Effects of sediment withdrawal

Description

The longitudinal profiles of Figures A to C visualize how sediment 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The intervention implies essentially that sediment is withdrawn continuously from the river at a certain location (A). This produces punctuated local erosion without immediate effects on the water levels along the river (B). This erosion advances downstream as a rarefaction wave. In a later stage, after sufficient erosion, the water levels decrease above the area of erosion and further upstream. The corresponding higher flow velocities cause erosion upstream of the intervention.

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 lower than at the start of the intervention.

If sediment is withdrawn during a limited period, only the initial response occurs. Eventually the river then restores its original profile.

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.

Related Pressures

• Sediment discharge from dredging
• Sand and gravel extraction

Related Measures

• Trap sediments
• Reduce undesired sediment input
• Remove sediments
• Remove non-native substratum

Related Hymo quality elements

• Structure and substrate of the river bed (Class: Morphological conditions)

Related Biological quality elements

No Biological Quality Elements apply to this tool.