Effects of weir removal
The longitudinal profiles of Figures A to C visualize how weir removal affects the main channel of a river reach. The vertical scale is exaggerated with respect to the horizontal scale. The reach can be kilometres to tens of kilometres long.
The intervention implies essentially that, at a certain location, a weir across a river is removed (A). The immediate effect is a decrease in water levels upstream (B). This decrease is not uniform, but varies from place to place. As a consequence, flow velocities differ from place to place too. The water levels and flow velocities can be calculated using 1D analytical models for gradually-varied flow or 1D numerical hydrodynamic models. The variations in flow velocity and, hence, the variations in the capacity to transport sediment give rise to a pattern of initial erosion and sedimentation along the river (B) that can be calculated using 1D analytical models for morphology on short time scales or 1D numerical morphodynamic models. Erosion over a relatively long distance occurs upstream of the intervention, whereas punctuated local sedimentation occurs downstream. This sedimentation advances downstream as a shock wave.
Eventually, in the long run, the river reaches a new morphological equilibrium without further trends of erosion or sedimentation (C). Upstream bed and water levels have become lower than at the start of the intervention. Downstream, the river resumes its original shape by erosion after an intermediate period of sedimentation.
The longitudinal profiles in the diagrams provide a simplified picture. They do not include the response of channel width, bed sediment composition or vegetation. Nonetheless, they offer the key to understanding the relation between local pressures or measures and their effects far upstream and downstream.
- No pressures apply to this tool.
Related Hymo quality elements
- Quantity and dynamics of water flow (Class: Hydrological regime)
- River continuity (Class: River continuity)
Related Biological quality elements
- No Biological Quality Elements apply to this tool.