Ineffective flows

[Anonymous]

Has anyone had the addition of ineffective flow boundaries actually reduce the WSE from an existing to a proposed geometry? When the ineffective flow boundaries are deleted, the anticipated results are seen (an increase in the WSE with adding fill to the site).

Thanks,
Brian

[Anonymous]

There are a couple of things that may be going on. Ineffective flows can reduce the amount of friction losses. Instead of the edge of the cross section contributing to the wetted perimeter/manning’s n friction, the edge of the cross section is in the ineffective flow zone. The boundary between ineffective flow and effective flow is not added to the wetted perimeter. You may want to use blocked obstructions instead.

However, even blocked obstructions can, occasionally, cause the water surface to drop. The energy grade goes up, but the velocity, and more importantly the velocity head, goes up even faster causing the water surface to drop.

[Marulke]

Brian,
it is often feasible to compare energy levels, not only water levels. In sub-critical flow the water level drops as it passes through a constriction (for example caused by a part of the cross section being ineffective) because the flow velocity increases. (Jarvus writes about this.)

The energy level (energy grade) will (almost) always drop in the downstream direction. If you display the energy grade line on the profile plot, it may help you understand what is going on.

Regards,
Marulke

[Anonymous]

Thanks guys. So here’s a summary of what I’m seeing:
The proposed with ineffective area has a lower WSE and higher EGL. The velocity is 6.1ft/s through the crossing instead of 4.93ft/s.

I understand why this is occurring, but as far as the ‘proper’ way to model this, I’m leaning towards removing the ineffective flows from the models and just allowing the ground surface and abutments to provide the reduction in flow-able area.

[Chris G.]

I would encourage you to leave the ineffective flow areas in, if you believe they are indeed ineffective flow areas. As pointed out above, you should expect to see a local decrease in water surface elevation through a constriction (at the cross section with the ineffective flow areas). The water surface rise I think you are expecting to see will happen upstream of the constricted cross section. That is where the energy loss through the constriction manifests itself as a higher water surface. Leave the ineffective flow areas in there and then compare the with and without project profiles for a few cross sections UPSTREAM of the constriction. I bet you’ll see an increase in water surface elevation (a backwater effect) from the proposed design with ineffective flow areas.

Good Luck
Chris
@RASModel

[Author]

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