Conformal Gmsh Meshing#

Architecture note: three concepts cooperate to produce a 2D mesh that conforms to geological zones and the hydrographic network.

  1. Hydrographic-network extraction from the DEM (Whitebox backend).

  2. In-memory river trace exposed to the mesher.

  3. Conformal Gmsh generation against the interfaces.

For the data structures shared by every mesh type, see Mesh Architecture Pivot.

1. Hydrographic-network extraction#

Module: hydromodpy/spatial/geographic/core/river_network.py.

Trigger: geographic.river_network.enabled = true. The geographic pipeline then produces a generated network that feeds the canonical HydrographicNetwork(role="generated") concept. Files written under results_stable/geographic/:

  • river_streams.tif: raster of stream cells.

  • river_streams_pruned.tif: same, after optional pruning.

  • river_stream_order_strahler.tif: Strahler order.

  • river_stream_link_id.tif: stream-link identifier.

  • river_network.shp: vector version clipped to the catchment (legacy filename kept).

  • river_network_summary.json: reproducibility metrics (legacy filename kept).

Contracts to keep distinct:

  • HydrographicNetwork: shared concept across storage, display, and comparison.

  • RiverNetworkProducts: technical bundle produced by the geographic preprocessing.

  • RiverMeshTrace: reduced projection consumed by the mesher.

Required inputs:

  • DEM (corrected via fill or breach).

  • Outlet, snapped on accumulation.

  • D8 direction and accumulation rasters.

Backend: WhiteboxWorkflowsBackend (hydromodpy/spatial/delineation/whitebox_workflows_backend.py). Other backends must register explicitly through register_backend().

TOML parameters:

[geographic.river_network]
enabled = true
threshold_mode = "area_km2"      # or "cells"
threshold_area_km2 = 0.5
prune_short_streams = false
min_stream_length_m = 0.0
compute_strahler_order = true
compute_stream_links = true

Internal conversion: threshold_cells = threshold_area_km2 * 1e6 / dem_res_m**2.

2. In-memory river trace#

Class: RiverMeshTrace (hydromodpy/spatial/geographic/core/river_mesh_trace.py).

Exposed through GeographicDerivedFeatures.rivers.river_mesh_trace or passed as an explicit river_trace to the meshing case. The trace is already reprojected into the domain CRS and clipped to the catchment. No disk re-read happens during meshing unless rivers.source = "file" is selected.

Fields:

  • source_kind: geographic_generated, hydrography_loaded, file.

  • crs_wkt: target CRS.

  • lines: tuple of Shapely LineString.

  • segment_count and total_length_m: control metrics.

Structuring rule: the hydrographic network is not a domain zone. Mesh generation stays in zone_meshing, the domain business logic stays in spatial/domain/, and the central concept shared across layers remains HydrographicNetwork.

3. Conformal Gmsh generation#

Module: hydromodpy/spatial/mesh/gmsh_grid/zone_meshing/.

Main entry point: conformal.py. Pipeline:

  1. Load the zone polygons and the domain.

  2. Geometric cleaning (_geometry_cleaning.py, _polygon_cleaning.py): make_valid, simplification, snapping, sliver removal.

  3. Non-overlapping planar partition (_partition_builder.py, _partition_split.py), with overlap resolution through a priority field.

  4. Translation to Gmsh OCC geometry (_gmsh_occ.py): points, curves, loops, surfaces.

  5. Size fields (_gmsh_fields.py): global, refinement around interfaces, around small zones, around rivers.

  6. .msh export (_gmsh_export.py) and metadata sidecar (_summary_sidecar.py).

Physical groups created:

  • zone::<zone_key> on surfaces.

  • interface::<zone_a>::<zone_b> on internal lines.

  • boundary::<name> on external contours.

Available modes:

  • river_only: hydrographic-network conformity, no lithology constraint.

  • river_plus_lithology: simultaneous conformity to lithology interfaces and rivers.

4. TOML mesh contract#

[mesh]
enabled = true
backend = "gmsh"
mode = "river_conformal"          # or "river_lithology_conformal"

[mesh.domain]
kind = "vector"                    # "bbox" | "polygon" | "vector"
path = "..."
id_field = "domain_id"
selected_id = "main"

[mesh.river.source]
origin = "geographic_generated"    # "hydrography_loaded" | "file"
path = null                        # required if origin="file"

[mesh.gmsh]
algorithm = "delaunay"
global_size = 250.0
min_size = 80.0
max_size = 500.0

[mesh.gmsh.river_refinement]
enabled = true
river_size = 60.0
river_distance = 400.0
river_sampling = 96

[mesh.runtime]
use_in_memory_river_trace = true
persist_mesh_inputs = false

[mesh.lithology]
enabled = false
source_mode = "geology_data_manager"

5. Reading and projection#

The produced mesh is read through GmshPlanarMesh2D.from_file(...). Field and FieldParam projection stays in the existing modules (Field.on_mesh, FieldParam.to_mesh_field). On a conformal mesh the number of mixed cells drops sharply; the projection logic remains unchanged so it can also handle non-strictly-conformal supports.

6. Quality criteria#

Output *_summary.json with:

  • River conformity: fraction of river length coincident with mesh edges.

  • Refinement gradient: median size in a near-river buffer compared to the size outside.

  • Domain coverage: |mesh_area - domain_area| under tolerance.

  • Numerical stability: total cell count within the expected range.

  • Lithology conformity, if enabled: drop in mixed-cell count.

  • Reproducibility: stable signature across identical runs.

7. Validation and tests#

Non-regression tests:

  • tests/unit/backends/test_whitebox_workflows_backend.py for extract_streams, raster_streams_to_vector, strahler_stream_order, stream_link_identifier, remove_short_streams.

  • tests/unit/geographic/test_river_network_*.py for the core layer and the signature golden.

  • tests/regression/extensive/ for end-to-end cases.

Deterministic helper: tests/support/whitebox.py::configure_whitebox_single_thread to stabilise CI runs.

8. Structuring decisions#

  • The hydrographic network is produced by geographic, exposed by DomainGeographicContext, and consumed by zone_meshing.

  • Domain does not carry meshing logic; it remains the business support for parameters (FieldParam).

  • The two paths (read an existing mesh, generate a conformal mesh) converge on GmshPlanarMesh2D plus a metadata sidecar.

  • Triangles are the target of the first iteration; quadrilateral recombination will follow if needed.

See also#

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