Shear Stress in 2D RAS

[Anonymous]

Does anyone know how shear stress is calculated in a 2D mesh? What equation is being used and whether it is an average shear?

[Anonymous]

There is just a little bit of information in the 2D Modeling User’s manual. It looks like it is average shear across a face and then interpolated between faces.

[rwq]

Hi!

Does anyone have more information on this? The manual provides a lack of detail on this. It makes sense that the Hydraulic Radius (R) is computed based on the rating curves between area, depth and hydraulic radius/wetted perimeter at each face. Yet, how is the friction slope computed at the face? Does it use the an average between the two nodes? I know in the 1D steady state computations, there are a couple of different methods to compute the friction slope (e.g. average conveyance). Or does it just use all the values that are already at the face? Or am I not understanding something correctly?

Thank you!

[primozr1]

I was able to exactly recalculate shear stress in some test faces. The hydraulic radius R is calculated as quotient of wetted area and wetted perimeter of the face. Then, friction slope is calculated between cell centers of the two adjacent cells of the face.

The results were completely same as those in hdf file. But still confused how RAS Mapper calculate and generate shear stress map. For example:

Face shear stress in 135278 is 47.5 N/m2
Face shear stress in 135279 is 17.9 N/m2
Face shear stress in 135278 is 21.5 N/m2
but !!!!
Shear stress in the face point 182411 is 146.1 N/m2 (read in the RAS Mapper). See also unusual overestimated area of the shear stress in the image.

[Anonymous]

I think what people are misunderstanding about RAS Mapper, is that it is a tool not the answer and that “Max” is not always the best representation of the desired results.

In some cases i plot peak shear peak depth and peak velocity. There should always be a correlation. Also beware of evaluating shear perpendicular to the direction of flow

[Author]

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