Culvert Lag

[Anonymous]

I’m attempting to build a HEC-RAS 2D model of a project that proposes to include a bypass pipe along a reach of channel. The bypass pipe is 1,500 feet long and the inflow hydrograph is short in duration and susceptible to effects of phasing. Does HEC-RAS lag the hydrograph routed through the bypass pipe if modeled using a SA/2D Connection (using pipe length and average velocity)? Thanks.

[Anonymous]

Using a simple model that includes a very long culvert in a SA/2D Area Connection, I’ve determined that travel time through the culvert is not considered. Any recommended workarounds that would allow me to delay the hydrograph routed through the culvert by a few minutes? Thanks.

[Anonymous]

You could try and model your pipe using 1D cross sections with lids. It would handle the travel time correctly. RAS can do pressurized pipe flow using cross sections with lids but it can be finicky to get it to be stable. That would also mean having a 1D/2D connection of some sort. Doable but another source of potential stability problems.

If you can compute the culvert hydrograph you could delay it for a few minutes using the advanced rules capability.

For instance, if the pipe is under inlet control you could dump the flow out of system and then use a separate structure with rules to bring it back in with a time delay.

But I’m not sure how you would do that if it is tailwater/outlet influenced.

[Anonymous]

Thanks Jarvus. I’ve been messing around with the lidded sections, but as you noted there are several issues there (addressing overflow above the bypass pipe being one). Unfortunately, lagging with advanced rules won’t work for me either as I have a few segments of pipe that include lateral inflows at the manholes (I’m modeling the manholes as depression in the terrain). I may have to move over to a different software platform if this project alternative wins out. Thanks again.

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