Dolgellau By-Pass

Fishguard to Bangor Trunk Road A470/A487

Re-alignment of Afon Wnion to allow for the construction of the new by-pass on the North Bank downstream of the bridge (Y Bont Fawr) leading to Dolgellau town centre.

To accommodate the new road, the channel bed was re-aligned by slanting it towards the south from a point 150 metres downstream of the bridge over a distance of 450 metres. This resulted in a lateral transposition of the bed by a matter of 30 metes over half the length. At the western end of the diversion, the existing sharp bend was replaced by a large radius curve.

To maintain the channel within its new course, walls 3.0 to 3.5 metres high, 440 metres long on the North Bank and 2.5 metres high, 340 metres long on the South Bank were required. The top of the wall on the South Bank stands about 1.5 metres above the existing ground level to form a flood bank to protect the recreation grounds behind.

Gwynedd County Council chose Maccaferri PVC-coated hexagonal woven mesh gabions filled with locally sourced stone for three reasons. Firstly, the importance of appearance which had to suit the surrounding area, and the natural stone in the gabions is admirable. Secondly, permeability to allow free drainage from the backfill and thirdly flexibility to take care of any settlement or movement under the walls.

The structure consists of 500mm thick gabions laid in 2 metres wide courses throughout, each course being stepped back 500mm to provide a 1:1 batter on the outer face.  The bottom course extends 3 metres onto the Riverbed beyond the toe to form a flexible apron protection against undermining.

The gabion mesh chosen by the County Council was 80mm x 100mm in black PVC coated wire of 2.7mm core diameter and an overall diameter of 3.8mm. I.C.I. Terran filter cloth was placed behind each wall to prevent the migration of fines. In all a total of £6,500 cubic metres of gabion work was installed in stages between February and August 1979. Since completion, the structures have performed satisfactorily, and vegetation has taken root in the aprons and lower courses which adds to the appearance of the scheme.