What’s causing heavy rains in Himachal, Uttarakhand?

There has been heavy rain over the Himalayan states and the northeastern states since August 7.

The northward movement of the monsoon trough, and its interaction with a weak western disturbance are responsible for the heavy rains in Himachal Pradesh and Uttarakhand that resulted in the deaths of several people in landslides, building collapses, and damage to roads and other infrastructure on Monday.

Damage due to rain and land slide in Mandi on Monday (HT)

The first is usual during lulls or weak phases in the monsoon. HT reported on August 7 that monsoon has entered a weak phase and that there would be subdued rainfall over the plains because the monsoon trough was moving northward from its normal position passing mostly over the Indo-Gangetic plains up to Bay of Bengal. The monsoon trough is an elongated low-pressure area which extends from a so-called “heat low” (a low pressure over the seas) over Pakistan to the head of the Bay of Bengal region (parts of Odisha, West Bengal, and Bangladesh). This is one of semi-permanent features of monsoon circulation according to the India Meteorological Department.

There has been heavy rain over the Himalayan states and the northeastern states since August 7 which is leading to loosening of soil, heavy erosion, and flash floods over Uttarakhand and Himachal Pradesh, experts said. Between August 1 and August 14, Uttarakhand recorded 232.5mm against normal of 191.40mm (21% excess) and Himachal Pradesh recorded 147.40mm against normal of 140.20mm (5% excess). Uttarakhand’s Dehradun recorded exceptionally heavy rain between Sunday and Monday (42 cm) and Rishikesh (42 cm). Several parts of Himachal Pradesh recorded very heavy and extremely heavy rain — Kangra 27 cm, Sujanpur Tira 25 cm, Dharmsala 25 cm, Palampur 22 cm, Guler 19 cm, Jogindarnagar 18 cm.

“The monsoon trough is north of its normal position. It’s over the Himalayan foothills. This region has been receiving heavy rain for a week now, so it’s also accumulated impact. Yesterday (Sunday), a feeble western disturbance also interacted with the monsoon trough, and it is continuing to interact today (Monday) also. The monsoon trough will gradually shift southward now temporarily which will lead to reduction of rainfall over the hills and increase in rainfall over east-central India,” said M Mohapatra, director general, IMD. He predicted that the rain over both states will reduce from Tuesday.

“During the monsoon breaks monsoon trough shifts north close to foothills which produces heavy rains over the hills and northeast India. Nepal also gets good rain. This was expected in general,” said M Rajeevan, former secretary, ministry of earth sciences.

“In hilly regions, the slope, asperity, rock strength, forest cover or the lack of it are very important in determining landslides. We have prepared landslide vulnerability map for the Himalayan region. In a deep slope for example, if there is a lot of loose weathered material it will come down with rainfall in the form of landslides. Continuous rainfall is obviously a trigger but there are other triggers also like tectonic activity and anthropogenic causes such as drawdown of groundwater which can create a vacuum under the surface, heavy construction activity and mining,” said Kalachand Sain, director of the Wadia Institute of Himalayan Geology.

“Forest and plant cover often play a very significant role in slowing down the impact of continuous rainfall on soil,” he added.

On Monday, HT reported that from 5% excess rain at the end of July, a weak monsoon spell in August has led to a 2% rain deficiency across the country as of August 13.

This is the second time during this monsoon that Himachal Pradesh and Uttarakhand are seeing floods and extreme rainfall. In July, floods, landslides and mudslides devastated Himachal Pradesh and Uttarakhand after an interaction of a western disturbance and monsoon. Experts said at the time that the damage was the natural outcome of three factors: the climate crisis; a young mountain range that is still geologically active; and mindless infrastructure projects.

“The Himalayas are a very young mountain range and there are a lot of subsurface and surface activities going on here. There is erosion going on due to heavy rainfall events and snowfall; there is exhumation of rocks; there is plate tectonics. It is geo-dynamically extremely active due to these ongoing processes,” Sain had said in July.