CIVIL ENGINEERING FORMULAS

CHAPTER 13

361

STORMWATER, SEWAGE,

SANITARY WASTEWATER,

AND ENVIRONMENTAL

PROTECTION*

DETERMINING STORM WATER FLOW

Sewers must handle storm water flows during, and after, rain storms, and snow-
and ice-melting events. The rational formula for peak storm-water runoff is

QCIA (13.1)

whereQpeak runoff, ft^3 /s
Adrainage area, acres
Ccoefficient of runoff of area
Iaverage rainfall rate, in/h of rain producing runoff

In heavily populated urban areas Cranges from 0.70 to 0.90. Residential areas
with gardens, shrubs, and extensive open land have Cvalues ranging from 0.30
to 0.40.

FLOW VELOCITY IN STRAIGHT SEWERS

Obstruction-free straight sewers will have a flow velocity of

V (13.2)

C

n

R2/3S1/2

*Hicks—Handbook of Civil Engineering Calculations, McGraw-Hill. The formulas and text in this
chapter, beginning with formula 13.11 and running through 13.77, and 13.84 through 13.88, are the
work of Kevin D. Wills, M.S.E., P.E., Consulting Engineer.