Desilt reservoirs, learn from our ancient irrigation systems

Silt buildup in reservoirs poses a significant threat to hydropower and irrigation systems, reducing their water storage capacity. This issue is exacerbated by poor soil management practices that deposit large amounts of silt into these bodies of water. In the case of the Polgolla reservoir, extensive draining revealed mountains of silt in its lower reaches.

This problem not only affects power generation and agriculture but also flood control. Our ancient irrigation systems placed a royal decree on desilting reservoirs during the dry season, diverting the collected clay to make bricks for constructing great stupas. This practice ensured that reservoirs maintained their full capacity for the upcoming cultivating season.

Ancient engineers understood the importance of not blocking main rivers when constructing reservoirs. The Minipe left canal is an example, where water was tapped from only the surface of the Mahaweli. Similarly, larger tanks like Nuwara Wewa and Parakrama Samudraya were fed by minor rivulets. Additional features, such as mud sluice canals and forest reserves between reservoirs, helped trap silt and regulate nutrient flow to prevent salinity buildup.

One engineering marvel is the Yoda Ela, an 87 km-long canal that carried water from Kalawewa to Nuwara Wewa and Tissa Wewa. This canal, constructed during King Dhatusena's reign around 459 AD, is a winding path featuring about 120 smaller reservoirs and has wet zone trees that absorb nutrients, making the water suitable for irrigation. However, the Mahaweli project replaced this natural, winding canal with a concrete one, removing the canal's ability to remove silt and nutrients.

The salinity issue faced by ancient Mesopotamian civilizations, who built dams along the Euphrates and Tigris rivers, serves as a cautionary tale. Continual irrigation over thousands of years led to increased salinity, rendering the land unsuitable for agriculture and causing famine. This highlights the dangers of blocking main rivers for agriculture.

Recent scientific evidence shows that the salinity of paddy soils in the Mahaweli C area is increasing. The opening of sluice gates in the Kotmale Reservoir during Cyclone Ditwah led to flooding in Kandy and Peradeniya. If the reservoir had more storage capacity, delaying the opening of the gates might have prevented such devastation. However, this argument may not resonate with modern engineers, as they prioritize hydropower generation and irrigation over flood control in reservoir design.

Considering the potential benefits of desilting reservoirs for flood management, restoring lost capacity should be a national priority.