Volume Ten - Pre-Victorian to the Present Day - Even more aspects - Investigation into the Condition of the Ely River Subway 1991 . . .

THE SURVEY

Introduction :

Following the removal of the cast iron linings at the Penarth end, the Subway was deliberately flooded. It is unclear if this was done to prevent casual use or to provide a reservoir of rapidly available water in the event of a fire at the nearby Celtic Oil Terminal.

In order that the Subway could be surveyed it first had to be dewatered. Northern Divers (Engineering) Limited of Hull were awarded the contract for dewatering the Subway and for providing attendant services to Brian Colquhoun and Partners throughout the survey.

Northern Divers commenced on site on 8th April 1991 and gained access to the north end of the Subway contained within the compound occupied by the Penarth Motor Boat and Sailing Club. The initial task was to dive along the length of the Subway to install an air line at the far (southern) end. The air line, driven by a compressor on the surface, was used to control pressure at the 'sealed' end of the Subway to prevent dangerous back pressures being developed during the pumping operation.

Pumping out commenced on 9th April 1991 using a 100mm outlet diameter submersible pump discharging via a lay-flat hose into Cardiff Bay adjacent to the Sailing Club. The National Rivers Authority had previously taken samples of the water within the Subway and confirmed their agreement for the water to be discharged in this manner at certain tidal conditions. The air pressure at the far end of the Subway was continually monitored in order to equalise pressure at both ends of the Subway.

During the pumping operation, the rate of drawdown was continually monitored to ensure that rate of pumping was greater than rate of ingress. In practice water ingress was so slight that drawdown was constant indicating that there were probably no substantial leaks in the Subway. Had the pumps not succeeded in dewatering the Subway due to excessive water ingress a decision would have been made as to whether to continue with the survey. The contract documents had been written to allow for this eventuality.