def drain_D(self):
        """ Returns depth of drain pipe.
        :returns: Depth
        :return: float
        """
        tank_A = 2 * self.channel_L * self.channel_W
        drain_D = (np.sqrt(8 * tank_A / (np.pi * self.drain_t) * np.sqrt(
           self.downstream_H * self.drain_K / (2 * u.standard_gravity)))).to_base_units()
        return drain_D
Returns depth of drain pipe.
        :returns: Depth
        :return: float