The iMOD Segment package defines the surface water system with an ISG-file which contains all relevant information used by surface water elements which are in direct relation with groundwater. The ISG-file stores:
• the actual outline of the surface water element;
• discharge/width/depth relation ships.
To store all these different types of information the ISG-file format consists of associated files that are connected by the ISG-file. The ISG package as is, is not available in MODFLOW, but it generates the input for the conventional RIV package. For the SFR package (see section Section 12.28), this type of an ISG file is expanded with more data, see section Section 9.9 for a detailed description of both types of ISG files.
The ISG package file format is based on vectors and time series and therefore has a much more efficient disk use than the RIV package. iMOD and iMODFLOW both, can handle those ISG files to generate model input. Within ISG Edit (see section Section 6.10.3) it is possible to compute IDF files from the ISG file for the different model parameters such as conductance, stage, bottom heights and infiltration resistances. Or, more efficient, it is possible to use the ISG directly in the RUN- and/or PRJ file and let iMOD/iMODFLOW grid the ISG file internally to a conventional/modified RIV-file, see section Section 10.11.
The way iMOD/iMODFLOW grids the vector based ISG file onto the simulation raster is as follows.
• Each segment, containing at least two nodes, is treated separately from the other segments and intersected with the model network;
• All intersected model cells are given the linear interpolated values for stage \(s\), bottom heights \(b\), infiltration factor \(f\) and resistances \(c\) in between all existing calculation nodes along the segment;
• For each location along the stream the appropriate cross-section is assigned. If a single cross-section is specified, that cross-section is valid for the entire stream. If more cross-sections are specified, the application of each cross-section is the stretch along the stream between the location of that cross-section up to the next specified cross-section - along the direction of the FROM- and TO-node. As a exception, the first specified cross-section is applied as well for the stretch between the FROM-node up to the first specified cross-section;
• The conductance is a function of the interpolated stage \(s\) and bottom height \(b\). The water depth \(d\) (difference between the stage and bottom height; \(d=s-b\)) is used to compute the wetted perimeter \(wp\) at each location along the segment with the appropriate cross-section. The conductance (m\(^2\)/d) is the product of the wetted perimeter \(wp\) times the length of the intersection in the particular model cell \(l\), divided by the resistance, so \(cond=\frac {wp \times l}{c}\). The infiltration factor \(f\) is used as a package entry and corrects the conductance iteratively whenever the stage is higher than the computed groundwater level \(h\), in that case the conductance becomes \(cond=\frac {wp \times l}{f\times c}\). The conductance has a lower values for short intersections than for longer ones, this is clearly seen in the following figure.
The cross-sections in a ISG can yield different appearances of the conductance and therefore the outline of the segment in the model network. The following configurations might occur:
• The cross-section is a 2D cross-section perpendicular to the stream and describes the bathymetry as a function of distance \(x\) and height \(z\). The distance \(x\) is at all times less than the width of current location in the model network;
• The cross-section is a 2D cross-section perpendicular to the stream and describes the bathymetry as a function of distance \(x\) and height \(z\). The distance \(x\) can be more than 1.5 times the width of current location in the model network. In this particular case, the influence of the segment will be distributed over more than the single intersected model cell. A ”brush“ (circle with a radius equal to the half the width at the current location along the segment) is used to compute the fraction in which each model cell is influenced by the segment. The fraction is computed as the number of points in a grid cell that occupied by a circle, a grid cell which is completely covered by a circle is given a fraction of 1.0, other location with less covering receive a fraction \(<1.0\), see the following figure.
• The cross-section is a 3D cross-section which describes the bathymetry on a local and regular \(x,y,z\) raster. The local regular raster is projected in the (ir)regular model network. The conductance is the sum of the overlying areas of the local regular bathymetry raster divided by the entered resistance. Whenever an indicator \(i\) is present in the ISG file - to denote inundation which depend on a reference height \(h_{\rm ref}\) - the final resistance is multiplied with the value of the indicator \(i\). This is only done whenever the stage \(s\) is higher than the river bathymetry \(b\) at that particular location.
Each of the above mentioned configuration yield a different assignment of conductances to the underlying model network, as shown in the following figure.
The ISG file format also makes it more easy to convert a surface water model data from SOBEK into iMOD using the SOBEK import tool.