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Hello BlueKenue-PSed users/developers,

I’ve been using BlueKenue-PSed to analyze tidal model results (converted to a .t3v file) for the USA-Canada border region of the Salish Sea, Washington State-British Columbia. The 2D (depth-averaged) simulation is nearly 59 days; consisting of 1-hour time-steps. In this study area, the central depth of the narrow passages between islands is typically ~50-200 m.

I’ve run a number of test simulations using PSed’s Lagrangian parcel transport model for the full-range of available grain sizes (silt through gravel). The modelled dispersal of sediment from all locations seems reasonable enough, but a note about selecting an appropriate time-step, found in the technical documentation (“5.9 Numerical Stability,” page 30, PSed V4.3 Document Release 1.3), has left me questioning the validity/realism of these simulations.

Fortunately, there are other tools offered by BlueKenue-PSed that can allow me to make confident predictions about sediment transport pathways based on a separate (3D not 2D) run of the tidal model: residual mobility vector maps and shear stress/mobility rose diagrams.

I would be grateful if someone is willing to share suggestions and clarifications with respect to the following:

1) Is a 1-hour time-step far too long to accurately model the dispersal of coarser sediments (sand/gravel) in 0-250 m deep marine channels using the Lagrangian parcel approach? Can any generalizations be made about the inaccuracies resulting from the use of longer time-steps?

The PSed technical documentation advises the following time-steps: coarse sand (~1 s); fine sand (<15 s); and coastal studies in general (<60 seconds)!

Vertical fall velocities (w) for different grain sizes (not based on the PSed settling equations):

w,coarse silt = 0.001 m/s  t,10 m = 2.7 h  t,50 m = 13.9 h  t,200 m = 55.6 h
w,f. sand = 0.01 m/s  t,10 m = 0.28 h  t,50 m = 1.39 h  t,200 m = 5.6 h
w,m. sand = 0.01 m/s  t,10 m = 0.21 h  t,50 m = 1.0 h  t,200 m = 4.27 h
w,v. coarse sand = 0.1 m/s  t,10 m = 0.03 h  t,50 m = 0.14 h  t,200 m = 0.56 h
w, pebbles = 0.12 m/s  t,10 m = 0.02 h  t,50 m = 0.12 h  t,200 m = 0.46 h

My understanding of how PSed calculates vertical parcel advection:

For Mobility (M) > 1 … Vertical advection rate = (w)(dt) = (fall velocity)(time-step) …The parcel moves toward the Rouse concentration-profile based centroid height (Zc) at this rate.

So for longer time-steps (dt) and M > 1, does this mean that the parcel will be more likely to reach the equilibrium centroid height and then stall there (if time is left over)? Overall, can it be said that this favours more/less lateral sediment dispersal?

For M < 1 … Vertical advection rate = (w)(M-1)  As the mobility (M) value gets closer and closer to zero, the vertical advection rate gets closer and closer to the magnitude of the fall velocity in still water (w) reducing the parcel’s centroid height  potentially leading to deposition, if the parcel mass becomes zero…

2) Can my use of a 1-hour time-step to model the Lagrangian tidal dispersal of sand/gravel in depth conditions (0-200 m) be justified with caveats?

I’d like to use this technique to describe the relative partitioning of sediment towards either side of a flow-constriction by ebb and flood currents. I’d also like to use the Lagrangian parcel technique to help predict general regional transport pathways. However, these pathways can principally be predicted/described using the residual vectors/rose diagrams (mobility and shear stress; each derived from the bottom layer velocities of a 3D, 11-layer, version of the tidal model).

I appreciate any thoughts and suggestions!

Thank you very much,

Sean Mullan
Marine geoscience grad student,
University of Victoria, British Columbia, Canada

Hello,

As Psed is not in the Telemac system, I'm afraid not sure you are not on the right forum, and I know nobody who could help you, except perhaps Martin Serrer, developer of BlueKenue.

With best regards,

Jean-Michel Hervouet

Hi Jean-Michel Hervouet,

Thank you for responding.

Since PSed is now included as a module within the BlueKenue software package, this seems to be the best place online to publicly post my questions. But, I could be mistaken. I'm a geology grad student with much to learn

A post about PSed-BlueKenue integration:
www.opentelemac.org/index.php/assistance...tasets-now-available

Perhaps some modelers with knowledge of my problem will encounter this thread. In the meantime, I'll contact several individuals I know to be involved with BlueKenue and PSed.

All the best,
Sean

Hi,

Sorry for slow response time.

PSed and Posm source code will both be included in the next public release of Blue Kenue.

Hope that helps!

Kindly,
Alan