A SECOND LOOK AT A SECOND LOOK AT

BRIMSTOWADBRIMSTOWAD

S. D. CHAWATHES. D. CHAWATHE

Hon. Director (Technical)Hon. Director (Technical)

IWWAIWWA

BRIMSTOWAD SAYSBRIMSTOWAD SAYS

CHAWATHE ET AL : RAINFALL ANALYSIS FOR THE DESIGN OF STORM CHAWATHE ET AL : RAINFALL ANALYSIS FOR THE DESIGN OF STORM

SEWERS IN BOMBAY SEWERS IN BOMBAY --19771977

ChawatheChawathe has provided the most comprehensive analysis yet for storm has provided the most comprehensive analysis yet for storm

durations of 5 to 60 minutes, and twice per year to once in ten durations of 5 to 60 minutes, and twice per year to once in ten years years

frequency . He used 1349 frequency . He used 1349 ColabaColaba storms between 1944 and 1972 and storms between 1944 and 1972 and

2708 2708 santacruzsantacruz storms between 1940 and 1972storms between 1940 and 1972

BRIMSTOWAD SAYSBRIMSTOWAD SAYS

NEW ANALYSISNEW ANALYSIS

We have We have analysedanalysed the hourly data available in digital form ( for the hourly data available in digital form ( for

the period 1969the period 1969--1979) 1979) seperatelyseperately for for ColabaColaba and and SantacruzSantacruz to to

produce Intensityproduce Intensity-- DurationsDurations-- Frequency (IDF) relationships for Frequency (IDF) relationships for

durations of 1 to 48 hrs and for a range of frequencies between durations of 1 to 48 hrs and for a range of frequencies between 1 1

and 50 years. We have also and 50 years. We have also analysedanalysed the daily data for the period the daily data for the period

19591959--1989 for durations of 1 to 8 days.1989 for durations of 1 to 8 days.

The analyses have been carried out by determining the extremThe analyses have been carried out by determining the extreme e

probability distribution on the annual maxima. probability distribution on the annual maxima.

BRIMSTOWAD SAYSBRIMSTOWAD SAYS

RAINFALL ANALYSISRAINFALL ANALYSIS

•• CHAWATHE ,1977, 5CHAWATHE ,1977, 5--60 min (observed) , 194060 min (observed) , 1940--7272

•• CWPRS PUNE FOR CIDCO, ( CWPRS PUNE FOR CIDCO, ( MahimMahim creek), 1954creek), 1954--7676

•• NEDECO (NEDECO (MahimMahim creek), 1 to 12 hrs , 80 yearscreek), 1 to 12 hrs , 80 years

•• WH + AIC, 1 to 48 hrs, 1969WH + AIC, 1 to 48 hrs, 1969--19791979

BRIMSTOWAD SAYSBRIMSTOWAD SAYS

STORM INTENSITIES FOR USE IN WALRUS MODEL STORM INTENSITIES FOR USE IN WALRUS MODEL Intensity in mms/hrIntensity in mms/hr

PERIOD PERIOD DURATION(hrsDURATION(hrs) )

1 2 1 2 4 8 124 8 12

1 in 10 yrs 109 89 61.5 1 in 10 yrs 109 89 61.5 35.5 2835.5 28

1 in 5 yrs 91.4 70.3 51 1 in 5 yrs 91.4 70.3 51 27.6 2427.6 24

1 in 2 yrs 74 53.3 38 1 in 2 yrs 74 53.3 38 22.5 19.422.5 19.4

1 in 1 yr 58 40.6 30.4 1 in 1 yr 58 40.6 30.4 18.1 16.518.1 16.5

2 in 1 yr 25 17 12 2 in 1 yr 25 17 12 7.6 5.9 7.6 5.9

BRIMSTOWAD USES EXTREME VALUE BRIMSTOWAD USES EXTREME VALUE

PROBABILITY METHODPROBABILITY METHOD

THIS METHOD IS UNRELIABLE AND DOES NOT REFLECT THIS METHOD IS UNRELIABLE AND DOES NOT REFLECT

THE ACTUAL FREQUENCIES OF STORMS IN THE PAST THE ACTUAL FREQUENCIES OF STORMS IN THE PAST

DATA.DATA.

THE METHOD OF RAINFALL ANALYSIS SUGGESTED IN THE THE METHOD OF RAINFALL ANALYSIS SUGGESTED IN THE

CELEBRATED REFERENCE BOOK CELEBRATED REFERENCE BOOK ““WATER SUPPLY AND WATER SUPPLY AND

WASTE WATER DISPOSALWASTE WATER DISPOSAL”” BY G. M. FAIR AND J. C. GEYER BY G. M. FAIR AND J. C. GEYER

IS MORE RELIABLE.IS MORE RELIABLE.

BRIMSTOWAD SAYSBRIMSTOWAD SAYS

To design the drainage system for 1 in 1 year storm a To design the drainage system for 1 in 1 year storm a

considerable increase in capacity is required and this results considerable increase in capacity is required and this results

in drains of substantially larger dimensions and a large in drains of substantially larger dimensions and a large

increase in the number of drains to be replaced or increase in the number of drains to be replaced or

augmentedaugmented…………………………There is 37 % increase in expenditure There is 37 % increase in expenditure

for for catchmentcatchment 129.129.

This clearly indicates that designing for the increased leveThis clearly indicates that designing for the increased level l

of service is no where justified.of service is no where justified.

BRIMSTOWAD SAYSBRIMSTOWAD SAYS

AS DISCUSSED THE COST OF DESIGNING FOR STORMS OF AS DISCUSSED THE COST OF DESIGNING FOR STORMS OF

GREATER INTENSITY THAN A 2 IN 1 YEAR EVENT IS NOT GREATER INTENSITY THAN A 2 IN 1 YEAR EVENT IS NOT

JUSTIFIED. JUSTIFIED.

IT IS THEREFORE PROPOSED TO CONSTRUCT WORKS TO A IT IS THEREFORE PROPOSED TO CONSTRUCT WORKS TO A

DASIGN CAPACITY TO ONLY COPE UP WITH A 2 IN 1 YEAR DASIGN CAPACITY TO ONLY COPE UP WITH A 2 IN 1 YEAR

EVENT.EVENT.

BRIMSTOWAD DESIGNS FOR TWICE IN A BRIMSTOWAD DESIGNS FOR TWICE IN A

YEAR STORMYEAR STORM

BRIMSTOWAD JUSTIFIES TWICE IN A YEAR FREQUENCY BRIMSTOWAD JUSTIFIES TWICE IN A YEAR FREQUENCY

FOR STORMS IN DESIGN.FOR STORMS IN DESIGN.

26/7 EXPERIENCE HAS TAUGHT US TO REVIEW THIS 26/7 EXPERIENCE HAS TAUGHT US TO REVIEW THIS

ASPECT AND ADOPT FREQUENCIES SUCH AS SAY ONCE IN ASPECT AND ADOPT FREQUENCIES SUCH AS SAY ONCE IN

TEN YEARS FOR DESIGN.TEN YEARS FOR DESIGN.

BRIMSTAWAD SAYSBRIMSTAWAD SAYS

CRITERIA FOR LOCAL AREA DRAINAGE SYSTEMCRITERIA FOR LOCAL AREA DRAINAGE SYSTEM

This Report , providing proposals to Master Plan and This Report , providing proposals to Master Plan and Preliminary Engineering , does not cover the detailed Preliminary Engineering , does not cover the detailed investigation and proposals for local drainage systems. investigation and proposals for local drainage systems.

The proposals contained in subsequent sections are the The proposals contained in subsequent sections are the result of analysis of the major network and give in outline result of analysis of the major network and give in outline the requirements to alleviate the major causes of flooding. the requirements to alleviate the major causes of flooding.

As with any analysis on this scale , not all branches were As with any analysis on this scale , not all branches were considered.considered.

BRIMSTOWAD SAYSBRIMSTOWAD SAYS

IN MANY CASES IN THE CITY AND SUBURBS,FLOODING IN MANY CASES IN THE CITY AND SUBURBS,FLOODING

WILL OCCUR DUE TO LOCAL DEFFICIENCIES WITHIN THE WILL OCCUR DUE TO LOCAL DEFFICIENCIES WITHIN THE

SYSTEM WHICH HAVE NOT BECOME EVIDENT DURING THE SYSTEM WHICH HAVE NOT BECOME EVIDENT DURING THE

COURSE OF OUR WORK.COURSE OF OUR WORK.

BRIMSTOWAD DOES NOT COVER LOCAL BRIMSTOWAD DOES NOT COVER LOCAL

STORM DRAINAGE SYSTEMSSTORM DRAINAGE SYSTEMS

THE STORM WATER DRAINAGE SYSTEM CAN BE DIVIDED INTO TWO SUBTHE STORM WATER DRAINAGE SYSTEM CAN BE DIVIDED INTO TWO SUB--

SYSTEMS. LOCAL DRAINAGE SYSTEM COVERING SMALL /MEDIUM/LARGE SYSTEMS. LOCAL DRAINAGE SYSTEM COVERING SMALL /MEDIUM/LARGE

ROADS AND SMALL NALLAS FORM THE UPROADS AND SMALL NALLAS FORM THE UP--STREAM SUB SYSTEM AND STREAM SUB SYSTEM AND

MEDIUM / LARGE NALLAS/RIVERS FORM THE DOWNSMEDIUM / LARGE NALLAS/RIVERS FORM THE DOWNS--TREAM SUBTREAM SUB--

SYSTEM.SYSTEM.

MOST OF THE AREA OF MUMBAI FORMS THE CATCHMENT AREA FOR THEMOST OF THE AREA OF MUMBAI FORMS THE CATCHMENT AREA FOR THE

LOCAL DRAINAGE SUB SYSTEM. SINCE THE UPLOCAL DRAINAGE SUB SYSTEM. SINCE THE UP--STREAM SUBSTREAM SUB--SYSTEM SYSTEM

INITIALLY COLLECTS STORM WATER RUNINITIALLY COLLECTS STORM WATER RUN--OFF AND SUBSEQUENTLY OFF AND SUBSEQUENTLY

PASSES THE SAME TO THE DOWNPASSES THE SAME TO THE DOWN--STREAM SUBSTREAM SUB--SYSTEM, IT IS SYSTEM, IT IS

NECESSARY TO DESIGN IT PROPERLY AND IDENTIFY CONSTRUCTION NECESSARY TO DESIGN IT PROPERLY AND IDENTIFY CONSTRUCTION

PROPOSALS FOR THE SAME.PROPOSALS FOR THE SAME.

BRIMSTOWAD DOES NOT COVER THESE PROPOSALS.BRIMSTOWAD DOES NOT COVER THESE PROPOSALS.

RECOMMENDATIONSRECOMMENDATIONS

�� DESIGN / PROPOSALS SHOULD BE PREPARED FOR INTENSITIES OF DESIGN / PROPOSALS SHOULD BE PREPARED FOR INTENSITIES OF STORMS FOR A FREQUENCY OF OCCURRENCE OF SAY ONCE IN TEN STORMS FOR A FREQUENCY OF OCCURRENCE OF SAY ONCE IN TEN YEARS.YEARS.

�� DESIGN/PROPOSALS SHOULD BE ADDED FOR LOCAL STORM WATER DESIGN/PROPOSALS SHOULD BE ADDED FOR LOCAL STORM WATER DRAINAGE SYSTEMS (SMALL/MEDIUM/LARGE ROADS , SMALL NALLAS DRAINAGE SYSTEMS (SMALL/MEDIUM/LARGE ROADS , SMALL NALLAS WITH CONCENTRATION TIME 10 MIN. TO 60 MIN.) WITH CONCENTRATION TIME 10 MIN. TO 60 MIN.)

�� USE RAINFALL INTENSITYUSE RAINFALL INTENSITY-- DURATIONDURATION-- FREQURENCY RELATIONSHIPS FREQURENCY RELATIONSHIPS DEVLOPED BY CHAWATHE ET AL FOR DESIGNING LOCAL STORM WATER DEVLOPED BY CHAWATHE ET AL FOR DESIGNING LOCAL STORM WATER DRAINAGE SYSTEMDRAINAGE SYSTEM

�� REDEVELOP RAINFALL INTENSITYREDEVELOP RAINFALL INTENSITY--DURATIONDURATION-- FREQUENCY RELATION FREQUENCY RELATION SHIPS BASED ON HOURLY RAINFALL DATA FOR DURATION OF STORMS SHIPS BASED ON HOURLY RAINFALL DATA FOR DURATION OF STORMS OF MORE THAN ONE HOUR DURATION BY USING METHODOLOGY USED OF MORE THAN ONE HOUR DURATION BY USING METHODOLOGY USED BY CHAWATHE ET AL . EXTREME VALUE PROBABILITY METHOD SHOULD BY CHAWATHE ET AL . EXTREME VALUE PROBABILITY METHOD SHOULD NOT BE USED FOR THIS PURPOSE. DESIGN/ PROPOSALS SHOULD BE NOT BE USED FOR THIS PURPOSE. DESIGN/ PROPOSALS SHOULD BE FRESHLY PREPARED FOR LARGE NALLAS/ RIVERS SUCH AS MITHI USING FRESHLY PREPARED FOR LARGE NALLAS/ RIVERS SUCH AS MITHI USING THESE RELATIONSHIPS.THESE RELATIONSHIPS.

�� MODIFY BRIMSTOWAD ON ABOVE LINES AND REVISE ALL COST MODIFY BRIMSTOWAD ON ABOVE LINES AND REVISE ALL COST ESTIMATESESTIMATES