Last week we discussed the origin, evolution and the significance of the ancient water technology in terms of tanks in Sri Lanka. Continuing the article, this week, we are going to study the technology associated with various parts of the tank and their usage.
In fact, a tank or a wewa is a massive construction and needs high knowledge of irrigation architecture since it holds an enormous amount of water, which has to be contained properly for people to use. Thus, special parts have been constructed in the tank to perform various tasks in this regard.
Tank bund (wew bamma)
Tank bund was constructed by pressing many layers of soil, mud, and clay on the top of the each other to ensure its strength, so that the tank bund can bear up the great force created on it by the water level. In the ancient days, animals like oxen, horses and elephants have been used to press the soil layers. The tank bunds of great ancient tanks are very strong even today.
The sluice gates or the sorowwa are two similar structures made at two levels of the tank, for the purpose of releasing water for canals and agricultural lands below the tank. The sluice at a higher level is called the upper sluice or goda sorowwa, and is used to release water to the paddy fields. The sluice at the bottom of the tank is called the bottom sluice or mada sorowwa, which is used to remove the mud accumulated at the bottom of the tank.
Bisokotuwa itself is a part of the sluice and functions to regulate the water let-out. When there are several sluices in a larger scale tank, each sluice is made with a bisokotuwa of its own. It is a square shaped, a well-like structure made with stones near the sluice and water is let into it through a tunnel. The moveable door of the bisokotuwa is moved up and down to regulate the flow of water into the spill. This water regulating system is called ‘Mohola’ which releases the right amount of water at the right time.
Breakwater or ralapanava is a stone lining on the surface of the inside of the dam or the wew bamma, to protect it. In large tanks, small waves of water are constantly created with the wind and when these waves crash on to the Wew Bamma unceasingly, it tends to wash away and breakdown eventually. Nonetheless, since the stones are water-resistant, the breakwater prevents it.
Inner spill and outer Spill
The inner spill or the athulvana has been constructed for the purpose of obtaining water to the tank from the outside sources. There is a small pit in which the slit that is carried in by the water flow is deposited, preventing it from entering into the tank. So, the inner spill lets in only clean water. It can be cleaned once the outside flow of water into the tank is ceased.
During the rainy season, the tanks receive much water and at some pint the tank becomes unable to withhold the total volume of water. In such situations, the excess water is drained out through the outer spill or the pitavana.
These unique technologies used in the construction of irrigation tanks in Sri Lanka have amazed the world and drawn much attention of the scholars. The aptness of the irrigation technology used in the ancient times can be understood as the huge tanks created ere many years still continue to function very well with the same efficiency. Hence, we should understand the significance of our irrigation heritage and try to protect them at the best we can.
By Induwara Athapattu