Rivers are essential for sustaining human life, preserving ecosystems, providing clean water, supporting energy production, offering recreation, and enabling transportation. Human interventions have led to the creation of so-called ‘engineered rivers’, such as the Dutch Rhine, which has undergone significant interventions over the past two centuries including straightening, dam construction, and the addition of fixed beds.

Fixed beds are found in the Dutch Rhine, such as in Nijmegen, St. Andries, and Spijk. Composed of non-erodible materials, they are strategically placed on the outer bends of rivers to enhance navigation by causing erosion in the inner bends, widening the river. Similar features worldwide include sediment nourishments and natural bedrock reaches.

This study investigates the large-scale morphodynamic effects of fixed beds, focusing on their influence on river slopes and sediment trapping. The research begins by examining the initial response of a fixed bed. A fixed bed results in (1) a sill-effect, (2) increased roughness, and (3) decreased mobility, and these effects are separated and treated individually. Conceptual models based on river dynamics theory are used to understand and predict how these effects contribute differently to the morphodynamic responses.

Following that, the study continues by looking into the transient and long-term response of a fixed bed using both conceptual and numerical models. These numerical models are created using the model system SOBEK-RE. The fixed bed-related effects are still considered separately with reference models created first and the effects integrated after. The reference model focuses on the transient state due to narrowing, where the slope decreases and the bed level increases. By doing this a comparison can be made of the fixed bed related effects with and without it. A similar process is repeated for a model run where the effects are all combined to assess their relative importance and the overall combined effect. The models reveal that all three effects contribute significantly to the fixed bed.

The model's key findings indicate that over a 50-year period, natural narrowing of the river reduces the slope by 4%. Introducing a fixed bed amplifies this effect: the upstream slope decreases by 3% and the downstream slope by 7%. This demonstrates that fixed beds alter the riverbed's slope, decreasing it downstream and increasing it upstream. At the upstream side of the fixed bed, it traps sediment caused by an M1-backwater curve. The height up to which this upstream sediment trapping continues is determined by two important parameters: the sill length and the sill height. However, the study acknowledges uncertainties related to model dimensions, sediment uniformity, discharge, and parameter choices. Real-world effects depend on the fixed bed's width, length, and protrusion relative to water level.

It is vital for water management authorities to recognize the importance of fixed bed structures, especially in extensively engineered rivers. This is because the fixed beds can result in significant and long-lasting changes to the riverbed.