HYDROCAD - Mounding for Infiltration Basins

[TheEnginerd]

Hello,
We usually do not get involved with mounding analysis for our infiltration basin designs - unless required by the reviewing engineer.
Our NJ state requirement for infiltration basins is to have a 2 foot separation to groundwater, and to "halve" the percolation rates of the soil for the design.
Our geotechnical engineer has followed the simple USGS mounding analysis spreadsheet available here

http://pubs.usgs.gov/sir/2010/5102/)and

tells me that the mound of water is 9 feet in the center of the basin and that the basin will overtop and it fails.
My question is, doesn't the HYDROCAD program take into account the impact of the underlying groundwater on the basin's ability to infiltrate? The HYDROCAD program asks me for the infiltration rate of the soil, the separation to seasonal high GW, and when I input the 100 year storm. Usually when the program tells me that the water reaches a certain level during the 100 year storm, that's as far as we go.
Does this mean that we've been designing basins without looking at the other half of the puzzle for all of these years, or is the geotechnical engineer completely wrong?
I feel that the two programs are calculating two different things, but I don't know enough to discuss intelligently with the geotech engineer.

 

[psmart]

HydroCAD provides an exfiltration option based on Darcy's law and the Saturated Hydraulic Conductivity. This procedure uses a fixed groundwater elevation as an input parameter. HydroCAD does not perform a GW mounding analysis. For details please see

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

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[fattdad]

I'm a big fan of the mounding analyses! You have to figure if there is a 2-ft separation to the water table, after about 6-in of infiltration the entire, "unsaturated" interval of soil above the water table will be saturated. That's only if it started out dry!

If you continue to hope for infiltration after this 6 inches has perc'd into the soil, then water has to get out of the way! That requires horizontal flow and a horizontal flow gradient - i.e., mound.

These infiltration facilities have all be conceptualized by drainfield designers with very little thought on ground water hydrology.

It's time to get it right!

f-d

ípapß gordo ainÆt no madre flaca!

 

[psmart]

Like H&H in general, there is a wide range of practices in the use of infiltration. Although infiltration can be beneficial for groundwater recharge, and for helping to reproduce the behavior of an undeveloped site, predicting the exact infiltration behavior is doubtful at best. When you factor in the probable loss of infiltration capability over time, you understand why many stormwater regulations do not allow the use of infiltration for peak-flow management. Using infiltration for gradual draw-down is another matter, because this can occur over days or even weeks. But for infiltration to have any significant effect on peak flow management, you have to be infiltrating a significant portion of your runoff in just a few hours - or less. In most sites this simply doesn't happen with any reliability or predictability.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[gbam]

I have a question regarding this water mounding:
Current testing uses a perc test where a pit is dug and filled with water. I have always thought that this approach over estimated infiltration rate. I prefer the double ring test that is used to derive the Infiltration curve. I presume the double ring includes influence from mounding that is typically derived from a double ring infiltration test. What do you thing, does the double ring account for mounding?

 

[fattdad]

The double ring has no relevance to mounding. By going to the double ring, you are being more precise on vertical hydraulic conductivity. When a mound develops the actual response is more governed by horizontal conductivity.

Fortunately, kh is usually 2 to 4 times greater than kv.

f-d

ípapß gordo ainÆt no madre flaca!

 

[fattdad]

I guess I should add that while the horizontal conductivity is typically 2 to 4 times greater than the vertical conductivity, the flow gradient vertically in unsaturated soils (i.e., under gravity) is 1.0 and the flow gradient under horizontal flow it likely to be much less than 0.05 (i.e., 5 percent gradient on the energy grade line of the mound).

Water just can't get out of the way fast enough to offset the vertical recharge - hence the mound. . .

f-d

ípapß gordo ainÆt no madre flaca!