Catastrophic failure of bolts securing the Prince George lock gate to its central support frame caused the entire 20t steel structure to break away from its seating.
Shoreham Port Authority port engineer, Tony Parker, said: "The immediate cause was failure of a set of bolts on one of the radial arms that support the gate and I think that will be at least in part due to fatigue."
The 14-year old gate is a 7.5m high single-leaf sector gate, curved through a 90° angle. It is sited at the seaward end of a ship lock in the port which uses a similar sector gate at its opposite end.
The gate was attached to a steel support frame mounted on a central pivot with bearings allowing it to rotate open and closed.
The bolts under investigation secured a lower horizontal radial member. This strutted between the gate and the support frame to prevent the gate from deforming under water pressure, which unusually for this design of gate, acted on the inside concave face when the gate was shut.
Failure occurred when the gate was in its closed position supporting a 5m head of water on its inside face. The sudden collapse saw sea water to force its way under the gate, pushing the structure upwards to contact a pedestrian walkway above.
He said that investigations now being carried out behind a temporary stop lock would identify the precise cause of the problem.
"In general, lack of maintenance can lead to faults of this sort," he said. "But the forensic scientists have been through our maintenance procedures, and their conclusion was that we've probably delayed the onset of the accident by a year or two by maintaining it so well."
The design of the sector gate at Prince George lock is of a similar basic design to lock gates across the country. Typically the member is secured using grade 8.8 bolts. However this type of gate is usually installed to resist water on it outer convex face.
Consultant Posford Duvivier, now part of Royal Haskoning originally advised Shoreham Port Authority on the construction of the lock gate. It will design and build the replacement.
Royal Haskoning principal engineer Rodney Woodhouse explained that the bolts most likely suffered a tension failure.
However, Kenneth Grubb Associates director Ken Grubb, who helped design the gate, told NCE that the gates should have lasted 50 to 60 years without any problems.
"The original design has been checked several times and nobody has picked out any flaws with it," he said. "We're waiting to see what comes out of the report."