Nepal flood : How a landslide triggered a flash flood that put rivers at risk

A disaster that began in the mountains

A landslide in Nepal’s mountainous region triggered a devastating flood that left the riverbanks littered with debris, washed away homes and bridges, and damaged critical infrastructure in Gorkha District. The flood started at around 4 am Nepal time on Sunday, 11 October. At least five people are missing, and the Nepal Army has so far rescued 49 people by Sunday afternoon. Authorities warned people downstream, and rescue teams are now trying to reach out to the affected communities.

The central question about this disaster is not only what damage the flood has caused but also how a landslide could trigger a dangerous situation in downstream areas.

Authorities initially speculated that the cause of the disaster was a rock slide near Chumchet in Gorkha district blocking a channel or a tributary. Water then came rushing down the river towards the affected communities.

According to Nepal’s Department of Hydrology and Meteorology, the satellite images showed that there was a huge landslide in the area. However, authorities were still trying to establish exactly what triggered the disaster. An avalanche was also suspected, but this had not been confirmed.

It is important to note that the link between the landslide and the flood has been established, but the mechanism that triggered the disaster is still under investigation.

The cascade of events that led to the flood is an important lesson about the risks that mountainous communities face.

What happened in Gorkha?

Gorkha district is located around 100 km to the west of the capital city, Kathmandu. The flood disaster impacted the communities along the path of the Budhi Gandaki river. The areas worst hit by the disaster were the Chumnubri rural municipality and the Dharche rural municipality.

Many of the houses were destroyed, and many others were damaged during the disaster. Bridges were also destroyed or damaged, and electricity and communication networks disrupted. The 341 MW Budhi Gandaki hydropower project’s construction site at Salleri was also affected. The damage assessment was still underway as of Sunday afternoon.

It is hard to say exactly how many houses were destroyed in the disaster. According to Associated Press, a house destruction estimate of about three dozen was made on Sunday. Reuters on the other hand reported about a dozen houses gone. This is because these were tentative figures that did not yet constitute a final damage assessment.

The flood disaster is a frightening reminder of the risks that mountainous communities live with.

As rescuers reach out to the affected populations, the immediate priority is to locate the five people who have so far been reported missing. Another crucial task is to get the affected communities to safer grounds and to assess the damage.

How can a landslide trigger a flash flood?

The mechanism by which a landslide can trigger a flash flood is relatively simple. When a landslide occurs in a river channel, the flow of water can be obstructed by the rocks and other debris coming down with the landslide. This causes water to back up until the dam formed by the debris breaks, and the water rushes down the channel at great speed. This rush of water often carries with it the debris that initially blocked the flow, creating a devastating current that can cause significant damage downstream.

This phenomenon is sometimes referred to as a landslide-dam failure or breach. The failure mode of the obstruction determines the type of flood that will be generated when the breach happens. If the flow of water is suddenly obstructed and then released, a surge flood results. On the other hand, if the obstruction gradually gives way, it causes a gradual flood.

The initial assessment indicates that the failure of the temporary obstruction created by the landslide triggered the flash floods. The flood has, however, highlighted the need to pay more attention to the unstable slopes and river systems in the region. The research institute observed that it is crucial to consider the interactions within steep mountain catchments when assessing potential flood risks.

The next step is to conduct a detailed analysis of exactly what triggered the disaster.

A landslide does not need to occur in close vicinity to a settlement to cause it to be ravaged by flooding. This is because the obstruction of a river by such a disaster can cause a surge that will impact all the structures along the banks of the river.

Five people missing: What do we know about the rescue operations?

Five people are reported to be missing as a result of the disaster: two women and three men. The Nepal Army has so far rescued 49 people, including a foreign national. The army and the police have been involved in the rescue and recovery operations, with helicopters playing a significant role in reaching out to the affected communities.

Authorities have advised people residing in the vicinity of the affected riverbanks to relocate to safer grounds. In addition, alerts have been sent to downstream communities as the water levels in some of the rivers started to rise.

Authorities have yet to confirm any fatalities as of Sunday afternoon. The search continues for the five people reported missing, and the damage assessment is ongoing. As such, the casualty numbers still remain tentative.

It is hard to say how long it will take for rescue teams to reach out to the affected populations. In addition to the fact that people who have so far been rescued are not yet out of danger, the difficult mountainous terrain presents unique challenges to such operations. The damage to critical infrastructure such as bridges has also slowed down rescue operations.

Were early warnings a factor in this disaster response?

One of the most important lessons about disaster management is that early warnings play a critical role in saving lives. In the case of the recent flood disaster in Nepal, it appears that mobile phone alerts played a significant role in getting the warnings to the affected populations.

Dharmaraj Uprety, the chief executive of Nepal’s National Disaster Risk Reduction and Management Authority, told The Kathmandu Post that authorities received the information about the flood at 6.20 am and sent out the SMS alerts to downstream communities about 20 minutes later. He further noted that the warnings saved many lives as residents were able to relocate to higher grounds just in time. His comments were made on Sunday afternoon after his agency received reports of no casualties.

The value of early warnings in disaster management cannot be underestimated.

When it comes to early warnings for floods, such alerts are only useful if they get to the affected populations in good time.

When it comes to flood disasters, the time to issue early warnings is long before the floodwaters arrive. As such, the information needs to be shared with the communities in a timely manner. Ideally, the communities should also know where to relocate when such a disaster strikes.

While the official account suggests that SMS played a significant role in the recent flood disaster in Nepal, it does not mean that every vulnerable community received the alerts. Nor does it suggest that Nepal’s early warning mechanisms are foolproof. The next step is to ensure that these alerts get to the most vulnerable communities during emergencies.

Why are damaged bridges and hydropower infrastructure significant?

The flood disaster is not only a tragedy for the families who have lost loved ones and homes. It is also a lesson about the critical role that bridges play in connecting mountainous communities. When bridges are destroyed, it becomes considerably more difficult to reach out to these populations, especially in an emergency. The same can be said about damage to power lines and other critical infrastructure.

The flash flood also affected the 341 MW Budhi Gandaki hydropower project’s construction site at Salleri. According to several reports, the damage assessment was ongoing as of Sunday afternoon. It is, however, important to note that the authorities did not confirm that an operational hydropower station was damaged in the disaster.

The flood disaster in Nepal is a wake-up call about the need to rethink infrastructure development in the Himalayan region.

It highlights the importance of taking into account various factors when constructing roads, bridges, and hydropower stations. It is not enough to build them in hard-to-reach areas because such solutions often come with risks. In the case of disasters such as flash floods, these risks can be considerable.

Is Nepal witnessing an increase in disaster activity?

The October flood disaster is just the latest in a series of devastating disasters that have affected Nepal in recent months. Earlier this year, on 26 August, a similar disaster struck the country, sparking catastrophic floods that triggered hundreds of fatalities. According to Reuters, 1,457 people have so far been reported dead, and thousands of others are still missing. The disaster was brought about by an unprecedented ice and rock disaster in the region close to the Nepal-China border. The event was entirely different from the Gorkha flood disaster in October.

It is difficult to say if such disasters are becoming more frequent in Nepal.

These are just two of the many destructive forces that can cause large-scale disasters in Nepal.

Earlier this year, in the Budhi Gandaki valley, ICIMOD registered another flood disaster. This was caused by an ice avalanche that triggered a sudden outflow of the water in Birendra Tal. Authorities noted that the disaster was not a typical glacial lake outburst flood.

What these disasters have in common is the fact that they are capable of causing large-scale damage. They are, however, triggered by distinctly different forces.

The series of disasters that have affected Nepal this year is yet to provide conclusive evidence that climate change has triggered warmer temperatures that have resulted in more frequent ice disasters. There is, however, a need for more research into the matter.

What should be done next?

The immediate priority in the wake of the flood disaster is to locate the five people reported missing and to get the affected communities to safety. In addition, it is crucial to conduct a damage assessment to establish the full extent of the impact.

Authorities will have to conduct a bridge inspection to determine how much damage has been done and whether it will take time for these structures to be reopened to traffic. The electricity and communication infrastructure will also need to be inspected before any repairs can be made. In addition, authorities will have to determine which communities have been most affected by the disaster.

There is a need to rethink the way disaster management works, and risk assessments need to take place.

Authorities must also think about how to ensure that early warnings get to the most vulnerable mountainous communities and how to ensure that the information is acted upon in good time. This is an essential lesson about the importance of taking timely action when a flash flood warning has been issued.

Repo have noted the need for authorities to think through the cascading hazards that can arise from such a disaster. The research agency has observed that it is critical to take into account the interaction between unstable geology and hydrological factors when assessing the risks. Technology will play a significant role in this process, but it cannot replace the importance of collaboration between various stakeholders.

The central lesson about the Gorkha flood disaster is that the danger goes far beyond the localised area where it begins. A landslide in the mountains can trigger a devastating flood that can affect homes, critical infrastructure, and the communities that depend on it. Nepal’s immediate task is to deal with the damage and to ensure that the most vulnerable populations are protected. The longer-term priority is to ensure that the next warning gets through to these communities in good time.