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11.17 Estimates of Storm Runoff 393
The unit-hydrograph method is useful in estimating magnitudes of unusual flood
flows, in forecasting flood crests during storms, and in the manipulation of storage on large
river systems. It has the important property of (a) tracing the full hydrograph resulting
from a storm rather than being confined to a determination of the peak flow alone, and (b)
producing useful results from short records. For small drainage areas, the method depends
on the readings of a recording rain gauge.

EXAMPLE 11.10 APPLICATION OF UNIT HYDROGRAPH METHOD
2
1. Given the rainfall and runoff records of a drainage area of 620 mi , determine the general-
ized distribution of runoff (the distribution graph) from isolated unit-time rainfalls.
2. Apply the average estimate of runoff distribution to the observed rainfall sequence ob-
tained above.

Solution:
This involves first of all a search for records of isolated rainfalls and for records of the resulting sur-
face runoff. The basic data for a typical storm are shown in Table 11.12, together with necessary
calculations. Development of this table is straightforward, except for column 4, which records the
estimated base flow and can be derived only from a study of the general hydrograph of the stream
in combination with all related hydrologic observations of the region.

Table 11.12 Observations and Calculations for Unit Hydrograph for Example 11.10
Average
Estimated Distribution
3
Runoff, ft /s Distribution
Sequence Observed Ratio for
of Time Rainfall, Observed Estimated of Surface Runoff 10 Storms,
3
Units in Total Base Flow ft /s % %
(5) (6)
(1) (2) (3) (4) (3) (4) 100(5)/6,200 (7)
1 1.20 1,830 870 960 15.5 16
2 0.03 3,590 800 2,790 45.0 46
3 0.00 2,370 690 1,680 27.1 26
4 0.00 1,220 600 620 10.0 10
5 0.00 640 510 130 2.1 1
6 0.00 430 410 20 0.3 1
7 0.00 350 350 0 0.0 0
— — —
Totals — . . . . . . 6,200 100.0 100
3
Conversion factors: 1 in. 25.4 mm; 1 ft /s 28.3 L/s
The calculations in Table 11.13 need little explanation except for column 3, the estimated loss of
rainfall caused principally by infiltration. This estimate rests on all available information for the re-
gion. Column 5 is identical to column 7 of Table 11.10. Column 6 is the net rain of 0.5 in. during
the first time unit multiplied by the distribution ratio of column 5. Columns 7, 8, and 9 are similarly
derived for the net rains during the subsequent time units. Column 10 gives the sums of Columns 6
3
to 9, and column 11 converts these sums from in. to ft /s. If the base flow is estimated and added to
the surface runoff shown in column 11, the hydrograph becomes complete.