How to approximate steady state water level of proposed pond on slope

Question

How to approximate steady state water level of proposed pond on slope

eng003 (Structural)

(OP)

27 Feb 13 20:40

I am trying to evaluate what the steady state water level will an irregular pond (but could be assumed circular) on a slope will stabilize at. I am assuming the soil is homogeneous sandy soil with a constant k everywhere. Groundwater is generally 3 ft below grade and the slope of the existing grade and assumed groundwater hydraulic gradient is approximately 0.007 ft/ft. There is no discharge planned, seepage from the uphill side and infiltration on the downhill side will be the only thing contributing to inflow/outflow. I have tried to come up with some rational argument/estimation using Darcy's basic equation Q=kia. In doing this I assume the half the perimeter of the pond is allowing inflow and the other half infiltrating out. I then set both inflow and outflow rate equal so kia=kia. Assuming k is constant you are left with ia=ia. I then assume that a = (half the perimeter of pond for both inflow outflow)x(existing groundwater level - bottom of pond). The existing hydraulic gradient is assumed equal at both inflow and outflow sides at i=0.007 ft/ft but will be increased or decreased by: (the vertical distance from the the pond water level above or below the existing groundwater level) divided by (the zone of influence of the mounding or drawdown). Assuming the mounding and drawdown are similar functions of the head differential ....the problem just reduces to finding the water level will be the exact average of the inflow and outflow side.

-Can someone please help me if my logic is sound?
-I know there are many equations for calculating the drawdown and mounding curves but the two generally end up similarly logarithmic in nature correct?
-I have made many simplifying assumptions but with setting all the variables that I have to constants and assuming symmetry would it theoretically be just the average?
-Apart from a finite element type groundwater software is there rational calculative method I could i.e. spreadsheet...to get a good estimate.
-If I need a software program...any recommendations?...

Thanks

beej67 (Civil/Environmental) · Answer 1 · 2013-03-06 20:09:41

Sounds like a sound argument for a perfectly equal pond .. just pick the groundwater level at the midpoint. However, I caution you that you're neglecting some other pretty important factors in your analysis, such as runoff into the pond, and seasonal changes in water table elevation.

Where's the project? If in Florida, for instance, state/district codes specify that you have an outfall set at the seasonal high water table, as measured in-situ via geotechnical investigation, or other historical control, and that dictates the water elevation of your pond.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

fattdad (Geotechnical) · Answer 2 · 2013-03-07 12:48:38

Surface water inflow (as mentioned by beej67) needs to be considered. Also on an annual basis if you receive 40/yr of rainfall and 35 inches of evaporation, that's an additional 5 inches of inflow.

So, if I understand correctly, there is no current pond. You will make some embankment to develop a rim and the water level in the pond will be uniform (duh). Won't that water table even on the 'uphill' side be higher than the current water table? If so, how would there be any inflow?

I'm not sure it's correct to assume isotropic homogeneous conditions. It's simple, yes, but it's also likely not accurate.

Are these fluvial soils, residual soils, man-placed fills, etc.? Typically the horizontal permeabilty is 4 to 10 times greater than the vertical permeabililty. If I was reviewing this project, the first question I'd ask is how do you justify a Kh/Kv value of 1? It goes against reason.

f-d

¡papá gordo ain’t no madre flaca!

gbam (Civil/Environmental) · Answer 3 · 2013-03-07 16:53:15

Sounds like you may want to consuct a water balance to evaluate the net volume. This can be done on whichever time basis makes sense; monthly, weekly, or ? My thought is the average level will be dynamic based on different times of year/season.

psmart (Civil) · Answer 4 · 2013-03-08 10:37:54

Even if you knew all the parameters with sufficient accuracy, the WSE will be subject to seasonal variations. Factor in short-term rainfall and the level will vary even more. It's like our local kettle holes with no defined inlet or outlet: The WSE varies by at least 5-10 feet. If your goal is to achieve a relatively stable WSE, you will probably need some kind of outlet control.

Peter Smart
HydroCAD Software
www.hydrocad.net

emmgjld (Geotechnical) · Answer 5 · 2013-03-08 11:35:54

You are over-simplifying an extremely complicated problem.
Step back -- and realize you need to do some serious definition of the problem for the modeling.

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How to approximate steady state water level of proposed pond on slope