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Bioretention Area - Modeling Outflow Through Underdrain

Bioretention Area - Modeling Outflow Through Underdrain

Bioretention Area - Modeling Outflow Through Underdrain

I have a bioretention area with an underdrain and biosoil media (40% voids) 2' above the underdrain. There is also some storage above the biosoil media that slopes outward for larger storm event storage (100% voids). The underdrain is 6" diameter that connects into an outlet, i.e. catch basin with a pipe outfall. The design permeability rate for biosoil media is 2.83 in/hr.

Is there a best way to model the exfiltration through the biosoil to the underdrain which then flows into the catch basin? The two ways I have seen are:
1. Multiply the 2.83 in/hr x surface area of biosoil media (sf) and after conversion, come up with a constant flow value (cfs). Then apply that constant flow to a 6" vertical orifice on the catch basin at the underdrain invert.
2. Route the exfiltration as based on the underdrain invert (and using surface area) to the primary outlet, which is the pipe.

Are both ways acceptable? Personally, I like #2 better, because if I have to change the biosoil media dimensions (length and or width), HydroCAD automatically takes that into account versus #1, in which I have to recalculate a flow rate.

Does surface area in #2 only account for a length and width at the elevation I specify or would I need to specify the biosoil minimum and maximum inverts? After reading through the HydroCAD help, I wasn't sure about the surface area exactly. I definitely don't want to use the horizontal or wetted area options.


RE: Bioretention Area - Modeling Outflow Through Underdrain

I would enter the exfiltration velocity (option 2) so HydroCAD can calculate the final flow based on the surface area.

For details on modeling bioretention areas please see www.hydrocad.net/raingarden.htm

Peter Smart
HydroCAD Software

RE: Bioretention Area - Modeling Outflow Through Underdrain

Looked through the help and mucho videos, but the one question that I couldn't find a definitive answer for (and maybe I'm just overlooking it) is:
When choosing surface area, does it only account for a length and width at the elevation I specify or would I need to specify a biosoil media minimum and maximum inverts?

RE: Bioretention Area - Modeling Outflow Through Underdrain

Also, under "Modeling Rain-Gardens", "Modeling Under-drains", the following statement is shown:
"B) If the growth media restricts the flow of water from the pond, then it becomes the effective outlet control, and the storage definition should include only the (open water) volume above this point."
I kind of get it from looking at the biosoil media as the "outlet" (even though the underdrain would be the 'real' outlet), but what about all the storage within the the media itself? Wouldn't I unnecessarily have to make the facility bigger, because of losing that storage?


RE: Bioretention Area - Modeling Outflow Through Underdrain

The voids in the media is not part of the "level pool" and generally should not be included in the pond storage. Although there may be some storage in the voids, it acts more like a sponge, and is generally modeled as an exfiltration outlet.

As to the exfiltration calcs, it will use the full area at each elevation, unless you restrict the area by setting the minimum and/or maximum elevation.

Peter Smart
HydroCAD Software

RE: Bioretention Area - Modeling Outflow Through Underdrain

Below is my version. One can ensure that the underdrain (device 1) has enough holes to pass the flow through the biosoil. The bottom of biosoil is at el. 10.00' and is 2 ft. deep.

Inflow Area = 2.000 ac, 65.00% Impervious, Inflow Depth > 5.95" for 100 YR event
Inflow = 16.74 cfs @ 12.05 hrs, Volume= 0.992 af
Outflow = 1.34 cfs @ 12.87 hrs, Volume= 0.819 af, Atten= 92%, Lag= 48.8 min
Primary = 1.34 cfs @ 12.87 hrs, Volume= 0.819 af
Secondary = 0.00 cfs @ 5.00 hrs, Volume= 0.000 af

Routing by Stor-Ind method, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs
Peak Elev= 14.80' @ 12.87 hrs Surf.Area= 10,088 sf Storage= 22,299 cf

Plug-Flow detention time= 173.8 min calculated for 0.819 af (83% of inflow) Center-of-Mass det.
time= 123.8 min ( 894.9 - 771.0 )

Volume Invert Avail.Storage Storage Description
#1 12.00' 24,336 cf 60.00'W x 100.00'L x 3.00'H Prismatoid Z=4.0

Device Routing Invert Outlet Devices
#0 Secondary 15.00' Automatic Storage Overflow (Discharged without head)
#1 Primary 10.00' 0.5" Vert. Orifice/Grate X 36.00 columns X 3 rows with 2.0" cc spacing C= 0.600
Limited to weir flow at low heads
#2 Device 1 12.00' 2.830 in/hr Biosoil over Horizontal area
Conductivity to Groundwater Elevation = 10.00'

Primary OutFlow Max=1.34 cfs @ 12.87 hrs HW=14.80' (Free Discharge)
^ 1=Orifice/Grate (Passes 1.34 cfs of 1.52 cfs potential flow)
^2=Biosoil ( Controls 1.34 cfs)
Secondary OutFlow Max=0.00 cfs @ 5.00 hrs HW=12.00' (Free Discharge)

RE: Bioretention Area - Modeling Outflow Through Underdrain

My real life setup (as it were) is a catch basin outlet with a pipe outfall (EL 14). Going to that pipe is an underdrain at EL 14, biosoil mix from EL 14 - 17, and the top of the catch basin (overflow) is at EL 18.

I set the model's pipe outlet as the primary and then tried modeling the exfiltration going to that pipe directly at EL 14 (with the limits of exfiltration from 14 - 17 based on surface area and constant velocity) and then the typical catch basin overflow (24" x 34" horizontal) at EL 18.

After running and bringing up the summary, the exfiltration is showing 0.0, even though I know it should be doing something.

I'm guessing I cannot route exfiltration directly to the primary outfall?
Would I have to run the exfiltration to maybe a 6" vertical orifice (i.e. where the underdrain connects to the catch basin side wall) first and then to the primary outfall?

I saw up above where TerryScan actually modeled the underdrain dimensions into the model. I was running mine to see if I could skip that step and just basically state that the exfiltration outlets directly to the primary outfall. The presumption is that the underdrain capacity will be more than adequate to handle what little bit of flow comes through the biosoil mix. Maybe a bad presumption on my part?

RE: Bioretention Area - Modeling Outflow Through Underdrain

Played around with the outlets and removed the exfiltration limits (between EL 14 and 17) and just left it as exfiltration above EL 14 and it appears to be giving better results.
Also, just routed the exfiltration straight to the primary and the results are holding.

From a very technicality aspect, wouldn't the model be looking at everything above EL 14 as exfiltration versus in reality, all above EL 17 is not exfiltrating, technically?

RE: Bioretention Area - Modeling Outflow Through Underdrain

You need to set the upper exfiltration limit slightly above the top of the media, otherwise it will exclude the entire area of the media. Note that the screen says "Only allow exfiltration above invert and BELOW maximum"

As for the exfiltration routing, you can (and should) route it to the outfall pipe rather than directly to primary. The results will be comparable, except at high flows where the pipe may start to limit the flow.

Peter Smart
HydroCAD Software

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