Heightening of Very High Gravity Dams: The Case Study of the Grande Dixence

Heightening very high gravity dams is one of the solutions considered as part of the energy transition and to mitigate the effects of climate change. The paper presents a stepwise approach to assess the potential for dam heightening and its impact on the hydroelectric scheme. A first diagnosis focusses on identifying and characterising the main site and operation constraints to dam heightening. Then alternative heightening concepts are screened for feasibility, in particular heightening concepts such as gravity dam, arch dam or multi-arch dam. The selected concept(s) are then developed to create several variants with diverse geometries. Subsequently several steps are undertaken to verify internal and external stability for given selected design criteria. This approach is applied to the case study of the Grande Dixence, the highest gravity dam in the world located in Switzerland, considering heightening solutions up to a maximum of 30 m. Within this heightening window the additional loads of reservoir water and dam weight should be in principle acceptable for the already known foundation conditions. The main site constraints are in fact due to the partial submergence of the main headwater conveyance tunnel and the need to adapt the downstream surge tank of the Fionnay’s power plant. Structurally, heightening the dam with a similar structural concept was preferred from inception when considering joint behaviour of the original and heightened structures. Four alternative heightening geometries were compared in terms of their overall stability for various heights of elevation. The results, obtained by analytical and computational models, showed acceptable values for all four variants. Regarding the economic analysis, a preliminary analysis of the Levelized Cost of Electricity (LCOE) computed considering the additional electricity and the construction costs is remarkably low in comparison with other projects within the Energy Transition and point out that a height increase within 10–15 m would likely be optimal. This study confirms the interest to pursue investigations and studies beyond the feasibility stage, in view of determining the optimal heightening design and further develop the business plan for high-value hydropower production.