Rational Routing Model
The Rational Routing Model is a triangular Unit Hydrograph Routing Model with the following properties:
Tp (time of peak) = Tc (time of concentration), Tb (base time) = 2 * Tp.
The "Rational" Unit hydrograph definition must be used on any subcatchment using a surface that uses the rational routing model.
The difference between the rational and unit routing models is that the input hydrograph is discretised at intervals of Tc instead of the run time.
See the Hydrograph method with rational formula section of the Japanese Runoff Methodology technical paper for further information.
The rational runoff formula is:
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where: Q is runoff (m3/s) C is the runoff coefficient ( 0< C <= 1) I is average rainfall over the time of concentration (m/s) A is subcatchment area (m2) |
The runoff routing model for a Runoff Surface is set in the Runoff Surface Grid Window of the Subcatchments Grid or in the Runoff Surface Property Sheet.
The easiest way to set up surfaces for subcatchment rational routing is to create a single surface with the following runoff surface properties:
| Runoff Surface Data Field |
Value |
|---|---|
| Surface type | Impervious |
| Runoff routing type | Abs(not used) |
| Runoff routing value | 1.0 (not used) |
| Ground slope | 0 |
| Runoff volume type | Fixed |
| Fixed runoff coefficient | 1.0 |
| Initial loss porosity | 1.0 |
| Initial loss type | Abs |
| Initial loss value | 0 |
| Routing model | Rational |
On a subcatchment using this surface, ensure that the Subcatchment properties are set to:
| Subcatchment Data Field |
Value |
|---|---|
| Area measurement type | Percent |
| Runoff rea 1 (%) | C * 100 |
