Geometrical and roughness effects on compound channel resistance

This paper presents an analysis of resistance coefficients derived from measurements taken in phases A, B and C of the research programme of the UK Flood Channel Facility (FCF). The FCF is a large-scale compound channel facility and the programme has included straight and meandering planforms, smooth and mobile main channel roughnesses and smooth and rod-roughened floodplain boundaries. Floodplain roughness consisted of 25 mm surface-penetrating dowel rods. Data were also obtained for straight trapezoidal channels for comparison by confining flow to the main channels. Stage–discharge relationships were derived which enabled calculation of Manning's roughness coefficient values for a range of selected relative depths for each geometry, planform and roughness case. Graphs are presented of the variation with depth of Manning's coefficient for each geometry and roughness expressed as a ratio of smooth straight compound, or straight trapezoidal channel data. These presentations quantify the increase or decrease of roughness coefficient resulting form cross-sectional geometry, planform and boundary roughness. It is suggested that this represents a possible valid approach to the estimation of Manning's roughness coefficient in compound channels and rivers.