Job of EIL Doc No.
:
MRPL Mangalore Rev No.:
DESIGN RAINFALL CALCULATION Page 1 of 3
Scope:
This document details the design rainfall calculation for Job of EIL at MRPL Mangalore.
Hourly Rainfall Calculation:
Annual daily maximum rainfall values are converted to 1 hour short duration rainfall using IMD Formula,
where, Pt = Rainfall in mm for t hours duration,
P24 = Daily Rainfall data in mm,
t = Shorter duration in hours.
Table of Historical Maximum Rainfall data (IMD) and corresponding hourly rainfall
Year Daily Rainfall (mm/day) 1 hour duration (mm/hr)
1970 146.1 50.65
1971 136.1 47.18
1972 190.2 65.94
1973 107.9 37.41
1974 218.5 75.75
1975 217.6 75.44
1976 127.8 44.31
1977 262.2 90.9
1978 170.1 58.97
1979 151.8 52.63
1980 190.7 66.11
1981 143.9 49.89
1982 235 81.47
1983 147.2 51.03
1984 187.2 64.9
1985 270.1 93.64
1986 180.9 62.71
1987 103.8 35.99
1988 189.3 65.63
1989 160.9 55.78
1990 116.6 40.42
1991 187.3 64.93
1992 144.3 50.03
1993 187.3 64.93
1994 293.1 101.61
Copyrights EIL - All rights reserved
Job of EIL Doc No.:
MRPL Mangalore Rev No.:
DESIGN RAINFALL CALCULATION Page 2 of 3
1995 168.9 58.55
1996 227.2 78.77
1997 182.8 63.37
1998 168.7 58.49
1999 244.1 84.62
2000 160.3 55.57
2001 191.5 66.39
2002 144.8 50.2
2003 294.3 102.03
2004 173.2 60.05
2005 201 69.68
2006 200.9 69.65
2007 133 46.11
2008 142.9 49.54
2009 132 45.76
2010 172.7 59.87
2011 112.1 38.86
2012 146.8 50.89
2013 157.8 54.71
2014 132.7 46.0
2015 215 74.54
2016 107 37.09
2017 137.6 47.7
2018 277.7 96.27
2019 159.7 55.36
2020 251 87.02
2021 173.4 60.11
2022 141.2 48.95
2023 206.7 71.66
Gumbel’s Extreme Value distribution method is used for calculating design rainfall intensity in mm/hr.
The rainfall (PT) corresponding of a given return period (T) using the Gumbel’s Distribution is given by,
where, Pave = Average Annual Daily Maximum Rainfall,
S = Standard Deviation of Annual Daily Maximum Rainfall.
The Mean and Standard Deviation are calculated using this formula,
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Job of EIL Doc No.:
MRPL Mangalore Rev No.:
DESIGN RAINFALL CALCULATION Page 3 of 3
and,
Mean of hourly rainfall from 1970 to 2023 for 1 hour duration = 61.78 mm
Standard Deviation of hourly rainfall from 1970 to 2023 for 1 hour duration = 16.63 mm
K = Gumbel Frequency Factor.
where, T = Return Period in years.
For 25 years Return Period,
Frequncy Factor, K = 2.044
Design Rainfall for 1 hour duration, Pt = 95.77 mm
Intensity of Rainfall, IT (in mm/hr) is given by,
where, Td = duration in hours. (1 hour)
Design Intensity from Gumbel's Distribution, IT = 95.77 mm/hr
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