For the first flush modelling, total incoming water volume Vtot and TSS load Ftot are computed for each rainfall event (including dry weather flow, inflow and sediment fraction). Afterwards accumulated load F(t)=(V(t)/Vtot)^b * Ftot is determined for each time step. TSS concentration can be calculated from the fraction load/volume c(TSS)=(F(t)-F(t-1))/(V(t)-V(t-1)). It corresponds to the slope of the curves in the figure below. If b=1 is set, no first flush is modelled. In this case TSS concentration is modelled to be constant during the whole rainfall event. For a normal concentration calculation without first flush, concentration would change as a result of mixing waste- and rainwater. But TSS concentration would fall during rainfall because the rainwater concentration is normally smaller than the wastewater concentration. It is questionable if this is realistic.
