Scientists looked at how dams have altered flows and flood occurrences
Over the last two decades, after 1995, there has been a steady increase in the number of flooding events in the Upper Ganga Basin. Catastrophic landslides and floods in the basin are more phenomenon.
This trend is unlikely to change with scientists predicting an increase in the magnitude of extreme flows and occurrence of flood. But advances in technology and new types of hydraulic structures can make a difference.
These were some of the findings of a study conducted by researchers from the Indian Institute of Science (IISc), Bengaluru, and Indian Institute of Technology, Kanpur (IITK) on how climate change and human activities like building dams affect basin.
They analysed the effects of past human activity on the mountainous regions, focusing on two major tributaries, Bhagirathi and Alaknanda, which merge at Devprayag to form the Ganga.
Their findings were published in the online peer-reviewed open access journal, Scientific Reports (published by Nature). “Our results indicate that low and moderate flows have been significantly altered, and the flood peaks have been attenuated by the operation of hydraulic structures in the Bhagirathi (western subbasin).
The Alaknanda (eastern subbasin) has experienced an increase in extreme rainfall and flows post-1995. The downstream reaches experience reservoir-induced moderate flow alterations during pre-and post-monsoon and increasing extreme flood magnitudes during monsoon,” said the researchers in the paper.
The team studied data on rainfall, water discharge in the rivers, and sediment load from weather stations across the Upper Ganga Basin (UGB) corresponding to the years 1971-2010.
When they analysed the data corresponding to two periods–pre-1995 and post-1995– they found a steady increase in the number of flooding events in both river basins after 1995.
“Further, the change in low flows and middle-level flows in Bhagirathi can be attributed to three major dams – Maneri, Tehri, and Koteshwar – on the river,” said IISc in a press release.
The Alaknanda basin saw a doubling of water flow from 1995 to 2005 at the Joshimath weather station, along with an increase in the rate of flow of water, termed extreme flow.
“We observed that Alaknanda basin has a high, statistically increasing rainfall trend, unlike the Bhagirathi basin. Most of these trends were observed in the downstream region of the Alaknanda. Therefore, we have also seen an increase in the magnitude of extreme flow in these regions,” said Somil Swarnkar, a postdoctoral fellow at the Interdisciplinary Centre for Water Research (ICWaR), IISc, and first author of the study.
The researchers suggested that after 2010, along with climatic changes, the building of dams in the Alaknanda region may have modified the water activity. “Dams and reservoirs have influenced the sediment transported by the rivers. Due to abrupt changes in water flow, sediment depositions in the upper reaches of the Ganga have led to changes in the sediment composition downstream,” stated the release.
It cited Tehri dam, which plays a crucial role in the UGB region. Being a large reservoir and flow control structure, it blocks sediment flow from upstream and controls the volume of water that flows downstream.
There are currently 11 new dam projects planned in the Bhagirathi basin and 26 in the Alaknanda basin. While they may provide the much-needed hydropower to the region, these structures will likely affect the water flow and sediment transport processes in these regions, says corresponding author Pradeep Mujumdar, Professor in ICWaR.
Regarding predictions of a further increase in the magnitude of extreme flows and occurrence of floods in the Ganga basin, the paper stated that advances in technology and new types of hydraulic structures can make a difference. For example, the Pashulok barrage in Rishikesh has helped prevent floods and reduce extreme flows in downstream regions. “We do not have control [over] what happens in the atmosphere. But on the ground, we have control. Flows can be predicted using hydrological models. With this knowledge, we can develop both structural and non-structural responses to mitigate [such] high flows,” said Prof Mujumdar.
The research teams hoped that the study will help in sustainable river basin management and encourage more serious work towards a better understanding of hydrology, ecology, and geomorphology in the Upper Ganga Basin.
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