A structure which is thought to be unique in the world, the Dalmuir Drop Lock was installed during the refurbishment of the Forth & Clyde Canal in 2000 to allow boats to pass under Dumbarton Road in Clydebank (West Dunbartonshire). It was thought unacceptable to build a new opening bridge over the canal for the road, which had been the previous configuration when the road was much less busy. After the canal closed in the 1960s, the original swing-bridge had been replaced by a fixed-span bridge which gave only 30cm / 12 inches of clearance beneath, rendering the canal un-navigable. Thus a drop-lock was designed, which allows the level of the water at this point to be dropped sufficiently to allow boats to pass beneath the existing bridge. The lock comprises two gates, one either side of the bridge, and an unusually long and deep lock chamber. Around 2000 cu. m (70,600 cu. feet) of water has to be drained to give 3m / 10 feet of clearance, a process which takes 40 minutes. The water is normally lost to a nearby burn, but when water supply is restricted it can instead be pumped out into the canal. Safety barriers prevent boats drifting under the bridge during pumping or damaging the lock-gates.
