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Monsoon rains cause deadly flooding

Worsening conditions for Nepal quake survivors. Here's why


Scott Sutherland
Meteorologist/Science Writer

Tuesday, April 28, 2015, 8:15 PM - Even with relief efforts underway in Nepal, recovering from Saturday's devastating earthquake could develop into a race against time, due to the expected impacts of the coming monsoon season.

The India Meteorological Department just issued their 2015 Monsoon Season forecast last week. Monsoon season typically lasts from mid-June to mid-September in the region. Based on conditions in the Pacific Ocean, the Indian Ocean and with the weak El Nino in the central Pacific, forecasters are predicting a below-normal season for rainfall this year.

Even a below-average monsoon is likely to complicate relief and recovery efforts after Saturday's 7.8 magnitude earthquake killed more than 5,000 people, injured more than 10,000, leveled homes, destroyed ancient monuments and set off deadly avalanches on Mount Everest and elsewhere.

Landslides

The mountainous terrain of Nepal already makes it prone to landslides, and these are always a concern for monsoon season.

As we've written about before, landslides happen when the pull of gravity becomes stronger than the force of friction that keeps a slope of topsoil in place.

"The reason that most hillsides remain in place is because of the force of friction acting between all the individual particles of soil, along with the rocks and the tree and plant roots. The tighter the soil is packed together, the stronger the force of friction is between the particles, and the more stable the hillside is going to be. However, add water to the mix and it changes everything."

With the heavy rainfall amounts during monsoon season, the potential for a mountain slope to destabilize becomes very high. The water not only adds more weight to the soil as it soaks in, but it also reduces the amount of friction between the soil particles and the friction with the rock face beneath the soil layer. The combination of these two factors raises the risk of landslides.

The tremendous jolt the region took during the initial quake and the repeated aftershocks have already touched off several landslides. This is a concern, even now, in the pre-monsoon season, since it will restrict access to parts of the area that may be very hard hit by this earthquake. That will make relief efforts go very slowly for those areas, resulting in more hardship for those affected.

The worry with the coming monsoon, though, is that the quake, the aftershocks and even the landslides already touched off, have likely destabilized slopes enough that the area is even more susceptible to landslides once the rain begins to fall.

FULL QUAKE COVERAGE

Another concern with the coming rains is that the landslides that have already occurred, coupled with the possible increased risk of further landslides during the monsoon, will result in devastating flooding.

In early August 2014, monsoon rains triggered a massive landslide in northern Nepal, which swept across the Sunkoshi River, effectively cutting off the flow of water through the valley. In the aftermath, as officials assessed the damage and began recovery operations, the water coursing through the river from days of torrential rainfall began to back up, forming a lake that kept growing as the rains kept falling.

It took nearly a month for the Nepal Army to break through the mud dam created by this landslide.

Thus, there is the worry that landslides triggered by the quake will cause further problems in the months to come.

As Professor David Petley, an expert on landslides at the University of East Anglia in the U.K., wrote in the AGU Landslides blog:

"Landslides are going to be a very serious problem. They will have caused losses; will have destroyed roads and other infrastructure, which will make the rescue and recovery operations very challenging; and they will continue to threaten the population. It is also likely that there are valley-blocking landslides in the mountains, which will be particularly problematic as the monsoon approaches. We need to find these very quickly."

While it's likely that operations now, in the pre-monsoon time, will be easier than what the army faced last year, they could be dealing with numerous landslides from this earthquake. If efforts to clear them persist into June, and are complicated by rainfall and flooding, and further landslides, which can cause even more flooding, the situation could become quite dire.

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WATCH BELOW: August 2014 flooding impacts Kathmandu, Nepal

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