Beating the bottleneck. Modelling for a new Dyfi Bridge.

The Grade-II-listed Pont-ar-Ddyfi is a bottleneck on the A487, a critical part of the Welsh north-south and east-west trunk road network. Floods of the River Dyfi regularly close the bridge and divert traffic 16 miles to get to and from Machynlleth. This can pose a significant problem during flood conditions for both residents and emergency services. A new viaduct across the Dyfi floodplain is proposed upstream.

A Flood Consequence Assessment, informed by detailed modelling of the River Dyfi floodplain was required to gain approval for the scheme.

Time for WHS

We completed a 1D/2D linked model, built on an existing Natural Resources Wales flood model by incorporating two upstream tributaries, the Dulas North and Dulas South, and the Garsiwn channels on the floodplain, recalculating peak flows in the catchment.

After modelling the baseline event, we modelled a range of mitigation options to protect the adjacent eco-park and manage impacts for properties on the A487 by the existing bridge, including:

  • construction of a bund next to the eco-park
  • channel widening downstream of the existing bridge
  • different bridge-pier configurations
  • providing an additional span to the proposed viaduct

Our modelling showed that:

  • a scheme could be developed that creates acceptable impacts to existing flood levels
  • significant benefits to the eco-park can be achieved with a strategically placed flood bund to protect the site from a 1-in-100-year-plus climate change event
  • there would be no significant changes to frequency, duration or depth of flooding in terms of the land’s agricultural usage.

We liaised closely with Natural Resources Wales through the development of the baseline and with scheme models. We agreed with NRW the mitigation measures to reduce the risk of objections to the project. NRW reviewed and approved our modelling with minimal changes along with our associated flood risk assessment.

Result: The scheme is currently awaiting formal publication of statutory orders to construct the viaduct. When implemented, the viaduct will significantly improve access to and from Machynlleth. We used our modelling to adapt the scheme to control and improve flood risk in the local area, minimising the risk of objections to the project when the orders are published.