Fixed PR Model
Runoff losses after initial losses can be defined as fixed independent of antecedent conditions. This type of representation is only advised for use with impervious areas, or pervious areas where runoff does not vary significantly with antecedent conditions. Alternative runoff models, such as the New UK PR Model should be used for permeable areas where runoff is affected by antecedent conditions.
The model defines a fixed percentage of the net rainfall, which becomes runoff. Different coefficients can be used for different areas of the catchment.
Defining a fixed runoff volume for the surface sets a constant runoff coefficient. A constant runoff coefficient is set in InfoWorks ICM either by choosing one of the pre-defined surface types, or by defining a fixed runoff volume for a surface on the Runoff Surfaces tab.
No definitive advice can be given for the fixed proportion of runoff appropriate for impermeable and permeable areas. However, a good first guess is 80% for roads and roofs and between 0% and 10% for permeable areas. In all cases it is important to collect as much rainfall and runoff data as possible to confirm that the correct values are being used. More information on appropriate values is given below
Fixed Runoff Advice
The model defines a fixed percentage of the net rainfall, which becomes runoff. Different coefficients can be used for different areas of the catchment.
In all cases it is important to collect as much rainfall and runoff data as possible so as to confirm that the correct values are being chosen.
Using fixed runoff coefficients for pervious areas is not recommended as the runoff from these areas does vary with the antecedent wetness of the catchment. In these situations it may be necessary to vary the coefficient for different storm conditions.
Defining a fixed runoff volume for the surface sets a constant runoff coefficient. Typical values for the runoff coefficient are given in the table below. It is advised that the use of these coefficients is supported either by field observations or by expert guidance.
Typical values of the runoff coefficient
| Surface Type | Description | Coefficient |
|---|---|---|
|
1 |
High quality paved roads with gullies < 100m apart |
1.00 |
|
2 |
High quality paved roads with gullies > 100m apart |
0.90 |
|
3 |
Medium quality paved roads |
0.85 |
|
4 |
Poor quality paved roads |
0.80 |
|
11 |
High density housing |
0.55 |
|
12 |
Medium density housing |
0.45 |
|
13 |
Low density housing or industrial areas |
0.35 |
|
14 |
Open areas |
0.25 |
A constant runoff coefficient is set in the software either by choosing one of the predefined surface types, or by defining a fixed runoff volume for a surface in the Runoff Surfaces tab.
Recommended values of IF
| Surface Condition | Effective impervious area factor, IF |
|---|---|
|
POOR |
0.45 |
|
FAIR |
0.60 |
|
GOOD |
0.75 |
The values in the above tables were derived in the days when runoff coefficient values referred to a whole subcatchment, before the ability to split a subcatchment into twelve surfaces was introduced. Therefore, for example, the quoted 55% runoff from high density housing is not just from the roofed areas, but includes the gardens etc that surround them.
Section 7.5 of the Wallingford Procedure states 'If the runoff volume were considered to derive from impervious surfaces alone, the average value of percentage runoff would be about 70 percent'. WaPUG user note 28 states "Roof surfaces are given a default minimum runoff of 80%. This is not based on analysis carried out as part of the development of the model, but on earlier published work on roof drainage".
It is therefore thought that values around 70 – 90% would be appropriate for impermeable (road and roof) areas.
