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Barrier


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I've build a 4 layers model (Modflow).

Boundary condition:

  • specified flow (in)
  • specified head (out, sea level, violet square)
  • no flow

A barrier (brown line) dividing the dock of the industrial port from the inland is to be modelled.

I set the Hyd. Cahr. to 1 10-11 m/sec for all layers and for the model domain length.

When I run modflow the pathlines pass throught the barrier (red lines with arrows ending in green circle).

Why?

Thank you

post-1028-0-36777900-1373985785_thumb.pn

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I would check the HFB package to make sure it is getting mapped correctly by selecting MODFLOW | Optional Packages | HFB - Horizontal Flow Barrier, and making sure the cells listed for the barrier make sense.

Another thing you could check is the flow budget for some of the barrier cells. If you select one of the barrier cells, right click on it, and select Flow Budget. Do the flows through the faces make sense? The front face is the face as you increase in i value, the right face is as you increase in j.

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Thank you for answering.

I've attached a new image summary the Flow Budget for two cells.

I'm not sure to understand front/right side of a cell and the flow budget.

From the flow budget for cell 1:

flow in is from the left face (from the specified head next cell, that's ok)

flow out is from the right face (where I've set the barrier!)

flow dicrepancy is -14% (too much)

From the flow budget for cell 2:

flow in is from the left face (from the barrier!)

flow out is from the right face (main) and front face (less)

flow dicrepancy is -0%

I check MODFLOW | Optional Packages | HFB - Horizontal Flow Barrier and edit the data in spreadsheet. The barrier is correctly mapped.

I look to simulation.out file in the modflow folder and I get the following "...barriers not defined by parameters" .

What does it mean?

HFB -- HORIZONTAL-FLOW BARRIER PACKAGE, VERSION 7, 5/2/2005.

INPUT READ FROM UNIT 707

0 PARAMETERS DEFINE A MAXIMUM OF 0 HORIZONTAL FLOW BARRIERS

654 HORIZONTAL FLOW BARRIERS NOT DEFINED BY PARAMETERS

0 HFB parameters

654 BARRIERS NOT DEFINED BY PARAMETERS

BARRIER LAYER IROW1 ICOL1 IROW2 ICOL2 HYDCHR

-------------------------------------------------------

1 1 40 14 40 15 1.0000E-11

post-1028-0-65602500-1374056113_thumb.pn

Edited by GMS User
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The flow between the two cells would be the 0.070 values you have highlighted. This would mean the path lines make sense given the output from the MODFLOW model, since that's the direction of the flow. I'm not sure the model is modeling what you want. What are the head values for cells across the barrier, and do they make sense? Is there a path for the flow to take around the barrier?

The message for no HFB parameters is nothing to worry about and means you aren't using any parameters for this model.

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Thank you for answering.

>The flow between the two cells would be the 0.070 values you have highlighted. This would mean the path >lines make sense given the output from the MODFLOW model, since that's the direction of the flow.

That's ok.

> I'm not sure the model is modeling what you want. What are the head values for cells across the barrier, and do they make sense?

I've attached a new image showing:

  • layer 1 (convertible), the top most layer (head in cells across the barrier is different: 0.1 where there is the specified head boundary and 0.5 inside the model area)
  • layer4 (confined), the bottom layer (head in cells across the barrier is quite the same: 0.13 where there is the specified head boundary and 0.16 inside the model area)

In the cross section the water level is dropping crossing the barrier (good, head loss) but I would expect a more raising before the barrier. The high head levels depends on recharge.

The material HK is between 1.0e-006 and 0.0001 m/sec while the barrier Hyd cond is 1e-11 m/sec.

Does it make sense?

>Is there a path for the flow to take around the barrier?

I've checked how the coverage describing the barrier is converted to modflow.

In the circle you can see the barrier edge. I think they are correct. But there is something wrong about the model which I don't understand.

post-1028-0-54496900-1374163699_thumb.pn

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It's kind of hard for me to see in the image, but it looks like the barrier goes from one boundary of the model to the other. To make the flow not go through the barrier it needs a path to be able to go around it like shown in the attached model screenshots. Since the top and bottom of the model are no flow boundaries, for the model on the left the flow has no where to go but through the barrier, although slowly. The model on the right allows the flow to go around the bottom since the barrier doesn't reach to the model boundary.

post-957-0-97029100-1374183457_thumb.png

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I replicate your model but I invert the specified head values (I set 450 left boundary, 500 right boundary).

I look at the cross section a it's as one would expect to be.

I change the (aquifer HK) / (barrier hydr cond) ratio from 1e+4 to 1e+2 and I get again what I expect.

I understand what you write me about the pathlines and the way the are drawn. In the pathlines report, the last tracking time (crossing the barrier) is greater the others: is it concerning the "slowly crossing barrier"?

Probably in my model the fault is that the specified head in layer 1-2 generates some flows entering layer 3-4 resulting in lowering head values before the barrier.

I suggest to myself to build a simpler model to perform flow analysis and flow budget consideration ;-).

Thank you

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