Flood 2D (1D-2D surface)

Build a 2D surface mesh over the ground from a DEM, couple it to the manholes, run the sewer and the surface together on SWMM, and map flood depth and hazard.

Tools > Flood 2D... opens the Flood 2D (1D-2D surface) window. It covers a study area with a mesh of 2D cells built from a DEM, joins each manhole to the cell above it with a bottom orifice, and runs the open project and the surface together on the SWMM engine. Water that overflows a manhole spreads over the ground instead of being lost, and the window maps the maximum depth and the flood hazard.

The 2D surface is modelled the way PCSWMM does it in SWMM: each cell is a junction at ground level and each shared cell edge a short, wide open rectangular conduit that carries sheet flow between cells. The run does not change the project. Add 2D mesh to project... adds the mesh to the model, as one edit, if you want to keep it.

What you need

• A DEM as an ESRI ASCII grid (.asc), in the same projected coordinates and length unit as the model (metres or feet), not longitude and latitude. Load it with the DEM (.asc grid) file box.
• A study area. Current map view uses the extent of the whole network (not the part of the map you have zoomed to), Drawn polygon uses the outline of the subcatchment selected on the map, and Whole DEM extent uses the DEM's rectangle. Select a subcatchment before opening the window to use its outline; a tip at the top reminds you.

Set up the mesh

Run it

1. Optionally tick Rain directly on the grid (no losses) and set the intensity (50 mm/h to start) and duration (1 h). This adds rain straight onto the cells, with no infiltration, on top of whatever the model's own subcatchments do.
2. Set the Routing step (s), 1 s to start. 2D meshes need short steps; Al Amin et al. (2024) used 0.5 s for 10 m hexagons.
3. Press Run 1D-2D. A progress bar shows the run; Cancel stops it. The run uses the project's own simulation period and reporting step, with dynamic wave routing.
4. When it finishes, the window says how many cells ran and how many manholes were linked; manholes outside the mesh are left unconnected and counted.

Read the flood map

• Colour the preview by Maximum depth (bands 0.05 to 0.15 m, 0.15 to 0.30 m, 0.30 to 0.60 m, 0.60 to 1.20 m, over 1.20 m) or by Hazard class (Australian, AIDR 7-3).
• Two tables give the cells and hectares in each depth band and each hazard class, and the total flooded area.
• Maximum depth comes from SWMM's Node Depth Summary, which sees every routing step; velocity and hazard are read at the reporting periods.
• PNG saves the picture, GeoJSON the cells with their results for GIS, and CSV one row per cell.
• View > Raster Layers... can export the last 2D run's maximum depth as a GeoTIFF (Flood max depth surface, m (last 2D run)).

Hazard classes follow AIDR Guideline 7-3 (Flood hazard), Table 2: combined hazard curves (Australian classification), combining depth, velocity and their product.