User Forum of Software BASEMENT

caroz

User

Registered: 2017-01-20

Posts: 1

Outflow Qsed has spurious unexpected variations

Hello all,
I'm trying to reproduce the results of simulations carried out in a meandering flume with constant sediment and water input. The bed of the flume is inerodible and has no sediment cover at the beginning of the simulation. The flume runs reached a steady state where the Qsed in output is equal to Qsed in input. I'm using mpm's formula with different bedload factors and different values of critical shear stress. The strange thing is that the Qsed in output has a generally smooth increasing trend but shows odd jumps(up to 3 or 4 times the previous value) and then comes back again, while the measured flume output has no such variations.
Both the flume experiments and the model develop point bars and sediment output fluctuations do not correspond to major changes in deposition patterns such as the complete erosion of a bar.
Has any of you encountered a similar problem? What parameters are most likely to cause these spurious fluctuations?

I'm including one example of the output file with two outliers highlighted

Thank you,
Carolina

  Qsed [g/s]
  0.000
  0.004
  0.008
  0.013
  0.023
  0.069
  0.320
  0.863
  1.296
  1.595
  1.790
  1.967
4.858
  2.197
9.068
  2.289
  2.332
  2.363
  2.418
  2.372
  2.359

Lukas Vonwiller

Developer

Registered: 2014-09-04

Posts: 134

Re: Outflow Qsed has spurious unexpected variations

Hi Carolina,
Is the output you show a 'boundary_history' or 'stringdef_history'? For the latter most values needs interpolation, whereas the 'boundary_history' gives exactly the values over the model boundary.
Note that sediment transport over inerodible/fixed bed is an artificial approach as most bed load formulas were developed for mobile bed.
Do you expect only point bars or as well free bars for your setup?

There is a new release of BASEMENT v2.7 soon, guess by end of this week.
Several things were improved including bed load transport on fixed bed (but mainly non-uniform sediment) and the correction of bed load transport direction due to river curvature.

Hope this helps!
Best, Lukas