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Assigning initial conditions for confined and unconfined conditions to Femwater


ALD
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Hi

Femwater requires initial conditions in a steady state run to be assigned as pressure heads. There is an option in GMS6.5 to generate the initial pressure heads using total heads.

However in a multiple aquifer model with a confined aquifer, one can have two different total heads at the same XY location resulting from a nest of piezometers at the same location, one being a water level in the confined lower aquifer and another being the water table level in the upper unconfined aquifer.

This situation therefore implies that the total head dataset should have a z value in it. For the lower confined aquifer that z value should be in the vicinity of the bores screens, and in the femwater model in the vicinity of the nodes within the material of interest over which the real bore is screened. For the upper aquifer the total head dataset should have a z value in it that is in the vicinity of nodes in the upper unconfined material.

What experience have others had with modelling this and what formats have they chosen for files as input into GMS. I think its tricky trying to get the z value elevation right to match the relevant material node and am not sure GMS can even handle this, although when i used it years ago I think it did.

Regards

ALD

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There is no tool in GMS to do exactly what you want, but here's a possible workaround. There is an interpolation option to "Assign default extrapolation value to hidden objects". You can use that in combination with hidden mesh elements and the data calculator to create the type of data set you want. What you would do is hide the elements pertaining to the upper aquifer, then interpolate from scatter points to the mesh using the option mentioned above and a default extrapolation value of something like -999 or some number much less than you have in your data. Then you repeat the process but this time hide the lower aquifer elements and interpolate to the upper aquifer. Then you use the data calculator and create a new data set that is the max of the two data sets you just created which will get rid of the -999 values. You can then export that data set and use it for your initial condition.

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  • 2 weeks later...

HI Michael

Thanks for that tip, let me try that.

Regards

There is no tool in GMS to do exactly what you want, but here's a possible workaround. There is an interpolation option to "Assign default extrapolation value to hidden objects". You can use that in combination with hidden mesh elements and the data calculator to create the type of data set you want. What you would do is hide the elements pertaining to the upper aquifer, then interpolate from scatter points to the mesh using the option mentioned above and a default extrapolation value of something like -999 or some number much less than you have in your data. Then you repeat the process but this time hide the lower aquifer elements and interpolate to the upper aquifer. Then you use the data calculator and create a new data set that is the max of the two data sets you just created which will get rid of the -999 values. You can then export that data set and use it for your initial condition.

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