The peak of a 4 hour flood hydrograph is 240 m3/sec . If the rainfall excess is 80 mm and base flow which is constant is 40 m3/sec, then the peak of 4-hours unit hydrograph will be A. 20 m3/sec B. 25 m3/sec C. 30 m3/sec D. 35 m3/sec

The correct answer is C. 30 m3/sec.

The peak of a 4 hour flood hydrograph is 240 m3/sec. This means that the total volume of water that flows through the hydrograph in 4 hours is 240 m3/sec. The rainfall excess is 80 mm. This means that 80 mm of rain fell in the area that the hydrograph represents. The base flow is 40 m3/sec. This means that 40 m3/sec of water is flowing through the hydrograph even when there is no rain.

The peak of a 4-hours unit hydrograph is the peak of the hydrograph divided by the rainfall excess. In this case, the peak of the 4-hours unit hydrograph is 240 m3/sec / 80 mm = 30 m3/sec.

The other options are incorrect because they do not represent the peak of the 4-hours unit hydrograph.