ground water flow into detention pond 1

[cimmeron]

I am working on a project where the normal water level of a detetnion pond is lower than the existing water table by about three feet. I need to compute the amount of water that will flow into the pond from the ground water. Any ideas?

 

[gbam]

Yes,
You can assume that the initial pond elevation is equal to the ground water elevation. This will reduce your storage volume by that amount. I do not beleive that you need to calculate the inflow because once the basin reaches equilibrium with the ground water you will have a pond.

I must qualify my comment by saying that I assume the ground water elevation is steady.

Research Aquifer levels in your region to determine how steady the levels are.

 

[bltseattle]

It will depend on the soils, check with the project geotech.

Depending on your circumstances, rather than trying to quantify the groundwater component, common practice is to either construct a cutoff "trench/wall" around the perimieter to prevent seepage into the pond or an underdrain/perimeter collection system to bypass groundwater around the flow control structure for the pond.

 

[sam74]

So the outlet on your detention pond is lower than the existing water table and you plan on having constant outflow? Are you looking to size your outlet to maintain a certain water level during times of no rainfall events? I'm just trying to get more information as this would be an unusual scenario in my locale but Darcy's Law is used to estimate groundwater flow in saturated soils.

 

[TerryScan]

I would be inclined to dig a test pit in the area in question and monitoring the inflow to come up with an in situ flow rate.

Also, have you considered side slope stablility? There is a likelihood of the inflow "blowing out" the slopes wherever it decides to concentrate.

 

[cvg]

A cutoff wall as recommended previously by Bltseattle will eliminate "blowing out" and will also prevent inflow/outflow through the sides of the basin if constructed properly

 

[LHA]

To use a "cutoff wall" as blt and cvg suggest, you will need a monolithic bottom and walls; a vertical wall around the sides alone will not keep groundwater out.

Forget the test pit, if you know where GW elev is, that will be the constant WS elevation in the basin; unless you have an underdrain and/or outlet. I agreee with Sam74, do Darcy analysis to get inflow rate, then size the outlet/u-drain for that.

Realize that, should a u-drain alone not suffice, you will have a hard time detaining smaller storms, because your outlet orifice will likely be too big. I would just count the GW elev as the 0 storage volume, and put your first outlet control orifice at that elevation. That way you will have a pond, though, so check with the reviewer first to verify this is acceptable.

 

[TerryScan]

I assumed that cimmeron is setting normal water with an outfall structure. (how else does one set normal water in a stormwater pond?)

While a vertical wall will not keep water out, it should keep it from flowing in the sides.

I suggested a test pit because it seemed just as easy to do as it would be to get soil tests to get an accurate coefficient of permeability for Darcy.

All that said, if it is at all possible, I aggree that the best solution is to not fight the groundwater and set pool at that elevation.

 

[Beantown89]

I work in the North East U.S. and designed a detention basin were the contractor blasted about 10 feet into exising ledge to construct the basin. A perimeter drain was installed to intercept ground water flow into the basin. The perimeter drain worked a little too well! The Grounder water flow is constant year round from the 6" pipe.

Erosion problem occured at the outlet because the rip-rap was improperly installed at the perimeter drain outlet.

My adivise is perimeter drains need construction monitoring after installation.

Groundwater Modeling programs exist, but I believe that's a little out of scope for an engineering service.

 

[queque]

To determine the amount of groundwater inflow, lower the outlet for a few weeks to allow the pond to reach steady state. The amounf of outflow from the pond will then equal the groundwater inflow, assuming part of the groundwater inflow to the pond does not bypass the outlet.

 

[Focht3]

Depending on the soil profile, a cutoff wall may be a very good solution. What does your geotechnical engineer say?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"