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Opportunities for strengthening sediment monitoring large watercourses in Nepal - Preliminary Report

  • NVE
  • Summary

The erosion rate in Nepal is very high compared to many other regions in the world, as indicated by the sediment loads of regional rivers. Sediment dynamics influence the landscape and infrastructure in Nepal. The country is vulnerable to mass wasting (i.e. landslides), glacial lake outburst floods and heavy monsoon rain which, combined with steep hillsides and gullies, mean that erosion dominates the landscape. Fundamentally, regular sediment monitoring has not been a priority in Nepal, despite many sectors, such as hydropower and disaster risk reduction, relying on such data to determine sediment generation, transport, deposition and overall budgeting. Thus, there is an urgent need to better understand the reasons behind this situation and to seek opportunities for how sediment monitoring can be strengthened. 

The major users of sediment data in Nepal are currently hydropower developers, particularly projects of medium and large scale. They need to prepare realistic forecasts of the sediment load passing the intake sites, the grain size distribution and the minerology of the finer fractions which are expected to pass through the turbines and cause damage. They will start by analysing historic data on the same river, but in most cases find that they need to start their own sampling and analysis program to be able to make such forecasts reliably. The time constraints of such projects means that there is often only 1- 2 years of data collected by the developer by the time they have to present their project and report its environmental impact to the licensing authorities and financiers. Such short programs may miss the major sediment movement events and will often underestimate sediment yield and forecast turbine erosion rates. It is therefore important that monitoring stations with some years of good record of sediment transport are prioritised and continue to deliver uninterrupted data of high reliability. These are the only stations which can document the variability over many decades and demonstrate any long-term trends in sediment yield and will provide vital data for the analysis of sustainability of recent and future hydropower projects.

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    Norwegian Water Resources and Energy Directorate (NVE)