§ 2.4.3. Rainfall Intensity (i)

Rainfall intensity (i) is the average rainfall rate in inches per hour, and is selected on the basis of design rainfall duration and design frequency of occurrence. The design duration is equal to the time of concentration for the drainage area under consideration. The design frequency of occurrence is a statistical variable which is established by design standards or chosen by the engineer as a design parameter.

The selection of the frequency criteria is necessary before applying any hydrologic method. Storm drainage improvements in Austin must be designed to intercept and carry the runoff from a 25 year frequency storm (4% annual chance event), with an auxiliary or overflow system capable of carrying a 100 year frequency storm (1% annual chance event).

The rainfall intensity used in the rational method can be read from the intensity-duration-frequency (IDF) curves based on the selected design frequency and design duration. The design engineer can also calculate the value of rainfall intensity from the best-fit IDF equation (2-8) to be discussed later in this sub-section with known Tc value for the entire drainage area of interest.

In 1998, William Asquith at the USGS Texas Office analyzed virtually all rainfall data available in the State of Texas using L-moment methodology and published the results in a USGS Water Resources Investigations Report (WRIR 98-4044). In November 2001, Dr. Asquith summarized his rainfall study of 1998 and generated the IDF and the DDF (depth-duration-frequency) values that are suitable for use in the City of Austin and Travis County. These DDF and IDF values are shown in Table 2-3 and Table 2-4.

An explanation of the derivation of the Austin intensity-duration-frequency curves is given in Appendix B.

The Austin intensity-duration-frequency curves are shown in Figure 2-2 in Appendix D of this manual.

Table 2-3. Depth-Duration-Frequency Table for Austin and Travis County>
Depth of Precipitation (in inches)
Recurrence Interval (year)	5 min*	15 min	30 min	1-hr	2-hr	3-hr	6-hr	12-hr	24-hr
2	0.48	0.98	1.32	1.72	2.16	2.32	2.67	3.06	3.44
5	0.62	1.26	1.71	2.28	2.89	3.13	3.56	4.07	4.99
10	0.71	1.47	1.98	2.68	3.42	3.71	4.21	4.81	6.1
25	0.84	1.76	2.36	3.28	4.2	4.55	5.14	5.9	7.64
50	0.94	2.01	2.68	3.79	4.88	5.28	5.94	6.86	8.87
100	1.05	2.29	3.04	4.37	5.66	6.11	6.85	7.96	10.2
250	1.21	2.73	3.57	5.26	6.86	7.38	8.24	9.67	12
500	1.33	3.11	4.02	6.06	7.94	8.51	9.47	11.2	13.5
* The 5-min rainfall depths were calculated using the 5-min rainfall intensity values from Table 2-4.

 

Table 2-4. Intensity-Duration-Frequency Table for Austin and Travis County
Intensity of Precipitation (inches per hour)
Recurrence Interval (year)	5 min*	15 min	30 min	1-hr	2-hr	3-hr	6-hr	12-hr	24-hr
2	5.76	3.92	2.64	1.72	1.08	0.773	0.445	0.255	0.143
5	7.39	5.04	3.42	2.28	1.45	1.04	0.593	0.339	0.208
10	8.57	5.88	3.96	2.68	1.71	1.24	0.702	0.401	0.254
25	10.1	7.04	4.72	3.28	2.10	1.52	0.857	0.492	0.318
50	11.2	8.04	5.36	3.79	2.44	1.76	0.990	0.572	0.370
100	12.5	9.16	6.08	4.37	2.83	2.04	1.14	0.663	0.424
250	14.5	10.9	7.14	5.26	3.43	2.46	1.37	0.806	0.501
500	15.9	12.4	8.04	6.06	3.97	2.84	1.58	0.934	0.564
* The 5-min rainfall intensity values were calculated using Equation 2-8 and the coefficients listed in Table 2-5 for the return periods of 2, 5, 10, 25, 100, 250, and 500 years.

 

The following equation mathematically represents the Austin area intensity-duration-frequency curves:

i = a/(t+b) ^c (Eq. 2-8)

Where,

i = Average rainfall intensity, inches per hour

t = Storm duration in minutes, which is equal to the time of concentration for the entire drainage area of interest

a, b and c = Coefficients for different storm frequencies.

The final best-fit coefficients of a, b, and c for equation (2-8) are listed in Table 2-5 below:

Table 2-5 Austin Intensity-Duration-Frequency Curve Coefficients
Return Period	Fitting parameters for IDF equation (2-8)
Year	a	b	c
2	54.767	11.051	0.8116
5	62.981	10.477	0.7820
10	70.820	10.396	0.7725
25	82.936	10.746	0.7634
50	100.60	12.172	0.7712
100	118.30	13.185	0.7736
250	150.10	14.892	0.7822
500	188.00	17.233	0.7959
Source: Asquith, W.H., "Depth-Duration Frequency and Intensity-Duration Frequency for Austin and Travis County, Texas, 2001".

 

The a, b and c parameters listed in Table 2-5 were derived using nonlinear regression methods and the data included in Table 2-4. The IDF curves and the IDF equations are applicable for all design frequencies shown. They are required for use in determining peak flows by the Rational Method or other appropriate methods.