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iMOD User Manual version 4.4 (html)


12.15HFB Horizontal flow barrier module

Horizontal barriers obstructing flow such as semi- or impermeable fault zone or a sheet pile wall are defined for each model layer by a *.GEN line file. The behaviour of this is twofold:

Note: A Factor \(f\) or Resistance \(r\) of 0.0, means that the fault is completely impermeable.

In iMOD faults can be simulated by entering GEN files in the runfile directly. iMOD will define automatically at which cell faces the permeabilities need to be adjusted based on the specifications of the fault.

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Figure 12.10: Example of a horizontal flow barrier parameterization in case of a uniform model network consisting of model cells of 25 x 25 m. Based on the location of an irregular shaped fault line (white line) the cell faces (thick black lines) are identified where the conductance between the cells is adjusted using the parameter values of the fault line. The computed hydraulic heads (thin black contour lines) illustrate the local effects of the barriers on groundwater flow.

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Figure 12.11: The same example as above, but now for a uniform model network consisting of model cells of 100 x 100 m.

The line *.GEN file defines the location of the barrier. The multiplication factor is used to create the obstruction by reducing the conductance between model cells. The HFB module corresponds with the MODFLOW HFB package.

Whenever GEN files are assigned to layer number 0, iMOD will assign the fault to the appropriate model layers automatically. In that case the GEN file needs to be a 3D GEN (see Section 9.11). Based on the elevation in the 3D GEN file and the TOP and BOT elevations of each model layer, the fault and the nett resistance will be assigned according the following method:

\begin{equation} \begin {array}{r|r|r|r|r|r} \rm {Total} & \rm {Thickness} & \rm {Resistance} & \rm {Weighted} & \rm {Nett} & \rm {Confined} \\ \rm {thickness} & & & \rm {harmonic} & \rm { Resistance} & \rm {Resistance} \\ m & m & d & md^{-1} & c & c \\ \hline & 2.5 & 100 & 0.025 & & \\ & 2.5 & 1,000 & 0.0025 & & \\ \hline 5.0 & 5.0 & & 0.0275 & 181.82 & 181.82 \\ \hline & 2.0 & 100 & 0.02 & & \\ & 2.0 & 1,000 & 0.002 & & \\ \hline 5.0 & 4.0 & & 0.022 & 227.27 & 4.89 \\ \hline \end {array} \end{equation}