Visual Objects

Figure 3.1 depicts how a collection of GeoSWMM’s visual objects might be arranged together to represent a stormwater drainage system. These objects can be displayed on a map in the GeoSWMM workspace. The following sections describe each of these objects.

Rain Gages

Rain Gages supply precipitation data for one or more subcatchment areas in a study region. The rainfall data can be either a user-defined time series or come from an external file. Several different popular rainfall file formats currently in use are supported, as well as a standard user-defined format.

The principal input properties of rain gages include:

• Rainfall data type (e.g., intensity, volume, or cumulative volume)
• Recording time interval (e.g., hourly, 15-minute, etc.)
• Source of rainfall data (input time series or external file)
• Name of rainfall data source

Subcatchments

Subcatchments are hydrologic units of land whose topography and drainage system elements direct surface runoff to a single discharge point. The user is responsible for dividing a study area into an appropriate number of subcatchments, and for identifying the outlet point of each subcatchment. Discharge outlet points can be either nodes of the drainage system or other subcatchments.

Subcatchments can be divided into pervious and impervious subareas. Surface runoff can infiltrate into the upper soil zone of the pervious subarea, but not through the impervious subarea. Impervious areas are themselves divided into two subareas - one that contains depression storage and another that does not. Runoff flow from one subarea in a subcatchment can be routed to the other subarea, or both subareas can drain to the subcatchment outlet.

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

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

To model the accumulation, re-distribution, and melting of precipitation that falls as snow on a subcatchment, it must be assigned a Snow Pack object. To model groundwater flow between an aquifer underneath the subcatchment and a node of the drainage system, the subcatchment must be assigned a set of Groundwater parameters. Pollutant buildup and washoff from subcatchments are associated with the Land Uses assigned to the subcatchment. Capture and retention of rainfall/runoff using different types of low impact development practices (such as bio-retention cells, infiltration trenches, porous pavement, vegetative swales, and rain barrels) can be modeled by assigning a set of predesigned LID controls to the subcatchment.

The other principal input parameters for subcatchments include:

• Assigned rain gage
• Outlet node ID
• Assigned land uses
• Tributary surface area
• Imperviousness
• Slope
• Characteristic width
• Manning's n for overland flow on both pervious and impervious areas
• Depression storage in both pervious and impervious areas
• Percent of impervious area with no depression storage

Junction Nodes

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 pressurized 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.

The principal input parameters for a junction are:

• Invert elevation
• Height to ground surface
• Ponded surface area when flooded (optional)
• External inflow data (optional)

Outfall Nodes

Outfalls are terminal nodes of the drainage system 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.

The principal input parameters for outfalls include:

• Invert elevation
• Boundary condition type and stage description
• Presence of a flap gate to prevent backflow through the outfall.

Flow Divider Nodes

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 Divider: Diverts all inflow above a defined cutoff value
• Overflow Divider: Diverts all inflow above the flow capacity of the non-diverted conduit
• Tabular Divider: Uses a table that expresses diverted flow as a function of total inflow
• Weir Divider: Uses a weir equation to compute diverted flow

The flow diverted through a weir divider is computed by the following equation:

Where is diverted flow, is Weir coefficient, is Weir height and is computed as:

Where is the inflow to the divider, is the flow at which diversion begins, and

The user-specified parameters for the weir divider are , and .

The principal input parameters for a flow divider are:

• Junction parameters (see above)
• Name of the link receiving the diverted flow
• Method used for computing the amount of diverted flow

Storage Units

Storage Units are drainage system nodes that provide storage volume. Physically they could represent storage facilities as small as a catch basin 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.

The principal input parameters for storage units include:

• Invert elevation
• Maximum depth
• Depth-surface area data
• Evaporation potential
• Ponded surface area when flooded (optional)
• External inflow data (optional)

Conduits

Conduits are pipes or channels in which water flows from one node to another in the conveyance system. They have a wide variety of cross-sectional shapes following some standard open and closed geometry as listed in Table 3.1.

Most open channels can be represented with a rectangular, trapezoidal, or user-defined irregular cross-section shape. For the latter, a Transect object is used to define how the channel’s depth varies with distance across the cross-section. The most common shapes for new drainage and sewer pipes are circular, elliptical, and arch pipes. They come in standard sizes that are published by the American Iron and Steel Institute in Modern Sewer Design and by the American Concrete Pipe Association in the Concrete Pipe Design Manual. The Filled Circular shape allows the bottom of a circular pipe to be filled with sediment and thus limit its flow capacity. The Custom Closed Shape allows any closed geometrical shape that is symmetrical about the centerline to be defined by supplying a Shape Curve for the cross section.

Table 3.1: Available Cross-Sections shape for Conduits

GeoSWMM uses the Manning’s equation to express the relationship among the flow rate (Q), cross-sectional area (A), hydraulic radius (R), and slope (S) in all conduits. For standard U.S. units:

Here  is the Manning roughness coefficient. The slope is interpreted as either the conduit slope or the friction slope (i.e., head loss per unit length), depending on the flow routing method used.

For pipes with Circular Force Main cross-sections, either the Hazen-Williams or Darcy-Weisbach formula is used in place of the Manning equation for fully pressurized flow. For U.S. units the Hazen-Williams formula is:

Where is the Hazen-Williams friction factor that varies inversely with surface roughness and is supplied as one of the cross-section’s parameters. The Darcy-Weisbach formula is:

Here  is the acceleration of gravity and is the Darcy-Weisbach friction factor. For turbulent flow, the latter is determined from the height of the roughness elements on the walls of the pipe (supplied as an input parameter) and the flow’s Reynolds Number using the Colebrook-White equation. The choice of which equation to use is a user-supplied option.

A conduit does not have to be assigned a Force Main shape for it to pressurize. Any of the closed cross- section shapes can potentially pressurize and thus function as force mains that use the Manning equation to compute friction losses. 

A conduit can also be designated to act as a culvert if a Culvert Inlet Geometry code number is assigned to it. These code numbers are listed in Table A.10 of Appendix A. Culvert conduits are checked continuously during dynamic wave flow routing to see if they operate under Inlet Control as defined in the Federal Highway Administration’s publication Hydraulic Design of Highway Culverts (Publication No. FHWA-NHI-01-020, May 2005). Under inlet control a culvert obeys a particular flow versus inlet depth rating curve whose shape depends on the culvert’s shape, size, slope, and inlet geometry.

The principal input parameters for conduits are:

• Names of the inlet and outlet nodes
• Offset height or elevation above the inlet and outlet node inverts
• Conduit length
• Manning's roughness
• Cross-sectional geometry
• Entrance/exit losses (optional)
• Presence of a flap gate to prevent reverse flow (optional)

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. Four different types of pump curves are supported:

• Type1: An off-line pump with a wet well where flow increases incrementally with available wet well volume
• Type2: An in-line pump where flow increases incrementally with inlet node depth
• Type3: An in-line pump where flow varies continuously with head difference between the inlet and outlet nodes
• Type4: A variable speed in-line pump where flow varies continuously with inlet node depth

Ideal: An "ideal" transfer pump whose flow rate equals the inflow rate at its inlet node. No curve is required. The pump must be the only outflow link from its inlet node. Used mainly for preliminary design.

The on/off status of pumps can be controlled dynamically by specifying startup and shutoff water depths at the inlet node or through user-defined Control Rules. Rules can also be used to simulate variable speed drives that modulate pump flow.

The principal input parameters for a pump include:

• Names of its inlet and outlet nodes
• Name of its pump curve
• Initial on/off status
• Startup and shutoff depths

Flow Regulators

Flow Regulators are structures or devices used to control and divert flows within a conveyance system. They are typically used to:

• Control releases from storage facilities
• Prevent unacceptable surcharging
• Divert flow to treatment facilities and interceptors

GeoSWMM can model the following types of flow regulators:

• Orifices
• Weirs
• Outlets

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 GeoSWMM 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 analyzed with Dynamic Wave flow routing.

The flow through a fully submerged orifice is computed as:

Where = flow rate, = discharge coefficient, = area of orifice opening, = acceleration of gravity, and= head difference across the orifice. The height of an orifice's opening can be controlled dynamically through user-defined Control Rules. This feature can be used to model gate openings and closings.

The principal input parameters for an orifice include:

• Names of its inlet and outlet nodes
• Configuration (bottom or side)
• Shape (circular or rectangular)
• Height or elevation above the inlet node invert
• Discharge coefficient
• Time to open or close

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 GeoSWMM 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 across the weir as listed in Table 3.2.

Table 3.2 : Available types of Weirs

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 analyzed 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.

The principal input parameters for a weir include:

• Names of its inlet and outlet nodes
• Shape and geometry
• Crest height or elevation above the inlet node invert
• Discharge coefficient

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 characterized by pumps, orifices, or weirs. Outlets are internally represented in GeoSWMM 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 analyzed with Dynamic Wave flow routing.

A user-defined rating curve determines an outlet's discharge flow as a function of either the freeboard depth above the outlet's opening or the head difference across it. Control Rules can be used to dynamically adjust this flow when certain conditions exist.

The principal input parameters for an outlet include:

• Names of its inlet and outlet nodes
• Height or elevation above the inlet node invert
• Function or table containing its head (or depth) - discharge relationship.