Exfiltration Only Above Invert/Phase-In
Exfiltration Only Above Invert/Phase-In
(OP)
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.
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.





RE: Exfiltration Only Above Invert/Phase-In
RE: Exfiltration Only Above Invert/Phase-In
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 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 www.hydrocad.net/overfilled.htm
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Exfiltration Only Above Invert/Phase-In
RE: Exfiltration Only Above Invert/Phase-In
Peter Smart
HydroCAD Software
www.hydrocad.net