Belfast underpass vulnerable to flooding

A multi-million pound underpass submerged under 20ft of water only a month after opening remains vulnerable to further flooding episodes, an inquiry team found today.

Motorists were forced to abandon their vehicles in the M1 underpass on the outskirts of Belfast when a nearby river overflowed and swamped the newly completed section of the £104m (€122m) city bypass upgrade.

An independent team of experts commissioned to investigate the incident in August at Broadway has warned it could be repeated if changes to the flood defence system are not made.

Amey Consulting said though the scale of the downpour that fell on the city had not been experienced since 1914, the underpass should have coped.

The culvert diverting the Clowney Water around the motorway was designed to withstand a one-in-100 years scale of flood.

The August storm was not as severe as that and rated at a level between a one-in-50 year and one-in-70 year flood.

The consultants have recommended that a £40,000 (€47,062) research project is now undertaken to establish what changes can be made to the drainage system.

“There remains a potential for further flooding of the underpass until the hydraulic efficiency of the system has been checked and any required changes and other mitigation measure are in place,” the report concluded.

The experts said there were a number of contributory factors that caused the flood, including:

:: A blockage of a trash screen at the mouth of the culvert

:: A valve to divert water exiting the culvert into another relief culvert was not fully open

:: Banks of the open section of the Clowney Water had not been built to the required level of 500mms above the anticipated water level of a one in 100 year flood. At the time of the August storm it was 300mms above this level. Amey said while the extra 200 mms would not have prevented the flood it would have delayed it.

As well as the modelling exercise to examine the drainage system, the consultants have recommended the raising of the banks, the introduction of a more robust early warning system with more liaison with the Met office and more regular inspections of the culverts.

The banks have already been raised to the 500mm level and a new monitoring process has been introduced.

The culvert system installed by contractors Highway Management City (HMC), which took the river round the bypass, was different to the suggested system on the original plan drawn up by the government’s Roads Service, which took pipes over the road.

However, the new method gained the required approval from the Rivers Agency and Amey said there was no evidence that the original design would have coped with the flood any better.

When plans for the upgrade of the city bypass were drawn up it was intended that the existing Broadway roundabout would be replaced with a flyover.

The decision to opt for an underpass was taken in 2000 after a public inquiry raised environmental concerns about the impact of a huge bridge over the road.

The Amey inquiry was initiated by Stormont Regional Development Minister Conor Murphy.

Outlining the findings of the report to the Assembly, the minister said the Roads Service fully accepted the findings.

“The report has highlighted that further work will be necessary as there are a number of complex contributory factors identified which may have impacted on the flooding,” said the Sinn Fein Newry and Armagh Assembly member.

He said his department would now be contributing to the modelling exercise.

“In partnership with the DBFO (Design, Build, Finance and Operate) company (HMG) and the Rivers Agency, my Department has begun the process of arranging a commission to build a model of the drainage system which will be used to determine exactly how each of these factors impact upon one another during periods of high water flows,” he added.

“Depending on the findings of this further analysis additional physical measures may be necessary to mitigate against a similar occurrence.”