How to estimate sewage and stormwater flow
Sewer mains, lift stations, and storm inlets are sized from two very different flows. Sanitary sewage is a fairly steady load driven by how much water people use each day, while stormwater arrives only during a storm and peaks hard and fast. This calculator produces both on one screen and adds a combined design flow for systems that carry them together.
Sewage starts from population times gallons per capita per day, a figure often taken as about 100 gpcd for residential service. That average is divided across the day and multiplied by a peaking factor to get the peak hour rate in gpm. Stormwater uses the rational method, Q = CiA, where Q is in cfs, i is rainfall intensity in in/hr, and A is the drainage area in acres, so the conversion constant is close enough to one to be dropped. A 2.5 acre site with C of 0.80 under a 3 in/hr storm sheds about 6.0 cfs, or roughly 2,690 gpm.
The rational method suits small catchments with short times of concentration and tends to overestimate once a basin gets large enough for storage effects to matter. Per capita flow, peaking factors, and allowances for infiltration and inflow are all set by the reviewing utility, so confirm the local design manual first and have a civil engineer review submittal calculations.
Frequently asked questions
Asphalt, concrete, and roof surfaces usually fall between 0.80 and 0.95, while gravel and lawn areas run 0.10 to 0.35. Mixed sites use an area-weighted average, and the value your reviewer accepts comes from the local drainage manual.
Read it from an IDF curve or the NOAA Atlas 14 precipitation frequency data for your location, using the design storm return period and the time of concentration. A 10-year and a 100-year value for the same site can differ sharply.