Need HEC-RAS Help 4

[Jessica2424]

When entering steady flow data for the different reaches, should you enter the "total" flow or should you enter the flow that will contribute to that reach upstream?

For example: If you have two reaches that are joined (one upstream (#100) and one downstream(#99)).

(#100)=181.00 cfs
(#99)=251.00 cfs (181.00cfs + 70cfs) (251.00cfs is the total flow going through #99 while 70cfs is the flow contributing only to #99)

Which flow values should you input into the steady flow data window?
181 and 251
or
181 and 70??????

Sorry if this is confusing... I couldn't think of any other way to describe it.

 

[sam74]

You can use this fema weblink to go directly to where you can get a copy of a flood study in your immediate area if one has been published

http://store.msc.fema.gov/webapp/wc...e=13049&cat_county=15403&cat_community=357474

if there is already a study in your location it will give you a realistic water surface elevation for your downstream condition (lake). Your stream may have already been studied as well.

This weblink will take you directly to where you can get a small flood map of your area if a study has been done.

http://store.msc.fema.gov/webapp/wc...&langId=-1&userType=G&type=1&dfirmCatId=12009

If you find your location has been studied we often print a copy and scan as a tiff file for use as a raster reference into Microstation. Find the correct scale and overlay on your surveyed sections for a rough look at how it compares with FEMAs study.

 

[Jessica2424]

Francesca...

I see what you are saying about modeling the junctions, about how you need to put the reach lengths in the junction editor versus the cross-section editor (of the last cross-section near the junction). My problem with that is that I have a culvert that connects one reach to the other and then flows donstream together. This culvert allows water to flow under railroad tracks into the main ditch on the other side of the tracks. The main ditch then flows downstream and comes into contact with another culvert with the same situation as the first.

The manual says that in order for me to model a culvert I have to have a 2 cross-section, one before the culvert and one after. Well, I have made the cross-section length before the culvert long enough to include the culvert length and I have made the cross-section after the culvert 1ft long, then it goes into the junction to combine with the main ditch. Am I doing this wrong?

 

[francesca]

Jessica,

I'm confused about your "cross-section lengths" -- do you mean your downstream reach lengths? Or your cross-sections?

You need a normal cross-section before the culvert and a normal one after. If your culvert is particularly long, especially if you have flow over the top, then you can include cross-sections over in between. (I've never done this myself, but I read it can be done.)

You might want to check your culvert modeling assumptions. Do you have pressure flow checked? Also check your culvert entry loss coefficients. On one of our projects we were getting results we didn't believe from the culvert modeling in HEC-RAS and so we checked our output by doing a Hydraflow detention basin analysis on the culvert. HEC-RAS was definitely more conservative, and our justification for going with the Hydraflow in this instance was that HEC-RAS is a one-dimensional model, while Hydraflow does stage-storage calculations.