Page 596 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch14_500-554.qxd 8/7/10 8:56 PM Page 552

552 Chapter 14 Design of Sewer Systems

Inlet 2
A B D
Length of storm sewers
Area 1
MH 1 - MH 2 120 m
MH 2 - MH 3 180 m
Sewer 1 Sewer 2
A1
MH 3 - Stream 100 m
Inlet
A2
Inlet 1
MH 1
C E
Area 2
Figure 14.41 Layout of Storm Drainage for Problem 14.24.
MH 2
Area From To Inlet Time, min
Inlet
A1 A Inlet 1 5
C Inlet 1 4
A2 B Inlet 2 10
C Inlet 2 12

The rainfall intensity i (mm/h) as a function of rainfall duration
MH 3
t (min) is:
Stream
2,500
i =
t + 18
Area 3
Determine the following:
Outlet sewer
Inlet (a) Overall runoff coefficient c for both areas combined.
(b) Concentration times Area 1 at Inlet 1
(c) Concentration time for Area 2 at Inlet 2
Figure 14.40 Layout of Storm Drainage for Problem 14.23.
Conversion factor: 1 m 3.2808 ft (d) Concentration time for Sewer 2
(e) Flow entering Inlet 1
(f) Flow entering Inlet 2
(g) Flow in Sewer 2
• Runoff coefficient c 0.30. (h) Minimum nominal diameter of Sewer 2
• The velocity in the storm sewers from MH 1 to MH 3 (i) Maximum nominal diameter of Sewer 2
is 0.75 m/s.
14.25 Determine the sizes of the three storm sewers needed to
• Manning coefficient n for sewers 0.013.
serve the 4.8 ha drainage areas shown in Fig. 14.42. Each area
• Ground elevation at MH 3 625.00 m
has an inlet time of 10 min. The distance between manholes is
• Invert elevation of outfall sewer at MH 3 622.00 m
• Ground elevation at edge of stream 624.00 m
• Bottom elevation of stream 614.00 m
• Maximum water level in stream 620.50 m MH 3 Housing
1 2 area
(a) Determine the required size of the outfall sewer so
Inlet
that no backflow of water from the stream into the
sewer is possible.
Inlet
(b) What size of outfall sewer will be required if exca- MH 2
Housing area
vation is to be kept to a minimum? In this case Park
assume that there is no minimum limit on depth of 3
cover.
14.24 Storm sewers 1 and 2 are planned to carry the
stormwater runoff contributed by the two areas A1 and A2 Inlet
2
(see Fig. 14.41). Area Al is 15,000 m and has a runoff coeffi- MH 1
2
cient c 1 0.65. Area A2 is 25,000 m and has a runoff coeffi-
Storm sewer
cient c 2 0.45. Flow time in Sewer 1 is 2 min. Inlet times are
as follows: Figure 14.42 Layout of Storm Drainage for Problem 14.25.