Disch. HWC:

Enter Discharge pipe “Hazen-Williams Constant” (HWC). This is a number usually between 115 to 155 which relates to the surface roughness of the pipe internal bore. Find Pipe Data at top of the page and enter HWC into the text boxes. Experiment with the HWC value to simulate old pipe (guess-work only, try 10 – 50 less than value for new pipe).