Shear Stress Calculation - Spillway

[DMcGrath]

I am looking at alternative linings (other than riprap) for an auxiliary spillway for a small embankment dam. I am thinking along the lines of a vegetated TRM. As a first look, I calculate the velocity and shear stress in the downstream section of the spillway to use as a guide for selecting a lining. For the spillway section across the embankment, however, the slope is zero, and as I understand it the shear stress would be zero by definition. Is this right or am I misguided here?

 

[civilperson]

If the velocity is zero then the shear stress is zero. Slope is only used to get the hydralic grade line and thus the velocity. The spill way with zero slope will have head to provide velocity and thus stress.

 

[cvg]

use the slope of the energy grade line

 

[blueoak]

You can review the NRCS NEH Part 628 Chapter 51. It provides a good description with equations and citations for a grass covered spillway. It can be downloaded from the Sites page at:

http://www.wcc.nrcs.usda.gov

and then go to Hand H and Tools.
Sites is a decent routing program, but can be used to predict spillway stability for vegetated earth and rock spillways. It can be a pain to use, but can give you some idea of expected damage from different flow events.

 

[bltseattle]

Like CVG said, use the EGL slope when the channel is flat and you are using the standard shear stress formula i.e. HEC-15.