Glossary

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A

Advection-Dispersion Equation – the partial differential equation that expresses conservation of mass for a water quality constituent with respect to time and space across an element of fluid.

Aquifer – as defined in SWMM, it is the underground water bearing layer below a land surface, containing both an upper unsaturated zone and a lower saturated zone.

Availability Factor – the fraction of buildup on a land use that is available for removal by street sweeping.

B

Best Management Practice - structural or engineered control devices and systems (e.g. retention ponds) as well as operational or procedural practices used to treat polluted stormwater.

Bio-Retention Cell – a LID control that contains vegetation grown in an engineered soil mixture placed above a gravel storage bed providing storage, infiltration and evaporation of both direct rainfall and runoff captured from surrounding areas.

BMP Removal Factor – the fractional reduction in runoff pollutant load achieved by implementing a specific BMP.

C

Capillary Suction Head - the soil water tension at the interface between a fully saturated and partly saturated soil.

Capture Ratio – the amount of the subcatchment's impervious area that is directly connected to an LID unit divided by the area of the LID unit itself.

Completely Mixed Reactor – a reactor where the concentration of all water quality constituents are uniform throughout the reactor's volume.

Continuous Simulation - refers to a simulation run that extends over more than just a single rainfall event.

Co-Pollutant – a pollutant whose runoff concentration is a fixed fraction of some other pollutant (e.g., phosphorus adsorbed onto suspended solids).

Curve Number - a factor, dependent on land cover, used to compute a soil's maximum moisture storage capacity.

Curve Number Method - a method that uses a soil's maximum moisture storage capacity as derived from its curve number to determine how cumulative infiltration changes with cumulative rainfall during a rainfall event. Not to be confused with the NRCS (formerly SCS) Curve Number runoff method as embodied in TR-55.

D

Darcy's Law - states that flow velocity of water through a porous media equals the hydraulic conductivity of the media times the gradient of the hydraulic head it experiences.

Depression Storage – the volume over a surface that must be filled prior to the occurrence of runoff. It represents such initial abstractions as surface ponding, interception by flat roofs and vegetation, and surface wetting.

Design Storm - a rainfall hyetograph of a specific duration whose total depth corresponds to a particular return period (or recurrence interval), usually chosen from an IDF curve.

Directly Connected Impervious Area - impervious area whose runoff flows directly into the collection system without the opportunity to run onto pervious areas such as lawns.

Drainage Mat - thin, multi-layer fabric mats with ribbed undersides that carries away any water that drains through the soil layer of a green roof.

Dry Deposition – pollutants deposited on land surfaces, typically in the form of particles, during periods of dry weather.

Dry Weather Flow - the continuous discharge of sanitary or industrial wastewater directly into a sewer system.

Dust and Dirt – street surface accumulation that passes through a quarter-inch mesh screen.

Dynamic Wave Flow Routing – a method of modeling non-uniform unsteady open channel flow that solves the full Saint Venant equations for both continuity and momentum. It can account for channel storage, backwater effects, and flow reversals.

E

Event Mean Concentration – the average concentration of a pollutant in the runoff produced by a single storm event.

F

Field Capacity - the amount of water a well-drained soil holds after free water has drained off, or the maximum soil moisture held against gravity. Usually defined as the moisture content at a tension of 1/3 atmospheres.

First Order Decay – a pollutant decay reaction whose rate is proportional to the concentration of pollutant remaining.

G

Green-Ampt Method - a method for computing infiltration of rainfall into soil that is based on Darcy's Law and assumes there is a sharp wetting front that moves downward from the surface, separating saturated soil above from drier soil below.

Green Roof – a type of bio-retention cell used on a roof that has a soil layer above a thin layer of synthetic drainage mat material that conveys excess water draining through the soil layer off of the roof.

H

Hydraulic Conductivity - the rate of water movement through soil under a unit gradient of hydraulic head. Its value increases with increasing soil moisture, up to a maximum for a completely saturated soil (known as the saturated hydraulic conductivity or K~sat~).

Hydraulic Residence Time - the average time that water has spent within a completely mixed reactor.

I

Impervious Surface – a surface that does not allow infiltration of rain water, such as a roof, roadway or parking lot.

Infiltration – the process by which rainfall penetrates the ground surface and fills the pores of the underlying soil.

Infiltration Trench – a narrow ditch filled with gravel that intercepts runoff from upslope impervious areas and provides storage volume and additional time for captured runoff to infiltrate into the native soil.

Initial Abstraction – precipitation that is captured on vegetative cover or within surface depressions that is not available to become runoff and is removed by either infiltration or evaporation.

L

Land Use Object - categories of development activities or land surface characteristics used to account for spatial variation in pollutant buildup and washoff rates.

LID Control – a low impact development practice that provides detention storage, enhanced infiltration and evapotranspiration of runoff from localized surrounding areas. Examples include rain gardens, rain barrels, green roofs, vegetative swales, and bio-retention cells.

Link – a connection between two nodes of a SWMM conveyance network that transports water. Channels, pipes, pumps, and regulators (weirs and orifices) are all represented as links in a SWMM model.

Longitudinal Dispersion – the process whereby a portion of a constituent's mass inside a parcel of water mixes with the contents of parcels on either side of it due to velocity and concentration gradients.

M

Manning Equation – the equation that relates flow rate to the slope of the hydraulic grade line for gravity flow in open channels.

Manning Roughness – a coefficient that accounts for friction losses in the Manning flow equation.

Moisture Deficit – the difference between a soil's current moisture content and its moisture content at saturation.

N

Node – a point in a runoff conveyance system that receives runoff and other inflows, that connects conveyance links together, or that discharges water out of the system. Nodes can be simple junctions, flow dividers, storage units, or outfalls. Every conveyance system link is attached to both an upstream and downstream node.

O

Overland Flow Path – the path that runoff follows as it flows over a surface until it reaches a collection channel or drain.

P

Permeable Pavement - street or parking areas paved with a porous concrete or asphalt mix that sits above a gravel storage layer allowing rainfall to pass through it into the storage layer where it can infiltrate into the site\'s native soil.

Pervious Surface – a surface that allows water to infiltrate into the soil below it, such as a natural undeveloped area, a lawn or a gravel roadway.

Pollutant Object – the representation of a water quality constituent within SWMM.

Pollutograph – a plot of the concentration of a pollutant in runoff versus time.

Porosity - the fraction of void (or air) space in a volume of soil.

Potency Factor – relates the concentration of the particulate form of a pollutant (such as phosphorous or heavy metals) to the concentration of total suspended solids.

R

Rainfall Dependent Inflow and Infiltration - stormwater flows that enter sanitary or combined sewers due to "inflow" from direct connections of downspouts, sump pumps, foundation drains, etc. as well as "infiltration" of subsurface water through cracked pipes, leaky joints, poor manhole connections, etc.

Rain Barrel – a container that collects roof runoff during storm events and can either release or re-use the rainwater during dry periods.

Rain Garden - a type of bio-retention cell consisting of just an engineered soil layer with no gravel bed below it.

Richards Equation – the nonlinear partial differential equation that describes the physics of water flow in unsaturated soil as a function of moisture content and moisture tension.

Rooftop Disconnection – the practice of directing roof downspouts onto pervious landscaped areas and lawns instead of directly into storm drains.

S

Steady Flow Routing – a method of modeling uniform steady open channel flow that translates inflow hydrographs at the upstream end of the channel to the downstream end, with no delay or change in shape.

Subcatchment – a sub-area of a larger catchment area whose runoff flows into a single drainage pipe or channel (or onto another subcatchment).

T

Tanks in Series Model – an approach to solving constituent transport where conduits are represented as completely mixed reactors connected together at junctions or at completely mixed storage nodes.

U

Underdrain – slotted pipes placed in the storage layer of an LID unit that conveys excess captured runoff off of the site and prevents the unit from flooding.

V

Vegetative Swale - channels or depressed areas with sloping sides covered with grass and other vegetation that slows down the conveyance of collected runoff and allows it more time to infiltrate into the native soil.

W

Wet Deposition - pollutant loads contributed by direct rainfall on a catchment.

Wilting Point - the soil moisture content at which plants can no longer extract moisture to meet their transpiration requirements. It is usually defined as the moisture content at a tension of 15 atmospheres.