Pre 2014 Blogs

Groundwater Calculations in SWMM

Hello Robert !

I am trying to incorporate groundwater in my model. Right now it is showing high negative groundwater error. Is the groundwater error calculated same as the continuity error ?

The Percent Error = 100.0 * (1.0 - totalOutflow / totalInflow)

What are the components and corresponding equations for inflow and outflow for groundwater ?I think the outflow is calculated as

QGW = A1(HGW - H*)B1 - A2(HSW - H*)B2 + A3(HGW HSW)

But how is the inflow calculated ? There are quite a few parameters used in aquifer data editor and groundwater data editor ( i.e. Conductivity slope, tension slope etc) I want to learn them in details. Can you suggest me any documentation for them?

I am following some of the regional modeling attempts for parameters who used XP-SWMM. And they used a lot of parameters like

' Fraction of evapo-transpiration assigned to upper zone, Maximum depth of significant
lower zone storage, Curve fitting parameter, Initial upper zone moisture , Coefficient for unquantified losses
Average slope of the tension versus soil moisture curve. '

It would be very helpful if I could  use this data in my SWMM model. Are they  denoted differently in SWMM? Please help.



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  • Thank you for the files. I appreciate your help.

  • Note: There are three sub flow components in the calculation of the groundwater flow from a SWMM 5 Subcatchment.
    1st Component: Flow = Groundwater Flow Coef. * (LowerDepth – Aquifer Bottom to Node Invert) ^ Groundwater Flow Exponent
    2 nd Component: Flow = SurfaceWater Flow Coef. * (Aquifer Bottom to Water Surface – Aquifer Bottom to Node Invert) ^ SurfaceWater Flow Exponent
    3rd Component: Flow = SurfaceWater-Groundwater Flow Coef. * (Aquifer LowerDepth * Aquifer Bottom to Node Invert)
    The total flow is the sum of all three components.

  • Subject:  SWMM 5 Fixed Surface Water Depth Boundary Condition

    A large difference between SWMM 5 and SWMM 4 is how the Groundwater Aquifer interacts with the drainage network.  In SWMM 4 since the hydrology was simulated in the Runoff Block, the results saved to an interface file and the hydraulics were simulated in the Extran Block it was not possible to have a time step to time step interaction between the Aquifer and the Open Channels.  SWMM 5 has integrated hydrology and hydraulics so it is possible to use either a Fixed Surface Water Depth for each Subcatchment or the Receiving Nodes Node Depth Invert Elevation – the Aquifer Bottom Elevation.  The groundwater thus flows either to a fixed boundary condition as in SWMM 4 or to a time varying boundary condition.

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