Lateral culvert between two parallel reaches in HEC RAS 1

[Jajosk]

I am new to lateral structures. That said, I am trying to model a lateral culvert between two minor drainage channels linked by a culvert. The flow in the channels are not inter-related and the channel that is at the downstream end of the culvert has negligible flow (0.3 m3/s) compared to the channel upstream (3.9 m3/s). The culvert has a positive slope of about 4%. I have used steady state subcritical flow in the first run, and since the WS reached critical, tried the mixed flow. Problem is that in both cases I am getting a negative flow through the culvert ('Q leaving Total') for the lateral structure. The Total Q upstream and downstream of the lateral structure is same in the upstream side of the culvert, which is also incorrect. I also cannot find a report in HEC RAS that will throw some light on the culvert hydraulics (flow profile, inlet/outlet control, even if only for theoretical interest), though I have seen such report for inline culverts.
As an aside, what does the HW positioning (L overbank, Next to L bank etc.) of the weir mean in reality? THe HEC-RAS literature has not been much help (probably it's too simple?). My culverts are connected to the trapezoidal channels at the floor level of the channels where the side walls start to rise. I have the lateral weir positioned (HW Position in Lateral Structure Editor) at the Next to Bank. How would it affect the calculations if I were to position it next to overbank? Weir elevation is set to match the top of the side wall of the channel upstream, on the side that is facing the channel downstream.
Thanks!!

 

[beej67]

Have you considered modeling this in SWMM?

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[Jajosk]

Thanks for the suggestion. Yes, I did try an initial run with SWMM with dynamic routing. The issue is that I need a node on the reaches where the culvert is connected (since I cannot draw a conduit unless there are u/s and d/s nodes). Two concerns with that - 1. I don't know what kind of node is appropriate given there is no manhole or catchbasin at those location, just a side entrance culvert. 2. Even if I specify a node with no surcharge I am not sure how using a node will change the result of WS and velocity profile through the culvert compared to HEC - RAS results. In short, HEC - RAS appears to be able to model the real set up more closely...but then I could be wrong. Thanks for any further suggestion you have with SWMM or HEC RAS. I am certainly willing to do this in SWMM if I know how

 

[gbam]

Jajosk - as far as i have ever been able to determine; the positioning of the weir is for graphic representation in the "plan". Hydraulics are based on the EGL or HGL (which ever you choose) and the geometry of the wier. HECRAS does a great job on modeling your scenario. You need to make sure that your coding is correct. You can always model this scenario using a rating curve for the culvert. If you chose to model with geometry HECRAS will determine the Tailwater condition and compare it to the Headwater to determine the direction of flow through the culvert. Hope this helps.

 

[LincolnPE]

Do you have Optimization turned on so HEC RAS can figure how much flow goes through the culvert and how much continues on downstream? It's an iterative calculation process.

 

[Jajosk]

LincolnPE, big yay for your suggestion! There is flow through the lateral culvert now, and I checked that the equal amount is subtracted from the U/S side of the culvert, and added to the D/S side...so that's all good.
The only other problem now is how to set a specific point on the reach downstream to tie the outlet of the culvert to. In the attached screenshot of the model, the wier seems to be connecting to two stations on the tailwater side of the culvert, but first - the culvert itself is not visible in the schematic. Is that normal?
Also strangely enough HEC-RAS doesn't report the velocity of the water through the culvert in the LAteral Structure Output table.

Attachment:

http://files.engineering.com/getfil...e95-8d68d18b4bc1&file=Lat_culvert_HEC-RAS.pdf