One unexpected consequence of moving the control valve to the upstream side of the dam was that the rectangular culvert which carried water between the reservoir and the original cut-off valve well, near the core of the dam, was now pressurised in a way for which it had not been designed, and the entrance to the culvert collapsed in 1964. A repair was made by welding oil drums together to form a circular tube, and then grouting the gap around the outside of the tube with concrete, to form a circular concrete pipe. As the steel drums corroded away, it became clear that not all the voids had been filled, and further grouting was carried out in 1977. In the 120 years since its construction, the clay core had consolidated, lowering its top to a point where water could soak through the fill above it, which might cause slippage of the downstream face of the dam. Rather than restoring the core to its former level, a new spillway was cut into the existing spillway. It was long, and lowered the maximum water level by . This reduced the storage capacity from to .