Selection of hyetograph shapes for generating synthetic hydrographs in a bivariate analysis for hydrological dam safety assessment.

In flood events, in order to evaluate the hydrological dam safety, it should be assessed the frequency curve of maximum reservoir water levels. For this purpose, a large set of inflow hydrographs are routed through the reservoir. In this approach, the result will depend on several variables, such as flood peak and hydrograph volume frequency curves and the dependence structure between variables. In addition, for this process several hydrometeorological simulations are performed in order to characterise the catchment response in flood events, obtaining hydrograph shapes that can be generated in the catchment. However, first, the hydrological model calibration requires, as input data, given hyetograph shapes.

This study presents the application of a bivariate analyses to assess the hydrological safety of dams based on hydrometeorological simulations. The analysis is carried out in the Cuerda del Pozo Dam in central Spain. In this catchment, flood hydrographs associated with annual maximum peak flows are usually generated by storms with a duration of several days. Consequently, hyetographs of several days obtained from intensity-duration-frequency curves are required, in order to obtain the runoff volumes given by the univariate frequency curve of hydrograph volumes. However, hydrological simulations with such long hyetographs present different problems. If a small time step is considered in the design hyetographs of several days, sharp hydrographs will be generated with peak flows greater than required. Unreasonable model parameters would be used to smooth the concentration and diffusion processes, reducing simulated flood peaks. On the other hand, if a large time step is considered, smooth hydrographs could be obtained with flood peaks smaller than quantiles in the flood frequency curve. Therefore, a detailed analysis is carried out to calibrate both flood peaks and hydrograph volumes, obtaining an appropriate hyetograph shape that will lead to acceptable values of the hydrological model parameters.

The calibrated rainfall-runoff model is used to generate a set of possible synthetic hydrograph shapes. A bivariate analysis is performed to generate random pairs of peak flow and hydrograph volume that fit the univariate frequency curves and keep the dependence structure between variables. A given synthetic hydrograph shape is assigned to each pair of peak flow and hydrograph volume. A long set of 500 000 inflow hydrographs is used. Hydrological safety of the Cuerda del Pozo Dam is assessed by using the frequency curve of maximum reservoir water levels.