At time, the time of excess precipitation in case of a unit hydrograph, keeping the unit precipitation the same, is required to be changed depending upon the storm period t.
This time of excess precipitation may be increased or decreased.
Suppose the unit hydrograph of 1 h-1 cm precipitation is available as follows:
A 2 h-1 cm unit hydrograph is required. (Intensity of precipitation will be 1/2 cm/ h for 2 h).
This can be derived by four methods as follows:
Method 1:
The step-by-step procedure is as follows:
i. Plot the given 1 h -1 cm unit hydrograph.
ii. Plot the same 1 h -1 cm unit hydrograph by a time lag of 1 h.
iii. Add together these who flood hydrograph coordinates. Thus, resultant hydrograph after addition will be a 2 h – 2 cm hydrograph (Fig 2.26).
iv. Divided the resultant hydrograph by 2 to arrive at the required 2 h-1 cm hydrograph. (The intensity of precipitation will be 1/2 cm per hour for 2 h.). The procedure will be clear form Table. 2.8.
The same procedure can also be followed graphically.
This new flood hydrograph will be 2 h -1 cm unit hydrograph with an intensity of 1/2 cm/h for 2 h, resulting into unit precipitation of 1 cm.
It should be noted that:
a. Maximum discharge has reduced.
b. The base period had increased by 1 h (2 – 1 = 1 h).
c. The total of the new unit hydrograph coordinates (2 h – 1 cm) remains the same (82 m3/s).
Method 2:
The same procedure can be followed by a different step-by-step procedure as follows:
i. Plot the given 1 h -1 cm unit hydrograph.
ii. Divide these unit hydrograph coordinates by 2, so that a 1 h -1/2 cm hydrograph is obtained.
iii. Plot this 1 h -1/2 cm hydrograph by a time lag of 1 h.
iv. Add these two 1 h -1/2 cm hydrographs to arrive at the required 2 h -1 cm unit hydrograph.
This procedure will be clear from Table 2.9.
This procedure can also be followed graphically as shown in Fig 2.27.
Use of Hydrograph:
S hydrograph can also be used to increase the time of precipitation.
Method 3:
The step-by-step procedure will be as follows:
i. Prepare an S hydrograph of 1 h -1 cm unit hydrograph and plot the S hydrograph.
ii. Plot the same S hydrograph by a time lag of 2 h, say S1.
iii. Deduct the second, i.e., S1 hydrograph from the first one S.
iv. After deducting S1 hydrograph from S hydrograph, divide the resultant hydrograph by 2 to arrive at the required 2 h—1/2 cm unit hydrograph. The intensity of precipitation will be 1/2 cm/h.
The procedure will be clear from Table 2.10.
The same procedure can also be followed graphically as shown in Fig. 2.28.
Method 4:
The same procedure as followed above in a different step-by-step procedure can be followed as follows:
i. Prepare the S hydrograph of the given 1 h -1 cm unit hydrograph and plot it.
ii. Divide these S hydrograph coordinates by 2 so that it will be a 1 h -1/2 cm S1 hydrograph. Plot this S1 hydrograph with a time lag of 2 h.
iii. Deduct this S1 hydrograph from the original S hydrograph to arrive at the required 2 h – 1/2 cm unit hydrograph. (The intensity of precipitation will be 1/2 cm for 2 h). The procedure will be clear form Table 2.11.
The same procedure can be followed graphically as shown in Fig. 2.29.
