Millom WwTW Storm Outfall
A series of new manholes were constructed and the new pipe line terminated in a large concrete outfall structure situated within the tidal estuary of Duddon Sands.
The project was challenging in many respects. The pipeline was excavated in poor ground at depths of up to 7.0m and the route went through marshland where the high water table needed to be controlled by the installation of well points which discharged back into the treatment works via settlement tanks. To complicate matters further, the whole of the pipeline cut through a Site of Special Scientific Interest as the marshland was a habitat for the protected Natterjack Toad.
The new outfall structure was constructed on the beach and, once again, environmental considerations were paramount as the Marram grass on the sand dunes had to be recorded, logged and individually bagged for storage before being replaced after the works in its exact location.
The outfall was constructed within a sheet piled cofferdam on the beach and works were undertaken in tidal conditions with a variance between high and low tide of up to 6.0m metres. The outfall was designed by Bethell and involved large precast components which were stitched together with insitu concrete joints which helped to significantly reduce the construction time.