DDM HydroLogic

From DEM to model-ready catchments in QGIS.

v2.0 now features:

1. Retro compatibility with QGIS 3.22 LTR (tested on v3.22.16)
2. Faster sub catchment processing and selection

INTRO

Setting up a hydrologic model usually means a few hours of GIS prep work before the modelling even starts. DDM HydroLogic condenses some of the initial steps into one interactive session: it traces D8 flow paths from a DEM, ranks them by Strahler order, and lets you pick the relevant drainage. Draw an outlet line, set a minimum subcatchment size, and DDM HydroLogic cuts the catchment into dissolved subcatchment polygons — then you have choices of exporting as:

• GeoPackage of flow paths and subcatchments,
• RORB .catg GE-ready catchment file,
• WBNM .wbn runfile,
• XP-RAFTS .xpx exchange file,
• or some of the most popular TUFLOW xxx_R.shp.

It is important to note that all hydrological/hydraulic choices, such as rainfall, losses, Manning's coefficients, subcatchment types, etc.. have been deliberately left blank or default values.

Current version: 2.0 · QGIS 3.22 LTR and 4.x

Workflow

1. Select a DEM lodaded in your instance. NOTE: A 5m DEM is a good sweet spot.
2. Choose the display flow-path accumulation threshold. The default is 10,000 (the lower the value the more effort & time).
3. Create a mask polygon (recommended) to confine the processing. Left-click to digitise, right-click to finish.
4. Press Compute to build temporary Strahler-ordered flow paths. If the mask is outside the DEM, the plugin shows a caution prompt. If NoData cells are detected in the active analysis domain, the areas are flagged as problematic.
5. Click flow paths to highlight upstream contributing subcatchment. CTRL + left-click to de-select an area.
6. (Optional) Draw an outlet line -> Recommended for hydrological modelling exports.
7. Enter a minimum subcatchment size in m2. (Default = 100,000 m²)
8. Press Process subcatchments. If successful, the plugin reports temporal layers with spatial representation of subcatchments and strahler-ordered flow lines.

Outputs

• Export flow paths and subcatchments to GeoPackage writes same-order Strahler reaches and subcatchment polygons. The default filename is DDM_HydroLogic_outputs_version_.gpkg.
• Export to RORB (.catg) writes a first-pass RORBwin/RORB GE .catg file using a self-contained connected node-link writer. The drawn outlet line is used to create the explicit RORB outlet node where available, and the export validates that every node drains to that outlet before writing (tested in RORB v6.52). DDM HydroLogic automatically loads temporary RORB nodes and RORB links.
• Export to WBNM 2025 (.wbn) writes a first-pass WBNM runfile (see notes below).
• Export to XP-RAFTS (.xpx) writes a first-pass XP-RAFTS exchange file (see notes below).
• Export TUFLOW files (.shp) writes TUFLOW regions shp into a chosen folder. The final catchment will be included the scaffolding of the following: 2d_code, 2d_loc, 2d_rf, 2d_po, 2d_mat, 2d_qnl and 2d_soil.

WBNM 2025 export notes

The WBNM exporter writes a scaffold runfile, not a finished model. It fills in only what the GIS can supply: the subarea topology, area in hectares, the catchment/outlet coordinates and EPSG, and which subareas need a natural stream segment. The file is laid out to the WBNM2023 runfile structure — exactly eight preamble lines, two blank lines between blocks, 12-character fixed fields, and the downstream subarea name in column 62 of each topology row — so it opens cleanly in WBNM and its GUI tools.

Rainfall is written as a single placeholder storm of zero depth, and the local/outlet structure blocks are empty. Open the runfile in WBNM, replace the rainfall, losses, imperviousness and structures with real values, and run WBNMCHCK/WBNMSORT before relying on any results.

XP-RAFTS export notes

DDM HydroLogic writes one RAFTS node per subcatchment (named, with easting/northing from the QGIS geometry), one link per drainage connection, and the sub-area area in hectares. Each node carries the five RAFTS sub-area slots, with slot 0 holding the real sub-area and slots 1–4 as inert placeholders. Manning's coeff, sub-area slope, channel routing, losses and storms are written as defaults — review them in XP-RAFTS before running. No design storms are selected, so the imported model has geometry only and waits for rainfall.

TUFLOW export notes

DDM HydroLogic dissolves all processed subcatchments into one topologically valid catchment polygon and writes it into each shapefile retaining the same CRS of the source DEM:

The filenames use the s1_s2_e1_e2_e3_EXG_001 scenario/event placeholder name. Field names, types, widths and precisions follow the TUFLOW 2026.0.0 data formats.

Scripting maintenance notes

• The RORB, WBNM, XP-RAFTS and TUFLOW exporters live in their own modules; the main just calls them.
• Startup and runtime failures report whether a Python module or a plugin file is missing, so hopefully it's clear what to install.
• WBNM export reads the engine's flow-accumulation values whether they are stored in a dict or a NumPy array.