Even with Sluice-Gate Nine seated and the downstream pool steadied, one critical task remained before the drainage system could be declared secure: the flushing of the main bypass scouring valve.
The scouring valve was located in the narrow, vaulted gallery of Entry hall 168, thirty paces to the west of the main gate-house. It was a four-foot disk of solid cast bronze, four inches thick, set into a circular stonework channel that connected the high-pressure water behind the gate to the deep overflow chimney pipes of the lower deep pit.
Its purpose was simple, brutal, and effective. Opened during a high-head differential. Then, the packed jet of water would sweep the upstream apron clean of leftover silt, scouring the stones with enough speed to prevent the magnetic iron sand from caking into a permanent crust.
Qiao Ming walked the narrow gallery of Entry hall 168, his horn lantern held low before his feet.
The floor here was a narrow stone narrow high walkway flanked by deep, dry drainage trenches. The air smelled of old grease, damp bronze, and the sharp, stone tang of sulfur from the outer water. At the end of the gallery, bolted to a massive granite inner wall, was the valve assembly.
