The formulation of determining the nonlinear forces and overturning moment on a submerged plate, located in shallow water, is accomplished by use of the Level I Green–Naghdi nonlinear-wave equations. The forces are calculated by integrating the pressure distribution on the plate. The top and bottom boundary conditions, as well as the body boundary condition around the plate, are satisfied exactly. Except in Region III, below the plate, the nonlinear and unsteady differential equations, along with the jump and matching conditions, are to be solved simultaneously and numerically. The procedure of solving the complete system of equations and results for solitary or cnoidal waves are given in Part II.