Ineffective horizontal areas

[Anonymous]

Hello, I have a question about a current project I am working on relating to the calculation of flow through a sedimentation pond. This pond is designed to be protected by a berm from a 10 year event. However in our 100 year event the flow is allowed to overtop the berm by 3ft. I had a question will I need to define an ineffective flow area at the bottom portion to the left of our berm or does HEC-RAS already consider the lower portion to be ineffective flows. Below is an image of one of my cross sections. The area I am acquiring about is the portion left of the berm. The 100 year wse is currently shown in the diagram.

http://s32.postimg.org/on9zoums5/sectionview.jpg

[Lonnie A]

Ben, You will want to set it to permanent ineffective. There will be some conveyance happening below the berm elevation but not full depth.
You may want to use a combination of a high vertical n value (0.99) to essentially block conveyance below say a foot or two of the berm and ineffective set at the berm elevation so that as water rises on the “wet side” of the berm it doesn’t calculate conveyance in the dry side. Once the berm is topped then it can start using say the top 1 to 2 feet as conveyance with a lower n-value (0.02?).
I would test it out first by just setting permanent ineffective at the berm elevation and then a few feet below the berm and see if it makes much of a difference in computed WSE. If it doesn’t just leave it at the top of the berm. I assume this is a steady state model?

[Author]

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