Many causes for floods, many causes for their devastation
Hydrologist Auroop Ganguly said the devastation from Wednesday’s flood in Nepal and Tibet was due to a “combination of factors” both natural and man-made.
The flood that swept through the border region of Nepal and Tibet this week killing hundreds and leaving thousands missing is believed to have been triggered by a massive piece of rock and ice that broke off from a mountain glacier in the Himalayas. But the devastation wrought by the deluge was likely the result of both natural and man-made factors, Northeastern University experts said.
“Almost none of the destructiveness of floods these days is fluid dynamics alone,” said Auroop Ganguly, a hydrologist and distinguished professor of civil and environmental engineering at Northeastern. He explained that the precipitous fall of the chunk of ice and rock caused landslides which were further exacerbated by the fact that rivers and watersheds in the region were already waterlogged as a result of the seasonal monsoon rains.
“The huge debris of the glacier chunk mixed with the monsoon sediments…and combined with the lack of preparedness and high vulnerability and exposure – especially in the transboundary mountain regions – led to the massive devastation,” said Ganguly, who in 2019 and 2023 led a Dialogue of Civilizations, a faculty-led intensive learning experience for students, which visited Nepal and areas of the Indian Himalayas.
Wednesday’s tragedy in Asia is just the latest mass flood event to make headlines in recent years. Parts of the Midwest were inundated by floodwaters following heavy rains earlier this month. In July last year, at least 100 people, including campers and counselors from the all-girls Camp Mystic near Kerrville, Texas, were swept away when the Guadalupe River rose from 1.73 feet to 34.29 feet in three hours, following record-breaking rainfall. Floods also devastated the Caribbean nation of Haiti in 2021 and 2023.
But not all floods are the same, Ganguly explained. For example, pluvial flooding comes from rainfall that exceeds the capacity of the drainage infrastructure or ground to accommodate it, while fluvial flooding or river flooding is often due to snowmelt or rain that exceeds the banks of a river system. Coastal flooding comes from tidal effects that can be intensified by storms, Ganguly said.
And while it alone is not responsible for a flood’s devastation, fluid dynamics plays a “significant role” in a flood event, Ganguly said. He listed everything from turbulence and storms in the atmosphere and oceans, to the way that water flows through channels, rivers and even through wet or dry soil, as impacting floods.
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Other factors include climate variability, from natural causes like the El Nino Southern Oscillation, a climate cycle characterized by fluctuations in ocean temperature and atmospheric pressure in the equatorial Pacific Ocean, and anthropogenic, that is human-caused global warming and land-use. These can enhance precipitation extremes and storm intensities, Ganguly said.
However, it’s difficult to attribute a specific flooding event to climate, the expert said, because while climate-related warming exacerbates the melting and dislodging of massive pieces of glaciers, these dense bodies of ice are dynamic and form and break as part of their lifecycles, he said.
And just as their cause may vary, so can the impact of a flood.
Ganguly pointed to factors such as the way that flooded land is covered and used. Human-induced changes such as deforestation and urbanization can impact the ability of soils to absorb water and how water moves across these soils.
Hydraulic infrastructures, such as dams and levees that restrict water from reaching sensitive areas, reservoirs that store water, and urban drainage systems can also impact the flow of a flood and its destructiveness, Ganguly said.
Design principles, such as allowing a river or coastal area “room to breathe” by not building along shores, degrading surrounding wetlands and more, leads to less human destruction and better ecosystem health, Ganguly said.
Daniel Aldrich, dean’s professor of resilience and director of the Master of Science Resilience Studies Program at Northeastern, said that human factors played a large role in the disaster that befell Nepal and Tibet.
“This flood is a case study in how infrastructure investment can quietly increase exposure,” Aldrich said.
Exposure refers to the people, property, systems, and other elements located in hazard-prone areas that are subject to potential loss and damage.
Aldrich said that China, through its Belt and Road infrastructure investment project, has built a road, a hydropower project, cross-border power lines and Beijing has plans for a railway through the flooded corridor.
“This connectivity has brought real economic benefits to the region,” Aldrich said. “It has also concentrated settlements, workers, tourists, and capital investment in a narrow, glacier-fed river valley that is prone to outburst floods – this is at least the third major flood on this same river system in roughly three years.”
But Aldrich said there is something that is more difficult to build in the region than roads – and that is trust.
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“Responding well to a disaster like this requires shared hydrological monitoring, coordinated warning systems, and a level of institutional trust that takes years to develop,” Aldrich said.
Cooperation has historically been lacking, with decisions about rebuilding after floods often made in Beijing or Kathmandu, the capital cities of China and Nepal, far away from the disaster site and with little local input, Aldrich added.
“Whether that changes this time, and whether affected communities get real influence over reconstruction, will say a lot about the region’s resilience going forward,” he said.
