code: def drain_D(self):
          
          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
docs: Returns depth of drain pipe.
          :returns: Depth
          :return: float