Oscillations causing trouble in HydroCAD

[hydroponder]

I'm having a bit of trouble getting my HydroCAD model to run without warning messages. I am modeling three infiltration basins that have underground rock storage with a 6" draintile outlet as well as an overflow catch basin outlet. The model is basically blowing up and I can't get rid of the oscillations in the hydrographs. Taka a look and let me know what you think. Thanks.

 

[TerryScan]

First, is there any flow that infiltrate into the ground? Your model has the underdrains capturing EVERYTHING that does not go to the overflow. Is flow rate through the rock to the underdrain really only .08 in/hr?

If this is not what you really intend (some flow infiltrates into the groudn), you need to model the basin differently. I can give you an example if this is the case.

I edited your model to make all the overflows the SECONDARY discharge. This makes it a lot easier to see what is going through the overflows.

I notice that the oscillations do not occur until the 100yr storm event. I noticed that your peak elevations exceed the flood elevations for several of your CB structures. (If this is the top of the structure, and there is more storage available after the rim/grate is exceeded, it can sometimes be helpful to add that storage to the model...or perhaps it flows elsewhere overland.) But, I digress...Manning's full flow for the 12" pipes is about 2 cfs. We are trying to put 7.4 cfs through them on the 100 yr event. I believe a combination of the "no storage" structures (the CBs), the pressure flow scenario, and the minimum dt limit are causing the oscillations.

 

[hydroponder]

I wasn't quite sure what type of infiltration I was going to get into the ground, so I was taking a conservative approach and assuming that everything would be exiting via the 6" draintile. The soil in the area is a silt loam and is in the HSG B/D. I suppose I could count some infiltration into the ground.

The infiltration basins will have approximately 2 feet of rock overlain by 1 foot of planting medium. Per our Minnesota Stormwater Manual, 0.8 in/hr is quoted for soils in the GM, SW and SP (USC). However, this would be pretty conservative for this case. It could probably be bumped up some.

CB 103, 104 and 105 are catch basin outlet structures for the basins. CB 102 is actually a manhole so there would be no addition storage. This would be the only one that would be worrisome if it were to overfop...flying manhole lids.

So, you're thinking I need to increase my pipe sizes and possibly count some infiltration into the ground?

 

[psmart]

Exfiltration is normally used to model losses into the surrounding ground, rather than flow within the stone storage. If this is a "loss", the exfiltration routing should be set to "discarded".

Otherwise, a pond simulation assumes a level pool within the storage, and since you're including the stone voids as part of the storage, it should not also be modeled as an outlet control. Although I'm not 100% sure of your intent, I suspect you can delete the exfiltration device. I would not expect a bed of stone with 30% voids to restrict the flow beyond what the 6" orifice is already doing.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[hydroponder]

Peter,

I revised my model as follows: exfilitration is set to discarded at 0.25 in/hour, assuming there will be some losses throughout the bottom of the stone. I then used the 6" orifice as the primary outlet and set the 12" horizontal orifice (catch basin overflow) as the secondary outlet so I could see where the flow was going. Running the model as described, the 100 year HWL are considerably less. These results seem to be a bit too liberal (if that makes sense).

I was using the exfiltration as a device because I wanted to include some kind of a lag time between when the water enters the basin and when it actually reaches the 6" draintile. Perhaps the prudent approach would be to ignore the underground storage all together (maybe a snow melt condition) and run the model that way. Or, at least ignore the 6" draintile outlet..

Thoughts?

I am attaching a copy of the model I described in the first paragraph.

 

[psmart]

It depends how you expect the stone to behave. If the outlet restriction is sufficiently small to create level-pool storage in the stone voids, the voids can be reasonably included as part of the pond storage. But if the stone will significantly restrict the flow, it should be treated only as an outlet control, and the voids should not be included in the pond storage.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[hydroponder]

That makes sense. So the first model I attached would have been correct if I had not included the stone void storage? And, I can still use exfiltration as an outlet and route it to the 6" orifice? The exfiltration is the controlling the device so I guess I could get rid of the 6" orifice all together. So, it would be exfiltration as the primary outlet and the 12" horizontal orifice as the secondary outlet. Does that all seem to make sense?

I know in the past there have been inconsistancies in the inflow and outflow volumes when I have tried to route exfiltration somewhere other than discarded...

 

[psmart]

Since CB104 and CB105 act primarily as control structures for their respective storage areas, I would include the culverts as part of each pond, rather than modeling as a separate CB. When doing this, adjust the device routing so both orifices are routed to the culvert.

A side note: When using stage voids, each value applies to the storage immediately below the associated elevation. This means that the first voids value (at the storage invert) is not used.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[psmart]

Regarding the inflow and outflow volumes, they will be the same, as long as there is a bottom outlet of some type (no retention), and the time span is sufficient to allow the pond to drain completely.

Although the infiltration volume is not subject to further routing, you will still get conservation of mass with the pond routing.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[hydroponder]

Okay, thanks.

Last question and I will stop bothering you. I was using Dynamic Storage Indication as the calculation method, but it appeared that there was a back flow condition between RG2 and RG3. Would it make sense to use the Storage Indication method so I can set tailwater elevations, thus forcing the HWL to level out? Or does the Dyn Stor Ind method handle this on it's own?

See attached HydroCAD model.

 

[psmart]

Setting the tailwater manually would only be appropriate in the case of a (relatively) constant tailwater.

The Dynamic Storage Indication method adjusts the discharge characteristics for each device based on the tailwater at each time step throughout the routing.

For details please see

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

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[TerryScan]

Your design sounds a bit like a shallow bioretention rather than infiltration.

I was not implying that your model approach of capturing everything in the underdrain was wrong, I was just making sure that was your intent since you said it was infiltration.

As Peter said, one should model the CBs that are outfall structures in the pond definitions. I have attached a copy of your file in which I edited pond 1 as an example.

With these heads, I don't think your going to have any flying manhole lids. I am not sure you even need to upsize the pipes once the model gets refined.

 

[hydroponder]

Terry,

You are right, a more appropriate term would be bioretention. I appreciate all the help.