Simplified Analytical Formula of Cubic Parabola-shaped Cross Section Normal Depth

As the cubic parabola-shaped cross section normal depth integral function and can not be involved in solving the transcendental equation calculation, using the analytical method can not be completed. By introducing the quadratic parabola approximation integration and optimization of fitting by successive approximation fitting, expressing simple form, the calculation process is simple, practical wide range of engineering staff to facilitate the practical application of the approximate calculation formula, error analysis and numerical examples calculations show that in practical engineering range, the formula for calculating the maximum relative error is only 0.941%, fully meet the accuracy requirements of the design, with application value.