Till Sun and Moon Shines [Part 13]By Eng. (Dr.) Chandana Jayawardana
Several past columns were dedicated to discuss the components of ancient small village tanks and how such components would have contributed to the sustainability of the system. Those manmade physical structures were complying with the natural water cycle, so that the principle of cyclicity is maintained. This discussion continues in the today’s column as well.
In line with the discussions in preceding chapter, the infiltration was considered as an important functionality achieved in ancient small tank cascades. Water thus infiltrated into the soil will be subjected to percolation, the movement through the soil and its layers by gravity or capillary forces. If water cyclicity to be achieved, there should be some measures adopted to harness this percolation flow. Water balance studies performed in certain small tanks are important in this respect as means to understand and quantifying the interception of upstream percolation flow by downstream tanks of the cascade. Jayathilaka et. al. recording the site measurements of two small tanks in Thirappane cascade system, Meegasgama and Alisthana notes Meegasgama tank received return flow due to upstream tank infiltration and water issue 12.5%. Alisthana tank, the immediate downstream to Meegasgama tank, received return flow of 2.8%. Therefore one measure of utilizing percolation is as the return flow for the tanks in downstream of the cascade.
Considering the morphological settings of small tank cascade systems, its contribution towards harnessing of the percolation flow is evident. Thennakoon highlighting the importance of such morphological characteristics notes two important natural formations in this respect. Sub water sheds which are the feeding grounds for small tank cascades are separated by inside low ridges or mounds running roughly parallel to the higher ranges, commonly known as heennas, and their summits known as mudunnas. Usually it is at the tail end of the valley bounded by two heennas that the largest tank in the cascade is located. Upstream of this tank are several medium size tanks. The side slope water courses leading to these tanks are dammed across to form the other minor tanks in that cascade. Those extents from the mudunnas of the heennas to the small tanks of the side slopes of the valley are the catchment of those small tanks.
Not only the surface runoff, but also the percolation flow is tapped in above settings, as revealed by Panabokke. Accordingly, the weathered hard rock surface of the North- Central dry zone is an ‘etched plain’. This is so referred because its close resemblance to etched metal plates on which artists make their ‘etched prints’. The water table is generally not horizontal and in order to reach equilibrium, water moves inside the ground from the high points of the water table to the points lower down. This internal flow is accumulated in etched points as illustrated in figure 1. By constructing tanks at such points, the ground water flow could be harnessed. The ancient tank builders took the advantage of this formations to make small tanks, developed not the isolated individual tanks at these hydraulically strategic points but entire cascade of tanks.
The tank bund structure is also important in the process of intercepting groundwater percolation. Elaborating the geological conditions considered in locating bunds in ancient tanks, Pattiarachchi notes that the bunds of most tanks, large and small, followed the geological strike of the country rocks. This implies that the bund is subsurface to the ground level and forms a barrier to groundwater percolation flow as indicated in figure 2. Bunds thus constructed forms an underground water reservoir, enhancing the groundwater availability for the tank formed by that bund. Apart from improved water availability, another positive aspect of series of bunds across the main stream is increasing deposition of silt locally without passing to the downstream tanks.
Considering the measures adopted to harness the groundwater percolation flow, it is further evident that the induced infiltration was to ensure the localized water cyclicity, delaying the conveyance either to natural or manmade water bodies.
Jayathilaka, C.J., Sakthivadivel, R., Shinogi, Y., Makin, I.W., Witharana, P., Predicting Water Availability in Irrigation Tank Cascade Systems; The Cascade Water Balance Model, Research Report 48, International Water management Institute, Colombo, pp. 19-21.
Panabokke, C.R., Small Village Tank Systems of Sri Lanka: Their Evolution, Setting, Distribution and Essential Functions, Hector Kobbekaduwa Agrarian Research & Training Institute, 2009, p. 22.
Pattiaratchi, D.B., Some Aspects of Engineering-Geology in Ceylon; Proceedings of the Twelfth Annual Session of the Ceylon association for Advancement of Science, Part II, Colombo, 1957, p. 142.
Thennakoon, M.U.A., Evolution and Role of Small Tank Cascade (Elangawa) Systems in Relation to the Traditional Settlement of the Rajarata, in Food Security and Small Tank Systems in Sri Lanka, ed. H.P.M Gunasena, National Science Foundation, Colombo, 2000, pp. 13-33.
Eng. Chandana Jayawardana has earned his first degree in Electrical Engineering from University of Moratuwa and then, post graduate qualifications in Industrial Engineering and Buddhist Studies. He is currently working as Design Manager, Balfour Beatty Ceylon (Pvt) Ltd, Katunayake.