Storm Water Management Model


The United States Environmental Protection Agency Storm Water Management Model is a dynamic rainfall–runoff–subsurface runoff simulation model used for single-event to long-term simulation of the surface/subsurface hydrology quantity and quality from primarily urban/suburban areas.
It can simulate the rainfall-runoff, runoff, evaporation, infiltration and groundwater connection for roots, streets, grassed areas, rain gardens and ditches and pipes, for example. The hydrology component of SWMM operates on a collection of subcatchment areas divided into impervious and pervious areas with and without depression storage to predict runoff and pollutant loads from precipitation, evaporation and infiltration losses from each of the subcatchment. Besides, low impact development and best management practice areas on the subcatchment can be modeled to reduce the impervious and pervious runoff. The routing or hydraulics section of SWMM transports this water and possible associated water quality constituents through a system of closed pipes, open channels, storage/treatment devices, ponds, storages, pumps, orifices, weirs, outlets, outfalls and other regulators.
SWMM tracks the quantity and quality of the flow generated within each subcatchment, and the flow rate, flow depth, and quality of water in each pipe and channel during a simulation period composed of multiple fixed or variable time steps. The water quality constituents such as water quality constituents can be simulated from buildup on the subcatchments through washoff to a hydraulic network with optional first order decay and linked pollutant removal, best management practice and low-impact development removal and treatment can be simulated at selected storage nodes. SWMM is one of the hydrology transport models which the EPA and other agencies have applied widely throughout North America and through consultants and universities throughout the world. The latest update notes and new features can be found on the EPA website in the download section. Recently added in November 2015 were the EPA SWMM 5.1 Hydrology Manual and in 2016 the EPA SWMM 5.1 Hydraulic Manual and EPA SWMM 5.1 Water Quality Volume + Errata.

Program description

The EPA storm water management model is a dynamic rainfall-runoff-routing simulation model used for single event or long-term 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 divided into multiple time steps.
SWMM accounts for various hydrologic processes that produce runoff from urban areas. These include:
  1. time-varying rainfall
  2. evaporation of standing surface water
  3. snow accumulation and melting
  4. rainfall interception from depression storage
  5. infiltration of rainfall into unsaturated soil layers
  6. percolation of infiltrated water into groundwater layers
  7. interflow between groundwater and the drainage system
  8. nonlinear reservoir routing of overland flow
  9. capture and retention of rainfall/runoff with various types of low impact development practices.
SWMM also 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:
  1. handle networks of unlimited size·
  2. use a wide variety of standard closed and open conduit shapes as well as natural channels·
  3. model special elements such as storage/treatment units, flow dividers, pumps, weirs, and orifices·
  4. apply external flows and water quality inputs from surface runoff, groundwater interflow, rainfall-dependent infiltration/inflow, dry weather sanitary flow, and user-defined inflows
  5. utilize either kinematic wave or full dynamic wave flow routing methods·
  6. model various flow regimes, such as backwater, surcharging, reverse flow, and surface ponding·
  7. apply user-defined dynamic control rules to simulate the operation of pumps, orifice openings, and weir crest levels.
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.
Since its inception, SWMM has been used in thousands of sewer and stormwater studies throughout the world. Typical applications include:
  1. design and sizing of drainage system components for flood control
  2. sizing of detention facilities and their appurtenances for flood control and water quality protection·
  3. flood plain mapping of natural channel systems, by modeling the river hydraulics and associated flooding problems using prismatic channels·
  4. designing control strategies for minimizing Combined Sewer Overflow and Sanitary Sewer Overflow ·
  5. evaluating the impact of inflow and infiltration on sanitary sewer overflows·
  6. generating non-point source pollutant loadings for waste load allocation studies·
  7. evaluating the effectiveness of BMPs and subcatchment LID's for reducing wet weather pollutant loadings. Rainfall-runoff modeling of urban and rural watersheds
  8. hydraulic and water quality analysis of storm, sanitary, and combined sewer systems
  9. master planning of sewer collection systems and urban watersheds
  10. system evaluations associated with USEPA's regulations including NPDES permits, CMOM, and TMDL
  11. 1D and 2D predictions of flood levels and flooding volume
EPA SWMM is public domain software that may be freely copied and distributed. The SWMM 5 public domain consists of C engine code and Delphi SWMM 5 graphical user interface code. The C code and Delphi code are easily edited and can be recompiled by students and professionals for custom features or extra output features.

History

SWMM was first developed between 1969–1971 and has undergone four major upgrades since those years. The major upgrades were: Version 2 in 1973-1975, Version 3 in 1979-1981, Version 4 in 1985-1988 and Version 5 in 2001-2004. A list of the major changes and post-2004 changes are shown in Table 1. The current SWMM edition, Version 5.2.3, is a complete re-write of the previous Fortran releases in the programming language C, and it can be run under Windows XP, Windows Vista, Windows 7, Windows 8, Windows 10 and also with a recompilation under Unix. The code for SWMM5 is open source and public domain code that can be downloaded from the EPA website.
EPA SWMM 5 provides an integrated graphical environment for editing watershed input data, running hydrologic, hydraulic, real time control and water quality simulations, and viewing the results in a variety of graphical formats. These include color-coded thematic drainage area maps, time series graphs and tables, profile plots, scatter plots and statistical frequency analyses.
The last rewrite of EPA 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 under a Cooperative Research and Development Agreement. SWMM 5 is used as the computational engine for many modeling packages plus components of SWMM5 are in other modeling packages. The major modeling packages that use all or some of the SWMM5 components are shown in the Vendor section. The update history of SWMM 5 from the original SWMM 5.0.001 to the current version SWMM 5.2.3 can be found at the EPA website. SWMM 5 was approved FEMA Model Approval Page in May 2005, with a note about the versions that are approved on the FEMA Approval Page SWMM 5 Version 5.0.005 and up for NFIP modeling. SWMM 5 is used as the computational engine for many modeling packages and some components of SWMM5 are in other modeling packages.

SWMM 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, 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. The rain gage objects can use time series, external text files or NOAA rainfall data files. The Rain Gage objects can use precipitation for thousands of years. Using the SWMM-CAT Addon to SWMM5 climate change can now be simulated using modified temperature, evaporation or rainfall.
The Land Surface compartment, which is represented by 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 low impact development controls are part of the subcatchments and store, infiltrate or evaporate the runoff.
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 connection to the Transport compartment can be either a static boundary or a dynamic depth in the channels. The links in the Transport compartment now also have seepage and evaporation.
The transport compartment contains a network of conveyance elements 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 to appear in a particular SWMM model. For example, one could model just the transport compartment, using pre-defined hydrographs as inputs. If kinematic wave routing is used, then the nodes do not need to contain an outfall.