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Independent Broadway underpass flooding report published

2 December, 2008


Sinn Féin Regional Development Minister, Conor Murphy, today welcomed an initial independent report into the flooding incident on 16 August 2008 at the Broadway Underpass.

The independent investigation carried out by Amey Consulting found that, following a period of heavy and prolonged rainfall, the Clowney Water river overtopped its banks at the inlet to a newly constructed culvert, resulting in the major flooding of the Broadway Underpass.

The Department will take forward the recommendations in partnership with the Design Build Finance and Operate (DBFO) Company for the M1/Westlink, and the Rivers Agency.

The Minister said:

"I welcome the publication of this independent report into the events surrounding the flooding of the Broadway Underpass. Roads Service fully accepts the report's conclusions and a number of the recommendations have already been put in place.

"Firstly, the DBFO Company has already raised the banks of the open section of the Clowney Water which will provide additional capacity in the system and secondly they have implemented an improved monitoring system which will ensure that in the unlikely event of any future events, that the underpass will be closed quickly to ensure public safety.

"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."

The independent report findings show that the new drainage system did not perform in accordance with its required design capacity of a 1in a 100 year flood event. It is likely the August storm event was between 1in 50 and 1 in 70 year flood event.

The findings showed a number of possible contributory factors, though no sole cause including: discharge from the Overflow Structure may have been restricted; a large valve called the Penstock Valve positioned at the entry to the Blackstaff Culvert may have restricted flow from the Overflow Structure; and that there was partial obstruction of the Trash Screen at the entrance of the Clowney Culvert by some debris.

The report also said that, completion of the works to raise the banks of the Clowney Water to their designed level may have delayed the flooding event, but would not have prevented it; the provision of the permanent pumps in the underpass would not have prevented the flooding incident; and that the hydraulic efficiency of the drainage system needs to be examined by developing a physical model.

The Minister said:

"In partnership with the DBFO Company 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. Depending on the findings of this further analysis additional physical measures may be necessary to mitigate against a similar occurrence.

The Independent Report is available on the Roads Service website www.roadsni.gov.uk

Note to Editors

  1. The trash screen was examined at 12.30pm on August 16 by the contractor and was found to be clear, however some debris was found later which is likely to have been swept from further upstream causing the trash screen to become partially blocked. It is considered that this partial blockage was a contributory cause but may not have been the sole cause.
  2. A model will be built on probably a scale of 1in 20 resulting in a size of approximately 10metres by 10metres. We would expect this to take about 6 months to complete.

The recommendations of the report and the Roads Service comments are outlined below:

Recommendation 1 - Completion of the works to raise the banks of the Clowney Water immediately upstream of the culvert to a level 500mm above the 1 in 100 year designed flood level.

This work has been completed. This will provide additional capacity in the system that would delay any future flooding event.

Recommendation 2 - Development of enhanced procedures to include more regular inspection on the Clowney Water and introduction of an early warning system to advise of impending high water levels to ensure the safety of the public until completion of the works.

The DBFO Company has implemented enhanced procedures which relate to MET Office forecasts and which have an escalating level of response associated with the severity of the MET Office warnings.

Recommendation 3 - Carry out an assessment of the hydraulic efficiency of the drainage system.

Roads Service has made enquiries to appoint a specialist hydraulic investigation team to construct and test flows using a physical model of the drainage system. This task will be lead by Roads Service in partnership with the DBFO Co and Rivers Agency. This work is estimated to take 6 months to complete.

On conclusion of Recommendation 3 the following should be considered:

  1. Develop procedures for setting the Penstock Valve.
  2. Investigate enhanced Trash Screen cleaning methods. This will be considered in association with Rivers Agency who will have ongoing responsibility for the efficient operation and cleaning of the Trash Screen.
  3. Consider measures to provide flood protection works in the upstream section of the Clowney Water Culvert at Broadway.

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