RAFTS Routing Model
In the RAFTS Routing Model (also known as the Laurenson Runoff Routing procedure) specified data is used to calculate the storage delay coefficient for the subcatchment in order to develop a non-linear runoff hydrograph.
Each subcatchment is divided into 10 sub-areas, each of which is treated as a cascading non-linear storage with the relationship:
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Where: S is the volume of storage (hr.m3/s) B is the storage delay time coefficient (RAFTS B). See below. n is the storage non-linear exponent (specified in the RAFTS n field in a subcatchment's properties ) |
During a simulation, rainfall is applied to each sub-area, an excess is calculated and converted into an instantaneous inflow. This instantaneous flow is then routed through the sub-area storages to develop an individual subcatchment outlet hydrograph.
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.
Storage delay time coefficient (RAFTS B)
The storage delay time coefficient (B in Equation 1) can either be calculated for the entire subcatchment or for each runoff surface referenced by the subcatchment, and can either be specified by the user or calculated by InfoWorks ICM.
To calculate RAFTS B on a subcatchment basis, the Per-surface RAFTS B box must be unchecked in a subcatchment's properties, and a value can either be entered by the user in the RAFTS B field in a subcatchment's properties, or if its default flag (#d) is set, InfoWorks ICM will calculate RAFTS B for the entire subcatchment.
To calculate RAFTS B for each runoff surface associated with a subcatchment, the Per-surface RAFTS B box must be checked in the relevant subcatchment's properties. If RAFTS B is to be specified by a user, the User-specified RAFTS B box must be checked in each applicable runoff surface's properties, and a RAFTS B value specified in the Runoff routing value field. If you want InfoWorks ICM to calculate RAFTS B, the User-specified RAFTS B box must be unchecked.
InfoWorks ICM calculates RAFTS B by first determining PERN from equivalent roughness (Equivalent Manning's n in either the subcatchment's properties (on a per subcatchment basis) or runoff surface's properties (on a per runoff surface basis) by linear interpolation as shown in the following table:
| n |
PERN |
|---|---|
| 0.01 | 0 .4 |
| 0.015 | 0.5 |
| 0.025 | 1.0 |
| 0.10 | 3.0 |
And then using the following equation:
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Where:
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Degree of urbanisation (U)
The degree of urbanisation (U) is used in the calculation of RAFTS B.
When RAFTS B is calculated on a per subcatchment basis, the degree of urbanisation can either be specified by the user in the Degree urbanisation field in a subcatchment's properties or, if its default flag (#d) is set, InfoWorks ICM will calculate it as a linear interpolation from the subcatchment's percentage impervious as shown in the following table:
| % Impervious |
Degree of Urbanisation (U) |
|---|---|
| 0 | 0 |
| 30 | 0.7 |
| 50 | 1 |
| 100 | 2 |
To calculate the percentage impervious, InfoWorks ICM checks all the runoff surfaces referenced by the subcatchment. A surface that does not have Runoff volume type of Fixed has 100% (set in the runoff surface properties), or 0% if its Surface type is Impervious or Pervious respectively. A fixed runoff surface's percentage impervious is set in the Fixed runoff coefficient field. The subcatchment's percentage impervious is the average of its referenced surfaces, weighted by area.
When RAFTS B is calculated on a per runoff surface basis, the value for the degree of urbanisation (U) depends on the setting of the Surface type field in the runoff surface's properties. If this is set to Impervious, InfoWorks ICM assumes the surface to be 100% impervious, and therefore a value of 2 is used for U. If the Surface type is set to any other option, the surface is assumed to be 0% impervious and a value of 0 is used for U.
