piles are probably 6-7 metres deep.’
Vibration and noise can both have a significant impact on local residents and businesses so they are closely monitored with strict guidelines adhered to. ‘We carried out noise surveys prior to the start of working,’ said Galliford Try’s environment officer Ed Tonks. ‘Two or three residential areas were highlighted as high-sensitivity for noise and vibration. We sent out fliers prior to working on site to alert residents to periods of high noise, relatively short periods for a few days here and there, and we monitored those levels. For high sensitivity areas we would use a silent pile press.
‘We look for a maximum of 75dB at the site boundary which is only around 5dB above the ambient background level. Around St Mary’s the traffic on the road gave a relatively high background level, so our noise was relatively low in comparison.’
The steel pile wall is encased in concrete and faced in brickwork in a style to complement existing local brickwork. Behind commercial units the wall is full height but glass panels are to be installed on top of the wall in residential and public areas to allow views of the river. The Waterside Inn will benefit from a new beer garden on the river side of the wall, accessed through a gate.
Coming down the west side of the river, Darley Park’s topography provides a natural defence, but from Handyside’s bridge a new wall will tie into some of the existing buildings to protect the housing along Duke Street. There is a combination of new flats with flood defence walls incorporated into their build and older properties which will require protection. Their rear garden walls will be replaced with the new wall which will also protect the Furnace Inn.
‘The boundary wall of the Furnace Inn will be replaced,’ says Dave Powner, ‘but the inn has a micro brewery. We are coming out by two metres so as not to disturb the business. Next door is the Britannia Court site with a planning application to develop the area. There will be a community space and the building will form part of the flood defence wall.’
The existing defence is part path/part wall which is now in a very poor condition and will be removed. There is also industrial archaeology in the area, Dave says, ‘All the way down we’ve excavated along the line of the wall under archaeological supervision, and they’ve recorded any industrial artefacts we’ve found, mainly foundations for buildings and walls. The trench has been filled with stone and we’ll pile through with a concrete and brick wall to go on top.’
Eventually we arrive at the junction of St Mary’s Bridge, Duke Street and Sowter Road across the river from the Waterside Inn. The barrier will cross Duke Street and tie in with the inner ring road bridge over Sowter Road. Throughout the scheme there are footpaths and roads to be crossed, including Duke Street and Haslam’s Lane next to the Rugby Club, with each requiring a water-tight gate to be closed in times of emergency.
The ground next to the river, where more trees have been removed, will be lowered to allow excess water to flow around St Mary’s Bridge Chapel and through the Sowter Road underpass. ‘Basically the bridges obstruct the flow of water,’ Dave explains, ‘so we are creating a path around the outside of the bridges that doesn’t increase the height of the water. The flow is faster on the outside of the bridges.
‘On the other side of the bridge the wall will continue to the properties while the ground adjacent to the Silk Mill heritage museum will be lowered. The water will come through the underpass and across the open space to return to the river. Historically the bottom of the Silk Mill was prone to flooding and designed with that in mind. There are proposals to look at additional flood defence on the building.’
This first stage of the Our City, Our River project should be completed in March 2018 by which time funding for the next stage will, hopefully, be in place. ‘We are widening the flood defences in the city centre as well, in Package 2,’ Dave adds. ‘At the Derwent Bridge next to the Council House, the water will flow around the other side of the bridge near to the Brewery Tap pub. There are no additional culverts but the ground will be lowered. These extreme weather events will be very infrequent so that would become a green open area with a better vista onto the river and a community asset making better use of the river.’ So far, it all looks promising for a future where Derby is able to exploit the scenic advantages of the river running through it.