# D2/PD2 HYDRAULICS

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```					                         B.Sc(HONS) IN CIVIL ENGINEERING
HYDRAULICS BLT 2012 - TUTORIAL No4
SEWERAGE DESIGN

1)        A 300mm diameter foul sewer is to be laid to serve a proposed development
of 2350 houses. If the expected occupancy is 3 persons per dwelling and the
anticipated water consumption is 200 litres/head/day determine the pipe
gradient to cater for 6DWF and give a minimum velocity of 1.0m/s for 1DWF.
(1 in 100)

2)        Design the foul sewer system shown below. Assume 3 persons per dwelling,
new system is to connect into a existing combined sewer at A4 which has an
invert level of 143.50m AOD.

A1                                                       C2
A2               A3

C1
B1

A4

Pipe run            Length          Number of        Ground level      Ground level
(m)            houses          upstream Mh       downstream
Mh
A1-A2                100               300              150.2            149.0
B1-A2                120               280              150.5            149.0
A2-A3                 95               350              149.0            147.8
C1-C2                130               290              150.9            149.5
C2-A3                 80               230              149.5            147.8
A3-A4                130               380              147.8            146.3

(150mm @ 1 in 71, 150mm @ 1 in 83, 225 @ 1 in 65, 150mm @ 1 in 77,
225mm @ 1 in 50, 300mm @ 1 in 77)

(A3 = 145.3m AOD, A2 = 146.8m AOD, A1 = 148.3m AOD,B1 = 148.3m AOD,
C2 = 147.0m AOD, C1 = 148.8m AOD)
5
Q3)

6
1                 2             4

7

3

The diagram above shows a storm water sewer system and the table overleaf gives
details of the sewer lengths and areas drained into each. Using the Rational Method
determine the flow rate for each pipe length and check the adequacy. Take the
storm return period as 1 year, Ks as 0.6mm and time of entry as 2 minutes.

Pipe                 Length            Fall across     Impermeable     Pipe Dia
(m)              the pipe (m)    Area (Ha)       (mm)
1-2                   80.0                1.40             0.11        150
2-4                   73.0                0.90             0.20        225
3-4                   94.0                1.10             0.09        150
4-6                   50.3                0.82             0.19        300
5-6                   45.8                0.75             0.10        150
6-7                  101.4                1.62             0.05        300
(20.3, 52.2, 15.9, 95.5, 21.5, 111.0 all in litres/sec)
Sewer run   Cumulative   Cumulative     Ground     Pipe       Min Pipe   Chosen pipe
(litres/sec)              (mm)       (1 in)     (1 in)
A1-A2

B1-A2

A2-A3

C1-C2

C2-A3

A3-A4
1        2            3        4         5        6       7       8         9       10         11        12
Pipe    Difference    Pipe    Pipe       Vel     Time    Time     Rate of   Imp.   Cumulative   Flow     Pipe
length   in level     length   gradient            of      of     rainfall   Area   Imp. Area     Q       dia.
ref No                                            flow    Conc.     i                 AP
(m)          (m)     (1 in )    (m/s)   (min)   (min)   (mm/hr)    (ha)    (ha)        (m3/s)   (mm)

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