User Forum of Software BASEMENT

DianaO

User

Registered: 2023-08-07

Posts: 4

weir: outflow & internal boundary leads to MASS BALANCE PROBLEM

Hi everyone,
I’m trying to simulate a weir with the height of 1.2m in a 35m wide channel with Basement V3. The weir is placed on the existing riverbed (upstream and downstream of the weir almost the same bed elevation). First, I simulated it as a non-erodible obstacle. For smaller uniform discharges (Q30=15m3/s) this worked fine but for higher discharges as (HQ100=300m3/s) the water depth upstream of the weir is ~4m and drops nearly vertically to ~1.5m after the weir.  Since the weir height is relatively small compared to the upstream water depth and the bed slope being quite small (< 1%), however, I would not expect a hydraulic jump to occur.
Then I’ve tried to simulate the weir with a linked boundary condition. The setup can be written successfully, and the simulation can be started. But then the message «MASS BALANCE PROBLEM: h = -38.6027 < 0.0» appears repeatedly with different numbers in the output console until the program crashes.  The same problem appears if I simplify the setup and only have a shorter straight river stream and put the weir at the end as outflow condition weir_out_constant, but the problem stays the same.

I’ve already tried the following:
-    Changing Simulation time and timestep
-    Weir_out_dynamic resp. weir_linked_dynamic
-    Safe mode off
-    Initial water level close to expected water level
-    Changing CFL number
-    Different discharges
-    Adjusting mesh that there are more nodes at the boundary (always same amount at in and outflow but still only one node string)

My Questions:
-    Is it reasonable to simulate a small weir like this on a plane surface as a dynamic weir. Simulating it as an elevation in the bed didn’t work for high flow conditions as explained before. What are other options?
-    How can I solve the Mass Balance Error?
-    In general, with a trapezoid weir and sloping riverbank walls: is the weir defined only over the riverbed but then how can I connect it to the sloping riverbanks?

Thank you very much in advance for your support.
Kind regards
Diana

Matteo Facchini

Developer

From: Trento

Registered: 2014-09-05

Posts: 278

Re: weir: outflow & internal boundary leads to MASS BALANCE PROBLEM

Hi Diana
your MASS BALANCE PROBLEM cannot be ignored!
The problem with linked boundary conditions was posted somewhere else in the forum.

I don't see a problem in simulating the weir as an 'obstacle', remember that the model does what the equations tell it to do. You say 'I would not expect a hydraulic jump to occur.' how did you come to such a conclusion?

If you really want to use a linked condition, I suggest you to carefully calibrate your data. What type are you using? hq-relation?

Cheers
Matteo

DianaO

User

Registered: 2023-08-07

Posts: 4

Re: weir: outflow & internal boundary leads to MASS BALANCE PROBLEM

Hi Matteo, thank you for your answer.

I’m happy to hear that you think I can simulate the weir as a fixed obstacle. I was unsure about it because the overflow depth is bigger than the weir height itself for the HQ100. I think these height relations cause the nearly vertical dop in the water surface. This seemed very unnatural for me, so I’ve questioned my approach.

I’ve checked all the posts I found about linked boundaries, but I could still not solve the problem. So far, I only know the total discharge of the river but I’m not sure about the hq-relation at the weir. Also, I am only interested in the peak flow so I let the model run with constant inflow (stationary situation).

About the hydraulic jump I’ve changed my opinion by doing some calculations by hand. But can the hydraulic jump be reasonably simulated in a 2D model without a linked boundary?
Cheers
Diana

Matteo Facchini

Developer

From: Trento

Registered: 2014-09-05

Posts: 278

Re: weir: outflow & internal boundary leads to MASS BALANCE PROBLEM

The hydraulic jump can be simulated by 2D models.

To properly tune your linked boundary, you could generate a rating curve with fixed bed, and use it afterwards in your mobile bed simulation.

DianaO

User

Registered: 2023-08-07

Posts: 4

Re: weir: outflow & internal boundary leads to MASS BALANCE PROBLEM

Okey, thank you. I'll try this approach.