Milwr Tunnel is a mine drainage adit extending 10 miles from Cadole near Loggerheads, Denbighshire to Bagillt on the Dee Estuary in North Wales.
The tunnel outfall is located approximately 100m inland from the banks of the River Dee and water flows to the Dee via a 18m wide 6m deep natural channel.
Erosion had occurred around the existing outfall structure and to the northwest bank in the vicinity of a piped outfall. The effect was to deepen the natural channel and undermine the outfall structure and adjacent public footpaths.
PaSCoE were commissioned by Eric Wright Civil Engineering to provide a permanent solution to stabilise the outfall and to reinstate the channel and surrounding land. The solution comprised a new retaining wall extension to the existing wingwall, a new outfall pipe, and erosion protection in the form of Rip-Rap to the channel.
The work area was within a Site of Special Scientific Interest (SSSI) and a Special Area of Conservation (SAC). Therefore the proposed scheme had to minimise the effect to the local mud flats and blend with the existing environment. Varying sizes of riprap were designed to channel the flow out into the estuary and the surrounding embankment was re-profiled and reinstated.
It was not possible to divert the flow from the tunnel entrance to undertake the remedial works. Therefore PaSCoE designed a novel temporary works solution which involved the addition of a temporary chute to outfall the water outside the working area; which allowed the void below the tunnel to be grouted and a new concrete apron installed.