Implications of discontinuous IDF equations in generation of runoff hydrographs. Case study: IDF-Porto Alegre (8º DISME)

ABSTRACT The intensity-duration-frequency (IDF) equations establish the relationship between the intensity and the duration of an extreme precipitation associated to the probability of its occurrence. Some studies have fitted multiple IDF equations per rain gauge, valid for certain rain duration ranges. An example is the IDF equation for the 8th District of Meteorology of Porto Alegre rain gauge, established and published by the CPRM in the Pluviometric Atlas Project. The main objective of the present study is to evaluate the implication of using this type of IDF equation, referred to as a “discontinuous IDF equation”, in the generation of runoff hydrographs, using the mentioned IDF as case study. The methodology consisted in comparing the peak flow and the runoff volume of hydrographs generated by the discontinuous IDF equation with the hydrographs obtained by using a single IDF equation. The runoff hydrographs were generated for hypothetical basins with the following characteristics: contribution areas of 5, 20 and 80 km2; CN of 70, 80 and 90; and time of concentration of 20, 40, 60, 100 and 200 minutes. A 24-hour rainfall event was considered with return period of 5, 10, 25, 50 and 100 years. As results, it was observed that, in the case studied, the multiple IDF equation present a better fit to the observed rainfall data than the single IDF equation. However, the discontinuity at the transition point between the equations, depending on its magnitude, may present some influence on the peak flow and on the runoff volume due to occurrence of secondary peaks on the runoff hydrographs. Therefore, it is recommended that a maximum limit of discontinuity must be observed between the multiple equations in order to avoid the occurrence of secondary peaks in the runoff hydrographs