Flood Inundation Modeling of the Gulin River Using HEC-RAS

Residents in hilly areas often select sites for housing on floodplains, riverbanks, and similar locations, which makes them highly vulnerable to casualties and property damage during flood events. Flood inundation analysis for mountainous rivers can provide critical data for urban flood prevention and disaster mitigation. This study takes the mountainous Gulin River as a case study, employing the hydrological analogy method and the inference formula method to calculate design peak discharges for recurrence intervals of 2, 5, 10, 20, and 50 years. Comparative analysis demonstrated that the hydrological analogy method yields results that are more representative of the design peak discharges across various river segments. Utilizing the derived design flood data and measured DEM data, a hydrodynamic model was developed using HEC-RAS for the Jinlan and Yongle reaches of Gulin River. The model computed key hydraulic parameters, including the inundation elevation and flow velocity under different recurrence intervals. The flood inundation extents were subsequently delineated using ArcGIS. The simulation results indicate that: (1) the study reaches begin to experience bank inundation under the 10-year flood scenario; (2) the maximum inundation depths for Jinlan reach reach 6.74 m, 7.81 m, and 8.92 m for 10-, 20-, and 50-year floods respectively; (3) the Yongle reach exhibits more severe flooding with maximum depths of 9.63 m, 11.45 m, and 16.77 m for the corresponding scenarios. The inundation depth in residential areas ranges from 0 to 2 m, with Xinhe Village and Yongle Town being the most severely affected; and (4) Building upon the one-dimensional model, two-dimensional hydrodynamic modeling was conducted for the river bend near Xinhe Village and the river reach near Yongle Town. The results indicate that particular attention should be given to floodplain inundation on the inner side of the bend at Xinhe Village, where flow deceleration may lead to extensive overbank flooding. In Yongle Town, riverside buildings and infrastructure are exposed to significant inundation risks under medium- to high-return period flood events. To mitigate these risks, structural flood protection measures such as levee construction along the riverbanks are recommended.