Robust design of dam heightening in a changing climate: a case study in the Swiss Alps

Climate change is expected to severely impact the hydrologic regimes of Alpine regions, with important implications for the operation of hydropower systems. Warming temperature will reduce seasonal snow cover and alter snow melt dynamics in the short term, and will boost glacier retreat in the medium and long term. As glacier retreat will result in a temporary increase of water availability, augmented storage capacity by dam heightening represents a promising adaptation option for temporarily managing the additional inflow volume, which can be used for increasing hydropower production and revenue while avoiding water losses due to unintended dam spills. In this work, we develop a framework for the robust design of dam heightening coupled with the optimal reoperation of the hydropower system under future scenarios of changing climate and energy prices. The framework is demonstrated on the Mattmark dam, an Alpine hydropower system located in the Visp Valley, Switzerland. As part of the national strategy for nuclear phase-out, Switzerland is indeed considering to increase hydropower production among the options called to compensate for both the missing nuclear power production and the increasing share of highly fluctuating renewable generation. Specifically, our framework is structured in four components: a spatially distributed weather generator model to stochastically downscale the most recent climate scenarios for Switzerland; a spatially distributed, physically based hydrologic model to transform projected climate into streamflow; a multi-objective joint design of dam heightening and associated re-operating policies; a stress-test of the designed solutions against a stochastic ensemble of future climate scenarios. Results show that dam heightening could be beneficial to both hydropower production and revenue under today climate, as a nonstationary trend in the reservoir inflow already manifested over the last 50 years. The potential for this infrastructural intervention decreases in the future, where it is offset by the system reoperation that becomes paramount in balancing production and revenue. Finally, dam heightening emerges a robust adaptation option to secure high hydropower production across multiple uncertain futures, while being less robust in terms of hydropower revenue.