Cost of most highway projects can be attributed to the design and construction ofrndrainage facilities such as bridges, culverts, storm drains etc. The design of thesernstructures involves hydrologic analyses to determine the design discharge andrnhydraulic analyses to determine the conveyance capacity. Although most hydrologicrnand hydraulic calculation procedures are now available as computer programs, whichrncan significantly reduce the mathematical effort involved, substantial effort is stillrnnecessary to manipulate the data required for input into these programs.rnTraditionally the data generated to support these programs have been extractedrnmanually from maps and cross-sections presented on paper drawings, however byrnbuilding a digital spatial database of the hydrologic features, a GIS in conjunction withrnthis database, the extraction of data and application of design procedures will becomernautomated and more efficient.rnIn this research study a GIS support system has been developed to assist in the designrnof highway drainage facilities. The selected case study is on tributary of the main AkakirnRiver which is located in Addis Ababa. By utilizing hydrologic spatial data to calculaternthe input parameters such as catchment area and flow direction for standardrnhydrologic software packages. This GIS support system reduces the analysis time andrnimproves the analysis accuracy by integrating digital spatial data that describe thernwatershed of interest with hydrologic theory. This helps in selecting methods ofrnhydrological analysis based on the existing situations.rnHydrology model is an essential tool for use in simulation, planning and managementrnof rainfall and runoff processes. It is designed in a simplified way for quantitative andrnqualitative modelling at the hydrological processes. In this study, HEC-HMS version 3.2rnis used to simulate stream flows for the tributary of Akaki river catchment which isrnlocated in Goro-Akaki road. Precipitation data used for calibration is collected from IDFrncurves of the region for 10, 25, 50 and 100 return periods the results obtained forrnreturn period of 100 is 504.3m3/sec and the high-water mark is 2164m which is lowerrnthan manually computed peak discharge and high water mark. The accuracy ofrnsimulation is highly dependable on the suitability of the parameters and accuracy ofrndata used in the calibration and validation process. Results of simulation can berngenerated in hydrograph, summary table and time series table. The correlation ofrncoefficient (R2) and root mean square (RMSE) are used to evaluate the prediction ofrnHEC-HMS model. Key words: GIS, hydrological analysis