Cwm Du outfall and overflow sluice was built in the late 18th century. This feature, 33m north of the aqueduct on the towing-path side of the canal, is one of the few remaining working outlet sluices. The water flows through an outlet tunnel at the level of the bottom of the canal, 1.6m below the sill at canal water-level.

A wall bisected the outlet and overflow chamber. The paddle apparatus being built on to the side of this wall which holds canal-water up to the level of the sill on top of the wall. A paddle or sluice gate could then be raised to drain the adjacent section of the canal. Excess water could spill over this central wall and when dry conditions made water conservation important stop planks could be inserted by the lengthmen on top of this central wall. When released the water ran down a 0.7m sloping weir into a bow-sided chamber end wall with a small arched brick-patched outlet. The vast amount of water brought into the canal after heavy rain on the steep hillsides necessitated a number of large overflow spill-ways along the length of the canal.
Outfall and overflow SN 7413 0577

Stephen Hughes, 14.08.2006