Culvert replacement-tidal region

[Crystalct1]

What can anyone tell me about design criteria for culvert replacement when the flow is tidal? The PM says we don't need a HEC RAS run for water surface elevations, but I need velocities to give to another department for scour calculations. Also, I need to reset/resize these culverts to aleviate flooding over a roadway. I am not sure what I'm governed by. Can anyone help?

 

[dicksewerrat]

You need to find your Q. then design the pipes to take it all with V>/+ 2.5fps. Don't use a pipe that reacts with the saltwater.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Crystalct1]

So I find my flows using HEC HMS as if the area were not tidal and then how do I account for the tidal changes? Aren't those changes area-specific? Don't I need high and low tide info for the project site? Once I find that, how do I use that data to account for the tidal flow changes?

 

[francesca]

You need to design for your worst-case. This involves checking your boundary conditions: high tide and low tide. You need to make sure that your flow can pass through the culvert without flooding at high tide, but you may get a higher flow rate at low tide when you have a lower tailwater elevation.

The PM may be right that you don't need a HEC-RAS, but you will need some kind of modeling. Ask your PM what he/she suggests. If they say you can do these calculations in your head, use whatever software you're most comfortable with.

You should be able to account for the tides with tailwater elevation set to the high/low tide mark. I'm not sure how this would affect your flow rate/velocity upstream of the culvert, however, and you may indeed want to use HEC-RAS with ineffective flow areas specified below the high/low water elevation.