A methodology to assess hydrological dam safety in dams with gated spillways under the effect of climate change.

Dam breaks can be driven by a flood that exceeds the design spillway capacity, causing important economic and human losses. Dam spillways are designed for a flood that is usually estimated through either hydrometeorological or statistical analyses with observed data. However, time series of observations are usually short, incomplete and recorded at a daily time step. Moreover, design floods have to be estimated for high return periods greater than 500 years, leading to high uncertainties. In addition, climate change is expected to increase the frequency and magnitude of floods in the future. Therefore, new methodologies are required to assess hydrological dam safety considering both short time series of observations and climate change.

A stochastic methodology is presented here to assess hydrological dam safety considering the impact of climate change on floods, by integrating a stochastic rainfall generator and a rainfall-runoff model. The methodology is applied to the Eugui Dam on the Arga River in the north of Spain. The Eugui Dam has a draining catchment area of 69 km², a reservoir volume of 22 hm³, and a gated spillway.

First, the stochastic rainfall generator STORAGE (De Luca and Petroselli, 2021) based on the Neymann-Scott Rectangular Pulse Model has been used to simulate long time series of 500 years of precipitation with a time step of 15 minutes. The generator has been calibrated with rainfall observations. In addition, the STORAGE model has been used to generate synthetic time series of precipitation considering climate change. Delta changes extracted from precipitation projections of 12 climate models, three periods (2011-2040, 2041-2070, 2071-2100) and two emission scenarios (RCP 4.5 and RCP 8.5) are considered (Garijo and Mediero, 2019).

Second, the stochastic precipitation time series are transformed into runoff time series by using the COSMO4SUB model (Grimaldi et al., 2021). COSMO4SUB is a continuous model that uses a high-resolution digital terrain model, land cover / soil type data, and precipitation supplied by the STORAGE model as input data, providing continuous runoff time series as output. The COSMO4SUB parameters have been calibrated with runoff observations by minimizing a set of objective functions.

Third, annual maximum hydrographs, peak flows and hydrograph volumes are extracted from the runoff time series generated by COSMO4SUB. The Volume Evaluation Method (MEV) (Girón, 1988) is used to simulate the operation of spillway gates in flood events, obtaining maximum water levels in the reservoir and outflow hydrographs. The MEV method specifies when the spillway gates are opened and closed to reach the target reservoir water level at the end of the flood event. Hydrological dam safety at the Eugui Dam is assessed by analysing the frequency curve of maximum water levels in the reservoir for the 12 climate models, three return periods and two emission scenarios mentioned above. Therefore, the methodology proposed allows practitioners and dam owners to check the hydrological dam safety requirements detailed in the regulations, accounting for climate change.

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