Fundamental Hydraulic Geometry Relations as an Accessible Tool for Identifying Channel Instability Over National Scales

ABSTRACT The geometry of alluvial river channels can give insight into their stability, which can inform predictions of morphological change, flood risk and ecological degradation. Fundamental hydraulic geometry relations can be used to estimate the equilibrium dimensions of stable river channels by evaluating the balance between the erodibility of bed material (sediment entrainment thresholds, τ * c ) and the erosivity of bankfull flows ( τ * bf ). These relations allow the possibility of identifying hotspots of (dis)equilibrium over large spatial scales. This paper tests for the first time estimations of channel disequilibrium against observations of contemporary morphological change at 51 reaches across the UK, using readily attainable field survey data and remote imagery. Deposition generally occurred where τ * bf / τ * c < 1.2, and coincided with overwidened, shallowed channels, whereas erosion occurred in underwidened deepened channels where τ * bf / τ * c > 1.2. Interpretations were further complicated by localised bed and bank properties, which control the erodibility and/or confinement of channel boundaries at the site scale, and differences in sediment supply at the catchment scale. Uncertainty analysis also showed that quantifications of disequilibrium geometries were sensitive to the resolution and quality of channel geometry data (e.g., biases in LiDAR imagery), potential error in bankfull discharge estimates derived from hydraulic models, and τ * c assumptions. Nevertheless, the fundamental hydraulic geometry theory was effective in distinguishing sites which could be at most risk of long‐term aggradation or degradation, and should therefore be used to inform selections for more targeted channel monitoring. We provide practical guidance and outline key considerations for their application by environmental managers.