Dynamic Wave Options
The Dynamic Wave page in the Simulation Options dialog sets several parameters that control how the dynamic wave flow routing computations are made. These parameters have no effect on the other flow routing methods.
The Inertial Terms indicate how the inertial terms in the St. Venant momentum equation will be handled. The choices are:
1. KEEP, which maintains these terms at their full value under all conditions.
2. DAMPEN, which reduces the terms as flow comes closer to being critical and ignores them when the flow is supercritical.
3. IGNORE, which drops the terms altogether from the momentum equation, producing what is essentially a Diffusion Wave solution.
The Define Supercritical Flow By option selects the basis used to determine when supercritical flow occurs in a conduit. The choices are:
1. Water surface slope only (i.e., water surface slope > conduit slope)
2. Froude number only (i.e., Froude number > 1.0)
3. Both water surface slope and Froude number
The first two choices were used in earlier versions of SWMM, while the third choice, which checks for either condition, is now the recommended one.
The option for Force Main Equation selects which equation will be used to compute friction losses during pressurized flow for conduits that have been assigned to a Circular Force Main cross-section. The choices are:
1. Hazen-Williams equation
2. Darcy-Weisbach equation.
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The Variable Time Step option checks the box if an internally computed variable time step should be used at each routing time period and selects an adjustment (or safety) factor to apply to this time step. The variable time step is computed so as to satisfy the Courant condition within each conduit. A typical adjustment factor would be 75% to provide some margin of conservatism. The computed variable time step will not be less than 0.5 seconds nor greater than the fixed time step specified on the Time Steps page of the dialog. If the latter was set to a time limit of 0.5 seconds, then the variable time step option would be ignored.
The Time Step for Conduit Lengthening option (time step in seconds) is 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 zero means that no conduits will be lengthened. The ratio of the artificial length to the original length for each conduit is listed in the Flow Classification table that appears in the simulation 's Status Report.
The Minimum Surface Area option specifies a minimum area that is used at nodes when computing changes in water depth. If 0 is entered, then the default value of 12.566 sqft (1.167 sqm) is used. This is the area of a 4-ft diameter manhole. The value entered should be in square feet for US units or square meters for SI units.