1
Question
A triangular unit hydrograph for 5 hr duration is having peak discharge of 45 cumecs occurring at 5 hours, and having a base width of 20 hours. If the value of ϕ - index is 0.5 cm/hr then the resulting peak discharge of direct runoff due to an average rainfall of 20 cm occurring for 10 hours duration is ______ m3/s.
- 562.5
- 562.5
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Solution
The correct option is A 562.5
Time
Ordinate of 5 hr UH
0
0
5
45
10
30
15
15
20
0
Rainfall excess = 20−0.5×10=15cm
A 10 hour hydrograph can be obtained by superimposing two 5 hours U.H lagged by 5 hours
Time
Ordinate of 5 hr UH
5 hr UH lagged by 5 hr
10 hr DRH
10 hr UH
0
0
-
0
0
5
45
0
45
22.5
10
30
45
75
37.5
15
15
30
45
22.5
20
0
15
15
7.5
25
0
0
0
Peak discharge of resulting direct runoff = QP=37.5×15=562.5m3/s
| Time | Ordinate of 5 hr UH |
| 0 | 0 |
| 5 | 45 |
| 10 | 30 |
| 15 | 15 |
| 20 | 0 |
Rainfall excess = 20−0.5×10=15cm
A 10 hour hydrograph can be obtained by superimposing two 5 hours U.H lagged by 5 hours
| Time | Ordinate of 5 hr UH | 5 hr UH lagged by 5 hr | 10 hr DRH | 10 hr UH |
| 0 | 0 | - | 0 | 0 |
| 5 | 45 | 0 | 45 | 22.5 |
| 10 | 30 | 45 | 75 | 37.5 |
| 15 | 15 | 30 | 45 | 22.5 |
| 20 | 0 | 15 | 15 | 7.5 |
| 25 | 0 | 0 | 0 |
Peak discharge of resulting direct runoff = QP=37.5×15=562.5m3/s
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