when completed in 1766, edwards' grand sluice consisted of three channels each wide, fitted with pointed gates on both sides, and a lock adjacent to the north bank, which could be used as an additional flood relief channel if required. the lock was originally very small, but was lengthened to its current in 1881. the pointed doors on the non-tidal side of the sluice were replaced by steel guillotine gates between 1979 and 1982.