The tank
Sri Lanka's irrigation is a unique water-management system, and the traditional tanks of our country can be presented as an outstanding creation of world irrigation technology. A reservoir built by damming a river, stream or branch of one to supply water for farming in an area of poor water supply is called a tank (wewa).
The large tanks spread over the dry zone hold a great capacity of rainwater. The main things considered were controlling the pressure of that volume of water so it could be held for a long time, and controlling the destructive force of the liquid pressure when water is let out.
Figure to be added
| Part | What it does |
|---|---|
| Tank bund (wew bamma) | Built of soil across the narrow neck of a valley; clay, soil, gravel and kiri-mati packed tightly in layers so it is strong and does not sink |
| Sluice (sorowwa) | Natural stone slabs under or through the bund; a stone pillar lifts and lowers them to release just enough water |
| Bisokotuwa | A square chamber in the sluice that releases water at each level, lowering the pressure before it flows out |
| Mud sluice (mada sorowwa) | At the lowest level; lets out the silt that collects after the rains |
| Ralapanawa | Stones laid on the inner face of the bund, stopping waves from eroding it |
| Iswetiya | Stops water carrying mud, sand or gravel from the upper slopes entering the tank |
| Catchment (wew ismaththa) | The area above that feeds the tank; felling trees, farming and building there were strictly banned |
| Wew thawulla | Land beside the tank at water level; a sanctuary for many plants and animals |
When the small tanks built in the catchment of a large tank (the kulu wew) fill, their water overflows through spillways and joins the main tank. So the tank is a human creation wonderfully in tune with nature.
Assignment 15.10
Carry out a scientific investigation of irrigation technology in Sri Lanka and prepare a report.
Why the bisokotuwa was so clever
Pressure in a liquid rises with depth, as you learned in grade 10. At the bottom of a deep tank, water released straight through the bund would rush out with enough force to scour away the soil and destroy the sluice. The bisokotuwa is a chamber where the water first gathers and loses much of its pressure before flowing gently out. Engineers elsewhere reached this idea — the valve tower — many centuries later.
An ancient, sustainable design
The tank stores rain for the dry season; its bund, sluice, bisokotuwa, ralapanawa and protected catchment manage water pressure, silt and erosion in harmony with nature.