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390 Chapter 11 Hydrology: Rainfall and Runoff

1000
900 35
800
700
600 30
500
Paved c 0.95
400 25
Character of ground 20
300

200 Poor grass c 0.55 Pivot line
Bare soil

Distance of flow, ft 100 Average c 0.44 0.5 Percent slope 15 Inlet concentration time, min
surface
grass
c 0.35
1.0
90
surface
2.0
80
70
grass
60 Dense c 0.28 5.0
10
50
20
40
10
30
9
8
20
7

10
6
Figure 11.10 Nomogram in U.S. Customary Units for Determining Overland Flow Time
(Nomogram in SI System is in Appendix 18). Conversion factor: 1 ft 0.3048 m

EXAMPLE 11.8 DETERMINATION OF INLET CONCENTRATION TIME
Determine the inlet concentration time for a land area sloping at 4.5% and covered with average
grass with a c value of 0.35. The furthest point from the inlet is 60 m (200 ft).

Solution:
Distance of flow 60 m 200 ft
Average grass surface, c 0.35
Slope of land 4.5%
From the nomogram of Fig. 11.10:
t 15 min