Effects of Rainfall Intensity - Duration - Frequency Curves Reformation on Urban Flood Characteristics in Semiarid Environment

Abstract

Background: A design storm is a theoretical storm event based on rainfall intensities associated with frequency of occurrence and having a set duration. Estimating design storm via rainfall intensity – duration – frequency (IDF) curves is important for hydrological planning of urban areas. Material and Methods: The impac t of changes in rainfall intensity – duration – frequency (IDF) curves on flood properties in an urban area of Zanjan city was investigated, using Storm Water Management Model (SWMM). For the IDF curve generation, Sherman and Ghahreman - Abkhezr methods were com pared. Results: According to results, the estimated rainfall depth and, consequently the peak runoff rate for different return periods had decreased in the recent years, except for 2 - year return period. Decrease in peak runoff rate was 30, 39, 41 and 42 p ercent for 5 - 10 - 20 and 50 - year return periods, respectively. Based on the results, for peak runoff evaluated in 50 - year return period using Sherman and Ghahreman - Abkhezr hyetograph, percent of flood that occurred before the peak runoff were 27 and 22 perce nt, respectively. Discussion and Conclusion: Design rainfall hyetograph showed that Sherman method gave larger rainfall intensity compared to Ghahreman - Abkhezr method. Estimated peak and total runoff volume follow trend of rainfall intensity. As Ghahreman - Abkhezr method use longer and newer rainfall data for creating IDF curves, we can conclude that climate change cause change in rainfall characteristics. The runoff modeling show that main urban drainage system had enough transfer capacity against the floo d condition, but survey information indicated several inundations in some flat areas, curbs and gutters. Inappropriate design and obstruction of the runoff paths via urban garbage and sediments are some parameters that could lead to such local inundation