Heritage of Sri Lanka :

Marvels of irrigation technology

Prof W I Siriweera

Large and small reservoirs are part of Sri Lankan heritage and some of their technological aspects are unique to the island. In the construction of reservoirs, the gaps along the low ridges running across the Dry Zone plains, were used efficiently to impound water flowing through them

Two different techniques were adopted after a careful examination of the physiographical and topographical conditions for impounding water. According to one, an embankment was built using natural rock formations across a valley where water was available from seasonal streams and rivers.

For example, the Kavudulla reservoir has been built by damming the Kavudulu Oya, Hurulu Wewa by damming the Yan Oya, Vahalkada Tank by damming the Tavalam Halmillava Oya, Kanadara Wewa by damming the Kana river, and the Naccaduva Tank by damming the Malvatu Oya.

Parakkrama Samudraya. Pic. courtesy: Google

According to the other, part of the water flowing down the rivers was blocked by an anicut and turned into excavated canals which conveyed water to more distant lands and reservoirs. The best two examples for this technique are the Giant’s Tank or Yoda Wewa in the Mannar District and the Parakkrama Samudra of the Polonnaruwa District. The Giant’s Tank built by Dhatusena (455-473) was designed to suit a flat terrain without rocky hills or elevated ground to which the extreme ends of the bunds could be connected. Therefore the tank bund is seven miles long and only 14 feet high. The water to the Parakkrama Samudra was obtained through a canal from a dam constructed at Angamedilla which impounded the water of the Ambangaga. This reservoir had a subsidiary water supply from the north-west through a canal from the Giritale Tank.

Construction of reservoirs

These constructions involved the use of accurate devices and instruments for levelling and surveying, good stone cutting implements such as jumpers, chisels, wedges, hand drills and pick hammers for quarrying and dressing of the stones and trowels for masonry work. The haulage of large stone blocks and enormous quantities of earth must have been done by pulling them along the ground using skids and rollers, with crews of men tugging ropes and with rough vehicular contrivances drawn by elephants. The constructions also required a detailed and planned layout for setting height, angle, distance and gradient.

The interconnection of intricate network of canals and reservoirs in order to ensure flowing excess water from one into another with a minimum gradient necessitated detailed plans. Such plans were perhaps drawn on perishable material such as ola leaves. Brohier has stated that most likely measuring cords and ranging poles or such simple means had been used for measuring distances in the past before making plans. He has further mentioned one example of Sinhala cartography of the seventeenth century, attempting to show the irrigation system near Elahera which has been discovered recently.

Selection of sites

The selection of sites for the construction of reservoirs had been carefully decided so that the location prevented the seepage of water into the tank-bed. For example Magalla Wewa of Mahasena (274-301), located at Nikaveratiya, had been constructed by building a dam across the river at a place where there were granite slabs. The Kavudulla reservoir of the same king has been established at a place where quartz stone layers were abundant.

The choice of sites for the construction of dams has also been carefully planned so that the constructional requirements were minimal. Most of the dams had been located in places where rivers meander and the speed of the flow of water was relatively minimal. For example the Elahera dam had been built in such a place at Ambanganga, a tributary of the Mahaweli.

An embankment only about one kilometre in length was necessary to create the gigantic Minneri reservoir which covered an area of 4,670 acres. In fact ancient designers utilized natural rock formations to serve as foundations for their earth works and as a result most of the embankments follow irregular lines. The Vahalkada dam constructed by building an embankment across the Tavalam Halmilleva river, a tributary of the Yan Oya is a good example for this type of construction.

Although Red-Brown and wet clay soils were found in abundance in the Dry Zone, these soils were avoided in the construction of dams as their water content was strong so that the dryness of the bund could not be maintained.

Ancient method

The wave action of large as well as small reservoirs had an erosive effect on the embankment. Therefore it was counteracted by covering the water face of the earthen embankments with a layer of rough stone boulders. This was and is known by the terms ralapanava or rala pannuma. It acts as a ‘wave-breaker’ and resists the action of wave play.

There is a misconception which attributes the use of elephants for hardening the soils in the dams. But it has to be noted that the bunds which were of decreasing size from bottom to top could be hardened only at the foundation level and at the bottom by the trampling of elephants. As they rose up and up, herds of goats and cattle had to be utilized by driving them over the bund to harden the soils with their hooves and not by elephants. The modern adaptation of this method is called the ‘sheep’s foot roller.’ In this machine a roller studded with imitation of a sheep’s foot is run in tandem by a tractor up and down the embankment. The cylinder of the roller is filled with sand to produce the light pressure natural to the ancient method.

Some of the bunds except their top most surfaces were built only with stone. For example, the outer wall of the Sukharanijjihara dam on the Deduru Oya, presently identified as the Ridibendi Ela dam, constructed by Parakkramabahu I (1153-1186) was built with well cut stone blocks some weighing up to ten tons. These were intricately fitted that the joints are only a quarter of an inch wide. The inner core was formed of undressed rubble laid in lime concrete. Scientific analysis has proved that concrete used in some of these ancient dams were superior to the Roman mortar, which had long been accepted as the best ancient concrete.

Waste-weir

Waste-weir (Pali: avarana or Sinhala: vana) was an essential element of the embankment for the safety of the reservoir. The waste-weir released excess water during heavy rains and when reservoirs were full. This enabled the control of the high water pressure so that the embankments could be kept intact. Larger reservoirs were provided with more than one waste-weir whereas small reservoirs had only one waste-weir.

These spillways were either made of stone blocks or located on natural rocks on the bund. When floods occur, excess water of the tank passes over the spillways. For instance, the spillway of the Kala Oya was on a natural granite rock chiselled by stonemasons. This ancient spillway is in its original position and is one of the most stupendous constructions. The later restorations have left the old spillway untouched. In the rudimentary small village reservoirs, bailing devices or stone walled canals leading through openings in the bunds were used for discharging water. Later cylindrical, burnt-clay pipes were laid under the bund. But such systems were totally inadequate to release the water from large reservoirs. The release of water had to be controlled without any damage to the embankment.

Christian era

Therefore, at least from the early centuries of the Christian era, outward flow of large reservoirs into canals had been regulated through intricately designed sluices. These sluices consisted of four essential parts. They are (a) a rectangular or square well or pit (bisokotuva) from a spot near the crest of the dam down to a certain depth, (b) an apparatus fitted to the bisokotuva by the raising or lowering of which flow of discharging water could be controlled; (c) an inlet culvert or two culverts through which water passed into the well; (d) a discharging culvert or two culverts from the well to the outer slope of the bank. To be continued