HydroCAD Exfiltrate Volume

[LisaG]

Hi All!!

Is there a way to exfiltrate a volume from the pond routing instead of the given parameters (conductivity, flow and velocity)?

Background: We have computed our net runoff volume that needs to be infiltrated in our ponds. The volume reduction is limited to 6 hours of infiltration only. As far as I can tell, HydroCAD uses the conductivity rate over the entire storm duration and results in higher infiltration volumes than targeted.

Thanks! Lisa

 

[psmart]

HydroCAD is intended for the simulation of real-world drainage systems. As such, there is no direct mechanism that would cause the exfiltration to artificially stop after an arbitrary 6 hour period. Although this may be the maximum exfiltration allowed by your stormwater regulations, the actual system will continue to exfiltrate throughout the entire storm.

However, you might be able to induce the cutoff by routing an exfiltration outlet (perhaps an orifice) to a link with a manual elevation table. The table can be used to define a tailwater that suddenly increases after 6 hours, thereby turning off the exfiltration outlet.

FYI, there are other outlet options that could be used to start the exfiltration at a specific time (including a pump, dam breach, or siphon) but getting the flow to stop as the WSE in the pond is still rising is more difficult. In reality, this just isn't going to happen.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[LisaG]

Thanks psmart! The theory of the 6 hour limitation is that the soil would reach saturation after this amount of time and the infiltration rate is reduced. I was hopeful that we could get the discarded value to be equal or close to the 6 hr calc.

 

[beej67]

What's the municipality? Just curious.

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

http://www.campbellcivil.com

 

[LisaG]

Pennsylvania - PADEP BMP Manual. The 6 hour time period is buried in an example, but it was recently brought to our attention that the infiltration volume calculation should be infiltration rate * infiltration area * infiltration period (assume 6 hours) * (1/12 ft/in) (PADEP BMP Manual Appendix D). We can't use the exfiltration discarded volume from HydroCAD because of this standard.

 

[psmart]

Just curious, when does the 6 hour time period start? If you start at zero hour with a 24-hour storm, there will be minimal runoff in the first 6 hours (depending on the CN), so any exfiltration is going to have very little effect on the overall pond routing. Exfiltration is more effective in handling low-depth events (like a 1" rainfall), where is can be expected to handle a larger fraction (perhaps all) of the event. In this situation cutting off the exfiltration makes no sense.

I know it's generally futile to argue with the regulations, but something doesn't seem right here.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

That's a good point. There's very little to no runoff in the first six hours of a 24 hour distribution.

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

http://www.campbellcivil.com

 

[LisaG]

The regs don't specify a particular 6 hour time period in the storm and the simple calculation doesn't take into account the hydrograph, storage, anything. We are having a hard time digesting this comment as well. In the end, the infiltration volume computed with the six hour period is isolated to a particular worksheet in the NPDES permit. After a run through of a few basin calculations, it hasn't effected the designs so we are just going with it.

You're right psmart! I don't argue the regs, I just try to bend to them. HA!

 

[TerryScan]

I am not familiar with PA, but just looked at the manual.

For an Infiltration Trench, section 6.4.4 states "*Infiltration Period is the time when bed is receiving runoff and capable of infiltration. Not to exceed 72 hours. "

The trench is used as an example in Appendix D where they "assume 6 hours". If one has infiltration testing that saturates the soil and then runs for a longer time, one should be able to justify periods supported by the site testing (up to 72h).

For an Infiltration Basin, section 6.4.2 states "*Infiltration Period is equal to 2 hours or tne time of concentration, whichever is larger. "