Till Sun and Moon Shines [Part 14]

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 man-made physical structures were complying with the natural water cycle, so that the principle of cyclicity is maintained. Last few columns were dedicated to discuss infiltration, percolation, and the evaporation, the passages of water in the water cycle. In this column, how the modern designs would deviate from these natural passages is discussed.

Classical hydraulics is sometimes referred to as `fixed boundary hydraulics' where physical principles can be applied in a direct manner. In this approach, the water boundary is assumed to be fixed with no interaction with the adjacent zones. Concrete lined canals and bunds are some of the structures developed based on this perspective, and efforts are made to “liberate” water from its natural setting in the process of utilizing for human activities. This line of design philosophy is well supported by the “point to point water conveyance” concept, as water is treated as a commodity and follows extraction, production, consumption and decomposition linage as discussed above.

But inherent behavioral pattern of water could neither be controlled nor demarcated by fixed boundary. The interactions between the tank and the catchment could not be fully liberated from each other, as catchment is the feeding source of water to the tank. Siltation process resulting from sediment inflow is inherent consequence of this interaction, of which current hydraulic engineering is struggling to cope with. In similar manner, the boundary between water mass in tank and down stream water could not be demarcated by the tank bund, due to the water seepage through the bund. Salinity caused due to this phenomenon has converted huge amount of land into arid waste. In these circumstances, fixed boundary hydraulics cannot predict the changes of natural water behavior because numerous interactions among related zones are not accounted for. In water management activities, zone boundaries should be treated as movable and `movable boundary hydraulics' approach should be adopted allowing interactive processes between water bodies.

Conceptual differences between `fixed boundary' and `movable boundary’ hydraulics could be represented in cyclicity perspectives also. As no cross boundary interactions are allowed, cyclicity is not applicable in `fixed boundary hydraulics’. But in `movable boundary’ approach, cyclicity is highly applicable as the inter-zonal movements could be converted into the original state, only by applying cyclic processes. Sediment flow from catchment to tank bed is moving the catchment boundary across the tank boundary. The reversal could only be accomplished by removal of sediments as well as maintaining gasgommana, perahana, potavetiys or isvetiya as detailed in previous columns. The below figure illustrates this concept of ‘moving boundary’ with regard to manmade water bodies and how that concept could be used in ensuring material cyclicity within the system.

Eng. (Dr.) 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.