In storm water drainage system design, estimation of runoff from the tributary catchment reaching various inlets of the drain is important. This can be estimated if Intensity Duration Frequency (IDF) curves are available. The IDF curve is drawn based on rainfall data analysis of the project area obtained from the daily rainfall charts of Self-recording Rain Gauge (SRRG) stations of Indian Meteorological Department (IMD) CPHEEO Manual Chapter 3 gives details of rainfall analysis
Measurement of Rainfall
Rainfall is measured by two types of gauges: 1.Self-recording type 1.Tipping Bucket Type Rain Gauge 2.Weighing Bucket Type Rain Gauge 3.Floating or Natural Syphon Type Rain Gauge 2.Non-recording type
Rain Gauge Density
The rain gauge density in a catchment is defined as the ratio of the total area of the catchment to the total number of rain gauge stations in the catchment.
NDMA (National Disaster Management Authority) has recommended ARGs (Automatic Rain Gauge Stations) should be installed in all urban cities (Class I, II and III) with a density of 1 per 4 sq km.
Steps for Analysis of Rainfall
STEP 1: The SRRG tabulated data may be obtained from IMD. In case, the tabulated data is not readily available then the SRRG Charts may be analysed to tabulate the data as explained with the help of a one day chart in the following Figure
STEP 2: Similarly, sort no of occurrences of rainfall intensities against corresponding duration for entire sample size of rainfall data obtained using MS Excel as shown in the Table
STEP 4: Add number of storms of all exceeding durations to the preceding / lesser duration storms. For instance in Table 3.4, number of storms corresponding to ≥ 5 mm/hr intensity are added as 111+ 326+ 367+ 12+ 134+ 262+ 83+ 188+ 168+ 106+ 97+ 60+ 51=1965. Similarly, number of storms are added vertically for each Intensity and tabulated in Table
STEP 5: As given in Chapter 4, choose Design Return Period for the project area. Accordingly, determine number of storms allowed to exceed the design rainfall intensity (mm/hr). For example, the required numbers of storm events having intensity equal to or more than design intensity for once in 5 year occurrence for 29 years rainfall data will be 29/5 i.e. 5.8 times on an average may exceed over a period of 29 years.
STEP 6: Draw a stepped line say for once in 5 year recurrence (5.8 no.) of occurrences occurring in intensity column and interpolate the corresponding duration (min) as shown in Table 3.6. Similarly, draw stepped lines for other return periods if required.
STEP 7: Intensity duration as interpolated in Step 6 is given in the Table
STEP 8: Establish Intensity Duration Frequency relationship
IDF relationship formulae are empirical ones that were developed based on the observation that as the time duration of storm increases the intensity of storm decreases. Bernard equation is commonly adopted i.e. 𝐼=𝑎/𝑡𝑛 for Indian conditions. The constants of the equation are found out by the curve fitting technique, which is described as follows
log 𝐼=log𝑎−𝑛log𝑡
Where,
I : Intensity of rainfall (mm/hr)
T : Rainfall duration (min)
a and n : Constant
Thus by plotting I and t on log-log graph paper, the trend line can be approximated to a straight line of best fit. The slope of this line will give the value of ‘n’ and its intercept on Y-axis will give the value of ‘a’
STEP 9: After the constants are determined, the intensities for various durations such as 5, 10, 15, 20, minutes and so on can be determined by Bernald Equation i.e. I = 𝑎/𝑡𝑛. Intensities and durations so computed are plotted on arithmetic graph paper and joined with smooth curve to trace the IDF curve of given frequency as carried out in Table
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Urban Infrastructure and Network Study notes for M. plan Sem-III
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