SWMM Network (pipe-system layer)

Part of Agentic SWMM — install the project first for the executable toolchain (aiswmm CLI, SWMM solver, MCP servers).

What this skill provides

• A stable JSON schema for SWMM drainage-network structure.
• Two complementary import paths:
  • Raw municipal shapefile path (prepare_storm_inputs → infer_outfall → reorient_pipes → import_city_network → qa) for typical city storm-pipe + manhole layers that arrive as bare LineString shapefiles.
  • Structured asset-DB path (import_city_network directly, or import_network for a fully field-mapped GeoJSON/CSV) when the source already contains explicit from/to nodes, inverts, and diameters.
• A subcatchment-to-network wiring step (assign_subcatchment_outlets) that ensures surface runoff actually enters the pipe network at a real upstream junction rather than dumping straight to the outfall.
• Topology / hydraulic-attribute QA (qa).
• Lightweight introspection (summary).
• Export from network JSON to core SWMM INP sections (export_inp).

When to use this skill

Use when a SWMM model needs a real pipe network. Specifically:

• You have municipal storm-pipe shapefile(s) and want them imported into a SWMM-ready network.json.
• You have a structured CAD/asset-DB export (CSV / GeoJSON with explicit topology) and want the same.
• You need to attach subcatchments to upstream junctions instead of letting them dump to the outfall.
• You need to QA an existing network.json before handing it to swmm-builder.

Do not use this skill when the user only wants subcatchment delineation (use swmm-gis) or only wants to run a finished INP (use swmm-runner).

MCP tools

mcp/swmm-network/server.js exposes nine tools. Pick by what stage of the pipeline you're at.

Raw-shapefile preparation chain

1. prepare_storm_inputs — clip raw \x3Cmunicipal>StormGravityMain.shp (+ optional \x3Cmunicipal>StormManhole.shp) to a basin polygon and emit pipes.geojson, manholes.geojson, and a filled mapping.json from a template.

   • Args: pipesShpPath, manholesShpPath (optional), basinClipGeojsonPath, mappingTemplatePath, outDir, caseName, sourceDescription, diameterPolicy (optional).
   • Use templates/city_mapping_raw_shapefile.template.json as the mapping template.
   • Does not pick the outfall, fix flow direction, or snap drifting endpoints (those are separate tools).

2. snap_pipe_endpoints — cluster nearby pipe endpoints (sub-millimetre to centimetre vertex drift) and snap each cluster to its centroid so adjacent pipes share identical endpoint coordinates. Without this, import_city_network infers separate junctions for drifting endpoints and the network ends up as disconnected fragments. Also drops pipes whose two endpoints collapse into the same cluster (self-loop conduits that SWMM rejects).

   • Args: pipesGeojsonPath, toleranceM, outPath.
   • Reports pipes_in / pipes_out / pipes_dropped_as_self_loops / clusters_merged / max_snap_distance_m.
   • Reasonable starting tolerance: 0.5–3 m for municipal storm pipe layers. Inspect the report before raising further.

3. infer_outfall — pick a single outfall point from pipe endpoints. Two modes:

   • endpoint_nearest_watercourse (default; needs a watercourse GeoJSON).
   • lowest_endpoint (uses min y, no watercourse needed; assumes a projected, north-positive CRS).
   • Args: pipesGeojsonPath, watercourseGeojsonPath (mode-dependent), mode, outPath.
   • Emits a single-Point outfalls.geojson (node_id=OUT1, type=FREE, invert_elev=0.0).

4. reorient_pipes — BFS from outfall vertices to flip LineString direction so it matches flow direction. Real municipal pipes are usually digitised arbitrarily and would otherwise produce bogus from_node/to_node assignments.

   • Args: pipesGeojsonPath, outfallsGeojsonPath, outPath, coordinatePrecision (default 3).
   • Reports pipes_reversed, pipes_unreached so connectivity gaps are visible.

Network assembly

4. import_city_network — main adapter. Takes the prepared pipes+outfalls geojsons (or any structured pipe table) plus a mapping.json and emits network.json with inferred junctions if needed.

   • Args: pipesCsvPath OR pipesGeojsonPath, outfallsCsvPath OR outfallsGeojsonPath, optional junctions, mappingPath, outputPath.
   • For mapping.json: see templates/README.md (raw-shapefile vs structured-export shapes).

5. import_network — older field-mapped import for GeoJSON/CSV when topology and inverts are explicit per row. Prefer import_city_network for new work.

Subcatchment wiring (REQUIRED for the pipe network to actually carry water)

6. assign_subcatchment_outlets — rewrite the outlet column of a subcatchments CSV so each subcatchment drains into a real upstream node (not the literal outfall). Without this step the pipe network sits idle in the SWMM model.
   • Args: subcatchmentsCsvIn, subcatchmentsGeojson, outCsv, mode.
   • Modes:
     • nearest_junction (default; needs networkJsonPath)
     • nearest_catch_basin (needs candidatesGeojsonPath + candidatesIdField)
     • manual_lookup (needs lookupCsvPath with columns subcatchment_id,outlet_node_id)

QA + export

7. qa — run topology + required-attribute checks on a network.json. Args: networkJsonPath. Returns a structured QA report (warnings include no_outfall_path, missing inverts, etc.).

8. export_inp — render a network.json to SWMM INP sections (junctions/outfalls/conduits/xsections/coordinates). Args: networkJsonPath. Used internally by swmm-builder; rarely called directly by an agent.

9. summary — quick counts (junctions, outfalls, conduits, total length, system_layers, dual-system-ready flag). Args: networkJsonPath. For diagnostics.

Recommended orchestration

For a raw municipal shapefile dataset, the canonical chain is:

prepare_storm_inputs       → pipes.geojson + manholes.geojson + mapping.json
snap_pipe_endpoints        → pipes_snapped.geojson  (heal vertex drift; also drops self-loop pipes)
infer_outfall              → outfalls.geojson
reorient_pipes             → pipes_oriented.geojson
import_city_network        → network.json
qa                         → ok / warnings
assign_subcatchment_outlets → subcatchments_routed.csv  (required if subcatchments came from swmm-gis basin_shp_to_subcatchments)
   ↓
hand off to swmm-builder.build_inp

For a structured CAD export with explicit from/to nodes, skip prepare_storm_inputs/infer_outfall/reorient_pipes and call import_city_network directly with the CSVs.

Templates and examples

• templates/city_mapping_raw_shapefile.template.json — fully-specified mapping for raw LineString-only pipe shapefiles. The adapter infers junctions from endpoints. Used by the prepare_storm_inputs chain.
• templates/README.md — decision walkthrough between the two mapping shapes.
• examples/city-dual-system/mapping.json — fully-specified mapping for structured exports with explicit from/to/x/y/invert columns.
• examples/import-mapping.json + examples/import-junctions.geojson etc. — example inputs for the older import_network path.

Scripts (Python implementations behind the MCP tools)

• scripts/prepare_storm_inputs.py — backs prepare_storm_inputs.
• scripts/infer_outfall.py — backs infer_outfall.
• scripts/reorient_pipes.py — backs reorient_pipes.
• scripts/city_network_adapter.py — backs import_city_network.
• scripts/network_import.py — backs import_network.
• scripts/assign_subcatchment_outlets.py — backs assign_subcatchment_outlets.
• scripts/network_qa.py — backs qa.
• scripts/network_to_inp.py — backs export_inp.
• scripts/minimal_stub_network.py — emits a 1-junction + 1-outfall stub network.json from a subcatchment shapefile that carries OUTLET/X/Y attrs. Use only for real-data smoke tests when no pipe-network geometry exists yet; the resulting network must not be treated as a calibrated drainage system.
• scripts/schema/network_model.schema.json — stable schema target.

Conventions

• Prefer explicit, machine-readable JSON in/out.
• Keep node/link IDs unique and stable; the adapter generates J_AUTO_\x3Cx>p\x3Cy> IDs for inferred junctions.
• MVP assumes gravity-network basics first (no pumps/weirs/orifices).
• Dual-system-ready currently means representation and QA metadata, not fully coupled 1D/2D hydraulics.
• All polygon area / distance calculations assume a projected CRS — the tools error early if a geographic CRS is supplied.

Known limitations

• Pipe inverts default to 0.0 m when not provided. A DEM-based invert inference tool is open as BACKLOG.md F12.
• infer_outfall always emits exactly one outfall (OUT1). Multi-outfall networks need a follow-up tool.
• snap_pipe_endpoints only heals vertex drift, not physically missing pipes. If a basin clip cuts out a trunk sewer that connects two sub-graphs, the sub-graphs remain disconnected. A future "buffered basin clip" feature in prepare_storm_inputs would address this.