Effects of water withdrawal
The longitudinal profiles of Figures A to C visualize how water withdrawal 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The intervention implies essentially that water is withdrawn continuously from the river at a certain location (A). The immediate effect is a decrease in water levels along the river (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. The decreased flows downstream will also produce sedimentation in the mouth of the river.
Eventually, in the long run, the river reaches a new morphological equilibrium without further trends of erosion or sedimentation (C). All bed levels and water levels have become higher than at the start of the intervention. This means that the response of the river bed is particularly complex upstream of the intervention: erosion on a short term but sedimentation on a long term.
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.
- Surface water abstraction
- Hydrological regime modification
- Interbasin flow transfers
- Discharge diversions and returns
- Lower river banks or floodplains to enlarge inundation and flooding
- Modify hydropeaking
- Water diversion and transfer
- Reduce surface water abstraction with return
- Retain floodwater
Related Hymo quality elements
Related Biological quality elements
- No Biological Quality Elements apply to this tool.