Building a network

Start a project, draw rain gages, subcatchments, junctions, outfalls, storage units, conduits, pumps and flow regulators, and set their properties.

Start a project in the right units

Choose the units before you draw anything, because every length, depth and flow you type is read in the project's units.

Command	Flow units	Lengths and map
File > New (SI units), the New toolbar button, or New project (SI) in the Start window	LPS (litres per second)	Metres; areas in hectares
File > New (US units)	CFS (cubic feet per second)	Feet; areas in acres

Both start with Horton infiltration, dynamic wave routing, depth offsets, a one day simulation from 01/01/2024, a 15 minute reporting step and a 20 second routing step. Change any of these in Project > Simulation Options..., described in Running a simulation.

Draw objects on the map

Pick a tool on the toolbar (or press Add under the Browser's list for that category), then click the map. A tool stays active, so you can place several objects in a row; click Select Object when you are done. Each new object gets the next free ID: J1, J2 for junctions, C1, C2 for conduits, S1 for subcatchments, RG1 for rain gages, and so on. Nodes share one set of IDs and links another, as in SWMM.

A conduit being drawn on the map. A solid line runs from the start node to a vertex, then a dashed line follows the mouse towards an outfall symbol. A hint at the bottom reads Click the end node, or empty map to add a vertex. Right-click or Esc cancels. — Drawing a conduit: click the start node, click the map to add bends, then click the end node.

Object	How to draw it
Junction, outfall, flow divider, storage unit, rain gage	One click where it goes.
Conduit, pump, orifice, weir, outlet	Click the upstream node, then the downstream node. For a conduit, clicks on empty map in between add bend points. Right-click or `Esc` cancels a link you have started.
Subcatchment	Click each corner of its outline. Double-click, press `Enter`, or click the first corner again to close it. `Esc` cancels.
Map label	Click where it goes, type the text, press `Enter`.

A hint at the bottom of the map says what the next click does while a link or outline is being drawn.

New objects take SWMM's default values, not values measured from the map. A new conduit is 400 long with a 1 diameter circular section and Manning n 0.01, in the project's length unit. A new subcatchment has an area of 5, a width of 500, 25 % impervious and 0.5 % slope, whatever outline you drew, and no outlet. A new junction has an invert of 0 and a maximum depth of 0. Type the real values in the Property Editor before you run.

Which object to use

Object	Use it for
Rain gage	The rainfall that falls on the subcatchments assigned to it. See Rainfall and design storms.
Subcatchment	A piece of land that turns rain into runoff and sends it to one outlet node (or another subcatchment).
Junction	A manhole, or any point where drains meet. Its invert is the lowest point; its maximum depth is invert to rim.
Outfall	The end of the system: a river, a nallah, an STP inlet. An outfall may have only one link.
Flow divider	A node that sends part of its inflow down a chosen link (CUTOFF, OVERFLOW, TABULAR or WEIR).
Storage unit	A wet well, pond or tank, with a shape that gives its surface area at each depth.
Conduit	A pipe or open drain.
Pump	A pump lifting flow from its inlet node to its outlet node, on a pump curve.
Orifice, weir, outlet	Flow regulators: an opening in a wall or floor, an overflow crest, or a rating curve between depth (or head) and flow.

Set properties

Click an object with Select Object and type its values in the Property Editor. The rows follow the SWMM 5.2 manual. The ones engineers change most:

The Property Editor for Conduit S-T03, from inlet node T03 to outlet node T04: shape CIRCULAR, maximum depth 0.2 m, 1 barrel, length 55 m, roughness 0.011, inlet offset 0, outlet offset 0.1668 m, initial and maximum flow 0 LPS, entry, exit and average loss coefficients 0, seepage loss rate 0, flap gate NO and a blank culvert code. Starred fields are required. — A conduit's properties. Starred fields must be filled in.

Object	Key properties
Junction	Invert El., Max. Depth (invert to rim), Initial Depth, Surcharge Depth, Ponded Area.
Outfall	Invert El., Tide Gate, Type: FREE, NORMAL, FIXED (with Fixed Stage), TIDAL (with a tidal curve) or TIMESERIES.
Storage unit	Max. Depth, Evap. Factor, Seepage with suction head, conductivity and initial deficit, and Storage Shape: FUNCTIONAL, TABULAR (a storage curve), CYLINDRICAL, CONICAL, PARABOLOID or PYRAMIDAL.
Conduit	Shape and its size, Barrels, Length, Roughness (Manning n), Inlet Offset and Outlet Offset, entry, exit and average loss coefficients, Flap Gate.
Pump	Pump Curve (`*` means an ideal pump with no curve), Initial Status ON or OFF, Startup Depth and Shutoff Depth at the inlet node.
Orifice	Type SIDE or BOTTOM, Shape CIRCULAR or RECT_CLOSED, Height and Width, Inlet Offset, Discharge Coeff., Flap Gate, Time to Open/Close.
Weir	Type TRANSVERSE, SIDEFLOW, ROADWAY, V-NOTCH or TRAPEZOIDAL, Height, Length, crest Inlet Offset, Discharge Coeff., end contractions, Can Surcharge.
Outlet	Inlet Offset, Flap Gate, Rating Curve: FUNCTIONAL/DEPTH or FUNCTIONAL/HEAD (coefficient and exponent) or TABULAR/DEPTH or TABULAR/HEAD (a rating curve).
Subcatchment	Rain Gage, Outlet, Area, Width, % Slope, % Imperv, Manning n and depression storage for the impervious and pervious parts, and the infiltration parameters for the method chosen.

New weirs start with SWMM's discharge coefficient of 3.33, which is in US units. In an SI project check it before you run.

Cross sections

A conduit's Shape list holds SWMM's shapes: CIRCULAR, FORCE_MAIN, FILLED_CIRCULAR, RECT_CLOSED, RECT_OPEN, TRAPEZOIDAL, TRIANGULAR, the ellipse, arch, egg and horseshoe families, CUSTOM (from a shape curve), IRREGULAR (from a transect) and STREET. Max. Depth is always the full height: the internal diameter of a circular pipe, the depth of an open drain. The rows under it change with the shape, for example Top Width for RECT_OPEN and Base Width, Left Slope and Right Slope for TRAPEZOIDAL.

Offsets are read the way Link Offsets in the simulation options says: as a height above the node invert (DEPTH, the default) or as an elevation (ELEVATION). The status bar shows which.

Inflows

The Inflows row of a node shows what it receives from the .inp file: direct inflows, dry weather flow and RDII. The row is read only: DrainPilot has no editor for inflows. To add or change dry weather flow or a direct inflow, edit the [DWF] or [INFLOWS] section of the .inp file in a text editor and open the file again.

Move, rename and delete

Drag a node, rain gage or label with Select Object to move it, or type its X-Coordinate and Y-Coordinate.
Rename an object by typing a new Name. Links, subcatchments and other objects that refer to it follow the new name; control rules are text and are not rewritten. An ID cannot contain spaces, quotes or semicolons.
Delete the selected object with Del, the Delete toolbar button or Edit > Delete. Deleting a node also deletes the links joined to it and clears it as the outlet of any subcatchment. There is no confirmation: use Edit > Undo (Ctrl+Z) to bring it back.

Title, notes and control rules

Project > Title/Notes... edits the project title (the first line) and notes. Project > Control Rules... edits SWMM control rules as text, in SWMM 5.2 syntax (RULE, IF, THEN, ELSE, PRIORITY). SWMM checks the rules when the simulation runs, so a mistake shows up as a run error.