A 4-hour unit hydrograph of a certain basin has a peak ordinate of 80 m³/sec. The peak ordinate of a 2-hour unit hydrograph for the same basin will be
- AEqual to 80 m³/sec
- BGreater than 80 m³/secCorrect
- CLess than 80 m³/sec
- DBetween 40 m³/sec to 80 m³/sec
Solution
Concept
Effect of storm duration on the peak of a unit hydrograph
Step-by-step solution
1. A 4-hour UH is the response to 1 cm of runoff generated uniformly in 4 hours. 2. A 2-hour UH is the response to 1 cm of runoff generated in a shorter time (2 hours) for the same volume. 3. The same volume in a shorter time means a higher intensity and a higher peak. 4. So the peak of the 2-hour UH is greater than 80 m³/s.
Explanation
The volume under every unit hydrograph is the same (1 cm over the catchment). A shorter-duration unit hydrograph is produced by a higher rainfall intensity, so its peak is higher and its base is a little shorter.
Shortcut
Shorter duration → higher and narrower UH; longer duration → lower and flatter UH.
Common mistake
Assuming the peak is halved because the duration is halved.
Correct answer: B) Greater than 80 m³/sec