‘Hungry water’ effect blamed for flood damage

Dams and reservoirs make rivers sediment-starved and menacing manifold downstream

The widespread damage caused by the recent floods in Kerala can be attributed to the ‘hungry water’ effect caused by the surge of sediment-starved water from the upstream reaches of a river, according to a leading scientist.

While heavy rainfall is also a key factor behind the floods, hungry water had a more pronounced effect, says D. Padmalal, Scientist and Head, Hydrological process group, National Centre for Earth Science Studies.

Dams and reservoirs trap the sediments eroded from rocks and soils, leaving the river starved of its sediment load. “When the sediment transport is interrupted, the potential energy of the hungry water released from dams will scour the river banks downstream, uprooting trees or riparian vegetation and damaging bridges and other engineering structures,” explains Dr. Padmalal.

Overloaded with silt and clay from the eroding river banks, the highly turbid and viscous water clogs drainage channels. Subsequent discharge of water from the dam will lead to inundation and waterlogging of large areas.

Sand mining

Hungry water can also develop in high gradient river channels devoid of adequate quantity of sand and gravel, especially during periods of high rainfall.

“Years of uncontrolled sand mining have left most of the rivers in Kerala depleted or exhausted of sand and gravel. This creates a situation similar to the release of hungry water from dams,” notes Dr. Padmalal.

When the river channel has adequate supply of sand and gravel, the potential energy of the water is used to transport the mixture. The water does not scour the banks or turn muddy.

Dr. Padmalal asserts that the hungry water effect has had a major stake in the flood hazards in areas close to dams like Ranni. “That is because the submerged areas remain inundated till the clay settles. In places like Aluva and Perumbavoor along the banks of the Periyar, the hungry water acted more vigorously because these areas are characterised by soil covered banks compared to the rocky banks upstream.”

He highlights the need to factor in the hungry water effect while developing flood control strategies. This, he feels, is more relevant for a State like Kerala with many hydel and irrigation dams in higher elevations.