Bisokotuwa

Sri Lanka with a written history over 1500 years has been practicing soil and hydraulic engineering from about the same period of time. Rice and grain being the islanders staple diet, its community has been rice farmers and needed to store rainwater to cultivate the dry zone. The storage of this excess rainwater needed earthen embankments built across the valleys.These expertise brought about a society that practiced soil engineering which created a unique hydraulic civilization. The growth of this hydraulic civilization expanded with enhanced knowledge and developed new techniques with state patronage. In a broader sense a practice of water management that became a culture.

Initially it was a technique to sustain the water table by storing water on the ground with earthen embankments built across valleys. The water so stored was made to percolate into the ground replenishing the ground water table. This water was re extracted and put back on the soil as irrigation water and the cycle was to continue. This simple water cycle was improved with enhanced knowledge on soil engineering by storing large volumes of water behind strong earthen embankments that created large lakes or tanks. These large tanks managed to irrigate large tracks of land producing rice at commercial level that boosted the state coffers.

Building such large and deep lakes brought about new challenges in controlling and handling water in a safe and non-destructive manner. The greatest problem faced was to release the stored water into earthen canals with controlled velocities while dissipating the enormous potential energy stored in the water. It was a requirement that the very earthworks that held these water needed to be protected. A breached embankment would wipe-out the entire community that lived besides it. The first technique used was a devised called the “Keta Sorowwa” presently known as a VT sluice or a Vertical Tower Sluice. [Sorowwa – Sinhala term for Sluice] The modern-day “Moring Glory” spillway is an advancement of the Keta Sorowwa. A type of structure adopted when natural ground conditions are not favourable to site the excess water spillovers in reservoirs. The Bomburu-Ella reservoir in Nuwara Eliya, Sri Lanka on the way to Horton Plains is facilitated with a Morning Glory spill. So named as the structure depicts the shape of the Morning Glory flower.

The Keta Sorowwa is a devise formed with a number of funnel type chute units of burnt clay placed one on top of the other to reach the water surface. This stack is connected to a burnt clay conduit placed at the bottom of the embankment leading to the canal which distributed the water to the paddies. The theory being that… the water pressure at the top of the surface is less and the stored energy is manageable. The surface water of the reservoir was transferred through the tower of chutes into the canal by the clay conduit. Once the water level of the reservoir reached the level of the uppermost chute unit the discharge ceased, and the top most chute was removed manually, re-activating the discharge of water to the depth of the second chute. This way one could control the water issue on a chute basis and the wastage could be monitored and managed. In an emergency the chute was to be plugged stopping the water flow.

This Keta Sorowwa was advantages over the water table management, but needed to be improved with the advancement of techniques and knowhow to building larger reservoirs that held water to depths over 15 meters. The Keta Sorowwa managed small tanks with depth up to 3-4 meters and the loose chutes needed to be supported with stakes once the reservoir dried up as it was the water pressure around the chutes that held them vertically when submerged in water. The larger reservoirs with water heads of over 10 meters required a different technique. The invention of the “Biso Kotuwa” [also known as the Cistern Sluice] solved the problem of high pressures and the excessive potential energies stored in the water at these depths.

Water in the reservoir was made to enter a large rectangular vertical tower chamber that was built with large chunks of granite dressed to sit tight sealed against each other. The water was guided into this tower chamber through a dressed stone conduit. The immense pressure and energy at these depths were dissipated within the chamber creating a calmer and lowered water level than the stored water level in the reservoir. Thus the somewhat tamed water was now released from the chamber out to the canals through two or more dressed stone conduits that were laid in level with the inlet conduit. The throat dimension of these outlet conduits were enlarged along its path to reduce the velocity of water and thereby dissipate the remaining energies. It was this invention by our Sri Lankan designers then,.. that revolutionized the development and design of water conveyance from large reservoirs through sophisticated concrete and steel sluices that are being design today.

However it should be noted that some of these ancient marvels have been incorporated in our present day irrigation works as the location for sighting sluices and spillways by the present day engineers have matched perfect with the thinking of our ancient designers. The mechanism of opening and closing the inflow to the Biso Kotuwa with such an enormous pressures is still a mystery. Though the granite walls of the Biso Kutuwas’ have survived the torrents of disaster any steel and timber works that may have been incorporated in the shutoff mechanism would have perished leaving no signs of any such gated structure.

Most of these structures are still found covered in the wilderness since our colonial rulers destroyed this water management culture during the Uva rebellion in 1817. Those in the dry zone seem to immerge from the depths of history when new schemes are designed and the most suitable locations for structures are excavated. Two such unique preservations of Biso-Kotuwas’ were unearthed at Urusita Wewa or Mahagama Wewa close to  Embilipitiya and the Maduru Oya Reservoir in the Uva Province of Sri Lanka recently. The calculations for the sluice for Maduru Oya by the present day engineers were exact to those of the ancient designer. The Maduru Oya sluice was located with its granite conduits intact in the 1970’s when the new scheme commenced. The brickwork above each conduit in the form of a corbelled arch distributed the load on to the walls on either side of the conduit reducing the stresses. This thick brickwork also acted as a defence against any seepage that would erode the fill material in the dam bringing destruction. This may have been the best possible solution to cope with the type of fill material they had to work with.

The best locations for Irrigation Management today were coincidentally the same that our ancient designers selected then. The sluice conduits of the ancient Bis-Kotuwa of the Kantale reservoir were enlarged from the upper end to the lower end by a factor of seven, which agrees exactly with modern day thinking. Impervious clay was used within the core of earthen embankments, while semi-pervious material provided the bulk of the embankment, as in the modern day designs. The only place the writer encountered an ancient Biso-Kotuwa to be incorporated in a modern day sluice design was that of the Buduruwagala Tank in Wellawaya. A section of the rectangular stone chamber is extended with reinforced concrete to form the sluice chamber where a cast iron sluice gate is installed. The canal following from the sluice gate is the same ancient canal hewed through a rock and that rock face has a carved image of a multi hooded cobra a symbol related to water then.

The water supply to Kalawewa had been insufficient then and needed supplementation from Amban ganga through Kala Oya. This was realized a necessity during the Bowatenna diversion in 1976. Today Kala Oya is augmented with waters from Amban Ganga through a tunnel.