Observation of the error at the inlet
Zoom on the height of water at different time in the middle
of the minor bed :
We can notice that inlet flow becomes equal to zero, in reality it is equal to 10 m3/s spread on a 2 km width !
Solutions for boundary conditions
The solution is to adapt the width of the inlet flow: width to the minor bed for 10 m3/s and adapt the with to the increase of flow by stopping calculation and restarting it with different inlet conditions.