Waterflooding oil reservoirs with a communicating ’bottom water’ zone or ’water leg’ is usually inefficient due to the flood water channelling into the bottom water zone. However, in some cases, waterflooding such reservoirs may still be feasible and economically viable. While there is no doubt that some of the injected water bypasses the oil-bearing formation through the bottom water zone, most of injected water may still displace the oil, depending on the reservoir conditions and the flooding technique used. Therefore, a mechanistic understanding of oil displacement by a waterflood in the presence of a bottom water zone is the basis for predicting recovery performance, and there is a need for developing a simple mathematical model to describe water channelling under such conditions. This paper presents a simple expression for water channelling under bottom water conditions. Previous experiments by other researchers that validate the analytical model are also discussed.