Hydrology Rainfall duration calculations

8/11/2019 Hydrology Rainfall duration calculations

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The return periods Trincluded are 1! years, 2" years, "! years and 1!! years.

For return period Tr# 1! years

$umber of records, n # 2"%ean of records # &1.&&'educed %ean (from Table 1, after interpolation), *n # !."&!+'educed tandard -eriation (from Table 2, after

interpolation), n# 1.!+1"

'eturn period as a function of the reduced arieties,

*Tr# 2.2"!2

Frequency Factor (from equation 1), K # (2.2"!2 / !."&!+) 1.!+1"

# 1."0"

For &! minutes rainfall duration

%ean, # &1.&& mmtandard -eiation, r # +.! mm

Therefore XTris X1! #&1.&& mm

1."0"(+.! mm)# "."0 mm

For 3 hr rainfall duration

%ean, # ".42 mm

tandard -eiation, r # 1.0 mm

Therefore XTris X1! #".42 mm

1."0"(1.0 mm)

# 45.5 mm

For 2 hrs rainfall duration

%ean, # 4&.04 mm

tandard -eiation, r # 2!.! mm

Therefore XTris X1! #4&.04 mm

1."0"(2!.! mm)# +".&2 mm


%ean, # 50.&5 mm

tandard -eiation, r # 2!.+2 mm

Therefore XTris X1! #50.&5 mm

1."0"(2!.+2 mm)# 12!.&& mm

For2 hrs rainfall duration

%ean, # +5.0 mm

tandard -eiation, r # 2".+ mm


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Therefore XTris X1! #+5.0 mm

1."0"(2".+ mm)# 1&5.5+ mm

For return period Tr# 2" years




*Tr# &.1+5"

Frequency Factor (from equation 1), K # (&.1+5" / !."&!+) 1.!+1"

# 2.


%ean, # &1.&& mm

tandard -eiation, r # +.! mm

Therefore XTris X2" #&1.&& mm

2.(+.! mm)# "&.2 mm


%ean, # ".42 mm


Therefore XTris X2" #".42 mm

2.(1.0 mm)# 51.4 mm


%ean, # 4&.04 mm


Therefore XTris X2" #4&.04 mm

2.(2!.! mm)# 112.0 mm


%ean, # 50.&5 mm


Therefore XTris X2" #50.&5 mm

2.(2!.+2 mm)# 1&5."1 mm


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%ean, # +5.0 mm


Therefore XTris X2" #+5.0 mm

2.(2".+ mm)

# 141.! mm

For return period Tr# "! years




*Tr# &.+!1+

Frequency Factor (from equation 1), K # (&.+!1+ / !."&!+) 1.!+1"

# &.!55


%ean, # &1.&& mm


Therefore XTris X"! #&1.&& mm

&.!55(+.! mm)# "+.2" mm


%ean, # ".42 mm


Therefore XTris X"! #".42 mm

&.!55(1.0 mm)# +1.1 mm


%ean, # 4&.04 mm


Therefore XTris X"! # 4&.04 mm &.!55(2!.! mm)

# 12".4 mm


%ean, # 50.&5 mm

tandard -eiation, r # 2!.+2 mmTherefore XTris X"! # 50.&5 mm


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&.!55(2!.+2 mm)# 1"1.+5 mm


%ean, # +5.0 mm


Therefore XTris X"! #+5.0 mm

&.!55(2".+ mm)# 100." mm

For return period Tr# 1!! years


interpolation), n # 1.!+1"


*Tr# .4!!1

Frequency Factor (from equation 1), K # (.4!!1 / !."&!+) 1.!+1"

# &.025


%ean, # &1.&& mm


Therefore XTris X1!! #

&1.&& mm

&.025(+.! mm)# 4".!& mm


%ean, # ".42 mm


Therefore XTris X1!! #".42 mm

&.025(1.0 mm)# 1!!."0 mm

For 2 hrs rainfall duration%ean, # 4&.04 mm


Therefore XTris X1!! #4&.04 mm

&.025(2!.! mm)# 1&5.0 mm



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%ean, # 50.&5 mm


Therefore XTris X1!! #50.&5 mm

&.025(2!.+2 mm)# 14".&0 mm


%ean, # +5.0 mm


Therefore XTris X1!! #+5.0 mm

&.025(2".+ mm)# 1+&.00 mm

The XTralues are now used to 6nd for intensity for the respectie return periods.

'ainfall 3ntensity for return period, Trof 1! years

-uration (hrs) 'ainfall (mm) 3ntensity (mm7hr)!." "."0 +1.11 45.5 45.52 +".&2 0.444 12!.&& 2!.!42 1&5.5+ ".0+

'ainfall 3ntensity for return period, Trof 2" years

-uration (hrs) 'ainfall (mm) 3ntensity (mm7hr)!." "&.2 1!4.51 51.4 51.42 112.0 "4.&04 1&5."1 2&.!+2 141.! 4.01

'ainfall 3ntensity for return period, Trof "! years

-uration (hrs) 'ainfall (mm) 3ntensity (mm7hr)!." "+.2" 115."1 +1.1 +1.12 12".4 42.524 1"1.+5 2".&&2 10"." 0.&1


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'ainfall 3ntensity for return period, Trof 1!! years

-uration (hrs) 'ainfall (mm) 3ntensity (mm7hr)

!." 4".!& 1&!.!41 1!!."0 1!!."02 1&5.0 4+.24 14".&0 20."42 1+&.00 5.!0

8 plot of these alues rainfall intensity against duration, for the respectie return

periods is shown below.

Hydrology Rainfall duration calculations

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