Modeling Overland Flow Above Culvert (HEC-RAS)? 2

[FloodCG]

I am trying to model the impact of structures to the 100-yr floodplain in the area above a culvert using HEC-RAS. Basically, an existing culvert is undersized for the 100-year storm discharge which causes about 25% of the total flow to by-pass the culvert and travel overland about 150 feet before it re-enters the channel at the downstream end of the culvert. A commercial developer is looking to place a couple of structures in the area between the inlet and outlet for the culvert and located in the floodplain (water surface elevation based on cross-sections at upstream and downstream ends of culvert). The community requires that the impact of the structures to the floodplain be determined.

I've modeled the overland flow using a separate model "plan" for the geometry that's above the culvert, inserted the proposed structures along with x-sections upstream and downstream of them. I used the excess flow from the culverts, calculated by HEC-RAS to be the weir flow (Q-total minus Q-culvert). For the downstream boundary condition, I used a known water surface elevation as the parameter. I got that by using the profile of the original HEC-RAS model for the culvert, selecting a point on the water surface (of the profile) at a distance upstream of the culvert outlet measured to the closest x-section (for the overland geometry). Here's my question: Is there a better approach to modeling the overland flow or a better method to establish the downstream boundary condition? See the attachment for the general site layout.

Thanks in advance for any help offered.

 

[francesca]

Firstly a note to others: the file downloads without an extension but it's a pdf. Either rename to add .pdf or open with Adobe Reader.

Hmm well this isn't going to answer your question but a few observations:
- your x-sections aren't perpendicular to the flow lines through the culvert.
- your downstream x-section is located at the mouth of a pipe and therefore in the middle of a downstream control so will not give you a reasonable downstream water surface elevation.

In fact, upon reflection, the entire system is going to be controlled by the downstream double bore pipes (assuming I'm interpreting the drawing properly) and I doubt that HEC-RAS is the best application for analyzing this system, particularly on the short reach length. Personally I'd be inclined to analyze the whole thing as a detention basin, or two detention basins and a reach if the upstream/proposed pipe modifies the flow rather than the two downstream pipes backing the water up through the upstream/proposed pipe.

 

[francesca]

Sorry, I just noticed the floodplain line - you will need to include those downstream pipes in your model - if you have their files you can copy and paste into your model as HEC-RAS data files are just plain text. Make sure your culverts are being analyzed with the "pressure flow" option.

 

[RWF7437]

Agreeing with Francesca, but can't help asking why your client wants to put his building in harm's way and possibly in the floodway ?

good luck