code: def drain_ND(self):
          
          drain_ND = pipes.ND_SDR_available(self.drain_D, self.SDR)
          return drain_ND
docs: Returns the diameter of the drain pipe.
          Each drain pipe will drain two channels because channels are connected by
          a port at the far end and the first channel can't have a drain because
          of the entrance tank. Need to review the design to see if this is a good
          assumption.
          D_{Pipe} = \sqrt{ \frac{8 A_{Tank}}{\pi t_{Drain}} \sqrt{ \frac{h_0 \sum K}{2g} } }
          :returns: list of designed values
          :rtype: float * centimeter