Flooding of the Melamchi was not due to increased flow of river water, as the rainfall measured in the catchment was well below the monsoon average, but most likely due to the formation of an artificial dam due to landslides that blocked the river for some time and then breached, leading to the propagation of large flood waves and debris flow downstream
Nepal, with 80 per cent of its area comprising fragile Himalayan mountains, has been facing a number of disasters of late.
Around 6,000 rivers and rivulets drain from north to south of the country, and the country gets more than 80 per cent of its annual precipitation within a period of about four months.
This leads to heavy floods each year. Moreover, climate change has resulted in an average temperature rise of about0.077 °C per decade in the Hindu Kush Himalayan region, according to ICIMOD. This rise in temperature increases glacier ice melting, adding extra flow in the streams.
Furthermore, additional flow from events like Glacier Lake Outburst Flood (GLOF) and permafrost melting will generate high discharge with very high velocity, resulting in excess stream power, leading to morphological extremes with larger bed and bank erosion and sediment transport. So, in such a scenario, development activities on the rivers and in their vicinity will be highly challenging, especially for a developing country like Nepal.
The general rule in water resources engineering, while building something above or within a river, is the concept of high flood.
This concept of high flood is based on probability estimate and risk assessment.
Engineers use the high flood concept to plan residential areas and hydropower dams and design bridges, culverts and floodwalls.
However, the impact of sediment during the design phase is usually ignored, which could be critical for our dynamic river basins.
The recent flood and sediment disaster in Melamchi on June 15isan example, which killed many people besides destroying human settlements and important infrastructure. A large amount of land was eroded or got covered with sediment, which resulted in property losses estimated at billions of rupees. The sediment-laden flooding also damaged the headwork of the Melamchi Water Supply Project.
Similarly, most of the river crossing structures like bridges on the Melamchi River have faced heavy sediment loading with decrease in vertical clearance.
The impact of the decrease in vertical clearance will be immediate and lead to water overtopping of the bridges in the future, leading to bridge collapse.
Beyond the general perspective, the flooding of the Melamchi was not due to increased flow of river water, as the rainfall measured in the catchment was well below the monsoon average, but most likely due to the formation of an artificial dam due to landslides that blocked the river for some time and then breached, leading to the propagation of large flood waves and debris flow downstream. The fatalities and losses due to the Melamchi disaster could have been minimised if human settlement development had not been extended to the lower-level alluvial deposit areas (inner side of bend).
The practice to tackle Landslide Dam Outburst Flood (LDOF), or sudden breach of a pool created by a landslide, leading to mass flow and/or large sediment concentration, is lacking in our country. Significant efforts are needed to cope with the mud/debris flow problem.
The earthquake of 2015, with its epicenter in Gorkha, resulted in several minor cracks in the neighbouring mountains. These cracks when filled with rain water may trigger numerous landslides. So, identification and risk assessment of such areas are necessary to manage any geomorphological catastrophe.
Moreover, they help to locate new settlements areas.
A settlement area should be above the highest level of mudflow and flash flood triggered by a landslide dam-break. Research on debris is gaining importance today. We can find out the possible length and depth of an affected area by mudflow routing.
Similarly, proper management of a river is necessary.
Our river management system is based on traditional methods, simply building structural measures, such as dams, levees and floodwalls, without proper study and risk assessment. In most of the river projects, the same method which was developed 20-30 years back is still followed. Before planning structural measures for river management, we should study their long and short-term impacts on a river's natural process, which will help to reduce disasters.
Nowadays nature-based solution is gaining importance in river engineering because it doesn't disturb the natural process of the river system and also fulfills the purpose. It's become essential to carry out a thorough study before the development of infrastructure and settlement zone that sustain the environment.
Planning of the settlement area based on floodplain zoning, considering the geomorphological hazards, is essential.
The government should set up a proper river management strategy, considering the hazards and risks like landslide-triggered flash flood based on state-ofthe-art knowledge and technology. Implementation of a real time flood forecasting system, use of remote sensing, computer-based modeling system and database management will serve as a backbone for minimising probable disasters.
For example, with the help of high-resolution multi-spectral satellite imagery, a mountainous catchment can be regularly monitored to detect incipient hill slope failures, ice mass, glaciers and even detachment of well entrenched permafrost.
Only through research and development can a disaster like the Melamchi flood be prevented. The government needs to establish research centres for disaster preparedness and rehabilitation of hydraulic infrastructures and provide appropriate resources for such endeavours. Such research centres can contribute to understanding the physical processes of our river systems as well as to quantify the hazards and risks that lead to disasters.
This will help government authorities to implement proper management and adaptation plans, thus minimising the damage to people's lives and property.
A version of this article appears in the print on July 6 2021, of The Himalayan Times.