Sri Lanka is the country that had most advanced early irrigation systems in Asia. The first settlers of Sri Lanka (Indo-Aryan) possessed of high technical ability, with wide vision and a highly developed “water” and topographical sense. The vastness of conception of Ceylon’s ancient irrigation system removes all doubt that the old engineer must have depended to a critical degree on a system of levelling and horizontal measurement to render the construction practicable. Anicut, spillways and bisokotuwa are several genius constructions of ancient engineers that worth to mention here.
Anicut of Malwatu Oya
In the Anicut (tekkam) built about thirteen hundred years earlier across the Malawatu Oya to divert water to the Giant tank and in other similar contemporary structures, a cement seems to have been used. It has been proved by analytics results published in the Chemical Trade Journal that this concrete prepared and used in ancient Sri Lanka 13 centuries ago, if not earlier, showed very superior properties to the Roman mortar, which had long been accepted as the best ancient product.
The spillways, some of them man-made and called vaan in Sinhalese, other natural rock called gal-vaan, over which the waters of the tank pass when surcharged by rain in flood time, range from simple contrivances in the small tank, to complex examples which must surely have involved study of the movement of water over many years, and considerable labor to build. Two or even more spillways were sometimes provided in the larger tanks.
One of the most spectacular examples of spillways is the ancient spill-wall of chiselled granite at Kala-wewa.it is a stupendous construction, approximately 216 feet wide and 170 feet long, completed about the5th century AD. The work impressively testifies to the craft of the stonemason. Each block of granite is meticulously fashioned to fit its neighbour and the whole is a wonderful monument of patient workmanship.
Another construction that shows a thorough knowledge of construction principal is the device known as sluice in English, and Sorowwa in Sinhala, whereby the water from the tank was led to the system of major and minor channels to be carried to fields near or far. In the smaller tanks, where pressure was by no means great, this was achieved by means of cylindrical burnt-clay pipes laid under the bund. What permitted the old-time engineer to proceed to boldly with the construction of better and bigger reservoirs was a structure found in tanks built earlier but perfected by the 3rd century AD, known as the bisokotuwa, meaning:”the enclosure where the water level lowers.” This outlet work built into the upstream face of the bund regulates or totally stopped the flow of the water into the discharge culverts, and also served as a silt trap. Engineers only surmise that the gates of this structure were of timber and that elephants furnished the motive power to lift them.
What is especially interesting is that even some of the earliest sluices are furnished with these triumphs of ingenuity which proves that the early engineers had mastered the problem so successfully that all others were content to copy their example. Thus, the builder of those Bisokotuwa has established a claim, which is 2,000 years old, to be considered the inventor of modern valve-pits and valve-towers.