iMOD User Manual version 4.4 (html)

12.29LAK Lake Package

The existing Lake package of MODFLOW2005 ((MerrittAndKonikow2000)) has been implemented in iMOD. With this package it is possible to compute lake stages in relation to the groundwater head. Given a bathymetry of the lake, iMOD will assign the lake to the appropriate model layers and assign the correct conductances to the lake. The SFR can be connected to locations where a lake is defined to route water in- and from a lake.

The implementation of the LAK package in iMOD is straightforward as several IDF files (and or constant values) need to be specified. iMOD computes the Lake Identifications and Lake conductances underneath and along the sides of the Lakes. In that same process, iMOD can adjust some of the geohydrologic parameters, such as top- and bottom elevation of model layers as well as permeability values. It is assumed that the position and spatial extent of the lake volume is defined by the specification of a volume of inactive cells within a three dimensional model grid. Because the model grid is used to define the lake volume, the lateral and vertical grid dimensions must be appropriately chosen so that the spatial extent and bathymetry of the lake are defined with the necessary accuracy. In some cases this may require a finer horizontal discretization in the vicinity of the lake and a finer vertical discretization than would be necessary to simulate heads in the aquifer. iMOD will modify the vertical discretization internally, this is explained the following figure.


Figure 12.17: Scheme of the implementation of the LAK package in iMOD.

Suppose a Lake is considered that intersects two model layers, the user enters the bathymetry of the lake (\(L_B\)). Whenever the lake bathymetry is lower than the top of a model layer (\(Z_T\)) and higher than the bottom of that model layer (\(Z_B\)) the bottom of the lake is supposed to be in that particular model layer. The bottom of a model layer contains an existing interbed with thickness \(D_C\) as well. For the Lake package it is essential that the elevation of the model layers describe the bathymetry of the Lake as these are used to compute the table with depth, area, and volume relations. iMOD will adjust the bottom of a model layer to reflect the bathymetry and corrects for the removed aquifer and/or increased thickness of the interbed. In case there is an interbed underneath the Lake, iMOD will increase the permeability of that interbed such that the vertical resistance of that interbed remains identical though its thickness has been increased (\(\underline {D}_C>D_C\)). In case the lake bathymetry intersects the interbed itself, nothing is changed.

In case there is no interbed, the permeability that existed underneath the Lake bathymetry and in the same model layer, will be added to the layer below such that the transmissivity for (\(\underline {D}_Z\))remains identical to the sum of the original two model layers. The permeability of the model layer underneath will be increased or decreased which depends on the permeability fraction between the two model layers. Furthermore, the vertical resistance of the removed aquifer (\(\underline {C}_L\)), is added to the Lake resistance.

The side conductances (\(L_C\)) are the sum of resistances of a) the lake itself and b) the resistance through the subsoil.

There are some build-in limitations and assumptions: