AbstractThe Koraiyar River basin is located in Tiruchirappalli city, one of the fastest-growing medium-sized cities in south India. Over the decades, the city has experienced sporadic flooding in vast areas due to the breaching of bunds when the river is in spate. The objective of this investigative study is to simulate rainfall–runoff in the Koraiyar basin watershed through the reliable Hydrologic Modelling System developed by the Hydrologic Engineering Center (HEC-HMS). Land-use and land-cover constantly change due to increasing population taken as one of parameter, its influence on CN, and their impact on surface runoff is analyzed in this study. The specified hyetograph method is adopted for the meteorological modelling; the Soil Conservation Services-Curve Number (SCS-CN) is selected to calculate the loss rate, and the SCS unit hydrograph method is adopted to simulate the runoff rate. Calibration and validation of the model are done to simulate assessed peak discharge values for comparison with actual observed values. The Nash–Sutcliffe efficiency coefficient is between 0.5 and 0.6, which indicates that the hydrological modelling results are satisfactory and acceptable for simulation of rainfall–runoff. The peak discharge is obtained for the single maximum rainfall event of about 100 mm in a day over the past 40 years, and hydrographs are generated.HighlightsIt is the first integrated rainfall–runoff modelling to be studied in Ungauged Koraiyar basin with scarce data.The methodology adopted for simulating selected extreme events can be used for similar hydrological and meteorological conditions.For ungauged catchments like Koraiyar basin characteristics where enough data is not available, HEC-HMS can be used as a reliable tool for rainfall runoff and flood flow simulation.