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OpenSWMM Engine
6.0.0-alpha.4
Data-oriented, plugin-extensible SWMM Engine (6.0.0-alpha.4)
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OpenSWMM Engine
6.0.0-alpha.4
Data-oriented, plugin-extensible SWMM Engine (6.0.0-alpha.4)
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This software is provided on an "as is" basis and the user assumes responsibility for its use. Although a reasonable effort has been made to assure that the results obtained are correct, the authors are not responsible and assume no liability whatsoever for any results or any use made of the results obtained from these programs, nor for any damages or litigation that result from the use of these programs for any purpose.
This reference manual was originally prepared by Lewis A. Rossman, Environmental Scientist Emeritus, U.S. Environmental Protection Agency, Office of Research and Development, National Risk Management Research Laboratory. His foundational work on the SWMM hydraulics model and its documentation is gratefully acknowledged.
See Authors & Contributors for the complete list of authors and contributors.
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. The reference manual for this edition of SWMM is comprised of three volumes. Volume I describes SWMM's hydrologic models, Volume II its hydraulic models, and Volume III its water quality and low impact development models.
Figure 1-1 Elements of a typical urban drainage system
Figure 1-2 Objects of an OpenSWMM model
Figure 1-3 Processes modelled by OpenSWMM
Figure 1-4 Block diagram of SWMM's state transition process
Figure 1-5 Flow chart of SWMM's simulation procedure
Figure 1-6 Interpolation of reported values from computed values
Figure 1-7 The hydraulics of this manual: the 1D network, the 2D surface and their coupling
Figure 2-1 Node-link representation of a sewer system
Figure 2-2 Comparison of dynamic wave and kinematic wave solutions (from Miller, 1984)
Figure 3-1 Node-link representation of a conveyance network in SWMM (from Roesner et al, 1992).
Figure 3-2 Special flow conditions for dynamic wave analysis
Figure 3-3 Illustration of a surcharged node
Figure 3-4 Ponding of excess water above a junction
Figure 3-5 Profile view of example rectangular conduit (not to scale)
Figure 3-6 Outflow hydrographs for example conduit -I
Figure 3-7 Outflow hydrographs for example conduit – II
Figure 3-8 Conceptual representation of the dynamic Preissmann slot
Figure 3-9 State transitions of the dynamic Preissmann slot at a conduit end (rendered diagram; states and transitions as implemented in updateDPSState)
Figure 3-10 Virtual junction representation of a conduit grade break
Figure 3-11 Workflow of one Anderson-accelerated iteration of the successive-approximation loop (rendered diagram)
Figure 4-1 Section factor versus area for a circular shape
Figure 4-2 Space-time grid for kinematic wave analysis
Figure 4-3 Outflow hydrograph for example conduit
Figure 5-1 Power law cross section shape
Figure 5-2 Geometric properties of a partly filled circular shape based on depth
Figure 5-3 Geometric properties of a partly filled circular shape based on area
Figure 5-4 Ellipsoid and arch pipe cross sectional shapes
Figure 5-5 Masonry sewer shapes
Figure 5-6 Composite cross section shapes
Figure 5-7 A Shape Curve with a depth segment shown
Figure 5-8 A natural channel transect
Figure 5-9 A transect depth increment with three compound segments
Figure 5-10 A one-sided street cross-section (not to scale)
Figure 5-11 Example of a storage curve and its section view
Figure 5-12 Finding the volume at a given depth for a storage curve
Figure 6-1 Orifice orientations
Figure 6-2 Determination of effective head for an orifice
Figure 6-3 Orifice with unsubmerged inlet
Figure 6-4 Transverse weir shapes
Figure 6-5 Coefficient for triangular weirs (from Brater and King, 1976)
Figure 6-6 Definitions of submerged and surcharged weir flow
Figure 6-7 Rating curve for a vortex device compared to an orifice
Figure 7-1 Depths used for computing seepage in storage units
Figure 7-2 Concrete box culvert (from FHWA, 2012)
Figure 7-3 Example of a culvert rating curve (from FHWA, 2012)
Figure 7-4 Roadway overtopping (from FHWA, 2012)
Figure 7-5 SWMM node-link representation of a culvert with a roadway weir
Figure 7-6 Discharge coefficients for roadway weirs (from FHWA, 2012)
Figure 7-7 Examples of storm drain inlets.
Figure 7-8 Standard types of curb and gutter inlets
Figure 7-9 Performance curve for a custom inlet
Figure 7-10 Representation of a dual drainage system
Figure 7-11 Street cross-section divided into gutter and roadway flow
Figure 7-12 Types of channel drop inlets
Figure 8-1 Node–link routing against the finite-volume view: one momentum balance per conduit versus a conduit cut into cells with the water surface resolved inside it
Figure 8-2 The tapered static slot for a 3 ft circular pipe: top width and celerity across the crown at four slot celerities
Figure 8-3 The Riemann fan at a face: HLL signal speeds for a wet–wet face and the rarefaction-tail estimate at a dry bed
Figure 8-4 Local time stepping: power-of-two tiers, the macro cycle and flux accumulation across a tier interface
Figure 8-5 Substep workflow of the explicit finite-volume solver, including the post-step census retry and local time stepping (rendered diagram)
Figure 8-6 Wet/dry and exception handling in one face flux evaluation (rendered diagram)
Figure 8-7 Hydrostatic reconstruction at a wet/dry front: an advancing front (left) and an emerged bank acting as a wall (right)
Figure 8-8 Node ghost-state construction at a coupling face
Figure 9-1 The one-dimensional network and the two-dimensional surface: exchange pathways
Figure 9-2 One 1D–2D co-advance batch and the explicit marcher's substep loop within it (rendered diagram)
Figure 9-3 Wetting cases of a planar-bed triangular cell and the wetted-edge face gate (placeholder — to be replaced by a final drawing)
Figure 9-4 A quadrilateral and a triangle sharing a face: local edge numbering, the padded slot layout and the centroid-to-edge distance
Figure 9-5 The flux-active set at a wetting front: the default one-ring halo, the front-rebuild halo and the breach that forces an early rebuild
Figure 9-6 Where a node spill is booked under COUPLING_IN_FLOODING: the same totals and the same continuity error, in two different rows
Figure 9-7 Coupled surface/depth reconstruction and the connected-wet-stencil decision at a shoreline. The drawing is schematic, not model output.
Figure 9-8 Final CPU cost and depth error relative to the earlier reconstruction, at different mesh resolutions. Points are measured pairs; bars are paired medians.
Table 1-1 Development history of SWMM
Table 1-2 SWMM's modeling objects
Table 1-3 State variables used by SWMM
Table 1-4 Units of expression used by SWMM
Table 2-1 Features and limitations of dynamic wave and kinematic wave solutions
Table 3-1 Surface area adjustments for various dynamic wave flow conditions
Table 3-2 Conditions under which Anderson acceleration reverts to standard iteration
Table 5-1 Geometric properties for open channel shapes as functions of water depth
Table 5-2 Geometric properties for open channel shapes as functions of flow area
Table 5-3 Geometric properties for the power law shape
Table 5-4 Geometric properties of a full circular cross section
Table 5-5 Full area and hydraulic radius of custom ellipsoid and arch pipe sections
Table 5-6 Number of entries in geometric property tables for masonry sewer shapes
Table 5-7 Geometric parameters of masonry sewer sections
Table 5-8 Geometric properties for a sediment filled circular cross section
Table 5-9 Properties of the rectangular section of a rectangular-triangular shape
Table 5-10 Geometric parameters for rectangular-round shapes
Table 5-11 Geometric properties for rectangular–round shapes
Table 5-12 Properties in the rounded top section of a modified basket handle shape
Table 5-13 Area at maximum flow to full area for standard closed conduits shapes
Table 5-14 Critical depth formulas for simple section shapes
Table 6-1 Pump curves recognized by SWMM
Table 6-2 Kindsvater-Carter constants for rectangular weir coefficient
Table 6-3 Rectangular broad-crested weir coefficients (ft1/2/sec)
Table 6-4 Formulas for flow derivatives of various types of weirs
Table 7-1 Relative depth at maximum width for select cross section shapes
Table 7-2 Types of minor losses in drainage systems (from Frost, 2006)
Table 7-3 Hazen-Williams C-factors for different pipe materials
Table 7-4 Darcy-Weisbach roughness heights for different pipe materials
Table 7-5 Description of grate inlet types1
Table 7-6 Splash-over velocity for different types of grate inlets1
Table 7-7 Effective depth for curb opening inlets under orifice flow
Table 8-1 Dry-state constants of the explicit finite-volume solver (internal US units)
Table 9-1 Wetting and drying thresholds and guards of the 2D solver (SI units)
Table 9-2 Momentum closures of the 2D marcher
Table 9-3 Diagonal selection of the two-plane quadrilateral closure (Begnudelli and Sanders, 2007, Cases 1–3)
Table 9-4 The fifteen REPORT_2D_VARIABLES tokens
Table 9-5 The forty-two live [2D_OPTIONS] keys and the theory each selects
Table 9-6 The sixteen retired [2D_OPTIONS] keys and values
Table 10-1 Formulations recorded in this chapter
Table C-1 Circular section properties as function of depth
Table C-2 Circular section properties as function of area
Table D-1 Standard elliptical pipe sizes
Table D-2 Elliptical section properties as function of depth
Table E-1 Standard arch pipe sizes
Table E-2 Arch pipe section properties as function of depth
Table F-1 Area of masonry sewers as function of depth
Table F-2 Width of masonry sewers as function of depth - I
Table F-3 Width of masonry sewers as function of depth - II
Table F-4 Hydraulic radius of masonry sewers as function of depth
Table F-5 Depth of masonry sewers as function of area - I
Table F-6 Depth of masonry sewers as function of area - II
Table F-7 Section factor for masonry sewers as function of area - I
Table F-8 Section factor for masonry sewers as function of area - II
Table G-1 Manning's roughness coefficient n for open channels
Table G-2 Manning's roughness coefficient n for closed conduits
Table G-3 Manning's roughness coefficient n for corrugated steel pipe
Table H-1 Culvert codes
Table H-2 Culvert coefficients
| Symbol | Description |
|---|---|
| A | cross section flow area within a conduit (ft²) |
| Ā | average cross section flow area along a conduit (ft²) |
| Ā̄ | average cross section flow area along a conduit over a time period (ft²) |
| Afull | full cross section area of a conduit (ft²) |
| Amax | cross section area at depth where a conduit's section factor is a maximum (ft²) |
| AO | area of an orifice opening (ft²) |
| ASP | surface area of water ponded above a node (ft²) |
| AS | surface area of a node and its connected links (ft²) |
| ASL | surface area of flow within a link (ft²) |
| ASlast | surface area of a node the last time it was not surcharged (ft²) |
| ASmin | minimum surface area associated with a node (ft²) |
| ASN | surface area associated with a storage node (ft²) |
| AW | area of a weir opening (ft²) |
| b | bottom or top width (depending on shape) of a conduit's cross section (ft) |
| c | wave celerity (ft/sec) |
| cI | inlet control constant for submerged culverts |
| cW | coefficient for a weir-type flow divider (ft1/2/sec) |
| Cd | orifice discharge coefficient (dimensionless) |
| CHW | Hazen-Williams C-factor coefficient (dimensionless) |
| CO | equivalent orifice constant for a surcharged weir (ft5/2/sec) |
| Cr | Courant number (dimensionless) |
| Cw | weir coefficient (ft1/2/sec) |
| D | circular pipe diameter (ft) |
| et | potential evaporation rate at time t (ft/sec) |
| E | elevation of a node's invert (ft) |
| EC | specific head at critical depth (ft) |
| f | Darcy-Weisbach friction factor (dimensionless) |
| fC | monthly climate adjustment factor (dimensionless) |
| fE | storage node evaporation factor (dimensionless) |
| fS | weir submergence adjustment factor (dimensionless) |
| F | cumulative depth of infiltrated water (ft) |
| Fr | Froude number (dimensionless) |
| g | acceleration of gravity (ft/sec²) |
| hL | minor head loss per unit length of a conduit (ft/ft) |
| hW | height of the opening for a weir-type flow divider node (ft) |
| H | hydraulic head (ft) |
| Hcrown | elevation of the crown of the highest conduit at a node (ft) |
| He | effective head seen by an orifice or weir (ft) |
| HIS | minimum head at a culvert's inlet for it to be submerged (ft) |
| HIU | maximum head at a culvert's inlet for it to be unsubmerged (ft) |
| Hmax | maximum head at a node before flooding occurs (ft) |
| HOutfall | head assigned to an outfall node (ft) |
| K | cross section flow conductance (cfs) (equal to \(nAR^{2/3}\)) |
| KI | inlet control constant for unsubmerged culverts |
| Km | minor loss coefficient (dimensionless) |
| KS | soil saturated hydraulic conductivity (ft/sec) |
| L | conduit length or weir crest length (ft) |
| Le | effective weir crest length (ft) |
| MI | inlet control exponent for unsubmerged culverts |
| n | Manning roughness coefficient (sec/m1/3) |
| P | wetted perimeter of a conduit's cross section (ft) |
| qE | uniformly distributed evaporation rate along a channel (cfs/ft) |
| qL | total uniformly distributed outflow rate along a conduit (cfs/ft) |
| qMIN | minimum flow needed to activate a flow divider node (cfs) |
| qS | uniformly distributed seepage rate along a conduit (cfs/ft) |
| qSN | seepage rate per unit area for a storage node (cfs/ft²) |
| Q | flow rate within a conduit, pump, or regulator link (cfs) |
| Qdiv | flow rate diverted to a second outflow conduit from a flow divider node (cfs) |
| QEN | evaporation loss rate from a storage unit node (cfs) |
| Qfull | normal uniform flow rate for a full conduit (cfs) |
| QIC | culvert flow rate under inlet control (cfs) |
| Qin | total inflow rate to a node (cfs) |
| QLN | total loss rate from a storage unit node (cfs) |
| Qnorm | normal uniform flow rate (cfs) |
| Qout | total outflow rate leaving a node (cfs) |
| Qovfl | excess flow that overflows a node (cfs) |
| Q̄net | average net inflow minus outflow over a time step (cfs) |
| QSN | seepage loss rate from a storage node (cfs) |
| R | hydraulic radius of flow cross section in a conduit (ft) |
| R̄ | average hydraulic radius of flow cross sections along a conduit (ft) |
| Re | Reynolds number (dimensionless) |
| Rfull | hydraulic radius of a conduit cross section when full (ft) |
| s | seepage rate per unit area for a conduit (ft/sec) |
| Scf | culvert slope correction factor |
| Sf | friction slope (ft/ft) |
| S0 | conduit slope (ft/ft) |
| t | time (sec) |
| U | flow velocity at a point along a conduit (ft/sec) |
| Ū | average flow velocity along a conduit (ft/sec) |
| V | node assembly volume (ft³) |
| VP | ponded volume (ft³) |
| VN | storage node volume (ft³) |
| VNfull | volume of a storage node when full (ft³) |
| W | top width of the water surface at a point along a conduit (ft) |
| W̄ | average top width of the water surface along a conduit (ft) |
| Wmax | maximum width of a conduit cross section (ft) |
| x | horizontal distance (ft) |
| y | vertical distance (ft) |
| yI | inlet control constant for submerged culverts |
| Y | depth of flow within a conduit or of water in a storage unit (ft) |
| Ȳ | average depth of flow along a conduit (ft) |
| Yc | critical depth within a conduit at a given flow rate (ft) |
| Yfull | full depth of a conduit, orifice opening or weir height (ft) |
| YN | normal flow depth (ft) |
| Y* | smaller of the critical and normal flow depth in a conduit (ft) |
| Z | elevation of a conduit's invert (ft) |
| ZO | elevation of the bottom of an orifice's opening (ft) |
| ZW | elevation of a weir's crest in its lowest position (ft) |
| α | generic coefficient |
| β | the square root of a conduit's slope divided by its roughness |
| ∆t | time step (sec) |
| ε | convergence tolerance |
| ε | Darcy-Weisbach roughness length (ft) |
| γ | exponent in power law cross section shape |
| η | Manning's roughness coefficient (sec/ft1/3) \(\left( equal\ to\ \frac{n}{1.486} \right)\) |
| σ | inertial damping factor |
| θ | time weighting factor, relaxation factor, or subtended angle |
| φ | distance weighting factor |
| θd | soil moisture deficit (dimensionless) |
| μ | kinematic viscosity (ft²/sec) |
| ω | pump speed setting or degree to which a regulator is opened |
| ψS | soil capillary suction head (ft) |
| Ψ | conduit section factor (equal to \(AR^{2/3}\)) (ft8/3) |
| Ψfull | section factor of a conduit at full depth (ft8/3) |
| Ψmax | maximum section factor for a conduit (ft8/3) |
An ordinary junction may specify a constant StorageArea in project area units. Zero preserves historical junction routing. For positive area A and depth y below the rim, physical node volume is A·y and its depth derivative is A; incident conduit volume remains owned by the conduits. The chamber remains active above connected pipe crowns. Above the rim, shaft inventory is capped and configured surface ponding uses Aponded separately. Extra surcharge depth is a pressure allowance, not additional shaft height.
MIN_SURFAREA supplies numerical stability under the compatibility path and must not substitute for a positive authored chamber area. Dynamic-wave water and quality balances, initial/final storage, and output volume include the physical chamber once. Finite-volume routing uses a physical node-volume closure, rather than an algebraic zero-storage junction. Unsupported routing modes/backends must refuse the configuration.
For dynamic wave, a chamber balances its change in physical volume plus its share of the actual change in adjacent conduit volume against the time-integrated net inflow. The conduit top-width tangent is used to converge head, and does not create extra inventory at the crown cutoff. This volume residual is used for positive chambers under both EXPLICIT and SEMI_IMPLICIT node-continuity settings. Zero-area junctions retain their existing algorithms.
For example, a junction with MaxDepth = 10 and StorageArea = 100 holds 400 ft³ at 4 ft depth in a US-unit model. An equivalent constant-area storage unit uses FUNCTIONAL 0 0 100. Both geometries have the same below-rim capacity; the ordinary junction retains its junction identity. Above-rim ponding, if enabled, uses the separately specified ponded area.
See OpenSWMM issue #10 and USEPA issue #200 for the physical-area proposal.
Physical junctions in finite-volume routing require the CPU backend and zero SurDepth; explicit ponding and rim flooding are supported. Above-rim pressure allowance requires dynamic wave. A selected device plugin refuses physical junctions until its closure is qualified. Positive chamber areas are not yet qualified with DYNAMIC_SLOT; select EXTRAN or SLOT instead.
Physical dynamic-wave chambers partition the actual conduit inventory according to its endpoint section areas. A dry end is not charged for water stored at the opposite wet end; the two shares sum to the conduit inventory. Numerical wetting geometry in a completely dry conduit supplies no stored water. Physical chamber iterations use a head increment tolerance of at most 1e-7 ft, while keeping the configured trial limit. If that limit and timestep cannot resolve the chamber head and water budget, the run stops with a diagnostic to reduce ROUTING_STEP or increase MAX_TRIALS. This can matter for chambers much smaller than the connected pipes; the authored area remains unchanged.
1.13.2