Distributed unit hydrograph (DUH) approach and mockus based analysis for hydraulic structure design in small watersheds under climate change conditions

In the Eastern Black Sea Region of Türkiye, streamflow gauging stations (AGS) are generally sparse and widely spaced in small watersheds; therefore, Synthetic Unit Hydrograph (SUH) methods are widely used for design discharge estimation. In particular, the Mockus (SCS) method is commonly adopted as the primary approach for hydraulic structure design in small and fast responding basins. However, under global warming conditions, the increasing frequency of short duration, high intensity rainfall events alters flood hydrograph characteristics, resulting in earlier peak times and higher peak discharges. In this study, the Altıntaş Stream watershed located in Rize, Türkiye, was considered as a representative small and steep basin of the Eastern Black Sea Region. In the first stage, SUH methods commonly used in Türkiye (Mockus, DSİ Synthetic, and Snyder) were applied, and the resulting hydrographs were analyzed. Based on this analysis, the limitations of these methods in representing physical processes, particularly in small and fast responding watersheds, were identified, and the necessity of examining the Distributed Unit Hydrograph (DUH) approach was justified within the preliminary analysis. Within this framework, a potential DUH scenario was developed based on literature findings, and a comparative analysis was carried out using a Mockus based DUH approach. The results indicate that SUH methods tend to underestimate peak discharge, delay the time to peak, and fail to adequately represent post flood processes, whereas the DUH approach provides earlier and higher peak discharges along with a more realistic representation of hydrograph behavior. These findings highlight the importance of the DUH approach for hydraulic structure design and operation in small watersheds.