Option

Description

Inertial Terms

Indicates how the inertial terms in the St. Venant momentum equation will be handled.

• Keep - Maintains these terms at their full value under all conditions.

• Dampen - Reduces the terms as flow comes closer to being critical, and ignores them when flow is supercritical.

• Ignore - Drops the terms altogether from the momentum equation, producing what is essentially a Diffusion Wave solution.

Variable Time Step Adjustment Factor

This is an adjustment factor, between 0 and 200%, applied when computing a variable time step at each routing period that satisfies the Courant criterion in each conduit and yet prevents an excessive change in water depth at each node.

A value of 0 indicates that the user-specified fixed routing time step should be used instead of a variable one. If a variable step is used, a typical adjustment factor would be 75% to provide some margin of conservatism.

Time Step for Conduit Lengthening

This is a time step, in seconds, used to artificially lengthen conduits so that they meet the Courant stability criterion under full-flow conditions i.e. the travel time of a wave will not be smaller than the specified conduit lengthening time step. As this value is decreased, fewer conduits will require lengthening.

A value of 0 means that no conduits will be lengthened. The ratio of the artificial length to the original length for each conduit is listed in the Conduit Flow Summary Table that appears in the simulation's Status Report.

Minimum Surface Area

This is a minimum surface area used at nodes when computing changes in water depth.

If 0 is entered, the default value of 1.167 sq. m (12.566 sq. ft) is used. This is the area of a 1.2 m (4-ft) diameter manhole. The value entered should be in square feet for US units, or square metres for SI units.

Use Normal Flow Limit

Selects which condition is used to make SWMM limit the flow in a conduit to the normal flow computed from the Manning equation:

• The water surface slope Inclination expressed as a vertical to horizontal ratio and where the vertical component is assumed to be 1. So "1:2" will be one unit vertically for every two units horizontally. is less than the conduit slope.

• the Froude number at either end of the conduit is greater than 1.0 i.e. the flow is supercritical.