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# MODFLOW 6 - LACP

September 8, 2022

MODFLOW 6 - LACP uses the MODFLOW 6.4.0 source code and modifies the Horizontal Flow Barrier (HFB) package to allow simulation of vertical hydraulic conductance (CV) by connection rather than by cell, an option available for previous versions of MODFLOW (USG).

The MODFLOW 6 - LACP HFB package is used to simulate thin, vertical, and low-permeability geologic features (barriers) and acts to supplement the Node Property Flow (NPF) Package by adjusting the horizontal hydraulic conductance computed by the internal flow package. The HFB requires as input the "hydraulic characteristic" for a barrier, which is defined as the hydraulic conductivity divided by the length of the barrier.

MODFLOW 6 calculates the CV as a function of the sum of half the cell thickness (t) divided by the cross sectional area (A) multiplied by vertical conductivity (VK) for both the top and bottom (bot) cell.

1/CV = t_top / (2 * A * VK_top) + t_bot / (2 * A * VK_bot)

In this calculation, the smaller of the two VK values will dominate the equation when there is an order of magnitude or greater difference between the two values. Therefore, problems arise when dealing with simulations of 4 or more layers where an order of magnitude lower VK is present in a middle layer. For example, an intermediate layer represents a fining-upward unit with a VK set to 1e-6 to account for the confining layer at the top of the unit and the connection between the overlying layer. If VK is set by cell, then the 1e-6 VK in the intermediate layer makes it impossible to have a vertically averaged VK of 100 between it and an underlying layer because vertically averaging 1e-6 with any number, however large, will at most produce a CV of 2e-6, which is not appropriate for a coarse-grained connection.

To address this problem, we have modified the HFB package to simulate VK by connection, like a low-permeability "vertical" geologic feature (barrier). This solution eliminates the uneven weighting of orders of magnitude lower VK values by simulating VK values by cell connection rather than by layer.

Publication Year 2022 MODFLOW 6 - LACP 10.5066/P92DJ8EF Daniel P Culling, Scott Paulinski Software Release USGS Digital Object Identifier Catalog Sacramento Projects Office (USGS California Water Science Center)