October 2007

Completion of outfalls at Dawdon Minewater Treatment Scheme for JN Bentley/The Coal Authority

When underground mining ceased in County Durham in 1993, the abandoned mines began to flood and rising waters contaminated with iron, manganese, chlorides and sulphates were at risk of polluting potable aquifers, local rivers and the coastline.
The Coal Authority are constructing a treatment system to abstract water from the Dawdon mine shaft and remove the iron content. LongboreTT were contracted to design and install an outfall capable of discharging 900 m3/hour of saline effluent, from the 32m high cliffs of the Heritage Coastline, to a discharge point some 115m offshore, in a water depth of 5m.
LongboreTT supervised a marine geophysical survey of the offshore area and, having identified potential discharge points, conducted a video survey to confirm seabed conditions.
LongboreTT’s hydraulic design specified two 250mm polyethylene SDR11 outfalls with twin port diffusers, designed to cope with the flow rates and discharge pressures associated with the dense saline solution, as well as providing duality for maintenance and improving offshore dispersion.
The 364m and 369m long bores were drilled in Magnesian Limestone, to very tight targets avoiding exposed rock ridges and areas of cobble and boulder. The outfall ducts were fabricated onshore in single lengths and floated off the beach before being ballasted and attached to the drilling assembly on the seabed for pulling into the bores.