Improving understanding of flooding and resilience in Nepal

October 12th, 2017

In Nepal, accurate understanding of flood risk is limited by inaccurate or outdated information. Five innovative techniques have been used by Practical Action and partners to address these challenges and share the information with communities and government authorities.

Rivers originating from the Himalaya are a vital resource for around 10% of the global population. The rivers irrigate land, supporting the agricultural livelihoods of communities living across the 255 million hectare wide Terai plain in which Western Nepal is located. However, heavy rainfall during the monsoon season can cause a rise in water levels that mean these rivers become a source of devastating floods.

Practical Action has been working with the Department of Hydrology and Meteorology in Nepal and an interdisciplinary team of geoscientists, engineers, social scientists and architects from the University of Edinburgh to better understand the flood risk faced by these communities.

An accurate understanding of the areas at risk of flooding is of critical importance to preparedness and resilience. Access to this information helps communities, local and national government agencies and civil society groups to act effectively to protect vulnerable people, livelihoods and infrastructure. Decisions on where to build emergency shelters and land use can be influenced by this information, for example.

The Karnali River is highly dynamic, changing pathways, shape and depth as it transports millions of tons of sediment along the river bed

1. Variable levels of sediment

Rivers which originate from the mountains in Nepal carry millions of tons of sediment during monsoon season. This sediment can change the shape of the underlying river channel, meaning that the water level alone does not indicate the amount of water in the river.

Solution: Using an Acoustic Doppler Current Profiler we measured the amount of water in the river at different times of year and at different cross-sections of the channel. We were also able to use this technology to measure flow velocities for the first time. We also measured the size of sediment grains on gravel bars and in filtered water samples. These measurements were then used to accurately calculate the quantity of water in the river so that flood levels could be predicted – enabling resilient land management, irrigation practices and crop usage.

2. Outdated topography maps

Satellite data is crucial for mapping flood risk. However, the maps currently being used in Nepal are based on data from 2001, but the course and elevation of the river has changed significantly since then.

Solution: We developed more detailed flood risk maps using up-to-date and in-depth satellite data. This allowed us to analyse how flood risk is affected by changes to the river shape and to identify areas at risk that were previously overlooked.

3. Channel migration and switching

Channel migration occurs when a river bank is eroded causing the river to move across its plain, while channel switching occurs when the accumulation of sediment causes a river to change course. If either occur during a flood, the new channel position may put unprepared communities at risk.

Solution: Using optical satellite imagery, we were able to map out the historical channel pathways of the Karnali River and to understand the frequency and patterns of river channel movements. This enabled the identification of areas most vulnerable to flooding.

4. Social dynamics

Communities often change as result of development or in response to disasters like flooding. This change can be caused by anything from new economic opportunities or business openings to new community arrivals, people moving to find work or whole families being displaced by flooding. These all contribute to changing demographics and vulnerabilities which authorities need to be aware of in order to protect communities.

Solution: We interviewed community members to determine how flood risk interacts with their day-to-day lives. We found that the region is undergoing major social transformation which, while increasing the availability of cash and opportunities, is exposing people to new vulnerabilities. For example, the rise in concrete housing construction creates a sense of security in communities, but means that homes are less mobile and more valuable, increasing the potential loss due to flooding.

5. Construction techniques

Nepal has many examples of traditional flood-resilient construction methods. However, these have been displaced by modern techniques and materials, like reinforced concrete. Although it is easier to verify the resilience of concrete buildings than comparable traditional designs, they require different skills and technical expertise to build and are more expensive. Therefore, modern construction is not always appropriate for vulnerable communities who may not have the resources to safely construct and maintain modern designs.

Solution: We studied the construction methods of homes and shelters in a range of settlements and proved the resilience of some traditional techniques such as using timber posts to elevate main living floors. We concluded that hybrid construction systems should be developed, which bring together both traditional and modern methods, and that evolving systems should only use concrete where essential to guarantee resilience without unnecessary costs for households or contributions to climate change.

Rising water levels put the communities who live near the river at risk of flooding

Next Steps

Rivers like the Karnali, which have high and variable sediment loads, directly impact the people living near them, and we need to better understand how their changing behaviour affects flood risk.

We also need to be able to share knowledge about safe and resilient modern building techniques so that investment in structural development effectively reduces the risks of flooding and other hazards.

The team conducting this research

project was formed from a range of disciplines across physical and social sciences. The benefits from working together and sharing knowledge and learning across the team became increasingly evident as the project progressed, resulting in greater insights into the challenges and potential solutions to those challenges related to flooding in the Karnali basin. Using an interdisciplinary approach to research, whilst challenging, can ultimately lead to unanticipated leaps in knowledge.

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