Hydrological dam safety analyses should assess the frequency curve of maximum reservoir water levels in flood events, by routing a large set of inflow hydrographs in the reservoir. Routing processes in the reservoir will depend on several variables, such as flood peak and hydrograph volume. Therefore, bivariate analyses are required for hydrological dam safety analyses. In addition, hydrometeorological simulations are required to generate possible hydrograph shapes that characterise the catchment response in flood events. Adequate hyetograph shapes will be required as input data for calibrating the model.

In this study, the Cuerda del Pozo Dam in central Spain is selected as case study. Hydrographs associated with the annual maximum peak flows lead to storm durations of several days. Therefore, hyetographs of several days based on intensity-duration-frequency curves are required to generate the runoff volumes given by the univariate frequency curve of hydrograph volumes. Design hyetographs of several days with a short time step can generate sharp hydrographs that require unreasonable model parameter values to smooth flood peaks. Design hyetographs with long time steps could lead to excessively smooth hydrographs with lower flood peaks than observed. A sensitivity analysis is carried out to identify the most suitable hyetograph shape to calibrate flood peaks and hydrograph volumes with reasonable model parameter values.

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. A given synthetic hydrograph shape is assigned to each pair of peak flow and hydrograph volume. Hydrological safety of the Cuerda del Pozo Dam is assessed by using a long set of 500 000 inflow hydrographs.