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 8 th 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 km 2 ; 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.Keywords: IDF; Hyetograph; Runoff hydrograph. RESUMOAs equações ou curvas intensidade, duração e frequência (IDF) estabelecem a relação entre a intensidade e a duração de uma precipitação extrema associada à sua probabilidade de ocorrência. A maioria dos ajustes destas curvas é realizado por meio de apenas uma equação; no entanto alguns autores estabelecem curvas IDF múltiplas, com duas ou mais equações distintas, válidas para determinadas faixas de duração de chuva. É o caso, por exemplo da IDF do 8º Distrito de Meteorologia de Porto Alegre, estabelecida pela CPRM no âmbito do Projeto Atlas Pluviométrico. O presente trabalho teve como objetivo avaliar a implicação da utilização desta IDF, chamada, neste trabalho, equação IDF descontínua, na geração de hidrogramas de escoamento superficial, utilizando-se como estudo de caso a IDF mencionada. A metodologia consistiu em comparar a vazão de pico e o volume dos hidrogramas de escoamento gerados a partir da IDF descontínua com hidrogramas obtidos a partir de uma IDF de equação única. Os hidrogramas foram gerados para bacias hipotéticas com áreas de 5, 20 e 80 km 2 , para cada qual variou-se o parâmetro CN em 70, 80 e 90 e o tempo de concentração em 20, 40, 60, 100 e 200 minutos. Considerou-se um...

Alternative model of intense rainfall equation obtained from daily rainfall disaggregation

“…Back et al (2011) used the limit with 120 minutes to adjust the equations of intense rainfall with rainfall data from Santa Catarina. Helfer et al (2017) point out that although the multiple IDF equations show a better fit to the observed rainfall data than the single IDF equation, the magnitude of the discontinuity at the transition point between the equations can cause changes in the values of the flow rate peak and the hydrographs volume.…”

“…Several studies comparing methodologies to obtain an IDF equation by rainfall disaggregation showed that the method using the coefficients of CETESB was the one that best represented the maximum rainfall intensities (Robaina, 1996;Damé et al, 2008;Garcia et al, 2011;Caleira et al, 2015). Helfer et al (2017) point out that most of the adjustments of these equations are performed using only one equation. However, in order to obtain better precision in rainfall intensity estimates, some authors establish multiple IDF curves, with two or more distinct equations, valid for certain rainfall duration ranges.…”