The HydroCAD culvert analysis is not just simple Manning's flow. It also considers headwater, tailwater, and entrance losses. If you apply enough head, you can push any amount of water through a culvert. Have a look at the stage-discharge curve and you can see the behavior as it transitions to pressure flow at about 2.4 cfs.

I you wanted to use strictly Manning's flow, and ignore the other factors, you could use a pipe reach. But your solution (a zero storage pond with culvert outlet) is much preferred, because it properly simulates the results under a much wider range of conditions.

For complete details please see www.hydrocad.net/pipes.htm

Peter Smart
HydroCAD Software
www.hydrocad.net

@psmart

Thanks for the info. My issue really is the fact that typically when the program typically forces the water through the pipe I am used to seeing the error that the culvert has surcharged and am not seeing that here. I see that the flow is pressurized upon reaching a 'flowing full' scenario and then starts jetting through the culvert.

So, because the outlet of this system is actually into an existing system, do you think I should utilize the reach node type? My ultimate goal is to ensure that the flow into the downstream system (downstream of the proposed detention pond) is being adequately controlled by the detention pond.

With the current design and output data, I am thinking that this 12" outlet pipe is insufficient to pass the flows being minimally controlled by the pond and thus:

1. The existing system needs to be upsized to handle the increased flows; Or,
   
2. The proposed detention pond needs revised to control more of the flow.
   

A submerged culvert inlet is properly handled by HydroCAD, so no warning message is generated. However, the user definitely needs to check the peak headwater elevation to ensure this meets your design criteria. That's one of the reasons for setting a flood elevation. You can use it to identify a submerged inlet - IF that's a condition that YOU would like to avoid. But it's not a problem for the software, which will continue to calculate properly for a submerged inlet.

To your question, I do not recommend a pipe reach, since it cannot handle potential tailwater effects. A culvert is better. If you want to avoid surcharging, simply check the headwater. The adequacy of the pipe depends entirely on your design criteria and the amount of headwater you choose to allow. If you want to reduce or eliminate the headwater, then you will probably want to use a larger culvert.

Since you are discharging into an existing system, you may also need to consider the tailwater at that confluence.

Peter Smart
HydroCAD Software
www.hydrocad.net

Ok, I understand, and thank you again for the assistance.

I switched the final reach node in my model to a pipe. I did this in order to model the run of the existing system to see this proposed system's effects.

HydroCAD was able to produce a message that node 33R peak inflow is 178% of Manning's capacity.

This is what I needed to see and be able to document.