Puzzled about basin outlflow results

[jj1212]

I have a basin with surveyed bottom elevation at 298. The as-builts show the basin bottom at 296 (there has been 2' of deposition accumulated on the basin bottom raising the bottom of basin elevation). The outlet structure consists of a riser structure with top elevation at 303.79', a 21" RCP pipe to the riser structure with invert elevation 296.07', and a 66" RCP outlet pipe from the riser structure with invert at 296'.

Therefore my current existing model has a basin with bottom elevation at 298'. Even though the 21" inflow pipe to the riser structure is blocked, I have still included it in the outlet configuration (I assumed that since both the invert and crown elevation are below the bottom of the basin, nothing would be routed through this pipe). So, my model shows the outlets as follows

# Invert Description Routing
1 295.60 66" RCP Primary
2 303.79 108" Concrete Riser Device 1
3 296.07 21" RCP Device 1

However, when I run the model, and review the discharge curve, it seems that water is being discharged immediately. How is that possible? I don't have any infiltration assigned to the outlets.

To double check, I took out device #3, and ran it again, and the curve appears as expected, with a horizontal line at discharge=0, upto elevation 303.79, at which point the discharge begins.

Is there something that I need to do to ensure the model doesn't include the 21" RCP pipe in the calculations. I am wondering if the model is creating an imaginary stage storage down to the elevation of the 21" pipe? I'd like to keep this component of the outlet structure in the model, so I can model other scenarios (ie, the as-built basin, an proposed design).

Any advice would be greatly appreciated. I have attached the hydrocad file

 

[psmart]

Water will discharge through an outlet even if there is no storage at that elevation. The model doesn't "know" that the lower pipe is blocked. In fact, it's not uncommon to have an outlet set below the bottom of a pond. If you want to prevent any flow through a specific outlet you can set it's discharge multiplier to zero.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[TerryScan]

And...for future reference, if you have a situation where the pipe is partially embedded, there is the option in the Hydrocad Culvert Outlet dialog for INSIDE FILL. (If, in you example, the pond bottom was at 297.07, the 21" pipe would have 12" of Inside Fill)

 

[jj1212]

thanks everyone for the helpful info

 

[Crystalct1]

"Water will discharge through an outlet even if there is no storage at that elevation", does not make any sense whatsoever. It absolutely should NOT discharge anything if there is nothing there stored to be discharged. The storage table determines the elevation of water at the inlet of the discharge device in order to determine the flow through said device. Your model is doing exactly what you thought it was: creating an imaginary stage storage down to the elevation of the 21" pipe. I have not downloaded your file and looked at it in the software, but this has happened to me before. I would imagine that you have defined your storage table close to the lowest elevation of the pond, but the model is assuming another data pair of 0,0 before your first line, so that it has interpolated an elevation for volumes between 0 and your first volume entry. Explained with simple numbers, if you say that elevation 10 has a volume of 1 and that is your first entry, the model thinks elevation 5 has a volume of 0.5, even though the bottom of the pond may be at elevation 7. You must first define the elevation that has 0 volume, so the model knows that there is NO water at all at that elevation to be routed anywhere.

 

[psmart]

HydroCAD assumes there is zero storage at any elevation below the storage volume you define. If you place an outlet below the defined storage, it does NOT extrapolate storage to the lower elevation - it keeps the storage at zero, exactly as you specified in the storage definition. You can confirm this by examining the stage-storage report.

Of course, the actual system will have some storage down to the lowest outlet, even if it is just a vertical feed pipe or drop inlet. But this volume is often insignificant in comparison to the primary storage volume, and can be omitted from the model, as done in the OP. This approach works just fine. If you need verification you can include the lower volume in the storage definition and compare the results.

Also have a look at the stage-discharge curve, and you will see that the discharge starts at the bottom of the storage, not at the depressed outlet.

Bottom line: Omitting the feed volume of a depressed outlet will not adversely effect the results, as long as the volume is minimal.



Peter Smart
HydroCAD Software

http://www.hydrocad.net