Contents preface (VII) introduction 1—37

Fig. 2.21 Average unit hydrograph
It is rare to find a suitable storm of desired characteristics (as outlined earlier) for the derivation of a unit hydrograph. More often than not, one is required to derive unit hydrographs using the data of two or more closely-spaced storms (i.e., complex storms) resulting into a hydrograph having two or more peaks. In such cases, it is necessary to separate the runoff caused by individual bursts of rainfall and, then, separate direct runoff from base flow. If individual bursts of rain in the complex storm result in well-defined peaks, it is possible to separate the hydrographs (i.e., separating the direct and base runoff). Averaging of the unit hydrographs would minimize the errors in the separation process. Consider the complex hydrograph shown in Fig. 2.22. Separation of runoff caused by the two bursts of rainfall is accomplished by extending (in a way similar to the recession curve TF) the small segment of recession (PQ) between the two peaks. Base flow separation can now be completed by drawing ABC and EF, Fig. 2.22. Obviously, the area ABCEFTRQPA equals the direct runoff volume from rainfalls J and K. One can now proceed to derive the unit hydrograph in the usual manner.



HYDROLOGY

3

Dischargeinm/s

73

Rain-J
Rain-K

P
R
Q
T

E

A F

B C

N = A^0.2 days

Time in days

Fig. 2.22 Analysis of complex hydrographs
A unit hydrograph can also be developed through sucessive approximations. A suitable D-hour unit hydrograph is assumed and used to obtain the storm hydrograph corresponding to a given rainfall of the duration D hours which should, obviously, match with the observed storm hydrograph for the given rainfall. If the two storm hydrographs do not match to the acceptable accuracy, the assumed unit hydrograph is suitably modified and the process repeated until a unit hydrograph that gives the best matching is obtained.