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Calculating Boundary Shear


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Using RMA -2 I know we can display velocities and depths but not shear stresses. Using a spreadsheet we are back-calculating slope using mannings N and then calculating shear ~ something like

Shear = p*g(n/1.49)^2 * (v^2/r^1/3) then importing the results back into SMS

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I can't think of a way to calculate the hydraulic radius in SMS at this time (that is what the r in your equation is if I'm not mistaken?).

Page 3-3 of the FST2DH (FESWMS) manual has an equation for Bed Shear Stresses you may be able to use. You would need to create a data set of your mannings roughness values and calculate some of the other inputs to the equation from your RMA2 output.

You may also want to consider running FESWMS and having FESWMS calculate the values for you. You may want to create an Initial Conditions file:

  1. Convert your RMA2 mesh to a scatter set
  2. Change the current model to FESWMS
  3. Reinterpolate RMA2 solution from the scatter set to the mesh
  4. Create the intial conditions file (FESWMS menu | Initial Conditions) using your interpolated RMA2 solution.
  5. You can now set the other FESWMS boundary conditions, model parameters and Hotstart the model with the RMA2 solution.

To have FESWMS output the Bed Shear:

In the FESWMS Model Control, on the "General" tab, select "Scalar Data Set file". Click the "Options" button and select "Bed shear stress magnitude".

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  • 2 years later...

@ Gene. I believe with the equation you posted the hydraulic radius term can be set equivalent to the water depth if you are looking at a wide channel rectangular. This is a convenient assumption in that depth is an output of RMA2 for each node.

@John. I am wondering if any other work around has been developed to determine shear stress in the RMA2 model since this post? Also, is there a way to include manning's roughness values in the RMA2 data calculator?

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