Hydrological principle behind the development of series of bunds in ancient tank cascades in small catchments, Sri Lanka

Utilisation of water harvesting systems of ancient origin for present-day requirements can be seen in many regions of the world. The adoption of ancient tank cascades as the smallest hydrological unit of river basin development in Sri Lanka is such a practice. However, these systems are associated with unsuccessful experiences in agricultural performance while being vulnerable to both floods and droughts. Therefore, the identification of principles behind the development of cascades is vital. This study reviewed historical evidence, definitions, terminologies and studies on the development and operation of these systems. A hypothesis was then developed and tested for a specimen tank cascade using a HEC-HMS model. The routing mechanism showed that the existence of a series of bunds increases the robustness of the tank systems for flood inflows. Accordingly, the development of tank cascades is a technique used to regulate experienced peak inflows with limited technologies. This principle and the new cascade definition developed should be prime considerations along with the focus on storage when restoring the tanks in cascades. The findings provide insights into the crucial need for identifying development principles behind the ancient hydrological systems in any region of the world to adapt them sustainably.