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Hi everyone,

I’m currently trying to simulate the sediment transport in a river stretch for my masters thesis. So far I used the equilibrium sediment discharge boundary condition at the inlet since there is no data for the sediment discharge. However I have trouble understanding the idea behind this boundary condition. I understand that the elevation at the inflow is fixed all time. Since there is no inflow and outflow of sediment, is it the case that only the amount of sediment can land on that previously (location wise) has been eroded, meaning there has to be erosion at the beginning and can’t be Sedimentation?
I would be very happy for some help or explanation since the manual doesn’t explain much in this case. Maybe you also have a paper in mind.

Thanks a lot
Severin

Hi Severin,

the bottom is fixed at the inflow boundary and the solid discharge is calculated according to the actual flow. So there is an inflow sediment discharge, but no sedimentation or erosion at the inflow boundary nodes. So it is important at which cross section you put your inflow boundary. It should be a morphological stable cross section where no sedimentation or erosion tendencies exist.

Best regards,
Rebekka

Hi Rebekka,

thanks so far. So is it than a recirculating condition, where everything leaving the model is fed to the inflow? I heard from another source that the first nodes after the inflow define the volume, by means if they get eroded heavily the sediment inflow will increase? And if so, is sedimentation right after the inflow even possible?

Thank you and best regards
Severin

Hi

It's not a recirculating condition in the sense of some laboratory experiments, but more just localised at the upstream boundary. You feed in what is pick up by de flow to keep the bed level and composition constant at the boundary nodes. This is a often used concept in sediment transport modelling.

Yes, erosion and sedimentation directly downstream of the boundary is possible and when this is significant will influence the influx of sediments. To use this condition it is important to select a stable river section for the upstream limit.

Hope that helps,

Patrick

Hi Rebekka,

I'm having the same question, too, as to how the sediment discharge is calculated when the inlet bed is at equilibrium. Could you elaborate more on this? Do you know in which routine the sediment discharge is calculated by any chance?

I could not find clue anywhere... This is bugging me because I found that the values gaia computes are orders of magnitude smaller than what I estimated using the same sediment transport formula with the same hydrodynamics and sediment properties.

Thank you so much,
Jason

Hi Jason,
sediment discharge QS is calculated in e.g. bedload_meyer.f this is printed in the results file as a capacity. If the sediment is limited (rigid bed, multiple sediments) the sediment fluxes are changed and limited to their availability. This is done in positive_depth_nerd.f (e.g.) So far as I understood is in case of a dirictlet boundary condition for the bed height, the sediment discharge is also changed in this subroutine in the subroutine FLBOR.

I have 2 ideas why your calculations don't fit to telemac results: The fluxes can be given by mass or by volumes with pores or volumes without pores. So if your factor is around 2650 or 1500 it could be this. FUrthermore, the shear stress in gaia is calculated from skin friction by default and not from the friction set in telemac. I can not guess how much this would change your values, but probably not so much.

Best regards,
Rebekka

Hi Rebekka,

Thanks to your insights, I've identified that it's the inaccurately estimated skin friction that resulted in the overestimation of the bedload sediment discharge at capacity at the inflow boundaries. I did a new round of calculation and it matches the telemac results. Thanks so much!

Jason