User Forum of Software BASEMENT

Rossella

User

Registered: 2014-09-11

Posts: 8

Inflow boundary condition

Hi,
I'm working on 2D simulation on alpine rivers. I'm using BASEMENT v.2.5 trying for the first time to perform simulation considering sediment
transport (bedload transport) and I have some questions about the  inflow boundary conditions. How is the "equilibrium flux" (as described in the reference manual) in the IOup condition calculated? Does the choice of the bedload transport formula have influence on it?
Moreover I would like to know if the parameter bedload_factor (morphology/bedload/parameter) affects also the inflow boundary condition or does it have influence only on the bedload transport calculation made for the inner cells of the domain? Does the parameter factor (morphology/bedload/boundary) have influence only on  the amount of sediment entering the domain?

Thanks
Rossella

Last edited by Rossella (2015-08-06 08:21:01)

Rossella

User

Registered: 2014-09-11

Posts: 8

Re: Inflow boundary condition

Hi,
I have some other question on the sediment inflow boundary conditions. I performed some trials with BASEMENT v.2.5.2 on different river reaches, with different river bed geometry, different inflow liquid discharge and
different boundary condition. For every group of simulations performed on the same river bed geometry, I used only one tipe of soil, constant diameter and other constant physical parameter (porosity, density, etc).
The first question is about the total amount of inflow sediment; in some tests with capacity_transport as boundary inflow condition I found that there was a difference between the boundary_history and the sediment_balance,
as in the boundary-history the inflow sediment discharge was 0 but in the sediment_balance the  inflow_sediment_volume was bigger than 0 (even if very small, and 10^2 times smaller than I expected). Could it depend
on some approximations made on writing the inflow sediment discharge, so if the values is very small it is written as 0?Moreover the inflow_sediment_volume seems to reach a limited value and doesn't grow in time as I expected.
I think it depends on the bed geometry changes due to deposit and scoures, could it be?
The second question is about the sediment discharge inflow condition. I expected to find in the boundary_history the same values of the sediment discharge I set as boundary condition and to recognize the integrated value of sediment discharge
in the volume_inflow_sediment in the sediment balance but in some cases this didn't happen. What could have such an effect on it? Could it depend on the inflow geometry?
Moreover, do you maybe have some useful hints to get a correct inflow geometry in order to achieve simulation stability for all inflow conditions? For example, should I be particularly careful on some aspects, such as the definition
of the sediment inflow nodes or should I set the river bed cells next to the inflow boundary to fixed bed?

Thanks for helping!

Rossella

Last edited by Rossella (2015-08-18 15:21:09)

Matteo Facchini

Developer

From: Trento

Registered: 2014-09-05

Posts: 281

Re: Inflow boundary condition

Hello Rossella,

Please be aware of the fact that with boundary_history, basement writes per default usually the data at the end of the simulation. So if your water discharge at the end of the simulation is not big enough, it will not trigger any sediment transport. Or did you use the parameter 'flush_all_num_steps'?

Anyway it is difficult to understand how you set up your simulation, could you please send us (i.e. attach to a next message or copy paste it) your .log file with the input hydrograph?

Thank you!

Bests,

Matteo

Rossella

User

Registered: 2014-09-11

Posts: 8

Re: Inflow boundary condition

Hi Matteo,
thanks for answering. My simulation setup and results (balance files) are avalaible at the link
https://www.dropbox.com/sh/oe4x5usgolci … db3va?dl=0
If you check the file "F_2D_TR4_balance.dat" the inflow sediment volume grows for the first time steps and then remains constant;  the sediment inflow is for every time step 0 (file F_2D_TR4_bnd_F_IN_SED_th.dat). Could it depend on the fact that all the volume enters the domain in a very short time, between the recording of two different time steps? The fact that the inflow sediment volume remains constant seems to me very strange if compared to the input hydrograph (file Ql.txt). Am I maybe setting up something wrong?

Thanks

Rossella

Matteo Facchini

Developer

From: Trento

Registered: 2014-09-05

Posts: 281

Re: Inflow boundary condition

Dear Rossella,

the problem that you found out was not so easy to discover, but it is relatively well documented in the BASEMENT parser. By clicking on the question mark close to transport_capacity you can read:"The sediment inflow is calculated for each element based on a given mixture by calculating the equilibrium transport capacity. Therefore the flow hydraulics at the inflow are used. It is required that the sediment inflow is defined on edges which have also an inflow hydrograph set as hydraulic boundary condition! ".
Basically the problem in your case is that you did not define a hydraulic boundary condition for your boundary F_IN_SED . To solve the problem you should either associate a hyraulic boundary condition to F_SED_IN or a morphologic boundary condition to F_IN.

A work around could be to calculate (outisde of BASEMENT) the transport capacity (e.g. with the mpm formula) associated to your hydraulic discharge and to feed your domain with a solid_discharge boundary condition. This one can be decoupled from the hydraulic boundary condition.

I hope this helps

Matteo

Rossella

User

Registered: 2014-09-11

Posts: 8

Re: Inflow boundary condition

Dear Matteo,
thanks for answering. To associate the morphologic boundary condition to F_IN I should define the hydraulic and morphologic boundary condition in the .bmc  file as:

HYDRAULICS {
            PARAMETER {
                minimum_water_depth = 0.05
                riemann_solver      = exact
                simulation_scheme   = exp
            }
            BOUNDARY {
                name        = F_IN
                string_name = F_IN
                type        = hydrograph
                slope       = 100
                file        = Ql.txt
            }
            BOUNDARY {
                name        = F_OUT
                string_name = F_OUT
                type        = zero_gradient
            }
        }

BEDLOAD {
                PARAMETER {
                    bedload_transport = mpm
                    bedload_factor    = 1.0
                }
                BOUNDARY {
                    name        = F_IN_SED
                    type        = transport_capacity
                    string_name = F_IN
                    mixture     = MIX1
                }
                BOUNDARY {
                    name        = F_OUT_SED
                    type        = IODown
                    string_name = F_OUT
                }
            }
        }

Is this the way to fix the problem?   

Thanks for helping!
Rossella

Matteo Facchini

Developer

From: Trento

Registered: 2014-09-05

Posts: 281

Re: Inflow boundary condition

Dear Rossella,

yes, it is. I would recommend you to check the slope of your in boundary condition. It seems to me that it is a little bit too sloping!

Cheers,

Matteo