User Forum of Software BASEMENT

Jules

User

Registered: 2026-07-15

Posts: 2

Sediment deposit downstream boundary condition lines

Good afternoon,
I need a piece of advice on boundary conditions. In particular, my model has several external boundary conditions along the reach simulating the contribution of the lateral basins. Having no info on the incoming sediment volume, I used the "equilibrium_in" condition. The results always show localized deposit areas right downstream the boundary condition lines. Is there any way to fix this behavior?

Thank you in advance,
J

Last edited by Jules (2026-07-27 14:41:29)

Matthias Bürgler

Developer

From: Zürich

Registered: 2019-04-04

Posts: 188

Re: Sediment deposit downstream boundary condition lines

Hi Jules,

I assume that with external boundary conditions you mean STANDARD boundary conditions in BASEHPC. My best guess is that the flow conditions change shortly after the boundary conditions, resulting in smaller transport capacities and deposition of sediment coming over the boundary. You could try the transport_capacity boundary instead, and if necessary combine with a boundary_factor < 1 to reduce the inflowing sediment.

Best regards,
Matthias

Jules

User

Registered: 2026-07-15

Posts: 2

Re: Sediment deposit downstream boundary condition lines

Hello Matthias,
Thank you very much.

I went and read what the manual says about the transport_capacity boundary condition. When explaining what the transport_capacity option does, it says "transport_capacity: the sediment inflow is defined by calculating the equilibrium transport capacity according to the hydraulic state at the boundary."

I have some questions about this explaination:
- what does "equilibrium transport capacity" mean theoretically and practically?
- what does it refer to when speaking about the "hydraulic state" at the boundary?

Also: if I got it right, I should set the boundary_factor as small as possibile (might 0.01 work?) to have the sediment trasport almost zero at the boundary condition, is that right?

Waiting for you reply.
Kind regards,
Jules

Matthias Bürgler

Developer

From: Zürich

Registered: 2019-04-04

Posts: 188

Re: Sediment deposit downstream boundary condition lines

Hi Jules,

Equilibrium transport capacity refers to the transport capacity calculated with the selected bedload formula type (e.g., MPM). What this actually does for the transport_capacity boundary condition in each time step:

1. Calculate the cumulative transport capacity Q_b over the boundary based on the transport capacity in each cell that borders that boundary
2. Calculate a specific transport capacity over the boundary by dividing by the nodestring length q_b = Q_b/L
3. Set the flux at each edge of the boundary to q_b

In this case, the hydraulic state just refers to the flow velocity and depth in the cells along the boundary.

For cases where the transport capacity varies strongly along the nodestring/boundary, this might result in deposition in cells along the boundary where the transport capacity is much smaller than the specific transport capacity q_b. If you want zero sediment transport over the upstream inflow boundary, you can simply not define a upstream boundary condition in the BEDLOAD block.

Best regards,
Matthias