In Sri Lanka's dry zone, earthen bunds impounded seasonal runoff in reservoirs (wewa) arranged as connected cascades (ellanga), so that spill and seepage from an upper tank fed the next one down the valley. The distinctive component is the bisokotuwa, a stone-lined valve pit set into the bund: it dissipates the pressure of the stored column of water so that outflow can be regulated without bursting the sluice. Cascades were paired with a village, a temple and paddy command area, giving a repeating settlement unit. Large works such as Kala Wewa (5th century CE) and the Parakrama Samudra extended the same logic to city scale at Anuradhapura and Polonnaruwa.
The valve pit turned reservoirs from simple ponds into controllable distribution systems, allowing dense agrarian settlement — and monastic capitals of 100,000-plus people — in a monsoon-dependent landscape with no perennial rivers. The cascade unit of tank, village, temple and field is one of the oldest surviving templates for a distributed, water-structured urban region. It anticipates modern catchment and resilience planning by roughly two millennia.
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