In response to changes in the upstream controls (i.e., the water discharge, the sediment supply rate, and the calibre of the load), engineered alluvial channels adjust their bed slope and bed surface texture to establish a new equilibrium state. Here we present and discuss various causes of degradational response of engineered channels to changes in the upstream controls and channel width. For that purpose, we apply a simplified 1D numerical research code to a schematic river reach of constant width consisting of mixed-size sediment, and assess its equilibrium state and transient response. We illustrate that the following perturbation to an initially equilibrium state lead to a degradational response: an increase of the water discharge, a decrease of the sediment supply rate, an increase of the sand content of the sediment supply, an increase of the gravel content of the sediment supply, and a decrease of the channel width. Degradational response under all conditions is associated with surface coarsening. The equilibrium states of the numerical simulations agree with analytical solutions. The results provide insight into the current degradational response of engineered rivers, such as the Rhine River, the Elbe River and the Danube River.

Engineered alluvial channels are dynamic systems and continuously adjust their bed slope (by aggradation and degradation) and bed surface texture in response to changes in the upstream controls i.e., the water discharge, the rate and calibre of the sediment supply (Mackin 1948, Blom et al. 2016, 2017a). These adjustment processes (the transient phase) proceed until a new equilibrium state is reached. The equilibrium state can be disturbed, for instance, by natural changes or measures such as river training, repeated sediment extraction and nourishment measures. The resulting changes in sediment transport capacity, sediment supply rate or caliber of load induces adjustment of equilibrium.

The main objective of this research is to  improve our understanding of the relative  contribution of the causes of long-term bed  degradation in Rhine and other degrading  rivers. That is, the research is intended to  quantify past channel adjustment processes,  mainly bed degradation and bed surface  coarsening over time and space, and to predict  future trends, in bed elevation and bed surface  texture, resulting from past interventions.