HEC-RAS Cross Section Requirements
HEC-RAS Cross Section Requirements
(OP)
Great forum, folks! Brand new here, and a HEC newbie, also, so please bear with me ...
I'm beginning a HEC-RAS analysis of a creek for a bridge design. What are the required limits (upstream and downstream) for the creek cross sections?
(I used the search function and noted that this question had been answered, but I didn't see the actual answer anywhere. So, a link to an old thread would be great, also.)
Thanks a lot.
I'm beginning a HEC-RAS analysis of a creek for a bridge design. What are the required limits (upstream and downstream) for the creek cross sections?
(I used the search function and noted that this question had been answered, but I didn't see the actual answer anywhere. So, a link to an old thread would be great, also.)
Thanks a lot.





RE: HEC-RAS Cross Section Requirements
RE: HEC-RAS Cross Section Requirements
Is there a concise, quantifiable requirement for the minimum limits of the cross sections?
RE: HEC-RAS Cross Section Requirements
RE: HEC-RAS Cross Section Requirements
RE: HEC-RAS Cross Section Requirements
You have to look if there is any control downstream of the bridge. There can be structures such as dams or waterfalls or even other bridges. Empirical water-surface elevation-discharge relations such as that for weir flow can be utilized for simulating rapidly-varying flow.
So if the creek is on subcritical flow, I suggest you take more sections downstream of the bridge than upstream.
I'm not talking about storm surges, tides or large tributary inflows here...
RE: HEC-RAS Cross Section Requirements
RE: HEC-RAS Cross Section Requirements
Your four bridge cross-sections need to be situated at (#1) the end of the expansion reach, (#2) base of downstream abutment, (#3) base of upstream abutment, and (#4) beginning of the contraction reach. XSs #s 1 and 4 will not be the same distance from the intermediate XSs; in fact, 4 will be half as far from 3 as 1 is from 2. (Contraction reach has a 1:1 slope, expansion reach has a 2:1 slope.) You need at least one cross-section upstream of #4 to show that the point of maximum backwater has been reached.
You need additional XSs (1-2) downstream to let the calculation settle from your downstream boundary condition and similarly upstream if you have mixed/supercritical flow.
Finally, what are your design criteria? On our job, we had two options: 0.14 ft of backwater at the ROW line or up to 1 ft of backwater at the ROW line with the purchase of flood easement upstream until the point that backwater had dissipated to 0.14 ft. (Those models required XSs a long way upstream.) If you have to dissipate your backwater by your ROW line, you'll need to model up to there and only beyond if the ROW line is inside the contraction reach/the backwater hasn't started to reduce. Obviously you need a cross-section at your ROW line, and if it's within the contraction reach, then you need to note that and make sure you're using the proper XS #4 for your approach section for your scour calculations.
For your survey, you need a good handle on top of bank, bank width, and an average cross-section. You also need a good handle on the average (linear regression) bottom slope. If you're using normal depth as your downstream boundary condition, that should really take precedence. Have the surveyor take 100s of photographs, especially if the site isn't on Google Street View.) A good aerial/GIS topo is essential because the majority of your flow will be on the overbanks. Outside of our topo (we had a 1,000 ft wide corridor and then 25-yr old Quad maps) we extrapolated cross-sections. (Same channel shape, vertical offset for average slope, widened/contracted the floodplain as measured off the quad map.)
Make sure your cross-sections #s 2 & 3 are the same under the 100-yr elevation as what's happening a the bridge site. The structural engineer will cut a cross-section under the bridge centerline and will need to design for the flow area your modes specifies. If the XS at the base of the abutments isn't representative of under the bridge, use the internal bridge XS option.
RE: HEC-RAS Cross Section Requirements
Question: Can you perform a simulation without the bridge and then add it to see what's the difference of WSE at the bridge (or a couple of feet upstream). If there is, for example, 1' between the two conditions, then you can work a scenario with a different bridge depending (widened, for example). Is such a way to do acceptable in a design study? When you don't necessarily want to access the values of the increase in WSE upstream of the bridge.
That's want I wanted to say.
RE: HEC-RAS Cross Section Requirements
if you have plenty of right of way, no environmental impacts and lots of money you can span the entire floodplain with a very long bridge and create no rise at all. However, I have seen very few bridges that actually do that. Most bridges are designed with very little skew and as short of spans as possible and create a reasonable rise in the WSE in order to save money on the bridge. so the answer to your question is yes, but probably not feasible.
RE: HEC-RAS Cross Section Requirements
In a perfect world, I'd like to have 5 miles of survey upstream of a bridge to see what's the increase on the WSE introduced by the bridge. But that is not economical and not necessary in most of my studies (except studies to access environmental impacts of a bridge, for example).
RE: HEC-RAS Cross Section Requirements
I typically find free or inexpensive sources of mapping (LIDAR) to perform preliminary analysis and then update with current topo-survey. This way you have an approximate need for survey. Of course, if the free or cheap information is not available then I use 500-1000 feet Upstream and the downstream control as my initial limits for survey. I also include a caveat for additional survey, if needed (T&M).
RE: HEC-RAS Cross Section Requirements
RE: HEC-RAS Cross Section Requirements