WMS:Rational Method Basin Data

If multiple basins are selected while the dialog is opened, the edit fields are grayed out (not dimmed) and follow the rules of multi-selection. For example if you wish to set the value of i (rainfall intensity) for all basins then you can select all basins, open the Rational Method dialog, click once on the edit field for i and specify the value. C (runoff coefficient) could be set in a similar fashion, while A (area) cannot be changed when multiple basins are selected.

All values used for computing peak flow with the rational method are stored with the basin attributes so that each time you select a basin the edit fields are filled with the values of that basin. This also makes it possible to use the areas computed from a feature object, DEM, or TIN using the Compute Basin Data command in the TIN or DEM Drainage module menus, or in the Feature Objects Map module menu.

Peak flow (Q) for each basin should update automatically, however the Update Q button may be used at any time to signify that you are finished entering the value for a given parameter (alternatively you can tab to the next field, or click in another edit field).

Peak flow values for all selected basins will be computed and displayed in the main text window of the Rational Method dialog.

The time of concentration, which is used to determine an appropriate rainfall intensity (i), can be determined in one of four ways:

1. The time of concentration can be computed “outside” of WMS and entered into the appropriate edit field.
2. The Compute Tc – Basin Data button can be chosen and one of the time of concentration (or lag time) equations specified (this option is only available when you have computed basin data from either a TIN or a DEM).
3. A series of time computation arcs may be used to define overland, sheet, and channel flow within a basin and then travel times for each arc are summed to compute the total travel time or time of concentration for the basin. The Compute Tc – Map Data button accesses the dialog that allows you to combine arcs within the currently selected basin to compute a time of concentration.
4. Finally, the kinematic wave equation can be used from within the IDF curves dialog.

Related Topics

• Outlet Data
• Rainfall Intensity
• Entering Rational Method Parameters