Taking back flood control
Source: RTM Aug/Sep 16
Network Rail’s commercial scheme sponsor, Joanna Grew, talks to RTM about the recent flood alleviation works carried out at Hinksey.
Over the last 14 years, flooding on the Didcot to Oxford railway line between Hinksey has led to 11 closures and severe disruption to passengers and businesses through the cancellation of services. But this looks all set to change.
Network Rail has just completed the main work on the £18m Hinksey project, which centred round raising the track height by 400mm and installing two culverts under the railway to allow water to flow from one side to the other.
The project, which went through a protracted planning application, involved closing the railway for 16 days and saw engineers install 44 culvert sections under the track.
The civils element of the work took place in the first five days of the closure, said Joanna Grew, Network Rail’s commercial scheme sponsor: “We had to undertake quite a large excavation and then install the flooding material and then the pre-cast concrete units were lifted in by a large crane.
“The culverts are 3.6m by 1.8m metres, and we have two side by side going across the length of the railway connecting the two watercourses.”
As well as installing the culverts under the line at the north section, near the Dairy Crest site, the team replaced the deck of Strouds underbridge to the south of Abingdon Road in order to raise the level of the track.
“We were putting quite a lot of additional ballast under the tracks in order to give us the 650mm increase at its highest point,” said Grew. “We also had to replace all of the S&C in the area: there were nine point ends, and also a track point that needed replacing as well.”
Future-proofed Grew added that the infrastructure owner believes the 11 closures of the past would have been mitigated against with the improvement measures taken. “If we have the most severe flooding we may still find that the area is affected in some circumstances,” she admitted, “but it would be for a much shorter duration and have a much lesser impact on the railway.”
AECOM were the designers for Network Rail on the scheme, and also did the work related to the project’s flood risk assessment.
One of the project’s challenges, Grew noted, was the planning application process, which saw the company’s plans thoroughly scrutinised by Oxford City Council, the Environmental Agency and the very vocal Oxford Flood Alliance.
“When we raised the railway we were creating, effectively, a dam across the floodplain, which is why we needed to install the culvert which balances out the water on either side,” she said. “But it is very difficult to not make any changes anywhere, and Oxford City Council was very mindful of any impact it might have on residents.
“We put a lot of work into the flood risk assessment to be able to satisfy the Environmental Agency and Oxford City Council.”
She added that the Environmental Agency also had a Flood Alleviation Scheme in the pipeline for Oxford, which needed to be considered in Network Rail’s work.
“They also have a need to move water from one side of the railway to another to create a channel that allows them to take water around the residential areas,” explained Grew. “We have worked with them very closely. In fact, it was about this time last year we signed an agreement with the Environmental Agency which allowed us to increase the size of the culverts to allow for passive provision of their requirements in the future.
“We have actually put in bigger culverts than we need, but we will, temporarily, block the ends up with orifice plates which restrict them back down to the size that we need them to be.”
Other challenges that faced the team included late ecological design changes, which meant undertaking some extra vegetation treatment; checking the site for potential archaeological remains; and coming across a raft of unexpected underground cables.
“We faced lots of underground cables that we didn’t necessarily know were there,” she said. “In some cases they were found to be redundant, and were removed. Any live cables though needed to be safely moved out of the way. In some cases, we had to erect specific cable support structures in order to lift the cables out of the way so we could work around them.”
Track renewal and remaining work
After completing the civils aspect of the programme, the track work was handed over to Network Rail’s contractor, Colas Rail, to renew 200m of plain line track.
“They had to replace quite a lot of plain line track in the whole stretch,” said Grew. “It is an area about 200m long, but there are four tracks in that area. We replaced all of that with brand new track. At the same time we raised it, so there was quite a lot of ballast that went down.”
Network Rail handed the line back on time, but the team still has some work to complete.
“We still have to connect the culverts up at either end with the watercourses,” noted Grew. “In the possession, we did all of the area underneath of the railway but we hadn’t ever planned to do the entire stretch – there was just too much to do in that time.
“The two ends need connecting up, which means installing dams at either end because they connect in to the watercourses, and installing the remaining culverts.”
This work, she added, would be done before the autumn, but wasn’t reliant on extended possessions.
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