During December 2015, the Conwy Valley in North Wales received over three times its average monthly rainfall, with over a metre being recorded at Capel Curig. As a result, the Conwy River finally burst its banks on 27 December and inundated the Conwy Valley railway line.
The combination of floodwater from the river to the west of the railway and the sheer volume of water coming off the mountains from the east, now unable to flow into the river, finally overtopped the railway in several places with floodwaters reaching platform level at North Llanrwst station.
Futureproof Washout Repairs
Working with Network Rail, several teams from Alun Griffiths (Contractors) were mobilised at first light on 28 December to assess the damage. North of Llanrwst, there were several areas of ballast washout totalling over a mile in length. The railway runs through a flood plain in this section and the flooding was made worse by breaches in the river flood defence embankments.
The damage was limited to the top and bottom ballast levels, leaving the earthworks largely unaffected. However, 10 foot-crossings were affected along with much of the lineside cabling.
Work commenced promptly with more than 1000 tonnes of ballast being delivered to site. Fortunately, there was a road-rail access point (RRAP) at Llanrwst North, so road-rail vehicles were used to run the ballast out to the affected areas progressively, allowing each section to be re-ballasted before moving on to the next site. The reinstatement of this section was straightforward and was completed in January, along with the level crossing repairs.
South of Llanrwst station, floodwater had overtopped the embankment and washed away a thirty-metre section, totally undermining the track. Access to the site was severally hampered because of flooding in the adjacent field so, working from track level, damaged sections had to be repaired in sequence.
As the floodwater subsided, access was gained across the flood plain in order to speed up repairs and take delivery of around 600 tonnes of rock armour.
Whilst initially asked to undertake a like-for- like repair, Griffiths also proposed a number of measures to build in future resilience. Short of raising the level of the railway, the embankment south of Llanrwst would always remain susceptible to overtopping during flood events and was prone to rapid erosion. The solution was to cover the embankment slopes with geotextile membrane, keyed into a trench at the toe of the embankment filled with free-draining material. Rock armour was then placed up to cess level.
This will slow the velocity of water during future overtopping events and, at the same time, protect the embankment slopes themselves.
Rapid Solution and Approval
Wing-wall scour had undermined the back of the abutments of Bridge 14, just south of Llanrwst station, causing the approach transition slab
to drop. The slab weighs 11 tonnes and, as the surrounding fields were totally saturated, it was impossible to use a crane from the side of the railway. A road-rail crane was brought in, but had to travel over the line affected by the washout. This meant that earthwork reconstruction had to be finished before the bridge slab could be lifted.
Arcadis, formally Hyder, was engaged to prepare general arrangement drawings and a Form 001 for both the earthworks and the permanent way design. Network Rail expedited the technical approvals process allowing construction work to get underway.
Griffiths proposed an extension to the bridge wing wall to provide anti-scour protection around the bridge and, for the embankment, designed a key trench with large 500mm stone wrapped in geotextile matting. The pitching stone and embankment slope was then reworked to a shallower angle such that the earthwork fill material would be better protected from future overtopping flood events.
Strengthened Overhead Flume
The fourth section of work revolved around a small aqueduct or flume that spanned the railway between Llanrwst and Betws-y-Coed. The parapet wall had failed, depositing silt and debris onto the track below.
To allow the failed parapet to be rebuilt, the watercourse was over-pumped. Griffiths took the opportunity to strengthen the works with the use of stainless steel dowels through the parapet. This work was completed in early January, preventing further water damage and debris affecting the line.
But nothing is ever easy. Just as all of the above work was being completed, more storms arrived, culminating in further flooding on 25 and 26 January. However, Griffiths’ two pronged methodology, by accessing the railway from adjacent land and using road rail machinery, provided the flexibility to keep working towards handing the railway back before the end of February.