Anuradhapura Kuttam Pokuna drain sluices: subterranean granite cleanout valves and silt clearing?

The 8th-century Kuttam Pokuna (Twin Ponds) in Anuradhapura incorporates sophisticated subterranean drainage engineering. The stone floors of both the northern pond (measuring 28m by 16m) and the larger southern pond (40m by 16m) slope gently toward the western boundary. At the lowest corner sits a perforated granite drain slab fitted with a vertical wooden or stone stopper plug. When monastic workers pulled the plug, water discharged by gravity through an underground masonry culvert passing beneath the boundary terrace into adjacent agricultural fields, completely emptying the bath so workers could scrub away slippery algal biofilms.