Ineffective Flow Areas

[mlc3n3]

Hi,

I’m still new with HECRAS. I have a question about x-secs with areas with low areas relative to the main channel. See the image below.

In this particular case and based on site characteristics, this low area right of the main channel doesn’t convey flow in real life. In addition, it doesn’t provide storage unless the stream exceeds the main channel and overflows into this valley. However, when modeling in HECRAS, it seems like the program “sends” water there because it’s lower or at a similar elevation to the channel. I tried to use ineffective flow areas to model this area as “not helpful” until the water reaches a specified elevation. But it still uses it as storage when below that elevation. I don’t want HECRAS to “send” there because it originates in the channel and overflows to this area.

Is there a better way to model this? I still want to include this portion of the x-sec because larger storm events will overflow and utilize this valley. Should I use a levee?

[Anonymous]

Yes,

HEC-RAS projects the calculated WSEL across the entire cross section no matter if its ineffective or blocked. I think it would benefit you greatly to read HEC-RAS definitions of an ineffective flow area, blocked obstructions and finally, Levees.

To accomplish what you are seeking you need to place a levee at that high point. This will not allow runoff into that area unless the WSEL exceeds the levees elevation.

[Author]

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