Harmon Peaking Factor

Sewers must be sized for the worst hour, not the average day, and William Harmon's 1918 expression is still the most used estimate of how much worse that hour is. Its logic is statistical: the more people on a pipe, the less their individual habits line up, so peaks flatten out with size. A hamlet of 1000 peaks at about 3.8 times its average; a town of 50,000 at 2.26; a city of a million at 1.44. Feed it a population and it hands you the multiplier.

Two competing forms sit alongside it. Ten States Standards publishes a curve that runs a little higher for small populations, and Babbitt's formula PF = 5/P^0.2 (with P in thousands) gives similar answers in the middle of the range. All of them describe DOMESTIC peaking only, so infiltration and inflow have to be added separately and are frequently the larger number — a peak wet-weather flow of six or eight times the dry-weather average is common in an old combined or leaky separate system. The other end of the calculation matters just as much and gets forgotten: the same pipe must carry the minimum night flow fast enough to stay self-cleansing, usually 0.6 m/s or 2 ft/s, or a generously sized sewer silts up and produces the sulfide corrosion and odour complaints it was meant to prevent.