A mathematical model is developed to determine the distance covered upstream by the monoclinal wave generated by instant closure of the sluice gate in a non-prismatic trapezoidal channel, where the distance is integrated form of a function depends on the initial flow parameter and wave velocity, wave height, channel geometry concerning the time where the wave simply ends. There is initial uniform flow in the channel from the inlet of the simulation modelling done by the volume of fluid (VOF) multiphase method with ANSYS FLUENT open channel hydraulic model. Pressure-based multiphase fluid flow with 1.14 m/s velocity equivalent discharge is set for computational modelling of the phenomenon. This study may help upstream structures resist the backwater effect in irrigation canals and hydropower plants. Computational modelling of linear and nonlinear stability of general monoclonal wave can define velocity with turbulence property at the particular grid upstream.