Equilibrium of Self-Formed, Single-Thread, Sand-Bed Rivers

Equilibrium geometry of single-thread rivers with fixed width (engineered rivers) is determined with a flow resistance relation and a sediment transport relation, if characteristic discharge, sediment caliber and supply are specified. In self-formed channels, however, channel width is not imposed, and one more relation is needed to predict equilibrium geometry. Specifying this relation remains an open problem. Here we present a new model that brings together a coherent train of research progress over 35 years to predict equilibrium geometry of single-thread rivers from the conservation of channel and floodplain material. Predicted channel geometries are comparable with field observations. In response to increasing floodplain width, sand load and grain size, the equilibrium slope increases, bankfull depth and width decrease. As the volume fraction content of mud in the sediment load increases, bankfull width-to-depth ratio and slope decrease suggesting that mud load has a strong control on channel patterns and bankfull geometry. Plain Language Summary Empirical relations expressing balance between sediment erosion and deposition, or linking channel properties such as bankfull discharge, channel width, depth and slope have long been used to model channel geometry owing to a lack of equilibrium solutions based on first principles. Here we present a solution for equilibrium geometry of single-thread, sand-bed rivers based on first principles that compatibly predicts measured field channel geometries. Empirical and theoretical formulations are used to compute sediment fluxes and bank migration respectively. The equilibrium solution can be used to predict long term (equilibrium) impacts of changes in hydrology and land use on alluvial rivers, and thus identify management and restoration practices based on how each river tends to evolve in space and time. Key Points Conservation of floodplain sediment allows us to predict the geometry of self-formed channels in agreement with field observations Bankfull geometry and discharge of alluvial rivers can be predicted if the hydrologic regime, sediment supply and caliber are specified Wash-load plays a prime control on channel geometry of single-thread rivers

Automated summary

The equilibrium geometry of single-thread rivers is determined by a flow resistance relation and a sediment transport relation for engineered rivers with fixed width. However, in self-formed channels where channel width is not fixed, an additional relation is needed. The new model presented in this study predicts equilibrium geometry based on the conservation of channel and floodplain material, showing comparable results with field observations. Mud load is found to have a strong control on channel patterns and bankfull geometry.