Page 436 - Civil Engineering Formulas

362 CHAPTER THIRTEEN

where n coefficient dependent on roughness of conduit surface
2
R hydraulic radius, ft area, ft , of fluid divided by wetted perimeter, ft
S energy loss, ft/ft of conduit length; approximately the slope of the
conduit invert for uniform flow
C 1.486 (conversion factor to account for change from metric units
used in development of the formula)
The value of n can range from 0.013 in smooth concrete sewers to 0.02 in plain
pipe. Further, n usually increases as a sewer pipe ages. Specific n values are
given in engineering handbooks such as those cited in the references section of
this book.
The water flow, cu ft/s is
Q AV (13.3)
2
where A cross-sectional area of flow, ft .

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:

0.59
2/3 1/2
V d S (13.4)
n
where d inside diameter of pipe, ft.
Flow quantity, cu ft/s, for a full-flowing sewer pipe is given by

0.463
8/3 1/2
Q d S (13.5)
n

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:
Q 0.7L(a y) 3/2 (13.6)

3
where Q quantity of runoff, ft /s
L length of opening, ft
a depression in curb inlet, ft
y depth of flow at inlet, ft