Dynamic tailwater on an orifice .. curious things happening

[beej67]

I have an underground stormwater vault, which discharges through two controls to the same receiving manhole. The low control is an orifice which then goes through a water quality device (neglected for the analysis) and the other is a higher slot weir which conveys the majority of the flow through a culvert to the same receiving manhole. I have the flows split into primary and secondary, but have both the primary and secondary flows routed to the same (manhole) node, which is modeled as a storage node with zero storage.

The receiving manhole also receives a lot of offsite flow. The manhole invert is 2 ft below the vault invert, but it does stage up above the vault invert during rain events. Everything in the model appears on the surface to work fine, utilizing dynamic storage indication method, and the stage-time graphs I pull from the raw data show that the vault stage is always higher than the manhole stage, indicating no conditions of reverse flow.

Here's the weird part.

There is only a small window of time when the manhole stage is above the vault invert. During this time I would expect to see the flow through the orifice slow, as it becomes based on differential head instead of purely upstream head. This isn't happening in the model. The orifice flow stays steady during the entire duration of the backwater, regardless of changes in the differential head.

I was under the impression that flow through a backwatered orifice in HydroCAD was based on differential head. Is that not the case? If it is, any insight as to what's going on in this model? An excel sheet showing the comparisons is attached.


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

http://www.campbellcivil.com

 

[beej67]

And the model..

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

http://www.campbellcivil.com

 

[psmart]

Looking at the HydroCAD project file, running a node comparison report for pond 12 (the vault) and pond 15 (the CB) indicates that the WSE in the vault is higher at all times during the routing. I don't see a reverse head at any time. The spreadsheet also shows a positive differential at all times up to 48 hours. (After that it only contains the downstream data, so the calculated difference is not valid.)

Are we looking at the same data?


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[psmart]

If you're asking about the orifice flow being reduced based on a reduction in the differential head, this DOES seem to be occurring in the model and in the spreadsheet, exactly as you would expect.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

Look closer Peter, at the time frame between 13.46 hours and 13.78 hours in the data.

This is the time frame where the tailwater in the manhole is over the vault invert. As you say, differential head never reaches zero, but it does drop off significantly. Over this span of time, the differential head starts at 4.62 ft, drops to 1.2 ft, and then rises back to 4.33 ft. Over this same time span, the flow through the orifice remains constant at 0.4 cfs. During this span, both sides of the orifice should be submerged and the orifice flow should vary with differential head.

That's the part that's got me scratching my head. Am I misinterpreting something?



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

http://www.campbellcivil.com

 

[psmart]

The elevation data in your spreadsheet for CB15 (A6-A1) is different than the HydroCAD results for that node. Are you sure you copied the correct data? I gather this is for the 100-year event?

For example, at 13.60 hours HydroCAD gives the elevations as 980.49' and 973.80, while the spreadsheet lists 980.49 and 978.92 - the second value is significantly higher.

You can view the elevations side-by-side in HydroCAD using the node comparison report:
1) Select the two nodes
2) Click View | Comparison Report
3) Select "Elevation" from the drop-down list
If you zoom in on the region of interest (or use the tabular view) you can see that the elevation differential is essentially constant during the interval when the orifice flow is around 0.40 CFS. The differential is constant, so the flow is constant. Looks correct to me.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

Time scales didn't line up in my excel cut/paste, leading to a graphing error. User error. Thanks.

That "comparison report" interface is pretty slick. I've been using your stuff half a decade and never realized it was even there.

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

http://www.campbellcivil.com

 

[psmart]

The node comparison report can be used for any number of nodes. It's especially useful for examining the water surface elevations for multiple nodes in a complex tailwater scenario. It can even be used to compare nodes in separate projects, such as existing and proposed scenarios.

For details please see

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

Peter Smart
HydroCAD Software

http://www.hydrocad.net