Why should I choose StormNET over SWMM?

StormNET is the most advanced, powerful, and comprehensive stormwater and wastewater modeling software available for analyzing and designing urban drainage systems, stormwater sewers and sanitary sewers. StormNET is the only model that combines complex hydrology, hydraulics and water quality in a completely graphical, easy-to-use interface.

By incorporating the SWMM computational engine, StormNET is able to offer all of the features of SWMM while adding enhanced methods and utilities. A complete description of StormNET’s capabilities can be found here.

What type of support is available for SWMM?

There is no formal support offered for SWMM. Unlike SWMM, BOSS International has dedicated engineers ready to help with any StormNET project.

Can StormNET import SWMM files?

StormNET can import EPA SWMM (v4.4 and earlier) and XPSWMM™ v10.52 files.

What are the conceptual compartments (components) of SWMM?

SWMM conceptualizes a drainage system as a series of water and material flows between several major environmental compartments. These compartments include:

• Atmosphere compartment, from which precipitation falls and pollutants are deposited onto the land surface compartment. SWMM uses rain gage objects to represent rainfall inputs to the system.
• 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.
• Groundwater compartment receives infiltration from the land surface compartment and transfers a portion of this inflow to the transport compartment. This compartment is modeled using aquifer objects.
• 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 modeled with node and link objects.
  
  Not all compartments need appear in a particular SWMM model.

What hydrologic processes are accounted for by SWMM?

SWMM accounts for various hydrologic processes that produce runoff from urban areas including:

• 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.

What hydraulic capabilities are included in SWMM?

SWMM contains a flexible set of hydraulic modeling 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 drainage 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
• Utilize 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

What are the water quality capabilities of SWMM?

In addition to modeling 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 modeled 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 best management practices
• 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

How is overland flow represented in SWMM?

The conceptual view of surface runoff used by SWMM is illustrated in the figure below. Each subcatchment surface is treated as a nonlinear reservoir. Inflow comes from precipitation and any designated upstream subcatchments. There are several outflows, including infiltration, evaporation and surface runoff. The capacity of this reservoir is the maximum depression storage, which is the maximum surface storage provided by ponding, surface wetting and interception. Surface runoff per unit area, Q, occurs only when the depth of water in the reservoir exceeds the maximum depression storage, dp, in which case the outflow is given by Manning's equation. Depth of water over the subcatchment (d) is continuously updated with time by solving numerically a water balance equation over the subcatchment.

What infiltration models are included in SWMM?

Infiltration of rainfall from the pervious area of a subcatchment into the unsaturated upper soil zone can be described using three different models:

• Horton infiltration
• Green-Ampt infiltration
• SCS Curve Number infiltration

How is groundwater simulated in SWMM?

SWMM uses a two-zone groundwater model. The upper zone is unsaturated while the lower zone is fully saturated. The following fluxes are accounted for: infiltration from the surface, evapotranspiration from the upper zone, percolation from the upper to lower zone, evapotranspiration from the lower zone, percolation from the lower zone to deep groundwater and lateral groundwater inflow to the drainage system. After computing the water fluxes for a given time step, a mass balance is written for the change in water volume stored in each zone so that a new water table depth and unsaturated zone moisture content can be computed for the next time step.

What type of flow routing methods are available in SWMM?

Flow routing within a conduit link in SWMM is governed by the conservation of mass and momentum equations for gradually varied, unsteady flow (i.e., the Saint Venant flow equations). The SWMM user has a choice on the level of sophistication used to solve these equations:

• Steady flow routing
• Kinematic wave routing
• Dynamic wave routing

Each of these routing methods employs the Manning equation to relate flow rate to flow depth and bed (or friction) slope. The one exception is for circular Force Main shapes, where the Hazen-Williams equation is used instead.

What is steady flow routing?

Steady flow routing represents the simplest type of routing possible (actually no routing) by assuming that within each computational time step flow is uniform and steady. This type of routing cannot account for channel storage, backwater effects, entrance/exit losses, flow reversal or pressurized flow. It is insensitive to the time step employed and is really only appropriate for preliminary analysis using long-term continuous simulations.

What is kinematic wave routing?

Kinematic wave routing solves the continuity equation along with a simplified form of the momentum equation in each conduit. The latter requires that the slope of the water surface equal the slope of the conduit.

The maximum flow that can be conveyed through a conduit is the full normal flow value. Any flow in excess of this entering the inlet node is either lost from the system or can pond atop the inlet node and be re-introduced into the conduit as capacity becomes available.

Kinematic wave routing allows flow and area to vary both spatially and temporally within a conduit. This can result in attenuated and delayed outflow hydrographs as inflow is routed through the channel. However this form of routing cannot account for backwater effects, entrance/exit losses, flow reversal or pressurized flow and is also restricted to dendritic network layouts. It can usually maintain numerical stability with moderately large time steps, on the order of 5 to 15 minutes. If the aforementioned effects are not expected to be significant then this alternative can be an accurate and efficient routing method, especially for long-term simulations.

What is dynamic wave routing?

Dynamic wave routing solves the complete one-dimensional Saint Venant flow equations and therefore produces the most theoretically accurate results. These equations consist of the continuity and momentum equations for conduits and a volume continuity equation at nodes.

With this form of routing it is possible to represent pressurized flow when a closed conduit becomes full, such that flows can exceed the full normal flow value. Flooding occurs when the water depth at a node exceeds the maximum available depth, and the excess flow is either lost from the system or can pond atop the node and re-enter the drainage system.

Dynamic wave routing can account for channel storage, backwater, entrance/exit losses, flow reversal and pressurized flow. Because it couples together the solution for both water levels at nodes and flow in conduits it can be applied to any general network layout, even those containing multiple downstream diversions and loops. It is the method of choice for systems subjected to significant backwater effects due to downstream flow restrictions and with flow regulation via weirs and orifices. This generality comes at a price of having to use much smaller time steps, on the order of a minute or less.

Can SWMM account for ponding?

Normally in flow routing, when the flow into a junction exceeds the capacity of the system to transport it further downstream, the excess volume overflows the system and is lost. An option exists to have instead the excess volume be stored atop the junction, in a ponded fashion, and be reintroduced into the system as capacity permits. Under steady and kinematic wave flow routing, the ponded water is stored simply as an excess volume. For dynamic wave routing, which is influenced by the water depths maintained at nodes, the excess volume is assumed to pond over the node with a constant surface area. This amount of surface area is an input parameter supplied for the junction.

Alternatively, the user may wish to represent the surface overflow system explicitly. In open channel systems this can include road overflows at bridges or culvert crossings as well as additional floodplain storage areas. In closed conduit systems, surface overflows may be conveyed down streets, alleys or other surface routes to the next available stormwater inlet or open channel. Overflows may also be impounded in surface depressions such as parking lots, back yards or other areas.

What are the visual elements of a SWMM stormwater drainage network?

The following is a list of visual components that can be used to represent a drainage network in SWMM:

• Conduits are pipes or channels that move water from one node to another in the conveyance system.
• Flow dividers are drainage system nodes that divert inflows to a specific conduit in a prescribed manner.
• Flow regulators are structures or devices used to control and divert flows within a conveyance system. SWMM can model the following types of flow regulators: orifices, weirs and outlets.
• Junctions are drainage system nodes where links join together.
• Outfalls are terminal nodes of the drainage system used to define final downstream boundaries under dynamic wave flow routing.
• Pumps are links used to lift water to higher elevations.
• Rain gages supply precipitation data for one or more subcatchment areas in a study region
• Storage units are drainage system nodes that provide storage volume.
• Subcatchments are hydrologic units of land whose topography and drainage system elements direct surface runoff to a single discharge point.