depth of flow in culverts

In EXTRAN output files, depth of flow is reported at junctions and discharge is reported for conduits. I am interested in depth of flow within a conduit, i.e. a culvert. If i understand correctly, averaging the upstream and downstream depths isn't appropriate, because in the case of a perched culvert, the depth reported for the downstream node will be reflective of the depth in the plunge pool, not the perched culvert. Is there an internal routine within EXTRAN that will return the depth of flow for a conduit for each time step via Manning's equation or equivelant?

Thanks,
Dave O'Rourke

DrDave,

You are right: the node depth does not represent the downstream or upstream culvert depth when the culvert or any other pipe is perched. Extran will assign the surface area of the culvert to upstream node when the downstream end of the culvert is perched and assign the surface area of the culvert to the downstream node when the upstream node is perched. The depth of water at the perched end of the pipe is the minimum of y critical or y normal from the Froude and Manning's equations, respectively. All of these calculations are performed in subroutine head(nhead) and were originally formulated or coded by Larry Roesner of WRE/CDM.

Now, on to your question: You can only get the node depths in the current version of SWMM 4.4h but the conduits depths are easy to add to the output and will be soon. If you want to do some programming you just need to alter the saving of the heads in subroutine TRANSX. I can show you where if you are interested. Note: The node and conduit depths are printed out in XP-SWMM.

Best Regards,
Robert Dickinson
dickinsonre@cdm.com

Robert,

Thanks for the response, I just have a follow-up question to clear up. In regards to your statement "The depth of water at the perched end of the pipe is the minimum of y critical or y normal from the Froude and Manning's equations, respectively." But what I'm seeing graphically, via the dynamic hydraulic grade line function in PCSWMM, is that the depth of flow reported at the perched outlet of the culvert is zero, and the depth of flow reported pertains to the pool below the perched outlet. This is in the case where ZP2 is positive but the plunge pool tailwater elevation is below the culvert outlet invert (see attached picture of DHGL screen capture).

So do I gather from your statement "Extran will assign the surface area of the culvert to upstream node when the downstream end of the culvert is perched" that the depth reported at the upstream node of the culvert conduit will be approximately representative of depth in the culvert (ignoring for the moment that it will pass through critical depth as it free falls into the plunge pool)?

Thanks,
Dave O'Rourke

Dave,

1. PCSWMM can only show what Extran exports. It shows the node depths or node elevations and not the actual model computed depths at the end of the perched pipe.

2. Usually, if the downstream end of the conduit is perched then the upstream node water surface elevation equals the water surface elevation of the upstream end of the conduit and vice versa when the upstream end of the conduit is perched. There might be some cases in which the water surface elevation of the node is above the water surface elevation of the conduit but don't quote me on this - I would have to look at the code. I will get back to you on this subject.

Best Regards,
Robert Dickinson
dickinsonre@cdm.com