Clarification of overbank flows
Overbank flows means flows that are greater than the channel can handle. Thus the water flow goes over the (river/stream) banks and onto the flood plain. 

Bankfull flow would be the maximum water flow that the channel could contain. 

Note that flow in the channel isn't necessarily the bankfull flow. Of course if there is overbank flow then there must be bankfull flow. 

total flow = Flow in channel + Overbank flow