The main objective of this study was to investigate the existing Hawassa city stormwater drainagernproblems with respect to its hydraulic performance. The methodological approaches employed inrnthis study include; the identification of the existing stormwater drainage network flow direction,rnand delineation of contributing areas. Field surveying work was conducted to inspect the existingrndrainage condition and to measure channel geometric dimensions. In order to develop the IntensityrnDuration Frequency curve (IDF) the rainfall intensities for different duration were analyzed for arnperiod of 1985-2016. Probability distribution functions were analyzed and identified usingrnMicrosoft Excel. rnThe analysis from the frequency distribution indicate that General Extreme Value Type I (GEV-I)rndistribution was fitted well reasonably for the rainfall intensities. Rainfall quantiles for a durationrnof 24hr with a magnitude of 52.75, 68.67, 79.20, 92.51, 102.4 and 112.2 mm/hr was predicted forrna return period of 2,5,10,25, 50 and 100 years respectively. Based on the flow direction of thernstudy area the existing drainage channel capacity were checked by using flow master software.rnEstablished on settlement of the city about 66, sub-catchments were delineated in the existingrnsystem and by using the intensity estimation (T=10 years) the hydrological peak flow determinedrnby using Bentley CivilStorm software. The rational formula is used to compute the peak floodrnestimation for (T= 10). rnThe result from model compared with existing hydraulic capacity of the channels and it shows thatrnat Wanza square stormwater drainage were not sufficient to carry the expected runoff during rnrainfall events. In general, the flood problems identified at Wanza square and Wanza square to Old rnbus station were mainly due to in adequacy of the drainage channel capacity and furtherrnsubstantiated by the waste disposal from different sources into the drainage channels. To resolvernthese problems BMPâ€™s have been recommended. Finally, the analysis from this study permitsrnfuture work on stormwater drainage design should strictly follow the standards of hydrologic andrnhydraulic design and social and environmental considerations.