CIVIL ENGINEERING FORMULAS

362 CHAPTER THIRTEEN

wherencoefficient dependent on roughness of conduit surface
Rhydraulic radius, ft area, ft^2 , of fluid divided by wetted perimeter, ft
Senergy loss, ft/ft of conduit length; approximately the slope of the
conduit invert for uniform flow
C1.486 (conversion factor to account for change from metric units
used in development of the formula)

The value of ncan range from 0.013 in smooth concrete sewers to 0.02 in plain
pipe. Further, nusually increases as a sewer pipe ages. Specific nvalues are
given in engineering handbooks such as those cited in the references section of
this book.
The water flow, cu ft/s is

QAV (13.3)

whereAcross-sectional area of flow, ft^2.

Required Flow Velocity

To ensure full carriage of the solids, soils, refuse, and sand conveyed by storm
and sanitary sewers, minimum flow velocities must be maintained. For storm
sewers, the minimum flow velocity should be 3 ft/s; for sanitary sewers, 2 ft/s
is recommended. Full-flow velocity of the liquid in a sewer is given by:

(13.4)

wheredinside diameter of pipe, ft.
Flow quantity, cu ft/s, for a full-flowing sewer pipe is given by

(13.5)

Disposing of Storm Water

Storm water must be allowed to drain off streets and roads to prevent pond-
ing and interference with pedestrian and vehicular traffic. Storm-water
inlets, Fig. 13.1, at a curb opening, with full gutter flow, have a capacity of:

(13.6)

whereQquantity of runoff, ft^3 /s
Llength of opening, ft
adepression in curb inlet, ft
ydepth of flow at inlet, ft

Q0.7L(ay)3/2

Q

0.463

n

d8/3S1/2

V

0.59

n

d2/3S1/2