The presently existing methods or calculating seepage in inclined ASEs have a limited area of use and often give substantial deviations from the actual values. At the same time, no analytical relations are known for calculating Iex in the backwater zone of the LP. The formulas proposed in the article permit determining all main parameters of the seepage flow, including separately for the zones of effluent seepage and backwater of the LP, with an accuracy sufficient for engineering calculations.