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Specified flow boundary condition with Modflow-USG

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Hi there,

I'm having a problem related with assigning specified flow boundary conditions to my UGrid / Modflow-USG model. The error appears when I do the '' Map to MODFLOW'' option while converting my conceptual model to UGrid numerical model. I have read the helping document  saying that If you want to use ''Specified flow boundary condition'' the UGrid must be a stacked grid. A stacked grid is one in which there is no vertical discritization of layers and the horizontal discritization of all layers is the same. (i.e. the Index of Vertical Sub-Discretization (IVSD) in the UnStructured Grid Discretization (DISU) package could be set to -1) (See the Figure attached). 

Firstly, I did not understand what this means and how to find the IVSD in the DISU package and set it to -1 to be able to use specified flow boundary condition. In my model there are 2 groundwater divide boundaries which must be zero flow and 2 formation boundaries where the aquifer feeds from neighboring formations with a specified flow value.  If I ll have to change my specified flow bound. cond. with something else or If there is a way to use them, could you please help me to find the solution?

Thank You!

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Hi there,

I am also interested in solving this issue. I have found out so far, that the ugrid has to be a "stacked" ugrid in order for the GMS spec. flow BC to map from the conceptual model. This means, that layer discretisation must not change from layer to layer. This sucks, because GMS Specified flow boundary condition utilizes WEL package and you can easily do the assignment manualy on the grid level even on a grid, that is not strictly "stacked". All you have to do is to ensure, that the cells in the column, where WEL is assigned have the same horizontal dimmensions through the vertical column of cells. Here is a short demonstration.

It is a simple test three layer model computed with MF-USG using quadtree geometry. It utilizes CHD for south BC and GHB for a river runnnig through the domain. This GHB river is in the top layer and it is densely discretized (see the first picture). There is also Recharge in the top layer. There are three pumping wells (WEL package), that withdraw water from the third bottom most layer. These are densely discretized in all three layers.

unstacked_L1.thumb.png.a7c088ed864aaf126da9147722defb25.png

The specified flow BC is in the third (bottom) layer as inflow 100 l/s. It was assigned to the north model boundary using grid aproach by dividing the flow equaly between cells. The cells with this spec. flow BC have all equal vertical column sizes. The point is, that the river running through the domain is discretized only in the top and second layer, not in the third one (see the picture below). This approach can save a lot of computation time as it reduces the number of cells significantly. The solution is the same as when everything is assigned on the stacked grid and the river is densely discretized through all layers.

unstacked_L3.thumb.png.852cd506568470551931accd36a5cfe5.png

The question is if there realy is necessary such a strict limitation on the grid geometry when mapping Spec. flow BC from the conceptual model? By my opinion only the vertical column sizes must have the same dimmensions and the grid doesnt need to be "stacked". The same thing as with specifying wells in GMS should apply to the Spec. flow BC.

 

 

Edited by Michal

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We will investigate the current limitations on the specified flow boundary with non-stacked grids and post what we find here.

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Michal,

It would be possible to change mapping with a layer range to work when the covered column of cells all have the same plan view discretization like you mention. I've added a feature request.

There would have to be the same discretization for the covered cells for the entire layer range. If mapping found mapped segments or areas that aren't identically discretized as a cell column, mapping would stop and give an error. It would also need to require that in a layer range, the top layer's cells completely cover all of the cells of the layers below. It would be complicated to intersect an arc or polygon with the cells in multiple layers and then make sure a portion didn't get mapped more than once.

Thanks,
Bill
 

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Hi Alan and Bill,

thank you for looking into it. Let me post just a few remarks.

Quote

There would have to be the same discretization for the covered cells for the entire layer range. If mapping found mapped segments or areas that aren't identically discretized as a cell column, mapping would stop and give an error.

That looks like exactly the correct behaviour, I think this is how the check is implemented in MODFLOW.

Quote

It would also need to require that in a layer range, the top layer's cells completely cover all of the cells of the layers below. It would be complicated to intersect an arc or polygon with the cells in multiple layers and then make sure a portion didn't get mapped more than once.

I may be wrong on this one. What if GMS just checked, that the layer above the top most spec. flow BC has the complete spatial XY coverage of the model region you mentioned? This would allow users to cover near-surface aquifers, that does not span across entire model with apropriate top layer in the multiaquifer system and still be able to apply spec. flow BC for the deeper aquifers/layers.

Edited by Michal

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