Results

In this part we are going to analyze the results given by simulations in order to find the best way to reduce pollution in the route of fishes. First of all, the boundary conditions are fixed. Then, we will try to increase the outlet flow rate of the canal to see its influence on the pollutant. At last, we will try to simulate the flow when a hydraulic work , a see wall,  aims at deviating the pollutant.

Fixed boundary conditions:

At first, let's repeat the boundary conditions:

- inlet flow rate of the Vienne = 100m2/s,
- inlet flow rate  of the Envigne = 1m2/s,
- outlet flow rate of the canal =2m2/s (-2m2/s),
- outlet heigth of water of the Vienne = 47.5m NGF
Before providing an accurate analysis of polllutant flow, we are going to have a look to some others results. The picture below represents the heigth of water in the domain studied for a final time equals to 1000 time steps that is to say 16min 40s.

As expected the height of water is higher in the Vienne than in the Envigne and the canal because of the higher depth of the bottom of the Vienne.

Concerning the Froude number, the flow is fluvial (the maximum Froude number is 0.28) . It is more important in two sections of the  mesh. First, at the inlet of the Envigne because of the shallow bottom of the river. Secondly, at the outlet of the Vienne because of the dam.

The picture below represents the velocities vectors in the domain:

We can do several remarks:

- the velocity vectors are bigger in the outlet section of the Vienne. This is due to the dam and the conservation of the mass law,
- the water which enters the canal comes rather entirely from the Envigne,
- there are few vortex at the level where the Vienne meets the Envigne.
The following step concerns the pollutant flow.
The first animation represents the trend of pollutant between 1min and 16 min 40s:

Click on the picture to see the animation

Thanks to this animation we could see that polluted water enters the canal and moves forward quickly.
The second animation shows what happens betweens 17 min and 2h 40 min:

This animation shows clearly the dissipation of the pollutant in the canal. We can notice than the canal is entirely polluted and that there is no longer clean  water in it. The fishes will be in contact with pollutant.

Consequently, we decided to increase the outlet flow rate of the canal. Indeed, we expected a narrowing of the polluted area which could allow the fishes to move forward in a clean water.

Influence of the outlet flow rate of the canal:

The main objective of these part is to reduce the polluted surface area and the concentration of pollutant in this zone. But we have to keep in mind that two other constraint must be respected:

- the flow rate at the level of the dam must not decrease considerably. Indeed, the production of electricity of EDF must not decrease only for fishes!
- the velocity in the canal needs to keep slow because the fishes have to go up the stream.
First we are going to have a look to the concentration of the tracer for an outlet flow rate of the canal equals to 15m2/s which is considerably more important than the previous one. The image is taken for a final time equals to about 4h:

We can observe that the polluted area has decreased and that there is a large clean passage for the fishes. But what about the flow rate?
We are now going to see its effect on the velocity vectorsc at the confluence:

The velocity reach 0.9m/s which is perhaps too high for the fishes.
Now, we will analyse more particularly the influence of the outlet flow rate of the canal.
First, we will study the profile of the flow rate in a section just before the dam:

As expected we can notice that when the outlet flow rate of the canal increases the flow rate at the dam for EDF is lower.
The next picture shows the concentration of pollutant in the outlet section of the canal:

Obviously, when the outlet flow rate of the canal increases the concentration of tracer decreases. We can also notice that the pollutant is preferably concentrated on the right of the river because of the place where the pollutant is released.
We decided to analyse also what happens just at the beginning of the canal:

Here, the influence of the flow rate is even more visible. The pollutant is more concentrated and more spread when the flow rate is lower.
To validate this method we have to check the velocity in the canal:

The maximal velocity in the canal is 0.65m/s =2.34km/h, fishes could easily go up the stream!
To conclude, the increase of the oulet flow rate of the canal seems to be a good method to reduce the polluted surface area.

Use of an hydraulic work:

The objective was to insert a see wall in oder to let the pollutant diffused in the Vienne and not in the canal. As you can see below we created lines with Matisse to represent the see wall:

Unfortunately, when we tried to mesh the domain aroud the see wall matisse gace us an errror message and we didn't have the time to solve it. Consequenly, we could not conclude about this second method.