UKWIR Model

The UKWIR equation was developed as a replacement to the New UK PR Model defined previously, primarily to overcome limitations of the old equation.

The UKWIR model incorporated within InfoWorks ICM is based on the model described in A Guide to the use of the UKWIR Equation (2014).

The UKWIR equation is described below:

(1) Where: IF - Effective impermeability factor PIMP - Percentage impermeability of the sub-catchment PIpv - Precipitation index - paved surface with rapid decay coefficient b - Power coefficient - paved surface PFpv - Soil store depth - paved surface NAPI - Antecedent Precipitation Index - Pervious surface with 30 day decay coefficient PIs - Precipitation index - Pervious surface with decay coefficient Cr - Power coefficient - pervious surface SPR - Standard Percentage Runoff (soils WRAP and HOST) PFs - Soil store depth - pervious surface

Using the UKWIR runoff volume model in InfoWorks

Paved surfaces

Each paved surface type should be modelled as a separate surface with its own set of parameters.

The paved surface type has a fixed runoff component and a variable runoff component based on the 'lost' rainfall on the paved surface.

From the UWKIR equation, the runoff coefficient for a paved surface is:

(2) Where: IF - Effective impermeability factor PIpv - Precipitation index with rapid decay coefficient b - Power coefficient PFpv - Soil store depth

To set up a paved surface in InfoWorks, set the Runoff volume type field of the Runoff Surface to UKWIRPaved.

The following parameters are enabled for the runoff surface:

• Effective impermeability, IF
• Precipitation decay coefficient, Cpv
• Power coefficient for PI, b
• Storage depth, PFpv

The initial value for PIpv is set in the Rainfall Event Editor, or in the Catchment Initial Conditions if using TSDB.

The precipitation index is a continuously calculated value, which is reported in the Subcatchment Results Data Fields.

Pervious surfaces

From the UWKIR equation, the runoff coefficient for a pervious surface is:

(3) Where: NAPI - Antecedent Precipitation Index (API30) Cr - Power coefficient SPR - Standard Percentage Runoff (soils WRAP and HOST) PFs - Soil store depth

To set up a pervious surface in InfoWorks, set the Runoff volume type field of the Runoff Surface to UKWIRPerv.

The following parameters are enabled for the runoff surface:

• Wetness decay for NAPI, Cs
• Power coefficient, Cr
• Storage depth, PFs
• Minimum NAPI

The initial value for NAPI is set in the Rainfall Event Editor, or in the Catchment Initial Conditions if using TSDB.

NAPI is a continuously calculated value, which is reported in the Subcatchment Results Data Fields. NAPI can be negative; the Minimum NAPI value defined in the runoff surface data fields is used as a cap to set a negative limit on the NAPI value.

Standard percentage runoff (SPR) is defined in the Subcatchment Data Fields. SPR may be user specified or determined from a specified soil class. To set SPR from soil class for a subcatchment:

• Set the flag for the Standard percentage runoff field to #D
• Select an option from the SPR calculation field. The options are:
  • WRAP
  • HOST_Soils (HOST predominant soils)
  • HOST_FEH (HOST FEH 1999)
• Enter the soil class in the WRAP soil type or HOST soil class field according to the SPR calculation type chosen

The Standard percentage runoff value will be set from the soil class specified.

Default values for SPR Calculation, WRAP soil type and HOST soil class may be specified for the network in the subcatchment User Defined Defaults.