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Modeling ponding, infiltration

Posted: Thu Apr 11, 2019 9:15 am
by jmb3uw
Hello all,
I am trying to model the de-watering of a coal ash pond, followed by a brief period of time it is left open to the atmosphere. The total scenario runs for 92 days with 5cm of ponded water removed weekly beginning Day 1.

Here is a brief overview of the inputs for my model:
Soil Hydraulic Model: van Genuchten - mualem
Soil Hydraulic Params: 1 Material, alpha= 0.036, n=1.64, l=-1
Time Variable Conditions: Precipitation, evaporation inputs, hCritA: 1000; this was calculated to be higher, however results are not sensitive to changes above this
Initial Condition: Fully saturated w/ h=0, and the top row of nodes h=-1 for stability purposes
Boundary Conditions:
- Top BC: Variable Head 1: 30 cm w/ 5cm decreasing every 7 days, then 0 until day 92
- Sides: No flux BC
- Bottom: Free drainage/unit gradient BC; groundwater is assumed to be far below

My concern is when I check the "Boundary Flux" results, there is nothing displayed for the Actual Atmospheric Flux, Surface Runoff, Evaporation or Infiltration. Is there another way to implement this scenario?

My goal is to take the final pressure head values from this "pre-closure" scenario as input into models with constant flux top BCs representing different closure scenarios.
TVAKSP6.zip
(571.8 KiB) Downloaded 33 times

Re: Modeling ponding, infiltration

Posted: Thu Apr 11, 2019 4:28 pm
by Jirka
The definition of this problem makes little physical sense to me.

Why would you expect to have any atmospheric fluxes, when you have no atmospheric boundary conditions specified anywhere on the domain boundary?

Also, your initial conditions make little physical sense. If "groundwater is far below" the transport domain, how can you have zero initial pressure heads everywhere in the domain?

J.

Re: Modeling ponding, infiltration

Posted: Thu Apr 11, 2019 6:53 pm
by jmb3uw
Hi Jirka,
Thanks for your quick response.
The initial condition is fully saturated to represent sluiced coal ash in a disposal pond, hence the zero initial pressure head.

As for the atmospheric flux, in the "Boundary conditions Options", I used the option to "Switch the boundary condition from Time-Variable Pressure Head to: Atmospheric BC when the specified nodal pressure head is negative".

Re: Modeling ponding, infiltration

Posted: Thu Apr 11, 2019 7:33 pm
by Jirka
I think you misunderstand this option. We have these two options, which are related to time-variable head/flux BCs and atmospheric BCs:
1. "Switch the boundary condition from Time-Variable Pressure head to" "Atmospheric BC when the specified nodal pressure head is negative". This is meant for conditions, such as an the boundary with fluctuating water level (e.g., furrow, stream, river). The Pressure Head BC is applied on the boundary with positive values of the pressure head (below the water level) and atmospheric BC is applied on the boundary with negative values of the pressure head (i.e., above the water level). Note that you have not specified any negative values.
2. Treat the Time-Variable Flux boundary condition as the atmospheric BC. This is for the time-variable flux (not pressure head) BC.

J.