Refactoring the SWMM 5 Help File – SWMM5 Introduction

Introducing EPA SWMM5

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 as well. The current edition, Version 5, is a complete re-write of the previous release. Running under Windows, SWMM 5 provides an integrated environment for editing study area input data, running hydrologic, hydraulic and water quality simulations, and viewing the results in a variety of formats. These include color-coded drainage area and conveyance system maps, time series graphs and tables, profile plots, and statistical frequency analyses.

This latest re-write of SWMM was produced by the Water Supply and Water Resources Division of the U.S. Environmental Protection Agency’s National Risk Management Research Laboratory with assistance from the consulting firm of CDM, Inc.

SWMM’s Conceptual Model

SWMM conceptualizes 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, which generates precipitation falls and deposits pollutants onto the land surface compartment. SWMM uses Rain Gage 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 modeled 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 modeled 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.

Computational Methods

SWMM is a physically based, discrete-time simulation model. It employs principles of conservation of mass, energy, and momentum wherever appropriate. This section briefly describes the methods SWMM uses to model stormwater runoff quantity and quality through the following physical processes:

· Surface Runoff
· Infiltration
· Groundwater
· Snowmelt
· Flow Routing
· Surface Ponding
· Water Quality Routing
Typical Applications of SWMM

Since its inception, SWMM has been used in thousands of sewer and 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.

What’s New in Release 5.1.013
· Individual subcatchments can be assigned time patterns that adjust depression storage, pervious surface roughness and infiltration capacity on a monthly basis (see Subcatchment Properties).
· LID controls can now treat a designated portion of a subcatchment’s pervious area runoff (previously they could only treat impervious area runoff). See LID Usage Editor.
· Permeable pavement LID units subjected to clogging over time can now have their permeability partly or fully restored at periodic time intervals (see LID Pavement Layer).
· The following options were added to control flow out of LID units through their underdrains (see LID Drain System):
i. A storage layer water depth above which a closed drain automatically opens.
ii. A storage layer water depth below which an open drain automatically closes.
iii. A control curve that specifies how the nominal drain flow rate is adjusted as a function of the head seen by the drain.
· Pollutant removal percentages can now be assigned to LID processes that have underdrains (see LID Pollutant Removal). The removals apply to flow leaving the unit through the drain and not to any surface overflow from the unit.
· The Subcatchment Runoff Summary Report now includes both pervious and impervious total runoff volumes (prior to any LID treatment) for each subcatchment.
· A choice of method used to handle surcharging has been added to the list of Dynamic Wave options. The EXTRAN method continues to use the traditional Surcharge Algorithm to update the head at surcharged nodes. The new SLOT option attaches a Preissmann Slot to closed conduits flowing more than 98.5% full that eliminates the need to switch to the Surcharge Algorithm for surcharged nodes.
· A closed vessel can now be modeled as a storage unit node that is allowed to pressurize up to a designated Surcharge Depth value (see Storage Unit Properties). If this depth is 0 then the unit is modeled as before as an open vessel.
· A weir’s discharge coefficient can now be allowed to vary with head across the weir by assigning it a Weir Curve (see Weir Properties). Weir curves tabulate coefficient values at specific head levels.
· Users can now choose to set a periodic time step for control rule evaluation (see Time Step Options). If this step is 0 then rules are tested as before at every routing time step.
· The option was added to have time series results for a project’s nodes and links be reported as average values computed over a reporting time step instead of being interpolated point values at the end of the reporting time step (see Reporting Options).
· Automatic raising of the upstream offset of a regulator link to match the invert of its downstream node is now only done under Dynamic Wave flow routing. A warning message is still issued whenever this condition is encountered.

Viewing All Project Data

A listing of all project data (with the exception of map coordinates) can be viewed in a non-editable window, formatted for input to SWMM’s computational engine. This can be useful for checking data consistency and to make sure that no key components are missing. To view such a listing, select Project >> Details. The format of the data in this listing is the same as that used when the file is saved to disk. It is described in detail in Appendix D of the SWMM 5 Users Manual.

Steps in Using SWMM

One typically carries out the following steps when using EPA SWMM to model a study area:

1. Specify a default set of options and object properties to use (see Setting Project Defaults).
2. Draw a network representation of the physical components of the study area (see Adding Objects).
3. Edit the properties of the objects that make up the system (see Editing Objects).
4. Select a set of analysis options (see Setting Analysis Options).
5. Run a simulation (see Initiating a Run).
6. View the results of the simulation (see Viewing Results).

For larger systems it will be more convenient to replace Step 2 by collecting study area data from various sources, such as CAD drawings or GIS files, and transferring these data into a SWMM input file whose format is described in the SWMM 5 User’s Manual.

Hydraulic Modeling Features

SWMM also contains a flexible set of hydraulic modeling capabilities used to route runoff and external inflows through a 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
· 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.

In addition to the nodes and links which characterize the physical aspects of a drainage system in a SWMM model, the following data objects can be used to augment the hydraulic description of the system:

· Transects
· Inflows
· Controls

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