SWMM Junctions and Conduits: Building the Pipe Network

After defining subcatchments that generate runoff, you need a conveyance system to transport that water. In SWMM, the pipe network consists of nodes (where water collects or changes direction) and links (that carry water between nodes). This tutorial focuses on junctions and conduits - the most common node and link types.

The terminology and property behavior here follow the EPA SWMM 5.2 User’s Manual. A SWMM model does not establish code compliance; confirm pipe sizes, slopes, cover, and allowable hydraulic grade against the criteria governing the project.

Understanding Nodes and Links

Node Types in SWMM

Link Types in SWMM

This tutorial focuses on junctions and conduits, which form the backbone of most drainage models.

Creating Junctions

Junctions represent points in your network where:

• Pipes connect
• Subcatchments discharge
• Flow changes direction
• Manholes or inlets are located

Adding a Junction

1. Click the Junction tool in the toolbar
2. Click on the map where you want the junction
3. The junction appears with an auto-generated ID

Key Junction Properties

Invert Elevation

The elevation of the bottom of the junction.

Setting Invert Elevation:

• Use survey data when available
• Estimate from topography for planning studies
• Maintain consistent datum (e.g., NAVD88, local datum)

Max Depth

The maximum depth of water at the junction (from invert to rim).

• For manholes: Typically 4-15 feet
• For inlets: Typically 1-4 feet
• Water above maximum depth overflows the node unless additional surcharge depth delays overflow

Initial Depth

Starting water depth at the beginning of simulation.

• Usually set to 0 for dry-start simulations
• Set > 0 for continuing a previous storm or wet conditions

Surcharge Depth

Additional depth above max depth before flooding begins.

• Models pressurized conditions in manholes
• Typically 0 for open systems

Ponded Area

Surface area available for ponding when the junction floods.

• Use a non-zero surface area only when the Allow Ponding analysis option is enabled and overflow should be stored on the surface and later returned to the system
• With zero ponded area, overflow is normally lost from the model; zero does not make water back up indefinitely inside the system

Creating Conduits

Conduits convey water between nodes. They can represent:

• Storm sewer pipes
• Culverts
• Open channels
• Ditches and swales

Adding a Conduit

1. Click the Conduit tool in the toolbar
2. Click on the upstream (inlet) node
3. Click on the downstream (outlet) node
4. The conduit appears connecting both nodes

Conduit Geometry Properties

Length

The physical length of the conduit in feet or meters.

• SWMM can calculate length from map coordinates if Auto-Length is enabled
• Always verify calculated lengths against design drawings

Roughness (Manning’s n)

The Manning’s roughness coefficient controls flow resistance.

Use our Manning’s Calculator to determine flow capacity

Inlet Offset

The vertical distance from the junction invert to the conduit invert at the inlet end.

• Offset = 0: Conduit invert matches junction invert
• Offset > 0: Conduit enters higher than junction invert

Outlet Offset

The vertical distance from the junction invert to the conduit invert at the outlet end.

Conduit Cross-Section

SWMM supports many conduit shapes. Set the shape in the Cross-Section property group.

Circular Pipes

The most common shape for storm sewers.

Rectangular Channels

Box culverts and channels.

Other Common Shapes

Irregular (Natural Channels)

For natural streams and channels with surveyed cross-sections:

1. Create a Transect (Project > Add Transect)
2. Enter station-elevation data
3. Reference the transect name in the conduit

If geometry originated in another model, verify the station/elevation convention, bank locations, roughness segmentation, units, and coordinate system before reproducing it as a SWMM transect.

Flow Direction and Slope

How SWMM Determines Flow Direction

Flow direction depends on:

1. Node heads and boundary conditions: These drive the computed hydraulic gradient
2. Conduit geometry and elevations: These establish the physical link and invert profile
3. Routing method: Dynamic Wave can represent backwater, surcharge, and reverse flow

Calculating Pipe Slope

In DEPTH link-offset mode, calculate the conduit-end elevations from each node invert and its corresponding conduit offset:

Here d_in and d_out are the conduit inlet and outlet offsets above their connected node inverts. If the project’s Link Offsets option is ELEVATION, the entered inlet and outlet values are absolute conduit-end elevations instead; do not add the node inverts a second time.

Example:

• Upstream node invert: 100.00 ft; inlet offset: 0.20 ft, so z_in = 100.20 ft
• Downstream node invert: 99.50 ft; outlet offset: 0.10 ft, so z_out = 99.60 ft
• Length: 200 ft
• Slope: (100.20 - 99.60) / 200 = 0.0030 = 0.30%

Design Slopes

SWMM accepts the geometry you enter; it does not choose a code-compliant minimum slope. Establish slopes from surveyed elevations and governing design criteria, then review velocity, depth, surcharge, and hydraulic grade over the required events. Dynamic Wave routing can represent reverse flow, so an adverse conduit is not automatically a software error, but it must be intentional and hydraulically justified.

Building a Pipe Network

Step-by-Step Process

1. Start at the outfall: Place your system outlet first
2. Work upstream: Add junctions moving toward subcatchments
3. Connect with conduits: Draw conduits from upstream to downstream
4. Set elevations: Enter surveyed or design inverts and confirm that any adverse reach is intentional
5. Verify connectivity: Check that all nodes are connected

Network Connectivity Rules

• Every conduit must connect two nodes
• Confirm that nodes and links form the intended connected network
• Confirm that modeled inflows have an intended boundary, storage, or discharge path
• Investigate isolated or unintended objects reported by review checks

Checking Connectivity

1. Run a simulation - SWMM reports disconnected nodes
2. Use Report > Status to see connectivity errors
3. Visual inspection - trace paths from subcatchments to outfall

Working with Multiple Pipes at Junctions

Pipes Entering a Junction

Multiple pipes can enter a single junction:

• Main trunk line from upstream
• Side branches from lateral areas
• Use inlet offsets to set relative elevations

Pipes Leaving a Junction

Normally one pipe leaves downstream, but:

• Dividers can split flow between outlets
• Multiple parallel conduits model redundant systems

Outfalls

Networks intended to discharge beyond the modeled domain need an outfall boundary. Closed or storage-only conceptual models require a different, explicitly documented mass-balance setup.

Creating an Outfall

1. Click the Outfall tool
2. Click on the map at the discharge location
3. Set properties

Outfall Types

Conduit Initial Conditions

Initial Flow

Starting flow in the conduit.

• Usually 0 for dry conditions
• Set > 0 for hot-start simulations

Maximum Flow

Optional constraint on conduit flow.

• Use zero for no flow restriction
• Enter a non-zero value only when an explicit maximum-flow constraint is intended

Validating Your Network

Before running simulations, check:

Elevation Consistency

1. Select each conduit in the Project Browser or on the map and inspect its Property Editor values
2. Review inverts, offsets, length, and the resulting slope against the source geometry
3. After a run, inspect warnings in Report > Status and the relevant link summary results
4. Investigate unexpected adverse, steep, or nearly flat reaches

Cross-Section Reasonableness

1. Verify pipe sizes match design intent
2. Confirm size transitions match the intended system
3. Ensure roughness values are appropriate

Common Errors

Practical Tips

Using a Backdrop Image

Import an aerial photo or CAD drawing:

1. Go to View > Backdrop > Load
2. Set the image bounds (coordinates)
3. Trace pipes and junctions over the image

Group Editing

SWMM 5.2 does not provide a spreadsheet-style object editor, property paste, or general Excel round trip. For a property that should have one value across a mapped group:

1. Use Edit > Select Region to outline the objects
2. Choose Edit > Group Edit
3. Select the object type, property, operator, and value
4. Review the affected objects after applying the change

For advanced data preparation, the User’s Manual documents the plain-text project-file format in Appendix D. Generate or edit that format only with validation, backups, and a subsequent SWMM input review; File > Export is not a generic object-to-Excel command.

Next Steps

With subcatchments generating runoff and a pipe network to convey it, you are ready to:

1. Run your model: Running Simulations
2. Add storage: Learn about detention ponds and tanks
3. Model pumps: Add pump stations to your network

Summary

Junctions and conduits form the conveyance system in SWMM:

• Junctions are connection points defined by invert elevation and depth
• Conduits are pipes or channels defined by shape, size, length, and roughness
• Flow direction follows the computed head gradient and can reverse under Dynamic Wave routing
• Outfalls define external discharge boundaries for models that release flow beyond the modeled domain

Proper network definition provides an intended path from each inflow to storage, a closed-system endpoint, or an external outfall boundary. Closed or storage-only models may have no outfall, but their retained volume, initial conditions, and mass balance must be explicitly reviewed.