User Forum of Software BASEMENT

AliceB

User

Registered: 2017-08-02

Posts: 1

Negative water depth at downstream BC

Hi,

I'm using Basement 2.7 to run a 2D simulation with sediment transport (multi grains).
The study area is a reach of a river where a local widening is planned. The downstream end of the domain is controlled by a weir. Thus, I used the downstream BC "weir".
Furthermore, I set a fix bed in the inflow channel as well as over the 5 last meters before the weir.

I got reasonable results when considering no sediment transport.

But as soon as I activate the mophology bloc, the following issues appear:

1. the downstream "weir" BC is not valid anymore and the simulation crashes down with the following error message: "negative discharge over weir, this should never happen".
   If I use a HQ relation instead, I don't have this issue anymore. Do you have any idea why it happens?

2. when I check the *bnd_combined_th.dat file, I have very strange and irregular outflow values, such as:

   time bnd_inflow  bnd_outflow
  ....        .....           .....
  400          1.5          0
  600          1.5          0
  800          1.5         10
1000         1.5          0
1200         1.5          0
1400         1.5          10

Moreover, I get the following error message: "warning -> Mass conservation is not guaranteed! Negative water depth in element XY", where the elements XY are the 2 elements located at the outflow BC.

Do you have any clues why this is happening? Is it caused by the BC condition I'm using? I tried with the downstream BC "zero_gradient" and I don't have this issue anymore. However, my BC IS a weir. So how to modelise it correctly?

3. Finally, I got the same issue as other users: all the sediments accumulate on the first cells of the domain. This phenomenon disapears when using the gravitational transport. But then, the *bnd_combined_th.dat file is empty. Do you have any idea why this is happening (I'm using exactly the same setting for both simulations, the only difference is that the gravitational transport bloc is activated).

Thanks a lot for your help.
Cheers