The A487 New Dyfi Bridge scheme; The A487 New Dyfi Bridge scheme;

A487 New Dyfi Bridge, Machynlleth, Wales

How do you improve the flood resilience of the A487?

The A487 New Dyfi Bridge scheme involves the provision of a new 725m long viaduct across the Afon Dyfi and associated flood plain north of Machynlleth. The scheme will improve the resilience of the A487 which is the principal strategic south to north route along the west coast of Wales. It will reduce the number of closures due to flooding and the collisions with existing narrow masonry arch bridge. 

Working for Alun Griffiths contractors and with Wallingford HydroSolutions Ltd, we provided technical services, including highway and bridge design, flood modelling, environmental and geotechnical engineering. The primary challenge faced by the design team was related to the impact on the river flooding and minimising any increases in flood levels.

We worked with key stakeholders such as National Resources Wales, to successfully develop a new river crossing scheme with minimum impact on flood levels. To make sure the scheme suited everyone, we led a successful Public Exhibition to engage the local community, listen to local feedback and incorporate additional works into the Scheme to maximise benefits to the community.

Project Summary


725m long viaduct

10 properties to be protected from flooding

74mlong main river span

Managing flood impacts

The initial flood modelling of the proposed viaduct and associated flood bunds showed large unacceptable increases in flood levels. We identified and tested a wide range of options to reduce the impact of the scheme. This assessment resulted in the adoption of a longer viaduct which had lesser increases in flood levels. Throughout the assessment process the team worked closely with Welsh Government and other key stakeholders to explain the range of options and their impacts. The extensive assessment of options and impacts provided meant NRW were comfortable with the scheme.


Reducing construction work within floodplain

Due to the relatively high frequency and quick onset of flooding of the floodplain, Arup and contractors Alun Griffiths Contractors Ltd proposed an alignment which enabled an innovative push-launch construction method. This construction method will reduce the amount of works within the active floodplain and effectively reduce the risk of negative environmental, safety, financial and programme impacts.


Visualising the scheme

In order to fully convey the proposed Scheme, we developed a comprehensive 3D visualization model. Fly-through videos and 360° viewpoints allowed local residents and groups to interact with the scheme and understand the proposals in a sensitive setting on the southern edge of Snowdonia National Park.

A flythrough visualisation of the A487 New Dyfi Bridge.