2008 – Geotechnical risk assessment and management for hydropower development sites in the Indian Himalaya

Robert Goldsmith

Abstract: Selection of suitable dam and associated structure sites for run-of-river hydropower projects in the Himalayan region requires a thorough assessment of the hazards and the consequences of their occurrence. Typical projects in this environment of deep ridge and ravine terrain comprise concrete gravity dams with steel flood or sluice gates. The steep abutment slopes generally require spillways to be over the dam crest and not as separate structures.

Hazards to be considered for such structures are linked mainly to geological and hydrological factors and include landslides both at the dam and in the reservoir, outwash floods from a variety of initiating causes (landslide dams or glacial melt), floods from storm events, weak or unstable foundations, seepage and pore pressures and earthquakes. Each hazard is related to a risk scenario and a matrix of events is evaluated for potential cause, consequence and impact. A measure of consequence to the project at various stages of design, construction and operation is obtained and provides a basis for preparing risk mitigation measures. Risk management plans can then be prepared using a forum process with stakeholders to achieve a satisfactory outcome.

Examples are provided from specific studies carried out for hydropower projects in the Indian Himalaya. They include a risk assessment of a floodwave overtopping a dam from debris torrent after breaching of a landslide formed dam, terrain studies and geomorphological assessment to locate landslides in dam abutments and design and construction issues relating to a project site astride a major tectonic thrust zone.

Keywords: geotechnical risk, hazard and risk matrix, risk management plan, landslides, landslide dams, hydropower dams and tunnel, Himalayas