Exfiltration Only Above Invert/Phase-In

[Twinkie]

I am working on a raingarden lined with an impermeable liner, with 4' of ammended soil (peat/organics) above the liner.

Bottom elevation = 100
Primary outfall 8" PVC = 100
Top of ammended soils = 104
Riser pipe = 104.50
Berm = 106

The calculations use an exfiltration rate of .5"/hr above elevation 100, with a phase in depth of 0.01'

There is a perforated pipe underdrain in the BMP.

HydroCAD is not showing the usual "Discarded" hydrograph.

I am struggling to visualise how HydroCAD does this calculation, and the applicability of using exfiltration in the routing of a lined basin.

 

[Twinkie]

This is actually routing to the primary culvert, that is why I am not seeing the brown hydrograph.

 

[psmart]

If you have the exfiltration routed to the culvert, there will not be any discarded (brown) outflow. The only discharge will be the culvert, which is presumably the primary outflow.

You need to be careful about the delineation between the amended soils and the open storage. Based on your description, the storage should start at 104. Below that you have exfiltration, and above you have storage. Although you may have some storage in the media, it will behave like a sponge rather than open storage, so it generally should not be included in the pond storage volume, which is assumed to be free-flowing.

Your outlets would be something like this:

Device#1 = 8" culvert at 100', routing=primary
Device#2 = top of riser - orifice at 104.5, routing=Device#1
Device#3 = exfiltration, routing=Device#1

This setup takes the exfiltration, as collected by the perforated pipe, and combines it with the riser overflow to create the final discharge through the 8" pipe. For further details please see

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

If the berm overflows you will also need a weir at that elevation, and appropriate storage data above the berm. For details see

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

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Twinkie]

Peter, thank you very much for the supply. I have typically included the media as storage, using a 17% void ratio. Are you saying that when exfiltation is used as stated above, that I should not consider the void space in the media as storage? I believe that is what the help link is saying. Thank you.

 

[psmart]

If the water can flow freely though the media and rapidly forms a level pool, (such as clean stone with 30-40% voids) then it's reasonable to treat it as part of the pond storage. Otherwise it's not really part of the level-pool volume and should not be treated as such. The media is tricky. That's the point of the link.

Peter Smart
HydroCAD Software

http://www.hydrocad.net