WMS:User Defined Time of Concentration

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Because it would be impossible to contain all possible equations used for computing travel times, a user defined equation may be defined for any arc segment. When “user defined” is selected for the arc type the Modify Equation button in the Time Computation Attributes dialog is active. By selecting this button, the Modify Equation dialog can be used to create/modify a suitable equation. User defined equations can be created by typing in the equation edit field using the following rules for precedence (order of operations):

  1. Parts of the equation in parentheses have the highest precedence.
  2. Multiplication and division have higher precedence than addition and subtraction.
  3. Equations are evaluated from left to right.

Besides typing in the equation, one of the pre-defined equations can be selected from the Sample equations drop-down list and add it to the equation line as a starting point to create a new equation. Any of the variables that WMS can manage can be used as part of the equation (e.g. length, slope, rainfall intensity, etc.). However, it is not possible to add a new "variable" since there is no way for WMS to manage it. If a variable that is not managed by WMS is used in the equation, determine what the appropriate value for the selected arc would be and enter it as a constant in the user defined equation. Variables are added to an equation by either typing the abbreviation or selecting the variable to use and clicking on the Add to Equation button. In a similar fashion, mathematical operators can either be typed or the corresponding button selected to add them to the equation.


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