The influence of roughness in the floodplain of the lower yellow river on water depth and flow velocity of overbank floods

The Jiahetan to Gaocun section floodplain of the Lower Yellow River (LYR). The LYR exhibits a typical wide-shallow compound channel morphology, with frequent floodplain flooding driven by geographical location and upstream hydrological variations. As roughness is a critical parameter governing floodplain flood evolution, this study developed a 2D hydrodynamic model using the finite volume method and unstructured triangular grids. The model incorporated Yellow River sediment effects and introduced frictional thickness in roughness calculations. Three roughness values were assigned based on floodplain land use, combined with five flood peaks to design 15 scenarios, aiming to quantify the mechanism by which roughness influences water depth, velocity, and their spatial distributions during floods. Concave banks are prone to overflow under terrain and scouring effects. When flood discharge ≤ 10,000 m³ /s, roughness significantly impacts water-depth distribution at 0–2.0 m. When discharge > 10,000 m³ /s, its influence on water-depth distribution above 2.0 m becomes prominent. Overbank velocities concentrate at 0–0.5 m/s, showing nonlinear relationships with roughness. Introducing frictional thickness improves roughness calculation accuracy for sediment-laden rivers and flood inundation modeling. Regulating hydrodynamic distributions based on roughness-flood relationships reduces flood losses, supporting coordinated flood control and socioeconomic development in the LYR floodplain. • Developed a 2D hydrodynamic model for the lower Yellow River with variable roughness. • Incorporated sediment-induced roughness changes using frictional thickness in calculations. • Simulated 15 flood scenarios with varying events and roughness values. • Quantified impacts of scenarios on flood inundation depth and velocity.