ReFH Unit Hydrograph Model

The ReFH (Revitalised Flood Hydrograph) unit routing model uses a kinked triangle unit hydrograph to calculate runoff resulting from subcatchment net rainfall.

For further information, see the R&D Technical Report FD1913/TR Revitalisation of the FSR/FEH Rainfall Runoff Method.

Time to peak, Tp, Unit hydrograph peak, Up and Degree of kink, Uk are specified in the Subcatchment Grid Window of the Subcatchments Grid or on the Subcatchment Property Sheet.

These fields are enabled when the Unit Hydrograph Definition field of the Subcatchment is set to ReFH.

Equations

The equations used by InfoWorks ICM are derived from the documentation of the ReFH Unit Hydrograph in the R&D Technical Report FD1913/TR Revitalisation of the FSR/FEH Rainfall Runoff Method. See the report for more details.

The ReFH standard instantaneous unit hydrograph (IUH) shape is a kinked triangle described by a time scaling parameter, T_p, and two dimensionless numbers, U_p and U_k.

Volume under IUH must equal unity: Up(TBt/Tp)/2 must equal 1 (1) Rearranging: Up=2 (Tp/TBt) (2)

The parameter uk is a multiplier applied to the ordinate U_c of the IUH at 2Tp:

Uc = Up(TBt - 2Tp) / (TBt - Tp) (3) If Uk = 1 the IUH is a simple triangle, but as Uk drops towards zero, the lost area is transferred into the IUH tail by extending the overall time base TB.  TB = (1+2(1-Up)/(Uk Uc ))Tp (4)

The S-curve method is used to convert the IUH into an equivalent hour unit hydrograph for the required timestep. See the Technical Report for further details.