Conduit Shapes Explained

Product(s): SewerGEMS, CivilStorm, SewerCAD, StormCAD
Version(s): CONNECT Edition, V8i
Area: Modeling

Background

There are many different shapes available to select when modeling a conduit in the above storm and sewer products. This article explains the assumptions and use of each. Input field names are shown in bold red.

Table of Contents

Arch

The cross sectional shape of an Arch conduit is loosely defined by its Span and Rise. The remaining attributes are derived based upon internal look-up tables native to the EPA-SWMM computations, and therefore, differs slightly from the "Pipe Arch" conduit shape. This section shape is currently supported with all four solvers.

Pipe-Arch

The Pipe-Arch shape originates from FHWA publications, and differs slightly from the more generically defined "Arch" shape which is based on EPA-SWMM and is defined with a series of additional input parameters (see below). Currently, a conduit may use a Pipe-Arch conduit shape only when the conduit type is a conduit catalog reference (it is not available in the list of shapes when choosing User Defined). In addition to Rise and Span, the cross sectional characteristics of a Pipe-Arch include:

  • Rc = Corner Radius
  • Rb = Bottom Radius
  • Rt = Top Radius
  • B = Bottom Distance
  • Full Area = cross sectional full flow area of the pipe.

Also note that when using the Implicit solver, the Pipe-Arch shape is internally converted to an equivalent regular Arch shape (see Arch above).

Basket-Handle

The required input to define the size of a Basket-Handle Arch is simply its Height or Rise (H).  Unique to basket-Handle shape is the ratio of the bottom width (W) of the Arch to be .99135 of the Height (H).  The remaining shape characteristics are derived based upon internal look up tables to the SWMM implementation.  These auxiliary attributes include:

See also: Modified Basket Handle

Box

The box shape is defined by the Rise and the Span. These are the height and the width (respectively) of a rectangular enclosed shape.

Catenary

The only user input to define a Catenary Arch is its Height or Rise (H). The geometric shape is principally with a cosine function,whose coefficients a and b are determined within hydraulic calculations. SWMM calculations estimate the geometry with a best-fit interpolation of geometric rating tables. 

For more information on how the effective shape or flow area is calculated when using the Catenary conduit section shape, see: Catenary conduit shape calculation

Circle

A circular pipe or channel is simply defined by its Diameter (D). This is the Inside Diameter (see more here). Optimal conveyance is achieved when the flow depth is about 0.938*D. (above this, there is an accelerating increase in surface area and friction).

Egg

The only user input to define a Egg Shaped Arch is its Height or Rise (H). The remaining attributes are derived based upon tables internal to the SWMM implementation. Its unique cross sectional characteristics are based upon a top radius equal to one-third of its height.  The definition of the bottom portion of the arch is more complex and based upon internal angles 

Ellipse

An Elliptical shaped pipe is defined by its Height or Rise (H).  The elliptical coefficients of A and B respectively correspond to H/2 and W/2. 

Gothic

A Gothic Arch is well defined based upon its Height or Rise (H).  At its greatest width (W) the unique proportion of W to H is 1.115.  Also unique to this arch type is the existence of the equilateral triangle with side lengths of W. 

Horseshoe

The required input to define the size of a Horseshoe Arch is simply its Height or Rise (H). The remaining attributes are derived based upon tables internal to the SWMM implementation. Sometimes referred to as “Boston Horseshoe Arch”, the unique cross sectional characteristics are based upon a top radius equal to half of its height.  The definition of the bottom half of the arch is more complex and based upon internal angles 

Irregular Channel (Irregular Open)

An irregular open channel is defined by a series of Station and Elevation points. Referenced when using a Bank Channel Roughness type, the Left and Right overbanks are highlighted in the above example.

This section type is used to model a custom shape with an open top, when you have data for station vs. elevation. Contrast with the Irregular closed channel option which is used to model a custom shape with a closed top when you have data for depth vs. width.

Note: the "Channel Weighting Method" and the ability to vary roughness by depth or flow, are only available with the Implicit solver. Only a single roughness coefficient can be modeled with the Explicit (SWMM) solver.

Irregular Closed Section

An irregular closed section is defined by a Depth Width curve which is a series of Relative Elevation and respective Width points. The elevation is relative to the invert, so it is actually a depth. The width is the width of flow at that respective depth.

This section type is used to model a custom shape with a closed top when you have data for depth vs. width. Contrast with the Irregular open channel option which is used to model a custom shape with an open top, when you have data for station vs. elevation.

Modified Basket-Handle

The Modified Basket Handle Arch is only defined by its Rise or Height (H) and its Span or Width (W).  The top portion is a semi-circular in shape with a radius equal in size to half its Span (W).

See also: Basket Handle

Parabolic Channel

The Parabola section type represents a channel defined with a general form parabolic function requires the input of the Rise or Height (H), Span or Width (W) and the quadratic coefficient (a) to the following equation:

y = ax^2

Power Channel

A channel defined with the power function requires the input of the exponent (n) to the following general form equation:

y = x^n

Rectangular Channel

The Rectangular Channel shape is defined by the Rise and the Span. These are the height and the width (respectively) of a rectangular enclosed shape.

Rectangular Round

A Rectangular-Rounded shape is completely defined by three attributes:

H = Height (Rise)

W = Width (Span)

D = Bottom Radius of the rounded portion of the channel

Rectangular Triangle

A Rectangular-Triangular shape is completely defined by three attributes:

H1 = Height (Rise) of the entire cross section

H2 = Triangle Height (height of the triangular shaped section of the channel)

W = Width (span) of the entire cross section

Semi Circle

A Semi-Circular Arch is sized only by its Rise or Height (H).  The semi-circular shape has a radius (R) equal to its Rise (H).  At its largest, at its base, its width is twice the Rise (H). 

Semi Ellipse

The required input to define the size of a Semi-Elliptical Arch is simply its Height or Rise (H).  Unique to this arch shape is the characteristic that at its widest dimension, at its base, the width is equal to its Rise (H).  The shape is geometrically defined with the function of an ellipsis, where the traditional elliptical coefficients of A and B respectively correspond to H/2 and H. 

Trapezoidal Channel

A trapezoidal channel requires the input of the Rise (H) and its bottom width (W).  Additionally, the left (Sl) and right (Sr) side slopes should be provided.

Triangular Channel

A triangular (or v-notch) shaped channel is based on it's input Rise (H) and Span (width). The channels left (SL) and right (SR) side slopes are assumed equivalent.

Virtual

The "Virtual" option (available for user defined conduits) is used to model connectivity between two nodes with no hydraulic impact. A virtual pipe basically means that the flow that comes into one end instantly goes to the other, with no hydraulic effects. It's essentially a way to connect elements without impacting the hydraulics. 

See more: What is a virtual pressure pipe and how virtual pipes are handled by each solver?

See Also

Modeling non-standard conduit or channel shapes 

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