Hec-Ras: Bridge perpendicular to cross sections

[ScottDunn]

Hi,
I'm having trouble modelling a bridge in hec-ras (check out attached file), it is perpendicular to the cross section layout. The problem is that hec-ras doesn't like a bridge/deck that is not connected to the cross sections. The bridge in reality is suspended by piers. The problem with modelling piers is that the width is centred around the centreline thereby making the top stick out further than reality. Anyone had similar problems?

Cheers
Scott

 

[Ryb01]

ScottDunn,

From the figure you've provided it would appear as the bridge is acting more like a lateral structure than a bridge. If this is the case then, you may want to explore the lateral structure option.

What is this bridge spanning? Is there another reach you need to consider? You may need to provide more detail for the forum to comment further.

 

[francesca]

If your bridge is 90 degrees to the flow direction with only the piers in the water, then trick HEC-RAS by modeling it as a single pier bridge with abutments outside of the flood plain. That will let you get a scour angle and will account for the flow area being blocked by the piers. One question, though; if the bridge is wholly inside the channel, what does it connect to? Could you possibly post lat/long coordinates so we can see the site in Google Earth?

 

[ScottDunn]

The bridge is a bike path, it is connected to the adjacent creek bank however in this case I'm not interested in modelling the connections. What I have done is I have modelled the area of the bridge as a complete obstruction (being conservative). Ideally I would have liked to have modelled two piers with a deck that wasn't connected to the creek bank, it appears that it can't be done.

Cheers