FLOOD06389

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Urban Storm Runoff Inlet Hydrograph Study - Volume 4
• Date: 5/1/1975
• Prepared By: Cheng-lung Chen
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

Table 2. Range of rainfall depths and corresponding intensities for various durations between 1 and 24
hours based on unit I-hour depth and intensity rainfall.
Upper limit Average Lower limit
(10% Ratio)' (40% Ratio)' (60% Ratio)'
Duration
(hours) Rainfall Rainfall Rainfall Rainfall Rainfall Rainfall
Depth Intensity Depth Intensity Depth Intensity
(inches) (in/hr) (inches) (in/hr) (inches) (in/hr)
1 1.0 1.000 1.0 1.000 1.0 1.0
2 2.0 1.000 1.25 0.625 1.1 0.55
3 3.0 1.000 1.4 0.467 1.2 0.400
6 5.5 0.917 1.75 0.292 1.4 0.233
12 7.75 0.646 2.1 0,175 1.55 0.129
24 10,0 0.417 2.5 0.104 1.667 0.0694

, Ratio of 1-hour to corresponding 24-hour rainfall depth

Table 3. Standard rainfall intensity-duration relationships for various ratios of I -hour to corresponding
24-hour rainfall depth.
Duration Ratio' (%)
10 15 20 30 40 60
5 min. 3.51 3.51 3.51 3.51 3.51 3.51
10 min. 2.70 2.70 2.70 2.70 2.70 2.70
15 min. 2.28 2.28 2.28 2.28 2.28 2.28
30 min. 1.58 1.58 1.58 1.58 1.58 1.58
60 min. 1.00 1.00 1.00 1.00 1.00 1.00
2 hrs, 1.00 0.850 0.750 0.650 0,625 0.550
3 hrs. 1.00 0.767 0.650 0.517 0.467 0.400
6llCs, 0,917 0.650 0.500 0.375 0.292 0.233
12 hrs. 0.646 0,438 0.333 0,229 0.175 0.129
24 11rS, 0.417 0.278 0.208 0.139 0.104 0.0694

'Ratio of I-hour to corresponding 24.hour rainfall depth

corresponding 24.hour rainfall depth, yields the
values of the "standard" stonn parameters, ai' bI,
and c I as shown in Table 4. Note that compu ation
of standard data points for the 10- and 15.percent
ratios resulted in a negative b I which must be treated
differently from the case of a positive bl' as
mentioned previously. The values of other relevant
parameters identified in the normalization of the
hyetograph equations are also computed and given in
Table 4. For general use in engineering practice, the
values of ai' bl, and c I are plotted against the ratio

of l.hour to corresponding 24-hour rainfall depth, as
shown in Fig. 4. The validity of these storm
parameters versus }. to 24-hour rainfall ratio curves
can be examined by comparing the a \' b), and cI
values obtained from these curves WIth those
estimated directly from the 49 isopluvial maps.

The standard intensity-duration relationship
(Appendix C) that seems independent of frequency
may be written in a mathematical form:

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