Flood control must begin at source, not at flood plains

FLOODS in Sri Lanka are often treated as downstream problems, to be managed at river mouths, river basins, embankments, or reservoirs.

- By Henry Gamage

A typical farm pond capturing runoff before it reaches streams Contour cultivation with lock-and-spill drains promoting soil moisture retention and reduced runoff A well-managed tea plantation where runoff is minimal despite heavy rainfall

Yet floods do not originate in floodplains. They begin where rain first falls, on rooftops, home gardens, plantations, forests, and steep hillsides in the upper catchments.While priority must be given to upstream watershed management, the role of floodplains as downstream sinks cannot be overlooked. In lowland areas, ecosystem-based approaches are essential, particularly the restoration of wetlands as sinks to absorb excess floodwater and the rehabilitation of natural drainage pathways to convey water safely without causing damage.

Harnessing floods before they become disasters

Sri Lanka’s 103 rivers discharge an estimated 39 billion cubic metres of water to the sea each year, nearly one-third of the country’s annual rainfall. This volume is equivalent to filling the Victoria Reservoir about 54 times. More than 90 per cent of this runoff comes from just three river basins, the Mahaweli, Kalu, and Kelani, with the Mahaweli alone contributing nearly 9 billion cubic metres annually. Much of this water reaches the sea as destructive runoff, remaining largely unutilised while causing severe land degradation in upper catchments.

During cyclone Ditwah. peak releases reportedly reached about 2,700 m³ per second from Victoria Reservoir and 250 m³ per second from Rajanganaya reservoir, reflecting the limits of reservoir-based flood moderation.

Poor land use and land use planning