Estimate of Design Hydrographs for the Angostura Dam, Sonora, Using Statistical and Spectral Methods

Large dams require not only the definition of peak flow but also description of the shape of the hydrograph because both aspects are considered in the design and operation of the spills. There are several methods to do this, but these are subjective and questionable, and rely on the time-base definition of the design hydrograph, and on the historical samples used for estimation. In this paper a new procedure is proposed to obtain the design hydrographs for this class of spills by means of daily mean inflow historical data, selecting the time base of the design hydrograph using spectral analysis of the historical floods and eliminating the subjectivity on its selection. Additionally, a procedure to give the shape of the flood is presented, that is different from the traditional procedure of alternating blocks or utilising Gamma or Hermitian Hydrographs because it is added a statistical analysis of the block to be alternated. Results were obtained using the different methods for a return period of 10,000 years, using the historical data of the Angostura dam at Sonora in Mexico. The design hydrograph obtained was compared with the Method of Institute of Engineering UNAM (IIUNAM) and a bivariate probabilistic method. Using the spectral method the time base of multi peak floods were obtained, similar to those which were recorded by the historical events.