A stochastic methodology to assess the impact of climate change on the Eugui hydrological dam safety (Spain) with an ensemble of climate projections

Climate change is increasing frequency and magnitude of precipitation extremes and floods. Increasing design floods in the future could lead to underestimated design capacities of current spillways, increasing the probability of dam overtopping. Therefore, new methodologies are required for assessing hydrological dam safety considering climate change. Moreover, uncertainty should be accounted to properly assess future changes in overtopping probabilities. This study presents a methodology to assess hydrological dam safety considering the impact of climate change on inflow hydrographs and initial reservoir water levels in flood events, quantifying the uncertainty in estimations. The methodology is applied to the Eugui Dam in the River Arga catchment (Spain), upstream the city of Pamplona.

The impact of climate change on inflow hydrographs in the Eugui reservoir is quantified by using the delta changes in precipitation quantiles extracted from climate projections in the Iberian Peninsula (Garijo & Mediero, Water 2019). The RIBS distributed hydrological model was used in Lompi et al. (Water, 2021) to quantify the expected changes in flood quantiles at the Eugui Dam for three-time windows (2011-2040, 2041-2070, and 2071-2100), two Representative Concentration Pathways (RCP 4.5 and RCP 8.5) and seven return periods (2, 5, 10, 50, 100, 500 and 1000 years).

The impact of climate change on initial reservoir water levels in flood events is assessed integrating the HBV continuous hydrological model with a reservoir operation model. HBV simulates daily inflow discharges in the Eugui reservoir by using rainfall and temperature climate projections as input data. HBV is calibrated with 13 years of precipitation, temperature, and reservoir inflow discharge observations. The reservoir operation model is developed to obtain daily reservoir water levels. It uses HBV inflow discharges as input data and considers reservoir operation rules, such as water supplies and environmental releases. Rainfall and temperature climate projections for an ensemble of 12 climate models are used to assess the changes in the expected daily reservoir water levels at the Eugui dam for each time window and emission scenario, including the control period.

A set of 10 000 peak inflow discharges are randomly generated from several GEV distribution functions fitted to the flood quantile outputs of the RIBS model. Given hydrograph shapes and initial reservoir water levels are assigned to each peak flow. The Volumetric Evaluation Method is used to simulate flow routing processes in the reservoir. The frequency curves of maximum reservoir water levels and maximum outflow discharges are obtained for each scenario, assessing the expected changes in the probability of exceedance of dam overtopping. In addition, a stochastic procedure quantifies the uncertainty chain of the methodology.

The results show an increase of both the maximum water level frequency and the probability of dam overtopping, especially in the period 2071-2100 for the RCP 8.5. Moreover, the maximum outflow discharge frequency also increases in all the time windows for the RCP 8.5, exacerbating the hydraulic risk for the downstream population in the Pamplona city.

Acknowledgments: This research is supported by the project SAFERDAMS (PID2019-107027RB-I00) funded by the Spanish Ministry of Science and Innovation.