Evaluation and analysis of the impact of factors affecting urban flooding using SCS-TR20 and SBUH hydrological models

Land use change and intense rainfall, particularly in urban areas, are among the key factors contributing to flood occurrences. This study investigates the impact of land use and rainfall changes on flood events using the Soil Conservation Service - Technical Release 20 (SCS-TR20) and the Santa Barbara Urban Hydrograph method (SBUH) models within the HydroCAD software. A key novelty of this study is the comparative application of the SBUH model-never before used in Iran for rainfall-runoff simulation-alongside SCS-TR20, combined with a detailed multi-temporal land use change analysis over a near-decade period (2010-2019) in the Doab-Veysian watershed. For this analysis, characteristics of five sub-basins (Bahramjo, Karganeh, Chenar-Khoshkeh, Cham-Anjir, and Doab-Veysian) including drainage area, time of concentration, curve number (CN), and cross-sectional geometry of river reaches were defined in HydroCAD. A 24-hour Type II rainfall pattern was selected as the optimal regional rainfall model. The models were then calibrated based on two rainfall events (24/04/2010 and 21/10/2014) and validated using a third rainfall event (01/04/2019). The results demonstrated that both the SCS-TR20 and SBUH models are capable of simulating floods with R² and Nash-Sutcliffe efficiency (NSE) values exceeding 90%, though these metrics were slightly lower for the SBUH model compared to SCS-TR20. An analysis of land use changes between 2010 and 2019 revealed that the expansion of dry farming (from 53,000 to 66,000 hectares), the decline of poor and moderate rangelands (from 50,000 to 43,000 hectares), and the reduction of forests (from 116,000 to 108,000 hectares) were additional factors increasing flood risks. Moreover, residential, commercial, and infrastructural developments in urban and rural areas expanded significantly, with such lands growing from 2,744 hectares in 2010 to 6,897 hectares in 2019. These changes led to an increase in the curve number (CN) from 81.72 to 83.47 in the Doab-Veysian watershed and from 91 to 92 in the urban area of Khorramabad. Combined with an increase in rainfall (from 21 mm to 116 mm), these changes resulted in a substantial rise in peak flood discharge (from 106 to 2,227 m³/s) and flood volume (from 3.83 to 98.33 million m³) according to the SCS-TR20 model. The SBUH model also indicated an increase in discharge (from 77.79 to 1,687 m³/s). These findings highlight the significant influence of land use changes and heavy rainfall on flood occurrences. In urban areas, flood discharge estimated by the SCS-TR20 model increased from 11.97 m³/s in 2010 to 135.87 m³/s in 2019, while flood volume rose from 0.146 to 2.42 million m³ over the same period. Correspondingly, the average flood depth across the urban watershed increased from 6 mm to 66 mm. These figures underscore the substantial impact of land use changes and increased rainfall in exacerbating floods in urban areas. In conclusion, the findings indicate that although both models perform effectively, SCS-TR20 simulates peak discharge more accurately than SBUH. Furthermore, rainfall was introduced as the major and dominant factor in flood generation in the region, although the role of antecedent soil moisture (AMC) land use changes and the CN should not be overlooked.