@article{article_1645774, title={Flood Discharge Estimation in Ungauged Basins Using Synthetic Unit Hydrographs and GIS}, journal={Firat University Journal of Experimental and Computational Engineering}, volume={4}, pages={375–392}, year={2025}, DOI={10.62520/fujece.1645774}, author={Kesgin, Erdal}, keywords={Coğrafi bilgi sistemleri (CBS), Sentetik birim hidrograf, Taşkın, Mockus, Snyder}, abstract={Flooding refers to the adverse effects caused by rivers overflowing their banks due to various reasons, affecting surrounding land, residential areas, and infrastructure. At the watershed scale, particularly in cases where flow monitoring stations are absent, hydrographs must be generated to analyze rainfall-runoff relationships for flood assessments. This study aims to generate synthetic hydrographs, analyze rainfall-runoff relationships, and estimate flood discharges for different return periods in a predominantly forested sub-watershed located in the Sarıyer district of Istanbul. The study analyzed extreme rainfall by calculating 24-hour maximum values for return periods of 2 to 100 years using four common probability distribution functions: Normal, Log-Normal, Log-Pearson Type III, and Gumbel. Among these methods, Log-Pearson Type III yielded higher rainfall values, and given the extreme nature of floods, it was preferred for discharge calculations. In the second stage of the study, flood hydrographs specific to the watershed were generated for different return periods using the DSI, Mockus, and Snyder unit hydrograph methods, incorporating watershed physical characteristics and dimensionless unit hydrograph coordinates. The results indicated that the DSI and Mockus methods produced similar and higher peak discharge values (Qₘₐₓ = 67.44 and 63.76 m³/s, T=100 years), whereas the Snyder method resulted in lower peak discharge (Qₘₐₓ = 32.17 m³/s for T = 100 years) but a longer hydrograph duration. Overall, it was concluded that the DSI and Mockus methods are more suitable for flood analysis in forested and relatively small watersheds (≈10 km²) due to their effectiveness in generating hydrographs for flood assessments. This study contributes to the literature by offering a comparative evaluation of three widely used synthetic unit hydrograph methods, specifically tailored for a forest-dominated ungauged basin in an urbanizing region of Istanbul, providing actionable insights for flood estimation in data-scarce, forested urban catchments.}, number={2}, publisher={Fırat Üniversitesi}