Evolution of Channel Roughness and Influencing Factors in the Wide Floodplain of the Lower Yellow River in Recent 20 Years

The key factor influencing flood evolution and the progression of riverbed erosion and silting in the wide floodplain of the Lower Yellow River (LYR) is channel roughness.The distribution law of channel roughness has changed since the implementation of water storage and sand control in Xiaolangdi Reservoir in 2002,and the change in roughness has had a number of effects on the design of flood control projects and ecological environment protection planning of the lower reaches.Manning’s formula was used to calculate the roughness based on cross-section data and hydrological data collected by typical hydrological stations in the wide floodplain of the LYR from 2002 to 2020.The evolution law and influencing factors of channel roughness of typical sections before and after floods were also examined.The findings indicate that:(1) The post-flood roughness was slightly higher than the pre-flood roughness in the same area.The post-flood roughness in the Huayuankou and Sunkou sections significantly increased compared to the pre-flood roughness,with an increase rate of 14.64% and 19.37%,respectively.(2) The time-to-space variation of roughness in the wandering section exhibits a decreasing trend,whereas the time-to-space variation of roughness in the transition section exhibits a slightly increasing trend.(3) The channel roughness decreases with an increase in Froude number,but there is no clear relationship between the roughness and sediment content.(4) Using Huayuankou and Gaocun stations as examples,the channel roughness is inversely proportional to the flow,the average sectional velocity,and the cross-section area of the flow.The channel roughness of the wandering section is inversely proportional to the ratio of width to depth,and the channel roughness of the transition section is proportional to the ratio of width to depth.Roughness and average velocity were determined to have the best-fitting connection,with R² values of 0.62 and 0.78,respectively.(5) The main elements impacting channel roughness in the wide floodplain of the LYR are section shape and flow condition.