Struggling undergrad trying to model pooling water in floodplain

[Anonymous]

Hello,

I’m an undergrad working on my capstone project trying to answer the question: how does flooding frequency effect what vegetation grows? I came across HEC-RAS a few months ago and have been trying to teach myself because none of my professors have heard of it. What a learning experience it has been!

My problem is that HEC-RAS assumes that all similar elevations receive the same water surface elevation, regardless if these are separate from the main channel. I see why this is normally what would be desired but in my case the way HEC-RAS models is making it impossible for me to answer my question. I need water to first flow over the bank, then fully fill up the first small pool it comes too, then flow into the next pool and keep flowing out laterally filling up each area it comes to before it flows out to the next.

I have been using a steady flow analysis method and have read up on the many cross section tools (levees, obstructions. ineffective areas, storage areas/connections) and the different steady flow plans (flood encroachment and flow distribution locations) but none of these seem to be the fix. Is there a way to get HEC-RAS to model the way I have specified above? Or thoughts on a different program or method that may help me accomplish this?

Please let me know if I can clarify anything. Thank you for your time and thought, any input would be very valuable to me.

-Kim

[Anonymous]

Unfortunately 1D HEC-RAS was not designed for what you are trying to get it to do. All 1D simulation will give you only one water surface elevation per cross section, no matter what you do. If you use a levee funtion, the water will fill up only in the area you’ve specified until it reaches the levee elevation and then it will spill over. But when it spills over, the water will not drop in the next area. You can still only have one elevation per cross section.

Depending on what your area looks like, you might be able to shorten your cross sections and draw lateral weirs along your high points (which will be the end points of your cross section). You would then connect your lateral weir to another reach with cross sections. So when your sections fill up, they would spill over to the next cross sections in your neighboring reach. Make sense? But I don’t know what your area looks like and if this would be appropraite.

You could also make your life way easier and just model everything in 2D using a 2D area in the HEC-RAS 5.0 Beta version (https://drive.google.com/file/d/0B0bpiyLiUeRXVEZyVWJVd0FNNUk/view?usp=sharing
) . All you need for this is to draw a 2D mesh, an upstream boundary condition line (specify a flow hydrograph), and a downstream boundary conditions line (usually a stage hydrograph, rating curve, or normal depth). Modeling in 2D will help you I think, but your output options are not as great. You go to RAS Mapper for your output, and one of the outputs is an elevation grid. You can also get a depth grid, velocities, etc.

Anyway, hope this helps.

[Anonymous]

Alright, thank you so much for all that info! I’ll try out the HEC-RAS 5.0. Glad to know what I’m trying to do should be possible.

Best,
-Kim

[Anonymous]

HEC-RAS 5.0 is still in beta. I truly recommend just working with 1d analysis HEC-RAS 4.1.0 in conjunction with arcmap. This allows you to import your floodplain extents from hec-ras into arcmap and it creates boundaries as to what elevation that flodplain water surface elevation has actually reached.

Download surface information for free here:
https://gdg.sc.egov.usda.gov/GDGOrder.aspx

and follow these steps outlined below and you should have zero problem getting a solution:

http://www.ce.utexas.edu/prof/maidment/CE374KSpr12/Ex4/Ex4.htm

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