Engineered rivers are often prone to channel bed incision. This decreases the channel-floodplain connection, hampers navigation where nonerodible reaches increasingly protrude from the bed, and can destabilize structures. Here we inventorize causes and characteristics of channel incision measures. We elaborate on how channel bed incision is a transient channel response toward a new equilibrium channel state. Causes of incision comprise base level fall, channel narrowing (e.g., due to river training), channel shortening (bend cut-offs), an increased channel-forming discharge (e.g. due to climate change), and a decrease (or fining or coarsening) of the sediment flux from the upstream part of the basin. Finally, we discuss two measures that may mitigate channel bed incision: sediment nourishments and longitudinal training walls.

Engineering modifications of rivers, e.g., dams or groynes, often induce long-term riverbed erosion, which can be mitigated with sediment nourishments. Here, we consider nourishments to mitigate channel bed erosion induced by channel narrowing, as opposed to the more common application downstream of dams. Our objective is to assess and quantify how dumping location, grainsize, and volume are important for mitigation efficacy. Our results show that erosion can be mitigated if nourishments change the sediment flux such that the corresponding equilibrium channel slope is increased. This is achieved by coarsening the sediment flux throughout the reach, increasing magnitude of the sediment flux, or both. Flux is coarsened via additions of sediment at or coarser than the bed surface and nourished sediment should be distributed throughout the incising reach. The second option is nourishing a large volume of relatively fine sediment to increase the equilibrium channel slope. Additions of fine sediment in small volumes decrease the equilibrium channel slope and enhance erosion, because the fine sediment flux makes the gravel more mobile.

The Waal Branch of the Rhine River has eroded over the last 150 years following channel straightening and narrowing. In 2014–2015 a pilot project replaced existing groynes over an 11 km long reach with three longitudinal training walls (LTWs) to mitigate channel bed erosion, among other purposes. Walls are lower than the river bank and split the flow between a primary and an auxiliary channel, which are hydraulically connected during floods. Water enters the auxiliary channel at three elevations (from bottom to top): via an entrance weir, through inter-wall notches, and over the wall. Bathymetry and discharge data were collected for 5 years after construction, which is a first indication that longitudinal dams can help mitigate channel bed erosion and analyzed to understand how the walls partition water and sediment and whether erosion is mitigated by LTWs. As the river discharge increases, a larger fraction of flow is diverted from the primary channel into the auxiliary channel. After a flood, sediment is deposited in the primary channel near the upstream end of each wall and localized scour occurs where the auxiliary channel rejoins the primary channel. Between floods, the accumulated sediment disperses and scour pits tend to fill. We observe a net-accumulation of sediment in the study domain 5 years after construction. Erosion is best mitigated when weir flow is minimized to keep bed material in the primary channel, but weir flow remains important at lower flows for ecological purposes.

While most of the world's large rivers are heavily engineered, channel response to engineering measures on decadal to century and several 100 km scales is scarcely documented. We investigate the response of the Lower Rhine River (Germany-Netherlands) to engineering measures, in terms of channel slope and bed surface grain size. Field data show domain-wide incision, primarily associated with extensive channel narrowing. Remarkably, the channel slope has increased in the upstream end, which is uncommon under degradational conditions. We attribute the observed response to two competing mechanisms: bedrock at the upstream boundary increases the channel slope over the upstream part of the alluvial reach to compensate for the reduction of net annual sediment mobility, and extensive channel narrowing reduces the equilibrium slope. Another striking feature is the advance and flattening of the gravel-sand transition, suggesting its gradual fading due to an increasingly reduced slope difference between the gravel and sand reaches.

The Rhine River, like many highly-engineered channels throughout the world, has experienced a long history of human-made modifications to suit stakeholders. While the implementations have changed over time, the goal has been generally consistent – to ensure flood safety and navigation while limiting environmental
effects.

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.

Laboratory experiments were conducted on a sand-gravel Gilbert delta to gain insight on its dynamics under varying base level. Base level rise results in intensified aggradation over the topset, as well as a decrease in topset slope and topset surface coarsening, the signals of which migrate in an upstream direction. Preferential deposition of coarse sediment in the topset results in a finer load at the topset-foreset break, which creates a fine signature in the foreset deposit. Base level fall has the opposite effects. Entrainment of the topset mobile armor causes a coarsening of the load at the topset-foreset break and so a coarse signature in the foreset deposit. The entrainment of the topset substrate and fine top part of the foreset may follow, which causes a fining of the load and a fine signature in the foreset deposit. The fact that the upstream sediment supply requires a certain slope and bed surface texture to be transported downstream under quasi-equilibrium conditions counteracts the effects of base level change. This information travels in the downstream direction. In nature base level change is likely so slow that the upstream sediment load maintains the topset slope and bed surface texture and so keeps the topset in a quasi-equilibrium state. Base level change is therefore not expected to leave a clear signal in a mixed-sediment Gilbert delta other than a change in elevation of the topset-foreset interface.

Downstream fining of bed sediment in alluvial rivers is usually gradual, but often an abrupt decrease in characteristic grain size occurs from about 10 to 1 mm, i.e., a gravel-sand transition (GST) or gravel front. Here we present an analytical model of GST migration that explicitly accounts for gravel and sand transport and deposition in the gravel reach, sea level change, subsidence, and delta progradation. The model shows that even a limited gravel supply to a sand bed reach induces progradation of a gravel wedge and predicts the circumstances required for the gravel front to advance, retreat, and halt. Predicted modern GST migration rates agree well with measured data at Allt Dubhaig and the Fraser River, and the model qualitatively captures the behavior of other documented gravel fronts. The analysis shows that sea level change, subsidence, and delta progradation have a significant impact on the GST position in lowland rivers.

Sediment management measures are  becoming increasingly popular as they are  considered sustainable from both economic  and environmental point of view. For example,  aimed at counteracting river bed degradation,  sediment nourishments have been carried out  in the German reaches of the Rhine river while  a nourishment pilot study has recently taken  place at the Dutch Rhine and a nourishment  project has been scheduled for the Danube by  the Austrian water management authorities.  Moreover, sediment management measures  are implemented in various ecological  restoration projects (e.g. Trinity river in U.S.  and Nunome river in Japan) as sediments and  their characteristics form the habitats of the  biota, as well as in river training projects  around the world.

A gravel-sand transition (GST) seems to be the result of a gravel wedge. Such a wedge can prograde, halt, and even retreat. It is typical of an ungraded or transient reach (Blom et al., 2016), where profile concavity and downstream fining can be much stronger than in a graded or equilibrium river reach. The GST migration speed decreases in time and is likely of the order of a few meters or less per year. The effect of abrasion on GST migration is limited to mainly affecting the sand and silt load transported into the sand-bed reach and so its slope. Our simple GST migration model describes GST dynamics fairly well. It provides an estimate of the time scale of GST migration and a criterion for the onset for a GST to halt or retreat. We recommend analysis of temporal change in bed elevation and bed surface texture in GST zones, and the development of new techniques to measure the bed surface texture to assess GST dynamics in the field

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. 

There has been quite some debate on the relative importance of particle abrasion and grain size selective transport regarding the river profile form and the associated grain size trends in a graded alluvial stream. Here we present new theoretical equations for the graded alluvial river profile that account for the effects of particle abrasion and grain size selective transport in the absence of subsidence, uplift, and sea level change. Under graded conditions we find that abrasion results in a mild profile concavity and downstream fining, whereas under aggradational conditions grain size selective transport can lead to large spatial changes in channel slope and bed surface mean grain size.