To improve the resilience of the principle A487 south to north route along Wales’s west coast, the New Dyfi Bridge scheme involves providing a new 725m long viaduct across the Afon Dyfi and associated flood plain north of Machynlleth. The bridge will reduce the number of closures due to flooding, and collisions with the existing narrow masonry arch bridge.

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

Arup worked with key stakeholders, such as National Resources Wales, to successfully develop a new river crossing scheme with minimal impact to flooding. To ensure 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 community benefits.

Bridge design and engineering 

The proposed viaduct’s initial flood modelling showed large unacceptable increases in flood levels. We identified and tested a variety of options to reduce the scheme’s impact, and the 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 a range of options and their impacts. The extensive going over of options and impacts provided meant NRW were comfortable with the scheme. 

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

3D modelling to visualise the scheme

To fully convey the proposed scheme, we developed a comprehensive 3D visualisation model. Fly-through videos and 360 viewpoints allowed local residents and groups to interact and understand the proposals in a sensitive setting, on the Snowdonia National Park’s southern edge.

Visualisation of the Dyfi Bridge scheme
A flythrough visualisation of the A487 New Dyfi Bridge.