EVALUATION OF EFFECT OF HYDROLOGIC OUTLIERS ON INTENSITY-DURATION-FREQUENCY (IDF) CURVES USING MIXED GUMBEL DISTRIBUTION

Abstract

Outlier is an observation in a data set which appears to be inconsistent with the remainder of that set of data. Hydrologic systems are sometimes crushed by extreme events which may be expressed by outliers in hydrologic observations. The presence of outliers in hydrologic data distorts the modeling efforts and need to be excluded from the set of rainfall data to be analyzed. Rainfall intensity, duration and frequency are important rainfall characteristics and interact with each other in hydrologic design problems. These three characteristic are graphically combined into rainfall intensity-duration-frequency (IDF) curve. The presence of outliers in rainfall data affects the IDF curves that are used to design dams, bridges, culverts and flood control structures. The IDF curves constructed using rainfall data with outliers could lead to inefficient designing of hydraulic structures in terms of over designing which will be not economically feasible. While the use of IDF curves developed using rainfall data after the deletion of outlier could lead to the underestimation which will not be safe. So the proper treatment of outliers was needed to construct IDF curves for safe and economical design. In this study, mixed Gumbel distribution is used to evaluate the effect of hydrologic outliers on intensity-duration-frequency curves and the results shows that the intensity-duration-frequency curves estimated using mixed Gumbel distribution are very helpful for safe and economical designing of hydraulic structures.