Sangroula/NTNU Kulekhani Reservoir Bathymetry and Sedimentation Studies

Summary

Multi-study compilation covering Kulekhani Reservoir sedimentation, including the primary DGPS bathymetric survey by Sangroula (NTNU/Hydro-Lab, 2009/2010), InVEST/RUSLE catchment modeling by Bokan et al. (2021–2024), the ADB/World Bank post-completion review, and Mandal's (2021) hydro-mechanical degradation study. Together these provide the most complete sedimentation evidence base for any single Nepali reservoir.

Primary DGPS Bathymetric Survey (Sangroula, NTNU/Hydro-Lab, 2009/2010)

Physical DGPS bathymetric mapping of Kulekhani Reservoir bed using echo sounders and differential GPS.

Total gross storage lost: 20.4 MCM (23.9% of 85.3 MCM original)
Active live storage lost: 14 MCM (19.1% of 73.3 MCM original)
Average annual loss: 0.56% of total reservoir capacity over 27 years of operation
Commissioning: December 1982 (Unit 1), May 1987 (Unit 2)

Source Document

Sangroula, D.P. (2006, updated with DGPS surveys 2009/2010). Sedimentation and sustainability of the Kulekhani Reservoir: A Himalayan Case. Doctoral dissertation, Norwegian University of Science and Technology (NTNU). Associated with Hydro Lab Nepal.

InVEST/RUSLE Catchment Modeling (Bokan et al., 2021–2024)

A DISTINCT study using spatial modeling, not physical bathymetry. Uses InVEST SDRmax model + RUSLE to calculate theoretical sediment yield from upstream catchment.

Sedimentation rate: 13.3 t/ha/yr (2002) → 6.6 t/ha/yr (2010) → 4.8 t/ha/yr (2018)
Total reservoir capacity measured at 61.66 MCM (April 2018)
Confirms declining delivery rate trend — watershed management works
Citation: Bokan, L.T. et al. Potential Sedimentation Rate and its Prioritization in Kulekhani Watershed Using InVEST Model (published 2024, modeling 2002–2018 data)

ADB Post-Completion Review (World Bank/ADB PCR)

Loan 528-NEP / Credit 600-NEP
80% cost overrun, 21-month commissioning delay
Key finding: NO sediment flushing system was designed — watershed management was completely unanticipated during appraisal
Kulekhani served as the lesson that forced ADB/IDA to mandate catchment management plans and sediment flushing gates in all subsequent projects (Marsyangdi Cr. 1478-NEP, Tanahu)
PCR URL: https://documents1.worldbank.org/curated/en/790631468289870058/pdf/multi-page.pdf

Hydro-Mechanical Degradation (Mandal 2021)

Mandal, N.K. (2021). Pelton Runner Erosion Due To Cavitation: A Case Study Of Storage Hydropower Plant, Kulekhani First Hydropower Station
Fine suspended quartz particles cause continuous abrasive pitting of Pelton turbine buckets
Altered blade geometries trigger secondary cavitation — efficiency deteriorates even under low sediment concentrations
Main inlet valves replaced multiple times since commissioning

Limitations

Bathymetric survey date. The primary Sangroula DGPS survey is from 2009/2010 — sedimentation rates may have shifted in the subsequent 15+ years of operation.
Multi-study compilation with different rigor levels. Combines a doctoral dissertation, spatial modeling (InVEST/RUSLE), an ADB/WB post-completion review, and a turbine-degradation case study — each with different methodologies and confidence levels.
InVEST/RUSLE modeled, not measured. The Bokan et al. sedimentation rates (13.3 → 6.6 → 4.8 t/ha/yr) are model outputs, not physical sediment trap measurements. The declining trend is directionally consistent with the bathymetric survey but the absolute rates are model-derived.
No post-2018 bathymetry. The most recent reservoir capacity measurement (61.66 MCM) is from April 2018. Current storage volume is uncertain.