Question

The ordinates of 3 hour unit hydrograph for a small catchment are given below if the design storm produces net rainfall depth of 4.6 cm and 3.5 cm in successive unit periods and if the base flow is 2m3/s, then the peak flood flow will nearly be

Answer 1

The peak of a 3 hour unit hydro graph is 105.5 m^3 / sec.

Explanation:

Correct statement:

The average rainfall for a 3 hour duration storm is 2.7 cm and the loss rate is 0.3 cm/hr. The flood hydro graph has a base flow of 20 m^3/s and produces a peak flow of 210 m^3/s. The peak of a 3-h unit hydro graph is

Solution:

Total depth of rainfall = 2.7 cm

Loss of 0.3 cm / hr×3 hr = 0.9 cm

Rainfall excess = 2.7 - 0.9 = 1.8 cm

Peak & base of flood hydro graph ⇒ 210 m^3 / sec & 20 m^3 / sec

Peak of DRH = 210 - 20 = 190 m^3 / sec

Peak of 3 hour unit hydro graph = Peak of DRH / Rainfall excess

Peak of 3 hour unit hydro graph =190 / 1.8

Peak of 3 hour unit hydrograph = 105.5 m^3 / sec

Thus the peak of a 3 hour unit hydro graph is 105.5 m^3 / sec