A Second Look at Brimstowad

8/7/2019 A Second Look at Brimstowad

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A SECOND LOOK ATA SECOND LOOK ATBRIMSTOWADBRIMSTOWAD

S. D. CHAWATHES. D. CHAWATHE

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

IWWAIWWA









BRIMSTOWAD SAYSBRIMSTOWAD SAYS

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

SEWERS IN BOMBAYSEWERS IN BOMBAY --19771977

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

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

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

27082708 santacruzsantacruz storms between 1940 and 1972storms between 1940 and 1972






















NEW ANALYSISNEW ANALYSIS

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

the period 1969the period 1969--1979)1979) seperatelyseperately forfor ColabaColaba andand SantacruzSantacruz toto

produce Intensityproduce Intensity-- DurationsDurations-- Frequency (IDF) relationships forFrequency (IDF) relationships fordurations of 1 to 48 hrs and for a range of frequencies betweendurations of 1 to 48 hrs and for a range of frequencies between 11

and 50 years. We have alsoand 50 years. We have also analysedanalysed the daily data for the periodthe 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 extremee

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




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




STORM INTENSITIES FOR USE IN WALRUS MODELSTORM INTENSITIES FOR USE IN WALRUS MODELIntensity in mms/hrIntensity in mms/hr

PERIODPERIOD DURATION(hrsDURATION(hrs))

1 21 2 4 8 124 8 12

1 in 10 yrs 109 89 61.51 in 10 yrs 109 89 61.5 35.5 2835.5 28

1 in 5 yrs 91.4 70.3 511 in 5 yrs 91.4 70.3 51 27.6 2427.6 24

1 in 2 yrs 74 53.3 381 in 2 yrs 74 53.3 38 22.5 19.422.5 19.4

1 in 1 yr 58 40.6 30.41 in 1 yr 58 40.6 30.4 18.1 16.518.1 16.5

2 in 1 yr 25 17 122 in 1 yr 25 17 12 7.6 5.97.6 5.9



BRIMSTOWAD USES EXTREME VALUEBRIMSTOWAD USES EXTREME VALUE

PROBABILITY METHODPROBABILITY METHOD

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

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

DATA.DATA.

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

CELEBRATED REFERENCE BOOKCELEBRATED REFERENCE BOOK ““WATER SUPPLY ANDWATER SUPPLY AND

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

IS MORE RELIABLE.IS MORE RELIABLE.








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

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

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

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

augmentedaugmented…………………………There is 37 % increase in expenditureThere is 37 % increase in expenditureforfor catchmentcatchment 129.129.

This clearly indicates that designing for the increased leveThis clearly indicates that designing for the increased levell

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




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

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

JUSTIFIED.JUSTIFIED.

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

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

EVENT.EVENT.



BRIMSTOWAD DESIGNS FOR TWICE IN ABRIMSTOWAD DESIGNS FOR TWICE IN A

YEAR STORMYEAR STORM

BRIMSTOWAD JUSTIFIES TWICE IN A YEAR FREQUENCYBRIMSTOWAD JUSTIFIES TWICE IN A YEAR FREQUENCYFOR STORMS IN DESIGN.FOR STORMS IN DESIGN.

26/7 EXPERIENCE HAS TAUGHT US TO REVIEW THIS26/7 EXPERIENCE HAS TAUGHT US TO REVIEW THIS

ASPECT AND ADOPT FREQUENCIES SUCH AS SAY ONCE INASPECT AND ADOPT FREQUENCIES SUCH AS SAY ONCE INTEN 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 andThis Report , providing proposals to Master Plan andPreliminary Engineering , does not cover the detailedPreliminary Engineering , does not cover the detailedinvestigation and proposals for local drainage systems.investigation and proposals for local drainage systems.

The proposals contained in subsequent sections are theThe proposals contained in subsequent sections are theresult of analysis of the major network and give in outlineresult of analysis of the major network and give in outlinethe 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 wereAs with any analysis on this scale , not all branches wereconsidered.considered.




IN MANY CASES IN THE CITY AND SUBURBS,FLOODINGIN MANY CASES IN THE CITY AND SUBURBS,FLOODINGWILL OCCUR DUE TO LOCAL DEFFICIENCIES WITHIN THEWILL OCCUR DUE TO LOCAL DEFFICIENCIES WITHIN THE

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

COURSE OF OUR WORK.COURSE OF OUR WORK.



BRIMSTOWAD DOES NOT COVER LOCALBRIMSTOWAD 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/LARGESYSTEMS. LOCAL DRAINAGE SYSTEM COVERING SMALL /MEDIUM/LARGE

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

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

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

NECESSARY TO DESIGN IT PROPERLY AND IDENTIFY CONSTRUCTIONNECESSARY 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 OFDESIGN / PROPOSALS SHOULD BE PREPARED FOR INTENSITIES OFSTORMS FOR A FREQUENCY OF OCCURRENCE OF SAY ONCE IN TENSTORMS FOR A FREQUENCY OF OCCURRENCE OF SAY ONCE IN TENYEARS.YEARS.

DESIGN/PROPOSALS SHOULD BE ADDED FOR LOCAL STORM WATERDESIGN/PROPOSALS SHOULD BE ADDED FOR LOCAL STORM WATERDRAINAGE SYSTEMS (SMALL/MEDIUM/LARGE ROADS , SMALL NALLASDRAINAGE SYSTEMS (SMALL/MEDIUM/LARGE ROADS , SMALL NALLASWITH CONCENTRATION TIME 10 MIN. TO 60 MIN.)WITH CONCENTRATION TIME 10 MIN. TO 60 MIN.)

USE RAINFALL INTENSITYUSE RAINFALL INTENSITY-- DURATIONDURATION-- FREQURENCY RELATIONSHIPSFREQURENCY RELATIONSHIPSDEVLOPED BY CHAWATHE ET AL FOR DESIGNING LOCAL STORM WATERDEVLOPED BY CHAWATHE ET AL FOR DESIGNING LOCAL STORM WATER

DRAINAGE SYSTEMDRAINAGE SYSTEM REDEVELOP RAINFALL INTENSITYREDEVELOP RAINFALL INTENSITY--DURATIONDURATION-- FREQUENCY RELATIONFREQUENCY RELATION

SHIPS BASED ON HOURLY RAINFALL DATA FOR DURATION OF STORMSSHIPS BASED ON HOURLY RAINFALL DATA FOR DURATION OF STORMSOF MORE THAN ONE HOUR DURATION BY USING METHODOLOGY USEDOF MORE THAN ONE HOUR DURATION BY USING METHODOLOGY USEDBY CHAWATHE ET AL . EXTREME VALUE PROBABILITY METHOD SHOULDBY CHAWATHE ET AL . EXTREME VALUE PROBABILITY METHOD SHOULDNOT BE USED FOR THIS PURPOSE. DESIGN/ PROPOSALS SHOULD BENOT BE USED FOR THIS PURPOSE. DESIGN/ PROPOSALS SHOULD BE

FRESHLY PREPARED FOR LARGE NALLAS/ RIVERS SUCH AS MITHI USINGFRESHLY PREPARED FOR LARGE NALLAS/ RIVERS SUCH AS MITHI USINGTHESE RELATIONSHIPS.THESE RELATIONSHIPS.

MODIFY BRIMSTOWAD ON ABOVE LINES AND REVISE ALL COSTMODIFY BRIMSTOWAD ON ABOVE LINES AND REVISE ALL COSTESTIMATESESTIMATES

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A Second Look at Brimstowad