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Hydchar in Horizontal Flow Barrier

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I understand that the Hydchar parameter is ment to adjust the length of the barrier that follows the stepped grid path in the model to match the length of the true (as the crow flys) barrier in real life. But I dont understand why that is important. To me it seems that the only important factor is the ability of the barrier to prevent flow across its length from the starting point to the end point. The path that it takes to get from the starting point to the ending point is generally not that important (within reason).

You can see how the Hydchar parameter begins to look rediculous as you make a smaller and smaller element grid. If your barrier makes a 45 degree angle across your grid, the Hydchar parameter will be the true barrier length times the Cos of 45 degrees. With a smaller and smaller grid, the stepped length of the grid gets closer and closer to the true length, but the Hydchar parameter remains the same "Cos(45)". It dosent seem to me that there should be any adjustment at all, a barrier is a barrier as long as the starting and ending points are correct and you take as straight a path as possible.

Any help understanding this would be appreciated, because I am sure better minds than mine have thought about this.

Update:

The above description of the HydChar parameter is from the U.S. GEOLOGICAL SURVEY, Open-File Report 99-184. I see that in the GMS reference manual they say that is is the hydraulic conductivity divided by the thickness of the barrier. This makes more sense, but does not match what the USGS says that it is. The GMS definition makes physical sense, but the USGS developed the model so you would think that they would know.

Edited by Hydrology

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In the original HFB documentation (http://water.usgs.gov/nrp/gwsoftware/modflow2000/ofr92_477_HFB.pdf) HYDCHR is defined as follows:

is a hydraulic characteristic of the horizontal-flow barrier. If the layer type is 0 or 2, it is the barrier transmissivity divided by the width of the horizontal-flow barrier. If the layer type is 1 or 3, it is the barrier hydraulic conductivity divided by the width of the horizontal-flow barrier.

In MODFLOW 2000 this changed slightly (http://water.usgs.gov/nrp/gwsoftware/modflow2000/ofr00-92.pdf):

The original version of the HFB Package required the input of the hydraulic characteristic either as barrier

transmissivity divided by the width of the horizontal-flow barrier (for layer types 0 and 2 in BCF) or as barrier

hydraulic conductivity divided by the width of the horizontal-flow barrier (for layer types 1 and 3 in BCF). In the

current HFB Package, the hydraulic characteristic is always the barrier hydraulic conductivity divided by the

width of the barrier, regardless of the layer type or flow package (BCF or LPF) used; thus, layer thickness is

always used in calculating the contribution to the conductance terms. The HFB Package uses cell elevations

specified in the discretization file to calculate cell thickness. Cell thickness is head dependent for layer types 1

and 3 in the BCF Package and for convertible layers in the Layer-Property Flow Package.

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I'm not sure to understand correctly the barrier hydraulic characteristic.

I've read from the previous post "in the current HFB Package, the hydraulic characteristic is always the barrier hydraulic conductivity divided by the width of the barrier,...".
What is the "width of the barrier"? Is it the length of the barrier coverage?

In my 100x100 grid model (10 m cell size), the sheet pile is 95 m length and the hydraulic characteristic is 5.00E-10 m s-1.
The barrier coverage hydraulic characteristic is to be set to 5.00E-10/95 m s-1 m-1?

Thank you.

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I build a very simple model to test the hydraulic characteristic value of a barrier.

The grid is 100x100 m, 10x10x1 cells.
Specified head (left) = 450 m.
Specified head (right) = 500 m.
Horizontal K = 1 m d-1.

I compared the head computed in two different scenarios:
1. inserting a barrier across the whole domain (10 cells, 100m , direction north-south)(coverage length 95 m).
2. modifing the property of the cells where the barrier is located (10 cells, 100 m).

To get comparable head levels I set the HK = 0.1 (m d-1) and the barrier hydraulic characteristic to 0.0105 (m d-1 m-1).
The hydraulic characteristic is obtained dividing HK (0.1) by the length of the coverage barrier (95) and multipling for the cell width (10).

Is it correct?


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The figures is doubtless clear, but I don't understand correctly the barrier hydraulic characteristic.

I attached a picture (cross section) of the simulated heads from a simple model and a chart of the head profile in different scenarios:

  • #1. the head is computed without any barrier (color: gray)
  • #2. the head is computed with a barrier. HC is set to 4.3 10-5 m d-1 (color: red). The HC value is obtained from technical data of the sheet pile which is set to be impermeable.
  • #3. the is head computed with a barrier. HC is set to 2.2 10-3 m d-1 (color: green). The HC value is obtained dividing the K (4.3 10-5 m d-1) by the sheet pile thickness (2 cm).

While #2 shows the impermeable behavior of the sheet pile (head is 500 m in the cell adjacent the barrier and 500 m is the right boundary condition), #3 shows a flow across the barrier.

The flow cause a drop in head values (7 m, from 500 to 493 m) and seems to me not impermeable.

post-1028-0-42770200-1389608274_thumb.pn

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It seems to me that neither case is 100% impermeable and it is a question of how much flow is going through the barrier as well as what the heads are. Have you used the flow budget tools to see the change in Q across the barrier?

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I checked the Flow Budget.
In the attached picture I updated the simple model head chart (previous post). I shifted the barrier position by 20 m (now the barrier divides the domain by 40% - 60%).

Both the "Total flow" from "all cells" and "cells across the barrier" Flow budget are the same.

Total flow computed without the barrier (steady state simulation) is: 26389, the same as computed by Darcy.
Total flow computed with the barrier decreases as the HC increases (see table in picture).

Did you refer to that value when you suggest me to take a look at the Flow Budget?

Is the barrier impermeable when the total flow (cells across the barrier) is 0?

Thank you for your support.

post-1028-0-68531900-1389694709_thumb.pn

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If the barrier goes across the entire domain and the HC is 0 then I don't think you will see flow crossing the grid cell faces where the barrier is applied even though there will be a head difference between the cells on either side of the barrier.

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The HC is set to 2.2 E-03 or 4.3 E-05 not to 0.

How can I check the flow budget to see if the sheet pile (HC) is impermeable?

In the table (picture previous post) I checked the total flow from the Flow Budget and compare it to the value without the barrier. Any difference greater than zero means to me there is a flow across the barrier.

Is it correct?

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You are correct, there will be flow across the barrier as long as the HC is not zero (having a barrier where HC really was zero would be very hard to accomplish in practice). An easy way to see the flow across the barrier is to assign zone budget ids to the cells. Assign 2 to the cells on the left side of the barrier and 3 to the cells on the other side. Then use the flow budget dialog to see the exchange in flow between the zones.

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