Analyzing the Impact of Riverbed Aggradation and Degradation on Flood Inundation Scenarios in an Ungauged River Using Hydrological and Hydraulic Model

ABSTRACT: Flooding is a recurrent and destructive natural hazard in Nepal, particularly in ungauged river basins where the lack of hydrological observations increases modeling uncertainty and where sediment-induced riverbed changes significantly influence flood behavior. Most flood models assume river courses remain constant. This is not true for rivers that constantly alter due to silt deposition and erosion. Ignoring these may lead to inaccurate flood predictions. This study examines the impact of long-term riverbed elevation changes on flood magnitude and risk in the Bakra River, a watershed in eastern Nepal characterized by limited data availability. The Soil and Water Assessment Tool was used to simulate runoff and sediment yield, and the one-dimensional Hydrologic Engineering Center–River Analysis System model was utilized to analyze hydraulic and sediment motion. The nearby Kankai River was used to calibrate and test the Soil and Water Assessment Tool model. The model performed well, with NSE = 0.77, R² = 0.79 during calibration (2010–2014), and NSE = 0.78, R² = 0.83 during validation (2015–2019). Simulating sediment with the same flow conditions yielded a good match (R² = 0.89). After that, calibrated parameters were calculated Bakra River water and sediment capacity. For return periods of 2, 25, 50, and 100 years, flood frequency analysis yielded design discharges of 78 m³/s, 245.7 m³/s, 328.2 m³/s, and 397 m³/s. Based on Digital Elevation Model terrain data and Manning’s roughness coefficient, the Hydrologic Engineering Center–River Analysis System hydraulic model was employed. The Hydrologic Engineering Center–River Analysis System sediment model showed 41 years of riverbed alteration using the same calibrated geometry. The data showed that degradation was the predominant process, with the river’s aggradation reaching 2.1 m and its degradation 4.0 m. Floods are modeled with varying return periods using new river morphologies. Changes to the riverbed demonstrated differences in flood area size, depth, and risk. Overall, flooded regions got smaller, but very high hazard zones got roughly three to five times bigger than when the bed didn’t alter. Aggradation raised water levels and decreased channel capacity, creating high-speed and scouring zones around bridges. The study may assist in planning and managing the Bakra River and other similar study reaches to prevent future floods.