WMS:Rational Method

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WMS includes an interface to the rational method which can be used for computing peak flows on small urban and rural watersheds. The interface includes the capability to combine runoff from multiple basins. Two different methods for determining peak flows/hydrographs at downstream confluences are available.

Traditionally a time of concentration is determined at a downstream confluence by determining the longest combination of time of concentration and routing travel time. Given a time of concentration for the outlet, a rainfall intensity can be determined from a rainfall-intensity-duration curve and a peak flow computed. The hydrograph for the confluence is then determined in the same manner they are determined for sub-basins; by using the peak flow, time of concentration, and a dimensionless hydrograph.

Alternatively, hydrographs for the sub-basins can be computed and then routed (lagged) and combined by summing at the confluence points. When using this method detention basins may be defined at confluence points in order to determine the effect of storage on the computations.

All of the computations for peak flows, hydrographs, and routing are done within WMS.

The Rational Method interface is included with all paid editions of WMS.

Rational Menu

The Rational menu, included in the Hydrologic Modeling module, has the following commands:

  • Read Simulation – Restore the tree and parameters to continue with a particular model. Imports the Rational Method File format (*.rat).
  • Save Simulation – Used to save the topologic tree structure with any rational method parameters which have been defined. Saved in the Rational Method File format (*.rat).
  • Run Simulation – Brings up the Rational Method dialog to set simulation parameters and run the model.

Important Limitations

Due to assumptions regarding homogeneity of rainfall and equilibrium conditions at the time of peak flow, the rational method should not be used on areas larger than about 1 square mile without subdividing the overall watershed into sub-basins including the effect of routing through any drainage channels.

WMS includes two different methods for determining runoff from larger watersheds, subdivided into smaller sub-basins, including the ability to account for routing and lag through drainage channels and detention basins.

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