A solution to water pressure
The Tabbova Wewa is an ancient reservoir built by damming the Nanneri Oya, a tributary of the Mee Oya. While its exact construction date is uncertain, the technology it uses is part of a advanced hydraulic system developed in Sri Lanka over two millennia ago. The reservoir's earthen dam, or "bund," is 1,910 meters long and 9 meters high, holding back 19 million cubic meters of water when full. The surface area of the water covers 461 hectares.
The main engineering problem for any large reservoir is releasing water without letting the immense pressure destroy the earthen dam. Ancient Sri Lankan engineers solved this with an invention called the bisokotuwa, or valve tower. This structure, a rectangular, well-like chamber made of stone slabs and bricks, is built into the dam at the point where water exits. Water from the reservoir enters the bisokotuwa through a conduit at the bottom, filling the chamber. The water's energy dissipates within this chamber before it flows out through a separate discharge tunnel to the irrigation canals. This design effectively reduces the water pressure and velocity, protecting the dam from erosion.