Incorporation of returning outflows during flood

Hello there, I am modelling a reservoir which during flood periods has water flowing back in from the outflow river (which backs up from downstream due to overland flow and overtops the dam and into the reservoir (i.e. sometimes the dam outflows are negative)). I have some calculations for these flows and it would be good to incorporate them in the model before applying the WBT to get an idea of unaccounted inflow/outflows. Considering I am planning to use the WBT so that will be using my distributed tributary option, what would be the best method of incorporating these flows? A normal tributary entering into the end segment? Has anybody else encountered a similar situation? Thanks Julie