Hydrocad Pond Flow Question. Is my Boss wrong???

[TheEnginerd]

Hello,
I have been modeling with hydrocad, hydraflow, and pond pack for years. I have occasionally encountered situations where the incoming flow is less that what is leaving the pond. This may occur during a large storm event when the water is flowing out of a low flow orifice and building up depth until it just spills over the crest of a large spillway. At that time i have seen spikes of peak flow be larger than the peak flow entering the pond.
My boss reviewed my latest project and tells me that this is NOT POSSIBLE and told me to look at the model. Can someone please let me know that this is possible and provide a scientific reason why? Thank you very much.

 

[psmart]

Are there any warning messages? Various conditions make it impossible for the software to perform an accurate simulation. Most of these cases will generate a warning message that identifies the situation and recommends various solutions. You can click each message for complete details.

As a wild guess, you have failed to provide storage data above the crest of the spillway. For details please see

http://www.hydrocad.net/overfilled.htm

If you have further questions please post your HydroCAD project file or email it to HydroCAD support.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

If we're just discussing peaks, your boss is right for a standard watershed -> pond -> discharge setup with no tailwater. I could probably manufacture a strange case where peak outflow is greater than peak inflow by monkeying with tailwater conditions, but it's not likely to show up in a model you craft for any 'real' case.

Now there are certainly times in many models where the instantaneous inflow is lower than the instantaneous outflow, but not peak to peak.

Echo checking to make sure you're not pegging your storage out somewhere, and there's no wobble in your hydrographs.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[TheEnginerd]

thanks very much to the both of you. it makes sense that this situation wouldn't happen in real life, so it must be an error in the model. there are several warning messages, but they pertain to the outlet pipes for this pond being under capacity (an existing condition). In fact, the outlet pipes are backpitched (so i had to assign a pretend .001% slope just to make the model work). Perhaps that has something to do with the unrealistic discharge rates.

 

[TerryScan]

WITH Tailwater, this can absolutely happen in certain timing scenarios.

a. upstream watersheds raise tailwater before substatial runoff for subject watershed
b. subject watershed fills pond which has outflow limited by tailwater (could even be zero or reverse flow)
c. storm event ends and tailwater quickly dissipates
d. surcharged pond is allowed to outflow freely

The peak inflow to the pond over the entire event could be a small fraction of the peak outflow in this scenario.

 

[TheEnginerd]

thanks very much terry. since my 3 ponds are connected in series, and the entire watershed is very flat, the downstream ponds could certainly have an affect on the upstream ponds (with tailwater). In addition, all 3 ponds have different TC's so i'm really relying upon the dynamic storage setting to understand where the tailwater is at any given time. What you described makes perfect sense, so maybe i'm not crazy after all.
andy

 

[psmart]

With tailwater effects it is definitely possible to have a peak pond outflow that exceeds the peak inflow. For example, you have a pond that fills gradually with a steady inflow, but the outflow is limited by the downstream tailwater. Then the tailwater drops suddenly, causing an increase in the outflow. In this scenario, the peak outflow can easily exceed the peak inflow.

Of course, we're all speculating on what might be happening in your model. The only way to know for sure is to look at your model. There are a lot of issues that could be involved.

BTW: There is no need to "assign a pretend 0.001% slope" to a culvert. You can use a zero or even a negative (adverse) slope for a culvert in HydroCAD. If the zero slope is causing you problems, there is probably another factor at work.



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[TheEnginerd]

Peter,
I must beg to differ regarding the pipe slope problem. I am using hydrocad 9.0 and when i enter a negative slope, the slope is highlighted in RED and it says X - Inlet must be higher than Outlet, and you cannot physically "APPLY" the data. You cannot close this window until you cancel or apply a Positive slope. Can you please try this and get back to me?
I have attached an image of the error message.

 

[psmart]

It sounds like you're using a pipe reach. Since a reach is modeled using normal Manning's flow, it requires a positive slope. (outlet lower than inlet)

But a culvert (outlet on a pond) definitely can have an adverse slope, and this is the way most pipes should be modeled.

For details see

http://www.hydrocad.net/pipes.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net