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Hello all,

I want to model model sediment transport in a straight channel.
Successfully tested the tutorial provided on www.hydro-informatics.com and on this basis i adapted some more details corresponding to my specific case:

-2 layers with specific vertical sediment distribution (user fortran user_bed_init.f)
- prescribed solid discharge at the inflow boundary

I´m really confused about my results and couldn´t identify the origin / reason.
So i really hope to find an expert to figure out the reason and what to adapt / change to get some logical results:



From the first result printing step on I can see a way to high deposition at the inflow of my model.

i have studied the result and possibly reasons for a quite a time now and basically could not eliminate the following ambiguities:

(1) with an inflow of i.e. 92 kg/s - is the solid inflow split up in the same classes as defined in .cas file with the keyword CLASSES INITIAL FRACTION ?
(2) with an inflow boundary defined as 4 5 5 ... 5(LIEBOR) 0.0(EBOR) .. it should be the equilibrium inflow condition (why hydro-info tutorial added nevertheless the Prescribed solid discharge keyword? ..perhaps mistake and you have to use 4 5 5 ...4..0.0.. for prescribing solid discharge)
(3) when a change the prescribed solid discharge to other values i often get a error message during simulation that I should give a boundary velocity profile or prescribe values (Q or H) on the inflow boundary
(4) is it mandatory or recommended to use a previous result file with the same order of Q (so to say also around 1130 m3s like in my morphological simulation, so that there is not a high change of imposed discharge)
(5) I used Meyer-Peter Müller which application range is for more finer sediment than I used here - can this be the reason for high deposition in the beginning?
(6) I used
SCHEME FOR ADVECTION OF VELOCITIES : 14
SCHEME FOR ADVECTION OF TRACERS : 14
SCHEME FOR ADVECTION OF K-EPSILON : 14

instead of
SCHEME FOR ADVECTION OF VELOCITIES : 14
SCHEME FOR ADVECTION OF TRACERS : 5
SCHEME FOR ADVECTION OF K-EPSILON : 14
according to hydro-informatics tutorial

Can this be the reason for unrealistic deposition and which influence have the parameters?




A lot of questions - I really hope that someone can bring some light into the dark and safe my day, weak , life



Also added my files in appendix

Hi,

for straight flumes it is always hard to have a stable inlet boundary in case you presribe a solid sediment discharge. Much easier is to use equilibrium solid discharge (no change of bottom at the inlet). Another possibility is to set the first part non-erodable and give the sediment to the flume with nestor (dredging/disposal) module. This procedure is also used in laboratory flume experiments.

The cli-file for the sediment should be:
Inlet sediment equilibrium:
4 4 5 0.0 0.0 0.0 0.0 5
Inlet sediment prescribed QS:
4 5 5 1.0 0.0 0.0 0.0 4
Outlet sediment :
5 4 4 0.0 0.0 0.0 0.0 4

In case of sediment equilibrium boundary condition the incoming fractions is set by the initial value of the boundary nodes. In case of prescribed QS it seems it is the same.

Error messages that you should change your velocity profile usually means that you have no water depth anymore at (one node of) the inflow boundary. This happens because there is no boundary condition implemented that the boundary water level is constant at the inlet. It helps using all the same 60° triangles at the boundaries.

We usually use a fully developed flow (as previous computation file in telemac) before starting the sediment transport. Another possibility is to use a hydrograph with a smooth increase of the hydrodynamic flow. We found it not necessary to do similar things for the sediment input.

MPM is fine even for finer sediments. It is the most stable formula and always a good start.
Do you have suspended load? Only for this the advection scheme of tracers would make a difference.

In case you still want to use solid discharge at the boundary I would advice a better grid, trying velocity profiles set to 4;4 and setting a sediment distribution at the inlet (4th column in cli-file) according to the shear stress distribution.

Best regards,
Rebekka