Hydraulic Geometry of ‘Equilibrium’ Channels: From Theory to Application at the National Scale

Interactions between sediment mobility and transport capacity are one of the key controls over the geometry and morphology of gravel bed rivers; these drivers are temporally variable, fluctuating in response to changes in channel hydrology - for example, with climate change - and sediment supply - for example, via land use change. Quasi-stable, equilibrium channels occur when transport capacity and bed mobility are in balance. In both field and experimental flume studies, various efforts have been made to predict the relationship between channel hydrology (bankfull discharge, Q_bf) and the equilibrium dimensions (channel width and depth). Similarly, recent studies seek to quantify equilibrium state by approximating the ratio of dimensionless bankfull shear stress to dimensionless critical shear stress (τ^*_bf/τ^*_c). If robust, these theories can offer a useful approach towards identifying channels that are sensitive to present and future aggradation and/or degradation, and can therefore be valuable tools in applications such as predicting the impacts of climate change and flood risk management. Despite their widespread use in the identification and comparison of channel stability at regional scales, these quantitative methods remain uncertain when investigating the equilibrium state of individual channels, particularly when applied to semi-managed reaches.

Through completing a UK-wide assessment of upland channel stability, this study aims to field-validate hydraulic geometry theories and critically evaluate their appropriateness in river management applications within a UK context. A dataset comprising 50 upland reaches of various sizes (Q_bf varied from ~2 to 270 m³s^-1) was collected through field survey. Observed evidence for recent aggradation and degradation was compared against hydraulic geometry theory. Where τ^*_bf << τ^*_c, channels are predicted to aggrade, typically resulting in geometries wider and shallower than expected (and vice versa for degradational regimes, where τ^*_bf >> τ^*_c). However, when compared against field observations, predictions do not always coincide with reality. Here, we identify case study exceptions, and explore process complexities (for example, sediment supply, confinement and bank reinforcement) that lead to deviation from the predicted aggradational/degradational regime. Additionally, to account for deviations from expected channel morphology, we consider temporal variations in bed structure and sediment mobility thresholds under different hydrological regimes.