import math


def manning_equation(hydraulic_radius, manning_coefficient, slope):
    """Manning formula estimating the average velocity of a liquid driven by gravity.

    :param hydraulic_radius: hydraulic radius of pipe or channel [m]
    :param manning_coefficient: Gauckler–Manning coefficient
    :param slope: slope of the hydraulic grade line [-]
    """
    return math.pow(hydraulic_radius, 2 / 3) * math.pow(slope, 0.5) / manning_coefficient