Levees versus Effective Areas 1

[Diana418]

I'm working on a large watershed model in HEC-RAS and having a discussion about whether it is better to use effective area to show areas of no flow or to use use levees to show where the flow should go. My question is "If you use effective area aren't you discounting HGLs in the main channel if you allow it to spread through out the cross-section. Some of my cross-sections are very large and I'm thinking levees are better.

 

[pinkpig]

ineffective area still has storage effect which may be substantial under a large watershed model. area behind levee does not get water unless the levee is over topped. also, using ineffective flow area will not increase your wetted perimeter or friction.
So you need to make the decision based on what will happen in reality during your modeled condition.

 

[Diana418]

How does the use of levees as top of bank impact FEMA submissions? I'm labeling edge of channel as levee and concerned it would be view as some type of extended levee to FEMA. The situation is the water should flow through the channel before over-banking. Then deal with flood plain areas where there is actual over-banking.

 

[pinkpig]

You may need to use levee instead of ineffective flow area per your description. also, you may need to consider using lateral structural to model levee overtopping rate.