Question 13.SQ.5: The data below refer to the sewer network in Fig. 13.18. Con......
The data below refer to the sewer network in Fig. 13.18. Continue the calculations in Example 13.8 and hence determine a suitable diameter for the remaining sewers.
| Sewer reference no. | Length (m) | Gradient | Impermeable area (km²) | Comments |
| 3 | 65 | 1 in 160 | 0.002 | Branch |
| 3.1 | 75 | 1 in 160 | 0.003 | Branch |
| 1.3 | 70 | 1 in 200 | 0.004 | Main sewer |
| 1.4 | 80 | 1 in 285 | 0.005 | Main sewer |
This question continues the calculations started in Example 13.8, which stopped at sewer 1.2. The remaining calculations are shown in Table STQ13.5. Note the following.
(a) Sewer 1.2 is a carrier sewer. The final t_{F} and t_{C} of 1.30 and 10.02 min repectively are not used for the design of this sewer, but are used to design sewer 1.3 where the t_{C} = 10.02 + 0.82 = 10.84 min.
(b) Sewer 3.0 is a branch, so the calculations effectively start afresh.
(c) Sewer 1.3 continues the calculations for the main branch. Since it is the first pipe downstream of branch 3.0/3.1, the longest of the upstream t_{C} values is used (i.e. 10.02, not 9.07 min). Its cumulative impermeable area is the total of everything upstream (= 0.021 km²). Note that the 0.450 m diameter pipe only just passes (Q_{\text {FULL }} = 0.230 > Q_P = 0.226 m ^3 / s). It could be argued that the rational method tends to overdesign so this is entirely satisfactory, or it could be argued that a larger – but more expensive – pipe should be used to allow for any future increase in flow.
(d) Sewer 1.4 obviously needs a larger diameter since it has an increased catchment area and a shallower gradient than sewer 1.3, which only just passed. Hence the calculations start with a 0.525 m pipe
| Table STQ13.5 | ||||||||||||
| 1
Sewer ref. no. |
2
Sewer length L (m) |
3
Gradient |
4
Guessed diameter (m) |
5
Q_{FULL}
(m³/s) |
6
Mean flow velocity V(m/s) |
7
t_{F} = L/60V (min) |
8
t_{C}
(min) |
9
i (mm/h) |
10
A_{IMP} for each sewer (km²) |
11
ΣA_{IMP} (km²) |
12
Q_{P} = 0.278 i \Sigma A_{ IMP }
(m³/s) |
13
Comments |
| 1.2 | 90 | 1 in 300
0.33 m/100 m |
0.375 | 0.115 | 1.03 | 1.46 | (10.18) | None | (0.012) | 0.146 | Fail | |
| 0.450 | 0.190 | 1.15 | 1.30 | (10.02) | None | 0.146 | OK | |||||
| 3.0 | 65 | 1 in 160
0.63 m/100 m |
0.150 | 0.014 | 0.79 | 1.37 | 8.37 | 44.9 | 0.002 | 0.002 | 0.025 | Fail |
| 0.225 | 0.042 | 1.03 | 1.05 | 8.05 | 45.8 | 0.025 | OK | |||||
| 3.1 | 75 | 1 in 160
0.63 m/100 m |
0.225 | 0.042 | 1.03 | 1.21 | 9.26 | 42.4 | 0.003 | 0.005 | 0.059 | Fail |
| 0.300 | 0.090 | 1.23 | 1.02 | 9.07 | 42.9 | 0.060 | OK | |||||
| 1.3 | 70 | 1 in 200
0.50 m/100 m |
0.450 | 0.230 | 1.43 | 0.82 | 10.84 | 38.7 | 0.004 | 0.021 | 0.226 | OK |
| 1.4 | 80 | 1 in 285
0.35 m/100 m |
0.525 | 0.285 | 1.32 | 1.01 | 11.85 | 36.6 | 0.005 | 0.026 | 0.265 | OK |