Inst of Engineers Seminar Rainfall Analysis Revised

7/30/2019 Inst of Engineers Seminar Rainfall Analysis Revised

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Prof. S. D. Chawathe, Senior ConsultantMWH India (P) Ltd. Mumbai19/1/2013

Institution Of Engineers (India)

RAINFALL ANALYSISFORSTORM WATER MANAGEMENTSYSTEMS

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RATIONAL METHOD

OFDESIGN

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Typical Storm Water Drainage System

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Rational Method of Design

DESIGN RUN-OFFThe Rational Method involves repeated use of the following equation:

Q = C x i x A

Where,Q is the design runoff of storm water,

C is the coefficient of runoff,

A is the area tributary on the storm water drain under design,

And i is the intensity of rainfall of a storm with a specificduration and specific frequency of occurrence.

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Concept of Concentration Time

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Type of Area Coefficient of Run-off

Commercial 0.70 to 0.90

Residential High Density 0.60 to 0.75

Residential low Density 0.35 to 0.60

Parks and undeveloped lands 0.10 to 0.20

Design values of coefficient of Run-off

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IDF Relationships

The most common IDF relationships used in the Rainfall Analysis are as follows:

a) Relationship 1 i = a / tn ----------------------- (1)

a) Relationship 2 i = a / (t + b)n -----------------------(2)

b) Relationship 3 i = CTm / (t + d)n -----------------------(3)

Where, i = intensity of rainfall (mm/hr)

t = duration of storm (min)

T = Frequency of occurrence (Return period in Years / months)

And, a, b, C , d, m and n are constants.

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Uses Annual Maximum Rainfall for VariousDurations over Past years

Annual Maxima Method

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Methods Based on Relationship - 1

The method uses the following relationship:

i = a / tn --------------------- (1)

Where, i = intensity of rainfall (mm/hr)

t = duration of storm (min)

And, a and n are constants for a given frequency of occurrence (Return period) (year).

Equation 1 can be converted into a straight line relation by taking logarithms of both sides asfollows :

log i = log a n log t --------------------- (1.1)

Or log i = -n log t + log a --------------------- (1.2)

The constants a and n can be determined as intercept on Y axis and slope of the linerespectively if we have a number of pairs of values for t and i. A straight line can then befitted to the points.

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Typical Automatic Rainguage

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Hyetogram

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Typical Conversion of a Hyetogram Segment

into Individual Storms

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Conversion of a Typical 4 Hrs. Rainfall Into

Individual Storms

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Records of Rainfall showing Number of Storms of Stated

Durations and Stated Intensities or more

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Records of Rainfall showing Number of Storms of Stated

Durations and Stated Intensities or more

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Interpolated Intensities of Rainfall

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Fitting of Log Log Relationship

It is now necessary to fit the followingequation to the interpolated intensities.

log i = -n log t + log a -----------(1.2)

For fitting a straight line to the data,plots of log i versus log t can now beproduced on log log paper one each forvarious frequencies of occurrence andconstants a and n can be determinedby reading intercepts on Y axis andslopes of the straight lines. A typical plot

is shown in the figure for the frequencyof twice in a year.

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IDF Curves

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Map showing Locations of Colaba & Santacruz

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S. D. CHAWATHE

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Banglore : P. Khageshan

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Amaravati : Dr. Tatekar Et al

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T= 1 YR 2.291 X 10 ** 6

i = ----------------------------------

( t + 188.85 ) ** 6.848

Also,

T = 1 YR i = 0.0004174(t -99.28) ** 2

T = 2 YR i = 0.0001177( t -181.9) ** 2

T = 3 YR i = 0.0003165(t129.18) ** 2

T = 4 YR i = 0.0004692( t

119.94)** 2T = 5 YR i = 0.0002755 ( t149 ) ** 2

New Delhi : Toshniwal C. T.

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Kolkata : Prof. V. Raman et al

T = 1 TO 3 MONTHS 6200 T ** 0.358

i = ------------------------------

( t +45 ) ** 1.75

T = 1 TO 2 YRS 953 T ** 0.157

i = --------------------------( t + 45 ) ** 1.34

T = 5 TO 20 YRS 290 T ** 0.119

i = -----------------------------

( t + 45 ) ** 1.05

T = 1 MONTH TO 20 YRS 810 T ** 0.126

i = -------------------------------

( t + 45 ) ** 1.31

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1 TO 1.75 YRS 5425 FREQ ** 0.4837i = --------------------------------- i in mm/hr

( DUR + 33.44)** 1.4019

2 TO 2.75 YRS107840 FREQ ** 0.564

i = ------------------------------------ Dur in min.( DUR + 69.08) ** 2.0029

3 TO 3.75 YRS135140 FREQ ** 0.6036

i = ------------------------------------- Frequency in months( DUR + 79.96 ) ** 2.0695

4 TO 5 YRS

1761900 FREQ ** 0.6709i = ----------------------------------------

( DUR + 115.58) ** 2.55

Pune : S. D. Chawathe

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1 TO 1.75 YRS 3660 FREQ ** 0.6338

i = ------------------------------------

( DUR + 42.32) ** 1.3054

2 TO 2.75 YRS

1524.3 FREQ ** 0.3203

i = --------------------------------------

( DUR + 29.69 ) ** 0.9531

3 TO 3.75 YRS

1005.6 FREQ ** 0.2514

i = --------------------------------------

( DUR + 18.67 ) ** 0.8374

i in mm/hr, Dur in min, Freq in Months.

Ahmedabad : S. D Chawathe

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Chennai : Mott McDonald

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Rambabu

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Akola

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Nomogram for Akola

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Frequencies of Occurrence as Recommended

by Authorities

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by Authorities

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by Authorities

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by Authorities

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Recommendations of FFC

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by Authorities

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Frequencies of Occurrence as Recommended by

Authorities

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Frequencies of Occurrence as Recommended by

Authorities

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DESIGN METHOD

BASED ON

CHICAGO CURVES

NEEDS SOFTWARE SUCH AS

INFOWORKS

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Typical Chicago Curve for a Location

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Recommendations of FFC

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Return Period for Mumbai

At present BRIMSTOWAD Master Plan is being updatedand the return period of twice in one year adopted earlier isbeing reconsidered in the light of the flooding observed on

26th July 2005 in Mumbai.

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SPATIAL VARIATION

OF

RAINFALL OVER MUMBAI

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THANK YOU

Inst of Engineers Seminar Rainfall Analysis Revised

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