Climax Flow Rate Jan 2012

8/2/2019 Climax Flow Rate Jan 2012

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EV301EngineeringHydrology

ClimaxFlowRate


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ClimaxFlowRateAnalysis Rational

Method,

,


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The rational method is used around the world for peak

flow estimation of small rural drainage basins and is

the most widely used method for urban drainage.

Q=kCiA Q

peak

flow

(cfs or

m3/s).

k conversionfactore ualto1.008 SI or.00278(metric).

C

dimensionless

runoff

coefficient.

i rainfallintensity(in/hr,mm/hr).

A catchmentarea(acres,ha).


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=

Where:

R=Runoffinmillionm3

= 2

C=Constant

=

verage

annua

prec p a on

n

me res.


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beanapproximatemodelforcomputingthe

oo pea resu t ng romag venra n a ,w t t e

runoffcoefficient

accounting

for

all

differences

betweentherainfallintensityandthefloodpeak.

Suchdifferencesresultfrominfiltration

temporarystorage,

and

other

losses.

AmericanSocietyofCivilEngineersandWater

o u o n on ro e era on s oun n a e . .


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RunoffCoefficient Coefficient

that

represents

thefractionofrunoffto

Characterofsoil

Previous

moisture

conditions

Slopeofwatershed

Amount

of

impervious

soil

Landuse

Duration

intensity Table

7.1


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perthetypementionedabove.Therunofffromeach

added

together

to

arrive

at

the

total

runoff.


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different

C. Use weig te average:


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Intensit

Averageintensityforaselectedfrequencyand

urat onover ra nagearea or urat ono storm.

Basedon

design

event

(i.e.

50

year

storm)

Overdesigniscostly

Underdesign maybeinadequate

Durationis

important

Tc =timeofconcentration


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TimeofConcentration Tc

Timeforwatertoflowfromhydraulicallymost

distantpointonthewatershedtothepointof

interest. Rationalmethodassumespeakrunoffrateoccurs

whenrainfallintensity(I)lasts>=Tc

Usedas

storm

duration

minutes.


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TimeofConcentration Tc

Dependson:

Sizeandshapeofdrainagearea

Slopeofdrainagearea

Rainfallintensity

Whetherflowisentirel overlandorwhethersomeis

channelized


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simplified analysis of runoff using isochrones, or

between them as illustrated in the figure below.

the basin, so the ratio between the peak runoff

ratio of the volumes of runoffand rainfall.

time t=0 and has a duration of the time of,

will reach an instantaneous peak at Ci.


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time after rainfall excess begins to when all

por ons o e wa ers e are con r u ng o

the peak flow at the outlet. If the duration is longer than Tc, the

reaching a value of Ci for a time period equal

o e erence o e ra n a ura on an

Tc. In either case the time of rise and time ofrecession are equal to Tc.


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of rainfall and basin size areaccounted for explicitly, while mostother physical characteristics of the

watershed are accounted for

concentration and runoff coefficient.

critical nature ofTc.

,area is not contributing.

no increase in contributing area, andtherefore no increase in peak flow.


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Exam le1: Theclassificationofacatchmentcoveringanarea

2

1.Rocky,impermeable=10%

.3.Forest=30%

Usingtherationalformulaandthecoefficient,

thecatchmentwhentheaverageannual


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R=CAP


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Exam le2:

Theclassificationofacatchmentcoveringanarea

of150km2 isasfollows:

1.Covered

with

vegetation

=30%

2.Cultivated=40%

3.Heav Forest=20%

4.Sandy

Soil

=10%

byRichard,findtheannualrunofffromthecatchment

whentheavera eannual reci itationis650mm.


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WhatistheModifiedRationalMethod

(MRM)

ser eso rapezo a s ape y rograp sarecrea e fordifferentStormDurations.

TheI

in

the

Rational

e uation

is

based

u on

the

duration

andnottheTimeofConcentration,

HoweverthehydrographsinitiallypeakattheoriginalTime.

Therunoff

volume

from

the

pre

development

hydrograph

issubtractedfromeachoftherunoffvolumes(areasundertheTrapezoid),foreachstormduration.

Thegreatestdifferenceinvolumebetweenthepreandrespectivecriticalstormduration.


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Desi nEventRecurrenceInterval

2yearinterval Designofintakesandspreadofwa eronpavemen orpr mary g waysan c y

streets. 10yearinterva Designo inta esan sprea o

wateronpavementforfreewaysandinterstate

g ways. 50 year Designofsubways(underpasses)and

sagverticalcurveswherestormsewerpipeisthe

only

outlet. 100 yearinterval Majorstormcheckonall

projects.


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Climax Flow Rate Jan 2012