Surface erosion and Gully Pot flushing calculations effectively take place in parallel because the two parts of the model are completely independent.
For more information, see Surface Washoff and Gully Pot Flushing.
Surface erosion
The three parts of the surface sediment calculation take place in parallel. They all use the initial sediment mass at the start of the timestep.
Surface erosion and the potency of attached pollutants are calculated using the rainfall intensity.
All the sediment washed off the surface is treated as Sediment Fraction 1.
The rate at which eroded sediment runs off into the system is calculated using the runoff model chosen for the hydraulic calculations.
The calculations use the runoff from Runoff Surface 1 and Runoff Surface 2 defined in the Land Use Definition. By convention these are both impervious surfaces - Runoff Surface 1 is the road surface and Runoff Surface 2 is the roof area.
Surface build-up continues during storm events and the build-up is calculated in the same way as during dry weather periods.
Gully pot flushing
The amount of pollutant flushed out of gully pots is calculated using a value for surface runoff.
The calculations use the runoff from Runoff Surface 1 defined in the Land Use Definition. By convention, Runoff Surface 1 is the road surface.
The runoff is calculated by the hydraulic engine using the runoff model you selected for hydraulic calculations.
After the gully pot flushing calculation, the software calculates the amount of build-up in gully pots. The calculation is the same as that used during dry weather periods.
Build-up
During periods of rainfall, build-up of sediment and pollutants continues, even though the amount is probably insignificant compare to wash-off.
The calculation is carried out in the same way as during dry weather flow periods. See Surface Pollutant Build-Up and Gully Pot Pollutant Build-Up.