Comprehensive flood risk identification and assessment in small mountainous watersheds using GF-7 satellite imagery and hydrological-hydrodynamic modeling

The Liucun Town watershed in Beijing, China, represents a typical small mountainous watershed prone to short-duration, high-intensity floods. This study establishes a comprehensive flood risk indicator, the Hazard Rating for Systems & Time (HR S&T ), by integrating land-surface types, and terrain slope from high-resolution Gaofen-7 (GF-7) satellite imagery with flood characteristics especially the duration simulated by a hydrological-hydrodynamic model, to assess flood risk in mountainous watersheds. Unlike conventional flood risk indicators such as the Hazard Rating (HR), HR S&T not only incorporates traditional flood hazard characteristics like inundation depth and flow velocity, but also takes into account land cover types, flood duration and terrain slope. The methodology was applied to simulate flood events driven by observed rainfall under five design rainstorm scenarios (2, 10, 50, 100, and 1000-year return periods) and calibrated using the July 2023 rainstorm. The study shows that land-surface types, terrain slope, and flood duration strongly shape flood risk evolution. Considering these factors can effectively improve the accuracy of risk assessment in cultivated and steep-slope areas, especially in mountainous areas prone to secondary hazards such as debris flows and landslides. Meanwhile, GF-7 satellite imagery effectively compensates for the lack of high-resolution Digital Elevation Models and land-use data, reducing uncertainties in hydrological simulation in mountainous areas. • A new flood risk indicator (HR S&T ) integrates GF-7 imagery and hydro-dynamic models. • Slope information refines risk assessment by excluding low-vulnerability areas. • Flood duration is critical for risk assessment of flash floods in mountain regions. • The new indicator’s results show the best consistency with field disaster surveys.