Contents preface (VII) introduction 1—37

Derivation of Unit Hydrograph

The unit hydrograph is best derived from the hydrograph of an isolated storm (i.e., occurring individually) of reasonably uniform intensity during its duration (of desired magnitude) of occurrence covering the entire catchment basin, and resulting into a relatively large runoff volume. The hydrograph of such a storm of duration, say, D hour is separated (Art. 2.7.1) and the direct runoff hydrograph (DRH) is obtained. From this DRH, volume of direct runoff (area under the hydrograph) is determined in terms of the depth (i.e., runoff depth) over the catchment basin. The ordinates of the DRH are divided by the runoff depth to obtain the ordinates of the D-hour unit hydrograph (for unit runoff depth). A unit hydrograph derived from a single storm may not be truly representative unit hydrograph for the given catchment basin. Therefore, for a given catchment basin, a number of unit hydrographs of a specified duration (say D hours) are derived using as many isolated storm hydrographs (which will not be identical due to variable storm characteristics) caused by storms of almost the same duration (0.9 to 1.1 D hours) and also satisfying the above-stated requirements as much as possible or feasible. These are, then, plotted and an average unit hydrograph, Fig. 2.21, is obtained. The ordinates of the averaged unit hydrograph should not be an arithmetic average of superposed ordinates because if peaks of different unit hydrographs do not occur at the same time, the average peak so determined will be lower than any individual peak (Fig. 2.21). The proper procedure is to compute average peak flow, average of times of occurrences of individual peak flows, and the

72 IRRIGATION AND WATER RESOURCES ENGINEERING
average of base lengths. Thereafter, the average unit hydrograph is sketched so as to conform to the shape of other unit hydrographs, passing through the average peak at the averaged time to peak and having base length equal to the averaged base length. The volume of direct runoff for this unit hydrograph should, obviously, be unit depth and any departure from unit depth is corrected by suitable adjustments (including adjusting the peak) to the unit hydrograph, Fig. 2.21. It is usual to draw average effective rainfall hydrograph of unit depth alongside the unit hydrograph to indicate the duration of rainfall producing the unit hydrograph.

3

Discharge(m/s)

60	0	6 h
	1 cm	Rainfall excess
50		Average peak
		Curve defined by
40		arithmetic average
		Average unit hydrograph
30
20
10

0

0 10 20 30 40 50 60 70 80

Time in hours