Effects of climate change on hydrological safety of dams with gated spillways.

Dam breaks can origin important economic and human losses. Failure of such infrastructure may be due to flooding that exceeds the design capacity of spillways. Design floods are estimated through a statistical analysis of observed data, but observations are usually short. Furthermore, high return periods are used for estimating design floods and climate change is expected to increase the frequency and magnitude of floods in the future. Therefore, new methodologies to assess hydrological dam safety considering short observed data and climate change are required. 
A stochastic methodology to assess hydrological dam safety considering climate change is presented. The methodology is applied to the Eugui Dam on the River Arga in the north of Spain that has a gated spillway. 
The stochastic model STORAGE (De Luca and Petroselli, 2021) based on the Neymann-Scott Rectangular Pulse Model has been used to simulate long time series of precipitation with a time step of 15 minutes. Delta changes extracted from precipitation projections of 12 climate models, three periods and two emission scenarios (Garijo and Mediero, 2019) are used to consider climate change.
The precipitation time series generated stochastically are transformed into runoff time series by using the COSMO4SUB model (Grimaldi et al., 2021). It is a continuous model that uses a high-resolution digital terrain model, land cover and the precipitation supplied by the STORAGE model as input data, providing 15-min continuous runoff time series as output. The curve number and time of concentration variables have been calibrated by minimizing a set of objective functions.
Inflow hydrographs are extracted from runoff time series simulated by COSMO4SUB. The Volume Evaluation Method (MEV) (Girón, 1988) is applied to simulate the operation of spillway gates, obtaining maximum water levels in the reservoir and outflow hydrographs. The MEV method specifies when the gates should be opened and closed to reach the target water level. Hydrological dam safety is assessed with the frequency curve of maximum water levels in the reservoir for the climate models. Therefore, the methodology proposed allows practitioners and dam owners to assess the hydrological dam safety requirements detailed in the regulations, accounting for climate change.