Bottom shear stress - steady currents

Question

Hi Chris,

The shear velocity of a steady current for resuspension is calculated as a function of friction factor fc, which in turn is fc=f(ks, h), h (mean water depth) is changing as the model evolve but I am not sure about ks. ks = 2.5D, with D the length scale.

1) Does the model calculate D, and how, or is it a paremeter defined by the user?

2) can I change ks in some way?

3) is it possible to change Tref (reference shear stress)?

Ricardo RB

Answer 1

Hi Ricardo,

The shear resuspension is calculated from the drag coefficient specified with drag_btm_cd in run_aem3d.dat.

The formula is Cd * U * U

It's not currently possible to change Tref

Comments

Hi Chris. I feel confused. The CAEDYM manual (pag95) explains shear velocity (steady currents) is u*c=sqr(fc*U*U/8) (eq 10.30). but it seems different from that you answered me. What did I miss?

Ricardo RB

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Hi Ricardo, sorry I was talking about the new WQ model which has simplified the stress calculations.  For CAEDYM D was taken as the dz grid size or the bottom cell which is not really appropriate in my opinion

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OK Chris, Thank you for your clarification. Just a last question: does the new WQ model still compute the shear stress due to oscillatory wave currents in the same way that CAEDYM works?  

BEst