HEC-RAS Question

[Stinja]

Hi,

Long time reader, first time poster.

I've been tasked to determine if a structure being built along the stream will have an affect to the BFE for the area. Luckily, a flood control channel was built within the stream to convey all of the flowing water. Unforunately, though, it doesn't look like the City or FEMA updated their flood maps as it still shows the flood zone boundaries AROUND the built channel. So that's weird.

My question, though, is that I created the channel using HEC-RAS and the information available to me (sections and flow rate). The sections included the channel and the flood zone boundaries outside the channel. The resulting water levels are below the top of of wall elevations of the channel and everything contained within the channel. So, Good. But then when I add my structure OUTSIDE of the channel, and run the same test, with the same values, the water level is now increased and my structure is now within it.

Am I missing something in my analysis? Or am I missing something with the final product (ie, the answer is correct and I am looking at it wrong).

Please let me know.

Thanks,

 

[gbam]

Can you provide a cross section of the pre-project condition? If the WSE is out of main channel and on the overbank what you have is a floodplain encroachment and your results may be true. Lacking all the definition of the situation and the model results it is hard to diagnose the potential issue. Not sure how much experience you have with RAS Modeling, maybe have a drainage/river hydraulics engineer perform the analysis to ensure proper coding of HECRAS?

 

[agendahispana]

It could be the wetted perimeter or the Mannings number but my guess is that you probably copy paste something twice -js

What I would do is:
1 Built a duplicate of the City model. Making sure the cross sections extend all the way to your structure eventhoug water doesn't get there. Run the Channel only model, Check it with original model
If models are NOT the same then talk to the city and telling them you will have to compare your results to the new duplicate model so that it can apples to apples.


If models stay the same then
2 Draw your structure, only in one cross section (the most downstream cross section if you are running steady flow analysis), using the "Graphical Cross Section Edit " under the "Tools" Tab in the "Geometric Data" Window

3. run the model, check your numbers...
if something change decrease the size of your structure a couple of points to match the existing channel model...run and repeat this step until the numbers don't change...until you find the culper
and a solution

if nothing changes...do step 2 for the next upstream cross sections.