Ineffective Flow Elevations

[AgE94]

What is the ineffective flow elevation based on? Is it the low point in the roadway profile? Is it the same as the Min Weir Flow El?

[MattZ]

The IFA elevation can be based on several things. It boils down to an elevation set by the modeler.

Where are you setting your IFAs? At a bridge, culvert, inline weir, or cross section?

There are many considerations to take into account when setting IFAs including:

1) permanent vs non-permanent
2) expansion/contraction ratios
3) elevation of IFA on U/S vs D/S side of structures
4) if they are even required at a particular area

Good luck.

[AgE94]

I have two models. One for an existing roadway over a box culvert. The proposed model is for a raised roadway and a 3-span bridge.
I have been using a low point elevation from the roadway profile in each model.
The section just upstream and just downstream of the structure is where these were applied using a 1:1 contraction and a 4:1 expansion from the structure ends.

[AbNormal_Depth]

IFA elevations should be set a bit below the min. top-of-deck elevation on hte U/S XS. Note that the EGL is used to turn the IFA off (NOT the WSE). For the D/S XS, the IFA elev. should be set to turn off with the U/S IFA.

Good practice is 1:1 for contraction, but more like 2:1 for expansion. You would need a very, very steep river for 4:1 to be approprate.