Habitat improvement stuctures are often buildt in many regulated rivers. The structures can be groynes made of stones or artificial pools. If the river carries sediments, there may be deposition around the structures. Also, groynes made of smaller stones may be eroded.
The purpose of the current project is to test a CFD model that computes the bed changes around the habitat structures.
The river Dalaa
The reach of the river with habitat improvements, looking downstream
In 1994, geometrical changes were made in this location, to improve the habitat for the salmon. A more narrow channel was excavated in the river, with varying depth. The channel was lined with rock from tunnel excavation. In 1995, the geometry of the reach was measured in detail for habitat assessment studies. The sediment composition was also measured.
This shows the depth of the reach. The red areas are where the channel was excavated.
A zoomed in picture of the reach
The habitat improvement structures changed over time, during floods, when sand deposited at one location in the excavated channel, where it turns to the right. The fine sand deposition deteriorates the habitat for the salmon, and is therefore an unwanted effect in the habitat improvement scheme. The purpose of the present study was to determine if a CFD model could predict the sediment deposition.
The location of the sediment deposition is shown in the image below:
Location of sediment deposition
The left bank at the middle of the reach, looking downstream. This is the location of the fine sand deposition.
The left bank of at the middle of the reach, looking downstream. Due to lowered water discharge, the left bank is seldom under water. A comparision of the bed levels in 1994 with measurements from 2002 showed that considerable amount of sediments had depositited here.
When the habitat improvement structures were made, modelling was only done for 0.5 m3/s, as this is the lowest allowed water discharge. This situation is then critical for the salmon. The modelled area only covered the excavated channel. However, the sediment movement mostly took place during floods, and the maximum flood was 30 m3/s. In the current study, this situation was modelled. A much larger grid was used in the lateral direction as more of the river banks were inundated. The effective bed shear stress is seen below:
The lowest shear is on the left bank, and in the area where deposition took place. This can be explained from the fact that during flood this reach of the river is in effect a large bend, and the left bank is on the inside of the bend. The water dpeth will therefore be lower here.
The CFD model also computed movement of fine material in the reach. The computed bed changes are shown below:
We see that the bed changes occur at the same location as observed in the field: on the left bank and in the right turn of the excavated channel.