| The 33 points used by NRCS* define the Dimensionless Hydrograph
better than the 19 points provided in Chapter 5 of SWCE/6. |
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| The duration of
the event is 100 time units, or 5Tp. Peak
discharge occurs at t=Tp=20. |
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| The area under
the curve is 2670
volume units. |
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| Editions 3, 4,
5, & 6 of Soil and Water Conservation
Engineering incorrectly gave the area as 2620
units. |
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| Point# |
t |
q |
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| 1 |
0 |
0.0 |
| 2 |
2 |
3.0 |
| 3 |
4 |
10.0 |
| 4 |
6 |
19.0 |
| 5 |
8 |
31.0 |
| 6 |
10 |
47.0 |
| 7 |
12 |
66.0 |
| 8 |
14 |
82.0 |
| 9 |
16 |
93.0 |
| 10 |
18 |
99.0 |
| 11 |
20 |
100.0 |
| 12 |
22 |
99.0 |
| 13 |
24 |
93.0 |
| 14 |
26 |
86.0 |
| 15 |
28 |
78.0 |
| 16 |
30 |
68.0 |
| 17 |
32 |
56.0 |
| 18 |
34 |
46.0 |
| 19 |
36 |
39.0 |
| 20 |
38 |
33.0 |
| 21 |
40 |
28.0 |
| 22 |
44 |
20.7 |
| 23 |
48 |
14.7 |
| 24 |
52 |
10.7 |
| 25 |
56 |
7.7 |
| 26 |
60 |
5.5 |
| 27 |
64 |
4.0 |
| 28 |
68 |
2.9 |
| 29 |
72 |
2.1 |
| 30 |
76 |
1.5 |
| 31 |
80 |
1.1 |
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| 32 |
90 |
0.5 |
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| 33 |
100 |
0.0 |
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| The Dimensionless Hydrograph has the same
total discharge as the Triangular Hydrograph (Figure 5.7 in the text). The
discharge prior to Tp is 3/8 of the total storm runoff. As can be seen in
Figure 5.7, that is represented by 1000 volume units. The remaining 5/8 of
the storm runoff occurs in the recession limb, so it is represented by 5/3 x
1000 volume units. The base of the recession limb must therefore be 5/3 x Tp. The total storm runoff is then represented by 8/3 x
1000, or approximately 2667 volume units. The difference of 3 volume units
between the Triangular and Dimensionless Hydrographs (about 0.1%) is
insignificant in practical terms. |
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| *Adapted from
NRCS National Engineering Handbook Part 630 Chapter 16 Hydrographs
(210-VI-NEH, March 2007). |
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