Approach and methodology for estimating combined glacial lake outburst flood (GLOF) and PMF design flood for Bajoli Holi hydro-electric project in the Indian Himalayas

In Indian Himalayas, many hydroelectric projects are now under construction due to the availability of a perennial water source and a natural head for hydropower generation. Hydropower plants often require significant investments, design lifetimes, and lengthy repayment. Indian Himalayan states are now developing State Action Plans on Climate Change, with policies for climate change mitigation and adaptation at the subnational level. These plans recognize GLOFs as a significant climate change-related flood to be considered for the safety of River Valley Projects. The snow-fed catchment area of these projects has many glacial lakes, and there is a high likelihood of breaching for lakes located at the glacier's snout. In general, potentially dangerous lakes are located near the end of a glacier in the lower part of the ablation area. A large mother glacier can create potentially hazardous lakes. These moraine dams could likely breach due   to   piping   or   overtopping   due   to   their porous soil content inside dam body. A sudden discharge of significant magnitude could endanger the safety of the downstream HE hydroelectric project. It is suggested, the glacial lake outburst flood (GLOF) and the design flood be simultaneously considered while assessing the spillway capacity of new hydropower projects to ensure that they are hydrologically secure.

Bajoli-Holi Hydroelectric Project, located on river Ravi in the Himachal Pradesh state of India, is studied, to analyze its spillway capacity considering both GLOF and Inflow Design flood. BIS published the guidelines for fixing spillway capacity. As per the codal provisions, the Bajoli-Holi dam qualifies for PMF as its Inflow design flood.

The hydrology of a particular basin or project undergoes certain changes due to factors such as climate change, urbanization, deforestation, soil erosion, a heavy spell of short-duration rainfall, etc. With the aid of the most recent methods, including hydrodynamic modeling and a hydro meteorological approach, the design flood and GLOF for the dam have been evaluated in this study.

There are a total of 83 glacial lakes identified and mapped in this catchment area. It is further critically analysed to find the effect of the most critical glacial lake which is glacial Lake-52 having an area of 14.5 ha at a distance of 26.5km from the project location. River cross sections spaced 400 m apart has been considered. The upper envelope of the PMF is calculated to be 15,303 cumecs, average envelope is 6247cumecs and the lower envelope value is 2551 cumecs. The combined GLOF peak attenuated after hydrodynamic channel routing at the project site and the PMF analysed, will be taken as the inflow flood for analyzing the spillway requirements for the Bajoli-Holi project. The study results can be applied to similar hydro-meteorologically similar basins of the Himalayas in India which are under the influence of glacial lake outbursts and PMF.