Project Details

Client: Gwynedd Consultancy & Network Rail Hochtief Construction (Main Contractor)

Categories: Detailed Design, Geomorphology, Hydrology

When: May 2013

Location: Gwynedd, UK

In May 2013 Hochtief Construction was successful in winning the design and build contract for the Pont Briwet Bridge in North Wales. Hewson Consulting Engineers (Bridge Design), Edenvale Young (marine & Fluvial), Beazley Sharp Railways and Nick Stilwell Associates (Highways) were appointed to undertake the design of the bridge.

More details

Pont Briwet is located in an SAC where the foreshore / estuary geomorphology is highly dynamic with frequent and rapid change in the estuary and foreshores. In order to address concerns associated with the impact of the temporary works Edenvale Young developed a geomorphological monitoring strategy which incorporated:

• Pre-construction survey including LiDAR, aerial survey, topographic and bathymetric survey
• Hydrodynamic and geomorphological modelling using TUFLOW FV
• Time lapse monitoring photography taken during construction
• Topographic survey monitoring of foreshore recession and scour hole development
• LiDAR and aerial photography during construction.

Geomorphological Monitoring Results

A TUFLOW FV model was constructed with a combination of LiDAR and bathymetric survey to investigate geomorphological and scour change during the construction of the bridge. This encompassed the immediate vicinity around the bridge site. The modelling indicated that a scour hole would develop on the downstream side of the bridge and that there were recessionary pressures on the northern bank which would have an impact on the salt marshes.

These conclusions were confirmed during the construction of the works as part of the monitoring strategy. The collection topographic data also allowed calibration of the TUFLOW FV model and a range of scenarios were tested to assess the impact on scour of installing a temporary and cofferdams within the estuary.

The scour hole was monitored throughout construction and the impact on geomorphological change was assessed in conjunction with Natural Resources Wales. Before removal of the temporary causeway a final assessment of morphological change was made to ensure that scour holes around the piles would not negatively impact the foundations as the morphology adjusted from the temporary to permanent works condition. This led to the design and installation of scour protection under the structure.

Edenvale Young’s other responsibilities also included the detailed design and specification of rock revetment embankment structures subject to protect the approach embankment from waves generated within the estuary. This included:

• Evaluation of rock quality sourced from local quarries and
• Analysis of rock deterioration and loss of mass during the lifetime of the bridge.
• Assessment of wave conditions against the approach embankments
• Optimisation of the size, cost and thickness of primary armour used for the works
• Design of falling toes at the base of the structure.
• Site inspection and supervision of placing of rock armour.

Pont Briwet Proposed Bridge showing Streamlines

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