Modeling an obstacle in HEC-RAS 1

[032157]

Upstream of a major bridge, a fender system was constructed to prevent damage to the bridge piers by the accidental barge collision. The system consists of individual cylinder shaped structure constructed of sheet piles driven to the river bed in a circular shape (30’ diameter). They are called DOLPHIN, and extend from the river bed to approx. 8’ above 100-yr W.S.E., and there are 4 of these giant structures guarding the piers. My question is:
What is appropriate way to model this kind of obstacles in HEC-RAS? One suggested modifying pertaining cross sections to create a section that represent these structures. I did that and after running the model the W.S.E. dropped, I suppose due to reduced x-sec area. Another suggestion was to model the obstacles as piers. I did that too, and the result was that the W.S.E., was raised. So what is the correct way to model this?
I thank you in advance for any suggestion you can provide.

 

[francesca]

Are they attached to the piers or upstream of them? If connected to the piers, I'd model the piers as being 30 ft wide, if independent, I'd put a cross-section in at the Dolphin structures and block out the area as ineffective.

Remember that HEC-RAS is a one-dimensional model. It might be better to use a two-dimensional model in this case.

If you post an image we could be more helpful.

 

[032157]

Thanks for the reply. I do not have an image. Additionally the Dolphin structurs are located approx. 100' to 80' U/S of the bridge. The original or pre-existing model (without the structures) was done in HEC-RAS, therefore I want to use the same program and model these structures to determine the impact.
Thanks again

 

[francesca]

If the structures create a wake effect to slow down the water you need to make the piers wider to block out the ineffective area under the bridge. For round piers, the rule-of-thumb of whether they function individually or as a composite is if spacing is less than 6x diameter. In this case, your spacing is about 3x diameter, so there is definitely some inter-relation going on.

From this (

http://onlinelibrary.wiley.com/doi/10.1002/stco.200910004/pdf)

I gather that the structure is to protect the pier from collision, not from scour. You will need to make sure you have adequately accounted for the effects on pier scour. Embrace HEC-18.

http://www.stream.fs.fed.us/fishxing/fplibrary/FHWA_2001_Evaluating_Scour_at_Bridges.pdf

 

[032157]

Does anyone knows why when we model an obstacle in HEC-RAS using just the pier option(as an obstacle) from bridge modeling without letting the water to touch deck, the W.S.E. increases, while if you model the same obstacle with the same dimensions as the pier using the ground geometry, the W.S.E. drops.

Please share your opinion.
Thanks

 

[francesca]

Two different modeling approaches. When you look on the Bridge Modeling options tab, you'll probably see that you have Energy checked on (i.e. use the energy equation) whereas regular cross-sections are aggregated into a single, one-dimensional "roughness" and Manning's is used. You really need to read the Hydraulic Reference Manual to make sure you're modeling these correctly.

 

[032157]

What about the obstacle option. Does this approch uses just manning's "roughness", or energy equation? Another word what is the most appropriate method to model an obstacle utilizing a one-dimensional, steady flow HEC-RAS program?
Thanks again

 

[francesca]

I think you're missing the point that you have to understand what the model is doing. You are the engineer and you can't rely on a "black box," you have to set up the model to account for these structures according to the laws of physics. You have to actually get out the Hydraulic Reference Manual and read it before setting up your model. Then you have to take the conservative answer, which may be the higher WSE of one model and the higher velocity of the other model. Right now you're putting garbage in and getting garbage out.

 

[IRstuff]

A PE is completely responsible for his calculations, whether done by hand or by whatever program. Anything that goes wrong with such calculations, it's still the responsibility of the PE, not the manual, not the program nor the programmer, unless it's the PE as well.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[032157]

Thank you.