Determining ground water elevations for dry ponds
Determining ground water elevations for dry ponds
(OP)
I'm a civil so I'm trying to figure out what the process is for the geotech establishing the shwt. I frequently get conflicting elevations for the shwt from the geotech and the environmental consultant. Currently I have a difference of 5 feet (it is a closed basin however). Just trying to learn what the thought process is and why there would be a difference. Thanks





RE: Determining ground water elevations for dry ponds
The geotechnical engineer may also be looking at the water table in a boring that extends 15 or 20 ft into the ground and into underlying water table conditions. A static water table may be present at 10 ft (or so) in some sand layer and a separate perched water condition may develop at some higher depth during the wet season.
An environmental consultant (or a soil scientist) will look at other factors. The presence of hydric soils (low chroma), the presence of oxidized rhysosphers (sp) or in some cases the population of hydrophytic vegetation. If you have a ground surface covered by facultative vegetation (or wetter), it's reasonable to assume that the high water table during the year is within 12 inches of the ground surface, irrespective of the well data (albiet it may be a perched condition).
The problem with dry pond design is that the typical design standard is to set the bottom grade 4 or 5 ft above the seasonal high water table. But, after you complete all your civil improvements you will ultimatly focus ALOT of infiltration in that very location. This in turn should (will) lead to a mound on the water table, which could ultimatly reduce the headroom between the design base grade and the pre-construction water table elevation. This is never addressed by design and where I believe is where the body is buried.
f-d
¡papá gordo ain't no madre flaca!
RE: Determining ground water elevations for dry ponds
also is the geotech giving the data from the exploration or are they giving you some estimation for design? a geotech must exercise much caution if we provide an estimation of where we think groundwater might be (cause we don't have a clue where the sucker will be in the future!)...mother nature is nasty, unpredictable and will always throw a curve ball. groundwater seems like a simple enough topic but i think it's much more complex and influential to what we do than many realize.
RE: Determining ground water elevations for dry ponds
One more thing- some folks whip out the NRCS soil book and recite the groundwater depth estimate for the site soil type verbatim. Don't trust that.
If you need very good data, consider installing piezometers and reading them at least through the rainy season
RE: Determining ground water elevations for dry ponds