ground water flow into detention pond
ground water flow into detention pond
(OP)
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?





RE: ground water flow into detention pond
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.
RE: ground water flow into detention pond
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.
RE: ground water flow into detention pond
RE: ground water flow into detention pond
Also, have you considered side slope stablility? There is a likelihood of the inflow "blowing out" the slopes wherever it decides to concentrate.
RE: ground water flow into detention pond
RE: ground water flow into detention pond
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.
RE: ground water flow into detention 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.
RE: ground water flow into detention pond
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.
RE: ground water flow into detention pond
RE: ground water flow into detention pond
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?"