QUESTION / ERROR
FEFLOW imports map files with polygons and polylines (e.g. shapefiles) delineating lakes or subdomains in a larger model area. As soon as these polygons are converted to supermesh items, FEFLOW throws the error message 'Polygon #2 dropped since overlapping with polygon #1' - see Fig. 1. Was does it mean and how can it be resolved?
Fig. 1 - Error message: 'Polygon #2 dropped since overlapping with polygon #1!'
SOLUTION
The error indicates that some of the polygons are partly or entirely located within another polygon (like an enclave). In FEFLOW all polygons need to be singular in terms of the area they cover. If several polygons cover the same area, they need to be cut from each other.
Two workflows are recommended to fix the issue:
Workflow #1: Preprocessing in GIS
Before converting polygons to supermesh items in FEFLOW, check and modify the geometries in your original shapefile in ArcGIS/QGIS to make sure there is no overlapping patterns.
Workflow #2: Redraw and split polygons directly in FEFLOW
Instead of converting shapefiles to supermesh items automatically, FEFLOW allows to draw supermesh geometries manually. Simply display the shapefile maps as a background in FEFLOW double-clicking the 'Default' text field under the map title in the Maps Panel. Make sure there are no overlaps by tracing every polygon and using the "Add Polygon" and "Split Polygon" tools. Avoid any polygon located completely within another polygon. Instead, split the polygons starting from the outer boundary and connecting the inner polygon to the outer one.
Fig. 3 shows an example of overlapping polygons without connection to each other.
Fig. 3 - Overlapping polygon
Fig. 4 shows an example of how to split the outer polygon - starting from upper left corner following the inner polygon and ending in lower left corner of the outer polygon.
Fig. 4 - The correct polygon
FURTHER INFORMATION & USEFUL LINKS
Manuals and Guidelines
FEFLOW 10.0 Documentation - The 2D Supermesh
FEFLOW 10.0 Documentation - The 2D Supermesh Design
Related Articles
Knowledge Base Article - Building 3D Meshes
Knowledge Base Article - Error message 'A self-intersection was detected' when Meshing with TetGen