The following Junction types are available in InfoDrainage, each type is processed differently during the Analysis.


A manhole is represented by a Storage node within the SWMM engine, with its dimensions set by the user.

Note that unlike in standard SWMM5, the Storage nodes are allowed to flood.

Volume passing through the inlet(s) is added to any volume already in the manhole. This generates a new depth in the node, with any excess water being allowed to flood into a 1000 m² plan area.

Note: Any water that floods out of the manhole can re-enter later once the water level has dropped back down.

The depth in the node is then used to determine the discharge through each of the outlets specified.


Where a Manhole is acting as an outfall with no outlets specified, the analysis will ignore the node dimensions as it is seen as free discharging (except where there is a surcharged level)

Simple Junctions

A Simple Junction is represented by a Junction node within the SWMM engine.

No dimensions can be set for this type of node. The invert level is derived from the connections incoming or leaving this node and the maximum depth is taken as the top of the highest connection. A Simple Junction can become pressurized.

Volume entering the simple junction is contained within a 1 m² plan area, which is used to generate the depths and therefore flows through the outlets. No flooding can occur at this locations due to their nature, but a depth is reported to expose how outlet flows are calculated.

Note the flow can come into a Junction either as a flow coming out of an incoming connection (positive discharge), or as flow coming out of an outgoing connection in the case of reverse flow (negative discharge). Reverse flow is driven by a greater hydraulic head at another Junction further downstream in the network.


Connections move water between Junctions (see above) or Stormwater Controls.

The following Connection Types are available in InfoDrainage, each type is processed differently during the Analysis.

Physical Connections

These are connections that have physical properties, i.e. pipes, box culverts, custom section types and channels.

Discharge in the Physical Connections is calculated from the water levels at each end, and from the physical properties (cross-section, friction, slope).

Note that InfoDrainage uses the 'Dynamic Wave' formulation within SWMM5, rather than the 'Kinematic Wave' formulation that is less sophisticated.

The engines solves the one-dimensional (1D) shallow water equations (gradually varied, unsteady flow), also called the Saint-Venant equations, to calculate the discharge in each physical connection.

Therefore the engine is able to represent different flow regimes (fast/slow flow, i.e. super-critical/sub-critical flow), backwater effects, flow reversal, flow on adverse slope, storage inside the connection and pressurized flow. The engine can also represent branched and looped networks, as well as bifurcations.

The Saint-Venant equations are a system of 2 non-linear equations that express the conservation of mass and momentum. They account for the following processes:

  • acceleration
  • gravity
  • pressure
  • friction

The friction term can be calculated using either the Manning equation or the Colebrook-White equation.


More detailed information can be found in Chapters 2 and 3 of the EPA SWMM Reference Manual Volume II — Hydraulics, including the actual form of the equations and the solution algorithm.

Notional Connections

These are non-physical connections that allow items to be connected without the need to specify the physical properties of the connection.

Notional Connections do not introduce a restriction to the flow. Reverse flow is not allowed through the Notional Connections.

Lagged Flow

With the Lagged Flow type, the routing through the connection is done as a simple translation (with no attenuation). The time of travel through the connection, i.e. the time it takes for water to flow through the connection, is calculated from the connection length and the flow velocity estimated by the user. The output hydrograph is equal to the input hydrograph with a delay equal to the time of travel. If the water level in the downstream structure is higher than the water level in the upstream structure, then there is no flow through the Lagged Flow connection.

Lagged Flow connections are not part of SWMM5 and have been developed specifically for use in InfoDrainage.

No Delay

In the case of the No Delay connection, the output hydrograph is equal to the input hydrograph without any delay, i.e. the flow is transferred instantly from the upstream end to the downstream end. If the water level in the downstream structure is higher than the water level in the upstream structure, then there is no flow through the No Delay connection.


Outlets are treated as a particular type of connection within the engine. The 1D shallow water equations do not apply for Outlets, however they are processed at the same time as the Physical Connections by the engine, using the head difference between the upstream side and the downstream side of the outlet.

Note that by selecting the Free Discharge outlet in a Junction or SWC, no actual outlet is used in the Analysis, i.e. the outgoing connection is directly connected to the Junction or SWC.

In the case of an outlet followed by a connection, although not visible in InfoDrainage, a Junction node is actually present to connect the end of the outlet and the start of the connection. The Water Level in this node is used to calculate the flow in the connection, indeed this level is closely linked to the Water Level in the Junction or SWC.


The SWMM5 engine is very sensitive to the size or capacity of an outlet.

If an outlet is specified in such a way that its flow capacity is very large (dimensions of orifice or gate, discharge of pump), this can lead to significant over-estimation of the outlet flow, and subsequently generate some flow continuity error if the volume removed from the Junction or SWC is greater than the volume present at the start of the time-step.

The main reason for this is that SWMM5 only accounts for the Physical Connections when calculating the suitable value of the computational time-step, and outlets are ignored.

This is something that InfoDrainage will address in the near future.

Reverse flow

Reverse flow can happen in any physical connection or outlet, but cannot happen in a notional connection or in an inlet.

Junctions are used to join together two or more Connections. Typically they mark a change in the connection type, slope, pipe diameter, open channel cross-section or direction or may represent a physical structure such as a manhole chamber.