Abstract
Background/Aim: This study aims to develop a systematic process model explaining how technical analyses performed during the preliminary design stage of flood protection projects are transformed into engineering design decisions. Within this framework, synthetic and statistical flood estimation methods, hydrological and hydraulic analyses, field investigations, institutional data, and multidisciplinary engineering evaluations were integrated to support reliable project discharge selection in medium sized watersheds.Methods: The Büyükköy Stream watershed was selected as the case study. Flood discharges were estimated using the Mockus, DSİ Synthetic Unit Hydrograph, Regional Flood Frequency Analysis, and Point Flood Frequency Analysis methods. The results were evaluated together with watershed characteristics, stream gauging station (SGS) data, field observations, hydraulic cross section analyses, and flood boundary assessments. The relationships between these technical analyses and engineering design decisions were systematically examined to develop the proposed interdisciplinary process model.Results: The comparative analyses showed that design flood discharge selection should not rely solely on the highest calculated discharge but also on watershed characteristics and the adequacy of SGS data. Although the Mockus method produced higher discharge estimates, the DSİ Synthetic Unit Hydrograph yielded results closer to the Regional Flood Frequency Analysis. Considering the adequacy of SGS records, Regional Flood Frequency Analysis was adopted as the basis for design discharge selection. Hydraulic analyses and field investigations further demonstrated that channel constrictions, bank erosion, sediment accumulation, bridge blockage, channel instability, and geological conditions significantly influenced engineering design decisions. These findings guided the determination of flood boundaries, protection structure locations and types, site plans, cross sections, and additional engineering investigations.Conclusion:The proposed process model demonstrates that the preliminary design stage of flood protection projects is an integrated engineering decision making process rather than merely a computational stage. Field investigations, institutional data, hydrological and hydraulic analyses, and multidisciplinary engineering evaluations are systematically transformed into engineering design decisions. The model also provides a practical framework for defining project scope, evaluating potential flood risks in upstream watersheds and tributaries, and identifying additional geological, sediment, hydraulic, and other technical investigations required during subsequent planning and detailed design stages.