SWMM Method

The EPA Storm Water Management Model (SWMM) is a dynamic rainfall-runoff simulation model used for single event or long-term (continuous) simulation of runoff quantity and quality from primarily urban areas. The runoff component of SWMM operates on a collection of subcatchment areas that receive precipitation and generate runoff and pollutant loads. The routing portion of SWMM transports this runoff through a system of pipes, channels, storage/treatment devices, pumps, and regulators. SWMM tracks the quantity and quality of runoff generated within each subcatchment, and the flow rate, flow depth, and quality of water in each pipe and channel, during a simulation period comprised of multiple time steps.

SWMM was first developed in 1971 and has undergone several major upgrades since then. It continues to be widely used throughout the world for planning, analysis and design related to storm water runoff, combined sewers, sanitary sewers, and other drainage systems in urban areas, with many applications in non-urban areas.

Hydrologic Modelling Features

SWMM accounts for various hydrologic processes that produce runoff from urban areas. These include:

Time-varying rainfall.
Evaporation of standing surface water.
Snow accumulation and melting.
Rainfall interception from depression storage.
Infiltration of rainfall into unsaturated soil layers.
Percolation of infiltrated water into groundwater layers.
Interflow between groundwater and the drainage system.
Nonlinear reservoir routing of overland flow.

Spatial variability in all of these processes is achieved by dividing a study area into a collection of smaller, homogeneous subcatchment areas, each containing its own fraction of pervious and impervious sub-areas. Overland flow can be routed between sub-areas, between subcatchments, or between entry points of a drainage system.

Hydraulic Modelling Features

SWMM also contains a flexible set of hydraulic modelling capabilities used to route runoff and external inflows through the drainage system network of pipes, channels, storage/treatment units and diversion structures. These include the ability to:

Handle networks of unlimited size.
Use a wide variety of standard closed and open conduit shapes as well as natural channels.
Model special elements such as storage/treatment units, flow dividers, pumps, weirs, and orifices.
Apply external flows and water quality inputs from surface runoff, groundwater interflow, rainfall-dependent infiltration/inflow, dry weather sanitary flow, and user-defined inflows.
Utilise either kinematic wave or full dynamic wave flow routing methods.
Model various flow regimes, such as backwater, surcharging, reverse flow, and surface ponding.
Apply user-defined dynamic control rules to simulate the operation of pumps, orifice openings, and weir crest levels.

The SWMM method supports the following node types.

Node Type	Description
Junctions	Junctions are drainage system nodes where links join together. Physically they can represent the confluence of natural surface channels, manholes in a sewer system, or pipe connection fittings. External inflows can enter the system at junctions. Excess water at a junction can become partially pressurised, while connecting conduits are surcharged and can either be lost from the system or be allowed to pond atop the junction and subsequently drain back into the junction.
Outfalls	Outfalls are terminal nodes of the drainage system, which are used to define final downstream boundaries under Dynamic Wave flow routing. For other types of flow routing, they behave as a junction. Only a single link can be connected to an outfall node. The boundary conditions at an outfall can be described by any one of the following stage relationships: The critical or normal flow depth in the connecting conduit. A fixed stage elevation. A tidal stage described in a table of tide height versus hour of the day. A user-defined time series of stage versus time.
Flow Dividers	Flow Dividers are drainage system nodes that divert inflows to a specific conduit in a prescribed manner. A flow divider can have no more than two conduit links on its discharge side. Flow dividers are only active under Kinematic Wave routing and are treated as simple junctions under Dynamic Wave routing. There are four types of flow dividers, defined by the manner in which inflows are diverted: Cutoff - Diverts all inflow above a defined cutoff value. Overflow - Diverts all inflow above the flow capacity of the non-diverted conduit. Tabular - Uses a table that expresses diverted flow as a function of total inflow. Weir - Treats diverted flow as linearly proportional to the inflow above a defined cutoff value.
Storage Units	Storage Units are drainage system nodes that provide storage volume. Physically they could represent storage facilities as small as a catchbasin, or as large as a lake. The volumetric properties of a storage unit are described by a function or table of surface area versus height.

It also supports the following link types.

Link Type	Description
Conduits	Conduits are pipes or channels that move water from one node to another in the conveyance system. Their cross-sectional shapes can be selected from a variety of standard open and closed geometries. Irregular natural cross-section shapes are also supported.
Pumps	Pumps are links used to lift water to higher elevations. A pump curve describes the relation between a pump's flow rate and conditions at its inlet and outlet nodes.
Orifices	Orifices are used to model outlet and diversion structures in drainage systems, which are typically openings in the wall of a manhole, storage facility, or control gate. They are internally represented in SWMM as a link connecting two nodes. An orifice can have either a circular or rectangular shape, be located either at the bottom or along the side of the upstream node, and have a flap gate to prevent backflow. Orifices can be used as storage unit outlets under all types of flow routing. If not attached to a storage unit node, they can only be used in drainage networks that are analysed with Dynamic Wave flow routing. The flow through an orifice is computed based on the area of its opening, its discharge coefficient, and the head difference across the orifice. The area of an orifice's opening can be controlled dynamically through user-defined Control Rules.
Weirs	Weirs, like orifices, are used to model outlet and diversion structures in a drainage system. Weirs are typically located in a manhole, along the side of a channel, or within a storage unit. They are internally represented in SWMM as a link connecting two nodes, where the weir itself is placed at the upstream node. A flap gate can be included to prevent backflow. Four varieties of weirs are available, each incorporating a different formula for computing flow as a function of area, discharge coefficient and head difference across the weir: Transverse - Rectangular shape. Side flow - Rectangular shape. V-notch - Triangular shape. Trapezoidal - Trapezoidal shape. Weirs can be used as storage unit outlets under all types of flow routing. If not attached to a storage unit, they can only be used in drainage networks that are analysed with Dynamic Wave flow routing. The height of the weir crest above the inlet node invert can be controlled dynamically through user-defined Control Rules. This feature can be used to model inflatable dams.
Outlets	Outlets are flow control devices that are typically used to control outflows from storage units. They are used to model special head-discharge relationships that cannot be characterised by pumps, orifices, or weirs. Outlets are internally represented in SWMM as a link connecting two nodes. An outlet can also have a flap gate that restricts flow to only one direction. Outlets attached to storage units are active under all types of flow routing. If not attached to a storage unit, they can only be used in drainage networks analysed with Dynamic Wave flow routing. A user-defined rating curve determines an outlet's discharge flow as a function of the head difference across it. Control Rules can be used to dynamically adjust this flow when certain conditions exist.

Water Quality Modelling Features

In addition to modelling the generation and transport of runoff flows, SWMM can also estimate the production of pollutant loads associated with this runoff. The following processes can be modelled for any number of user-defined water quality constituents:

Dry-weather pollutant buildup over different land uses.
Pollutant washoff from specific land uses during storm events.
Direct contribution of rainfall deposition.
Reduction in dry-weather buildup due to street cleaning.
Reduction in washoff load due to BMPs.
Entry of dry weather sanitary flows and user-specified external inflows at any point in the drainage system.
Routing of water quality constituents through the drainage system.
Reduction in constituent concentration through treatment in storage units or by natural processes in pipes and channels.

Typical Applications of SWMM

Since its inception, SWMM has been used in thousands of stormwater studies throughout the world. Typical applications include:

Design and sizing of drainage system components for flood control.
Sizing of detention facilities and their appurtenances for flood control and water quality protection.
Flood plain mapping of natural channel systems.
Designing control strategies for minimizing combined sewer overflows.
Evaluating the impact of inflow and infiltration on sanitary sewer overflows.
Generating non-point source pollutant loadings for waste load allocation studies.
Evaluating the effectiveness of BMPs for reducing wet weather pollutant loadings.

SWMM's Conceptual Model

SWMM conceptualises a drainage system as a series of water and material flows between several major environmental compartments. These compartments and the SWMM objects they contain include:

The Atmosphere compartment, from which precipitation falls and pollutants are deposited onto the land surface compartment. SWMM uses Rain Gauge objects to represent rainfall inputs to the system.
The Land Surface compartment, which is represented through one or more Subcatchment objects. It receives precipitation from the Atmospheric compartment in the form of rain or snow; it sends outflow in the form of infiltration to the Groundwater compartment and also as surface runoff and pollutant loadings to the Transport compartment.
The Groundwater compartment receives infiltration from the Land Surface compartment and transfers a portion of this inflow to the Transport compartment. This compartment is modelled using Aquifer objects.
The Transport compartment contains a network of conveyance elements (channels, pipes, pumps, and regulators) and storage/treatment units that transport water to outfalls or to treatment facilities. Inflows to this compartment can come from surface runoff, groundwater interflow, sanitary dry weather flow, or from user-defined hydrographs. The components of the Transport compartment are modelled with Node and Link objects.

Not all compartments need appear in a particular SWMM model. For example, one could model just the transport compartment, using pre-defined hydrographs as inputs.