Conduits

Conduits are pipes or channels where water flows from one node to another in the conveyance system. They have various cross-sectional shapes following some standard open and closed geometry as listed in Table 7.

Most open channels can be represented with a rectangular, trapezoidal, or user-defined irregular cross-section shape. For the latter, a Transect object defines 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 7: Available Cross-Sections shape for Conduits

Name	Parameter	Shape	Name	Parameter	Shape
Circular	Full Height		Circular Force Main	Full Height, Roughness
Filled Circular	Full Height, Filled Depth		Rectangular - Closed	Full Height, Width
Rectangular Open	Full Height, Width		Trapezoidal	Full Height, Base Width, Side Slopes
Triangular	Full Height, Top Width		Horizontal Ellipse	Full Height, Max. Width
Vertical Ellipse	Full Height, Max. Width		Arch Full Height	Max. Width
Parabolic	Full Height, Top Width		Power	Full Height, Top Width, Exponent
Rectangular- Triangular	Full Height, Top Width, Triangle Height		Rectangular- Round	Full Height, Top Width, Bottom Radius
Modified Baskethandle	Full Height, Bottom Width		Egg	Full Height
Horseshoe	Full Height		Gothic	Full Height
Catenary	Full Height		Semi- Elliptical	Full Height
Basket handle	Full Height		Semi-Circular	Full Height
Irregular Natural Channel	Transect Coordinates		Custom Closed Shape	Full Height, Shape Curve Coordinates

GeoSWMM uses 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:

$Q = \frac{1.49}{n} AR^{\frac{2}{3}}S^{\frac{1}{2}}$

Here n is the Manning roughness coefficient. The slope S 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:

$Q = 1.318CAR^{0.63} S^{0.54}$

Where C 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:

$Q = \sqrt{\frac{8g}{f}} A R^{1/2} S^{1/2}$

Here g is the acceleration of gravity and f 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)