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Cross Sections

A cross section is the 2D profile of a reach, perpendicular to the flow direction. The cross section defines for a given water level how much water the reach contains and controls how much and how fast water flows through the reach. Having sound and reliable cross section data is essential for high fidelity simulations.

  • Geometry; defines the width and volume of water of the reach for a given water level.
  • Resistance; defines how easily water flows through the reach.

There are two ways of representing the properties of a cross section:

  • Raw data
  • Processed data

Raw data describes the geometric shape of a cross section. The geometric shape is specified by a number of (x,z) coordinates, where x is the transverse distance from some point, often the left bank, and z is the corresponding bed elevation. The x coordinate is positive from left to right. The left/right sides of the cross section are defined relative to the downstream direction (direction of natural flow), i.e. when walking downstream. Usually, raw data is measured on location.

Processed data is an abstraction of a cross section. It describes properties of a reach for a number of water levels in tabular form. For each water levels, it includes:

  • Storage/surface width, bs
  • Cross sectional area, as a geometric area Ag and an effective flow area Afl
  • Resistance number
  • Hydraulic radius, R

For water levels, which are not present in the processed data table, values are estimated by interpolation.

The resistance number depends on the resistance formulation, being Chezy, Manning, etc, and is used differently for each formulation. Section 3.3 contains details on the resistance specifications and formulations.

A cross section can be specified using either raw or processed data. However, the engine works with processed data. For a cross section specified in raw data, the processed data is automatically calculated from the raw data.

The number of cross sections required for a model to perform well is determined from both physical and mathematical requirements. Physically there should be a sufficient number of cross sections to adequately define the variation in channel shape along each reach.

The cross sections are conceptually split into two different types:

  • Pre-defined cross sections.
  • User specified cross sections, either open or closed.

An open cross section typically defines a river bed. Closed cross sections typically define pipes in sewers.

The different types of cross sections are discussed below.