Saturday, 16 November 2013 00:00
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By D.C. Ranatunga
A museum demonstrating the marvels of Sri Lankan engineers in the field of water management was recently opened in Colombo. It is the Irrigation Museum located at the Mahaweli Centre on the Nelum Pokuna Mawatha (formerly Ananda Coomaraswamy Mawatha).
A model of the Victoria Dam indicates the entrance to the museum. Being the sole visitor at the time I went, I had the opportunity of spending my time leisurely having a good look at the exhibits and the accompanying explanatory notes.
Rich history of our irrigation network
Only Stage 1 of the museum has been completed so far. Here, the rich history of our irrigation network has been comprehensively covered. Much effort has been taken to present the numerous aspects of the irrigation technology developed locally by the technologists who received unstinted support of the ancient kings.
Irrigation technology has been defined as the technological processes connected with agriculture and other requirements such as collecting and storing water, conservation, and release of water to fields. Irrigation technology is considered as a valuable contribution by Sri Lanka to the world.
Original settlers of Sri Lanka utilised water from the natural ‘villus’ for agriculture and gradually formed village reservoirs to store water for continuous use. These were turned into big reservoirs later.
At village level, water was collected by constructing a reservoir crossing a small stream of water. The water thus collected was sent down to the fields. The excess water in the fields again got collected to the same stream. A second reservoir was built and then a third and the process continued thus forming a cascade. The reservoirs tended to become bigger and bigger thereby collecting large quantities of water as the stream flowed own.
Village lakes were constructed in association with the smaller tributaries of the upper basins of rivers. The location of the lake was determined by the lowest area of the basin where the water got collected. The village lake was considered the common property of the village. The villagers were responsible for its construction as well as the protection of the environmental system created with it.
Anicuts
Collection of rain water, and water collected from rivers, canals and streams was done through anicuts (‘amuna’) built across them. Storage of water was done in reservoirs which needed reservoir construction technology relating to the positioning and building bunds.
The release of water involved cisterns (‘bisokotu’), sluice (‘sorovu’), spills (‘pitavana’), and water level measurement stones (‘diya-keta-pahan’). Moving of water to needy places was done through anicuts of irrigation canals while water was supplied to fields through subsidiary canals. The usage of water was through cross bars (‘karahan’), building enclosure bunds in fields, and water logs (‘vakkadas’).Protective bunds were built to prevent wastage.
The anicut (‘amuna’) is the bund built across a natural stream to divert the water to another location. At first these were temporary bunds built across ‘villus’ but by the 1st century A.D they became permanent structures built across large rivers using stone blocks. The anicut was built to obtain water to a single or several reservoirs and also to move water to areas where water was scant or needed.
Special engineering technological creation
Bisokotuva is a special engineering technological creation of Sri Lankans on water management. It constitutes the water control centre of the reservoir without damaging the reservoir bund. The entrance of the water control centre would have been located nearby. The stone water channels which took in and released the water through stone slabs serving as entrances and exits, were built across the inner base of the bund in order to avoid the water seeping into the earth bund. The water conduits were covered with bricks. The ‘bisokotu’ of the Anuradhapura period were rectangular in shape while those in the Polonnaruwa period were square in form.
Water is sent through a covered stone gutter known as the Sluice (‘sorovuva’) In large reservoirs there were two types of sluices – ‘godasorovuva’ and the ‘madasorovva’. The former supplied water to distant fields while the latter supplied water to nearby fields.
The Spillway (‘pitavana’) is the technological device built at the bund to spill away the extra water when the reservoir gets filled. This is built at a lower level than the lake bund. A spill was usually positioned on a rocky surface. Both sides of the bund at the spillway were protected with stone pavements in order to avoid seepage of a bund at the time of floods.
The reservoir bund (‘vakandiya/vavubamma’) is the main device built to store the water. The size of the reservoir is determined by the geo-physical state of the ground, the magnitude of the reservoir and the water capacity. By joining the high level ground of stone strata, the bund is well protected. The strength of the bund was maintained by digging a ditch further down and laying earth mixed with clay and gravel strata.
Another technological device known as ‘ralapanaava’ was used to avoid washing off or seepage of the bund. It was constructed by covering the inner slope of the bund with stone blocks methodically.
