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528 Chapter 14 Design of Sewer Systems
14.11 STORM-PATTERN ANALYSIS

Different from the averaging procedures associated in practice with the rational method of
runoff analysis is the development of a generalized chronological storm pattern or hyeto-
graph and its translation into a design runoff pattern by subtracting rates of (a) surface in-
filtration, (b) depression storage, and (c) surface detention during overland flow. The
runoff hydrograph obtained is routed through overland flow, gutter flow, and flow in build-
ing drains, catch basins, and component sewers. Generalization of rainfall information by
converting an intensity-duration-frequency curve into a hyetograph is illustrated in Fig. 14.18.
An advanced peak in rainfall intensity is assumed in this case at 3/8 the time distance or
storm duration from the beginning of appreciable precipitation, selection of a suitable frac-
tion being based on specific rainfall experiences.
Figure 14.18 illustrates results obtained in the application of conversion and routing
procedures developed by Tholin and Keifer (1960) for the city of Chicago. Necessary calcu-
lations are based on (a) the infiltration-capacity curves shown in Fig. 14.18; (b) depth of de-
pression storage assumed at 1>4 and 1>2 in. (6.35 and 12.7 mm), for example, and normally
distributed about this mean depth, 50% of the area covered by depressions lying within 20%
of the mean depth (
14%); and (c) surface detention computed by Izzard’s equation:
1/3
–4
D 0.342[(7 10 i c r )>s ](LQ) 1/3 (U.S. Customary Units) (14.7)
where D is the surface detention, in inches of depth; s is the slope of the ground; L is the
3
distance of overland flow, in ft; Q is the overland supply, in in./h (ft /s per acre); i is the
intensity of rainfall, in in./h; and c r is a coefficient of roughness varying downward from
6.0 10 –2 for pervious areas of turf, through 3.2 10 –2 for bare, packed pervious areas,
and 1.2 10 –2 for pavements, to 1.7 10 –2 for flat, gravel roofs. The following equation
adopts the SI system:
1/3
–4
D 4.39[(0.276 10 i c r )>s ](LQ) 1/3 (SI Units) (14.8)

14
13 3 8 t d 5 8 t d
12 Maximum i 12.49 in./h at
3 t 3 8 100 37.5 min
8 d
11
Intensity-duration curve Hyetograph for 5-year storm
for 5-year storm
10
Rainfall intensity, i, in./h 8 i 19.7/(t 2) 0.66 Highest 2.5-min intensity, 7.21 in./h
9
7
6
5
4
5
t
3 8 t Highest 10-min intensity, 3.82 in./h
8
d
3 d
Average intensity,
2 0.93 in./h
1
0 10 20 30 40 50 60 70 80 90 100
Time, minutes from start of rainfall, t, and duration t d
Figure 14.18 Hyetograph Derived from Rainfall Intensity-Duration Frequency Curve.
Conversion factor: 1 in./h 25.4 mm/h