Exfiltration modeling with Conductivity - Allowing exfiltration at all elevations or Only above inve 1

[swood19]

I've modeled a bio-retention system that has a measured infiltration capability at a specific invert of 4 in/hr. I'll call the invert of my bio-retention system at elevation 100. Seasonal high water table is located at invert elevation 98. Per the stormwater regulations of my delegated agency, I must half my measured infiltration rate, and my invert of the infiltration practice must be 2' above the seasonal high water table.

I'm having trouble finding the answer as to whether I should model the exfiltration at all elevations with a manual input for the groundwater elevation or if I should enter allowing exfiltration only above the invert of the system. I find it somewhat deceiving in that the outflow that's discarded via the exfiltration is much higher as the groundwater elevation increases closer to the system's invert. Does that make sense?


What it boils down to is that I'm getting higher discharge rates than I expected with the 'At all elevations' selection with the groundwater only 2' below my basin invert while setting groundwater to 0 gives me a lower Q output. Selecting a groundwater elevation of 0 actually gives me the Q I'd expect (surface area x conductivity rate) whereas with the high groundwater, I'm experiencing discarded rates almost triple that amount.

 

[psmart]

Since you have a specific exfiltration velocity of 4 in/hr, I would use the velocity option, in conjunction with a minimum and/or maximum elevation.

If you specify the saturated hydraulic conductivity, the exfiltration flow is determined by Darcy's law, where the flow if is proportional to the gradient across the mdeia. This means that the flow will increase as the groundwater rises and the media thickness (the distance from the bottom of the pond to ground water) decreases.

For further details please see

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

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[swood19]

Thanks Peter. I have it trained in my thought process that the outflow would decrease with a higher seasonal high or ground water elevation. Typical practice for most of our smaller bio-retention/rain garden practices is a flat bottom, so conductivity seems the most applicable.

Thanks again

 

[swood19]

Peter,

In response to utilizing the velocity, I also have a question about selection of where I'm allowed to 'allow exfiltration'. The flow rate difference when using the option 'At all elevations' as compared to 'Only above invert' are substantial. In my particular scenario, my seasonal high water table is Elevation 253.50 whereas my invert of my Bio-retention area is 256.00. Per stormwater regulations in my county, I'm allowed to utilize 50% of my measured infiltration rate with a >2' separation between facility invert and seasonal high water. I'm still having difficulty choosing whether to allow exfiltration 'at all elevations' or whether to allow it only above an invert. If I select 'only above invert', would I define that as the groundwater elevation or the invert of my facility?

 

[psmart]

When using exfiltration, the elevation restriction specifies the portion of the pond wall that is permeable. So if you set a range of 101 to 102, only the surface (or wetted) area between 101 and 102 will be utilized for exfiltration, and the remainder of the pond area (above and below the range) will be impervious. The restriction is used to specify which portions of the pond are permeable vs. impermeable.

If you select "all elevations", the area calculations will include the bottom of the pond, which is often a substantial portion of the total area. Whereas if you specify a minimum elevation (above the bottom of the pond) you are immediately excluding the bottom of the pond, so the difference can be substantial.

Note that the exfiltration range must be within the storage range. You cannot set a minimum that is below the bottom of the pond because there is no area there to exclude.

In your case, it sounds like you want to allow exfiltration at all elevations.

For further details please see

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

Peter Smart
HydroCAD Software

http://www.hydrocad.net