For each layer in the domain, you should include a single instance of the following instruction:

## New layer...End

Causes grok to begin reading a group of new layer instructions until it encounters an End instruction.

By default, the top elevation of the layer will be set to zero by assigning all nodes in the lowermost 2-D slice to have a z-coordinate value of zero. You can change the layer top elevation using the instructions described in Section 5.3.5.4.

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The following instructions are all optional:

## Layer name

1. layer_name Layer name.

Changes the layer name, which defaults to Layer n, where n is the current layer number.

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## Minimum layer thickness

This instruction causes grok to enforce a minimum thickness constraint for the current layer. At nodes where the computed layer top elevation is less than or equal to the current base elevation, z_added will be added to the current base elevation to get the top elevation.

If this constraint is not enforced, grok will stop and issue a warning message if the computed top elevation is less than or equal to the current base elevation.

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By default, a new layer will not be subdivided vertically unless one of the following two instructions is issued:

## Uniform sublayering

1. nsublayer Number of sublayers.

This instruction divides the layer vertically into nsublayer elements, which will each have the same height, equal to the top elevation minus the current base elevation divided by nsublayer.

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## Proportional sublayering

1. nsublayer Number of proportional sublayers.
2. sub_thick(i),i=1,nsublayer Proportional thicknesses in order from top to bottom.

This instruction can be used if you want to refine the mesh vertically, for example in the subsurface near the surface flow domain or a fracture.

It is important to understand that the variable sub_thick is not a true thickness, but is instead a relative thickness, which is used along with the layer thickness to determine the element heights in the current column.

For example, these instructions would subdivide the current layer vertically into three elements, between the current base and top elevation, with element height proportions of .1, 1 and 10 from top to bottom:

```Proportional sublayering
3
0.1
1.0
10.0
end
```

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## Offset top

1. value Thickness value (L) by which to offset the layer top elevation.

This instruction causes the elevation of the top layer to be offset vertically by the given value. This can be used to create a surface a given distance below another surface.

For example, these instructions create a layer with a top elevation 1 metre below the elevation defined in the raster file gs.asc:

```new layer
Uniform sublayering
10

elevation from raster file
gs.asc

offset top
-1.0

end

new layer

Uniform sublayering
3

elevation from raster file
gs.asc

end
```

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