02 Precipitation

7/23/2019 02 Precipitation

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LECTURE-2 LECTURE-2

PrecipitationPrecipitation

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22

18

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Lifting cools airLifting cools airmasses so moisturemasses so moisturecondensescondenses

Condensation nucleiCondensation nuclei Aerosols ( Aerosols (suspension of particles in gas:suspension of particles in gas:

a suspension of solid or liquid particles in a gaseousa suspension of solid or liquid particles in a gaseous

medium)medium) (10(10-3-3 – 10– 10 µµm)m) water molecules attachwater molecules attach

Rising growingRising growing Critical si!e ("0#1 mm)Critical si!e ("0#1 mm) $ra%it& o%ercomes and$ra%it& o%ercomes and

drop fallsdrop falls

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Terminal Velocity Terminal Velocity

'hree forces'hree forces uo&anc& *riction $ra%it&uo&anc& *riction $ra%it&

Accelerate until terminal %elocit& Accelerate until terminal %elocit&V V tt +here forces ,alance+here forces ,alance

to.es Lawto.es Law

32

23

6246

0

D g

V

DC D g

W F F F

wad a

D Bvert

π

ρ

π

ρ

π

ρ −+=

−+==∑

332

2

6624 D g D g

V DC

W F F

wat

ad

B D

π ρ

π ρ

π ρ −=

−=

−= 13

4

a

w

d t

C

gDV

ρ

ρ

Re

24=d C

a

aVD

µ

ρ =Re

W

F B F D

/

V

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Precipitation – various forms

Rain (most important and de%astating)

Snow (signicant in cold countries - Canada northern 2urope –

and mountain areas) Hail (pellets of ice: small ,alls of ice and hardened snow that fall li.e rain) (de%astating ,u

conned to short periods of time)

C$ 314001/17/11 10:10 PM 18



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!lo"al Precipitation!lo"al Precipitation

566geograph&#uoregon#edu6en%change6clim7animations68$lo,al940+ater940alance01/17/11 10:10 PM 26



Precipitation VariationPrecipitation Variation

:n;uenced ,&:n;uenced ,& /istance from the sea5/istance from the sea5 'he sea a<ects the'he sea a<ects the

climate of a place# Coastal areas areclimate of a place# Coastal areas are

cooler and wetter than inland areas#cooler and wetter than inland areas# >cean currents5>cean currents5

/irection of pre%ailing winds/irection of pre%ailing winds5 +inds that5 +inds that

,low from the sea often ,ring rain to the,low from the sea often ,ring rain to the

coast and dr& weather to inland areas#coast and dr& weather to inland areas# Relief5Relief5 ?ountains recei%e more rainfall?ountains recei%e more rainfall

than low l&ing areasthan low l&ing areas

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#easurement of rainfall – Re$uire% parameters

1 &ept' of precipitation (in cm or mm)

1 &uration (min 'rs)

1 Rainfall intensity (in*'r cm*'r)

1 Space+time %istri"ution of precipitation

#easurement of rainfall – Types of Recor%ings

@oint measurements (Locali!ed)

– on-recording (standard) gages – measure onl& (1)

– Recording gages – tipping ,uc.et weighing-t&pe ;oat recording-t&pe

- measure (1) to (B)

Area measurements (o%er a certain area)

– Radar measurements (L:/AR 2RA/)

– $auge networ.

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Tipping ,uc-et Rain !age

1 Recor%ing gage

2 .ollector an%

/unnel

0 ,uc-et an%

Recor%er

ccurate to 31 ft

4 Telemetry+

computer

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(Source: NationaL Wweather Service - US, 2000)

ainfall measurement + Ra%ar

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1 Recent 5nnovation

2 &igital %ata is measure%

every 4 min over eac'

gri% cell as storm

a%vances ( -m 6 -m

cells)

0 T'e ra%ar %ata can "e

summe% over a storm to

provi%e total rainfall

%ept's "y su"+area

ccurate to 143+243

-m

4 Provi%es spatial %etail

"etter t'an gages

ainfall measurement + Ra%ar

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Raingauge networ- Raingauge networ-

ince the catching area of raingauge is %er&ince the catching area of raingauge is %er&

small compared to areal eDtent of a storm itsmall compared to areal eDtent of a storm it

is o,%ious that to get a representati%eis o,%ious that to get a representati%e

picture of a storm o%er a catchment thepicture of a storm o%er a catchment the

num,er of raingauges should ,e as large asnum,er of raingauges should ,e as large as

possi,lepossi,le

>n the other hand economic considerations>n the other hand economic considerations

to a large eDtent and other considerationsto a large eDtent and other considerationssuch as topograph& accessi,ilit& etc restrictsuch as topograph& accessi,ilit& etc restrict

the num,er of gauges to ,e maintained#the num,er of gauges to ,e maintained#

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Eence one aims at the optimum densit& ofEence one aims at the optimum densit& of

gauges from which reasona,l& accurategauges from which reasona,l& accurate

information a,out the storms can ,einformation a,out the storms can ,e

o,tainedo,tained +?> recommends the following densities5+?> recommends the following densities5

:n ;at regions of temperature ?editerranean and:n ;at regions of temperature ?editerranean and

tropical !ones5 ideal -1 station for F00-G00.mtropical !ones5 ideal -1 station for F00-G00.m44HHaccepta,le5 1 station for G00 – 3000.maccepta,le5 1 station for G00 – 3000.m44##

:n mountainous regions of temperate ?editerranean:n mountainous regions of temperate ?editerranean

and tropical !ones5 :deal – 1 station for 100-4I0.mand tropical !ones5 :deal – 1 station for 100-4I0.m44HH

accepta,le5 - 1 station for 4I-1000.maccepta,le5 - 1 station for 4I-1000.m44

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+?> recommends the following+?> recommends the following

densities5densities5 :n arid and polar !ones5 1 station for 1I00 –:n arid and polar !ones5 1 station for 1I00 –

10000.m10000.m44 depending on the feasi,ilit&depending on the feasi,ilit& Adequac& of raingauge stations5 Adequac& of raingauge stations5

:f there are alread& some raingauge:f there are alread& some raingauge

stations in a catchment the optimalstations in a catchment the optimal

num,er of stations should eDist to ha%e annum,er of stations should eDist to ha%e an

assigned percentage of error in theassigned percentage of error in the

estimation of mean rainfall#estimation of mean rainfall#

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J optimal num,er of stationsK J allowa,le degree of error in theestimate of mean rainfall andC % J coe<icient of %ariation of the

rainfall %alues

2

= ε

vC

N



Analysis of Temporal Distribution ofAnalysis of Temporal Distribution of

Rainstorm EventRainstorm Event

- On! "ea#i$e "or %ata o$taine% "ro& recor%in' 'au'e#

- Rainfall Mass Curve : *ot #howin' the cu&uative rain"a

%e*th over the #tor& %uration

- Rainfall Hyetogragh : *ot o" rain"a %e*th or

inten#it! with re#*ect to ti&e

+i&e

, e * t h o r

- n t e n

# i t !

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!rap'ical Representation of Rainfall &ata



!rap'ical Representation of Rainfall &ata+ #ass curves rainfall 'yetograp's +

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Example of Rainfall AnalysisExample of Rainfall Analysis

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Double Mass Curve AnalysisDouble Mass Curve Analysis

• Shi"tin' o" a rain'au'e #tation to a new ocation, e.*o#ure,

in#tru&entation, or o$#ervationa error "ro& a certain %ate &a! cau#ereative chan'e in the *reci*itation catch +hi# in"or&ation i# not u#ua!

incu%e% in the *u$i#he% recor%#

• ou$e/&a## curve ana!#i# te#t# the con#i#tenc! o" the recor% at a 'a'e

$! co&*arin' it# accu&uate% annua or #ea#ona *reci*itation with theconcurrent cu&uate% vaue# o" &ean *reci*itation "or a 'rou* o"

#urroun%in' #tation# +hi# techniue i# $a#e% on the *rinci*e that when

each recor%e% %ata co&e# "ro& the #a&e *arent *o*uation, the! are

con#i#tent

• $ru*t chan'e# or %i#continuitie# in the re#utin' &a## curve re"ect

#o&e chan'e# at the tar'et 'a'e ra%ua chan'e# in the #o*e o" the &a##

curve re"ect *ro're##ive chan'e# in the vicinit! o" the tar'et 'a'e, #uch a#

the 'rowth o" tree# aroun% a rain 'a'e

• +he #o*e# o" %i""erent *ortion# o" the &a## curve can $e u#e% a# a $a#i#

"or correctin' the recor% o" the tar'et 'a'e01/17/11 10:10 PM 39



Operation of Double Mass AnalysisOperation of Double Mass Analysis

∑i,t or ∑

i,t n

∑.,t

%u#t&ent "actor "or %ata

a"ter 1516 S1 S

2, ie,

., t

., t× S

1S

2, t 7 1516

S2

S1

1516

• chan'e o" #o*e #hou% not $e con#i%ere% #i'ni"icant une## it *er#i#t#

"or at ea#t 8 !ear#

• ue to the "act that the %ata &a! have #o&e #catter, an in%icate% chan'e in

#o*e #hou% $e con"ir&e% $! other evi%ence une## the chan'e in #o*e i#

#u$#tantia (#a!, 'reater than 109)

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ExampleExample

– Double– Double

MassMassAnalysisAnalysis

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real Precipitationreal Precipitation



real Precipitation real Precipitation9stimates:9stimates:

rit'metic #ean rit'metic #ean+hen the area is ph&sicall& and+hen the area is ph&sicall& and

climaticall& homogenous and theclimaticall& homogenous and the

required accurac& is small therequired accurac& is small thea%erage rainfall ( ) for a ,asina%erage rainfall ( ) for a ,asin

can ,e o,tained as the arithmeticcan ,e o,tained as the arithmetic

mean of themean of the hhii %alues recorded at %alues recorded at

%arious stations# %arious stations#

P

∑=

=++++

= N

i

ini P

N N

P P P P P

1

21 1((((((((((

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rit'metic #ean rit'metic #ean∑==

J

j j P

J P

1

1

Station Observed Rainfall

mm

2 20

3 30

4 40

8 80

140

ve Rainfall 13* 04 mm

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T'iessen Polygon #et'o%T'iessen Polygon #et'o%

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T'i l



T'iessen polygons #

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T'iessen polygons



T'iessen polygons #

1 2

0

4

;

<

8 P1

P2

P0

P

P4

P;

P<

P8

01/17/11 10:10 PM 46

T'iessen polygons



( )m

mm

A A A

A P A P A P

P +++

+++

= (((((

(((((

21

2211

∑∑

=

= == M

i

i

i

total

i

M

i

i

A

A P

A

A P

P

1

1

T'iessen polygons #

!enerally for # station

T'e ratio is calle% t'e weig'tage factor ofstation i

A

Ai

01/17/11 10:10 PM 47





T'iessen Polygon #et'o%T'iessen Polygon #et'o%∑==

J

j j j P A

A P

1

1

Station ObservedRainfall

Area WeightedRainfall

mm km2 mm

1 1 !22 2!2

2 2 "!2 #!"

3 $ 1!$% "!%

4 " 1!& &"!

8 % 1!'% '(!%

'!1" 2#"!&

ve Rainfall 28;*=1 011 mm 01/17/11 10:10 PM 48





T'iessen Polygon #et'o%T'iessen Polygon #et'o%

01/17/11 10:10 PM 49

rea Prec p tat onrea rec p a on



rea Prec p tat onrea rec p a on9stimates:9stimates:

5so'yetal #et'o%5so'yetal #et'o%

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5so'yetal #et'o%



An isoh&et is a line Moining points of equal rainfall magnitude#

5so'yetal #et'o%

/

,

9

.

&

12

=2

3

<3

<2

=1133

133

12

8

8

;

;

a1

a1

a2

a0

a

a4

01/17/11 10:10 PM 51

5so'yetal #et'o%



P1 P2 P0 > Pn – t'e values of t'e iso'ytes

a1 a2 a0 > a – are t'e inter iso'ytes area respectively

– t'e total catc'ment area

+ t'e mean precipitation over t'e catc'ment

5so'yetal #et'o%

P

A

P P a P P a P P a

P

nnn

+++

++

+

=

−−

2(((

22

1

1

32

2

21

1

The isohyet method is superior to the other two methodsespecially when the stations are large in number.

?@T9

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5so'yetal #et'o%5so'yetal #et'o%

ve Rainfall 2442*=1 2<= mm

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i i ' % l&i A i ' % # l



&istance+Aeig'te% #ean real&istance+Aeig'te% #ean real


01/17/11 10:10 PM 54

&i A i ' % # l&i t A i 't % # l



&istance+Aeig'te% #ean real&istance+Aeig'te% #ean realPrecipitationPrecipitation

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#ean areal precipitation1 it' ti



p p1 rit'metic

#et'o%

1 T'iessen Polyg

1 5so'yetal #et'

N

P

P

N

i

i∑== 1

∑=

=

N

i T

ii A

A P P

1

E:$E2R ACCNRACO

∑=

=

N

i w

ii A

A P P

1

01/17/11 10:10 PM 56

9 ti t9 ti t



9stimates9stimates

T'ree #et'o%sT'ree #et'o%s rit'metic verage rit'metic verage

$ages must ,e uniforml& distri,uted$ages must ,e uniforml& distri,uted :ndi%idual %ariations must not ,e far from mean:ndi%idual %ariations must not ,e far from mean

rainfallrainfall

ot accurate for large area where rainfallot accurate for large area where rainfalldistri,ution is %aria,ledistri,ution is %aria,le

T'iessen PolygonT'iessen Polygon Areal weighting of rainfall from each gage Areal weighting of rainfall from each gage /oes not capture orographic e<ects/oes not capture orographic e<ects

?ost widel& used method?ost widel& used method 5so'eytal5so'eytal

?ost accurate method?ost accurate method 2Dtensi%e gage networ. required2Dtensi%e gage networ. required Can include orographic e<ects and stormCan include orographic e<ects and storm

morpholog&morpholog&01/17/11 10:10 PM 57

/illing+in missing rainfall recor%s



/illing+in missing rainfall recor%s

1 @ften rainfall %ata are missing over various

perio%s of time

2 @ne 'as to estimate t'e missing %ata "ase% on

information

provi%e% "y surroun%ing gages( ) 1 e wher

11

== ∑∑==

N

i

i

N

i

ii x aa P P

1 rit'metic veragemet'o%

N

iai =

A'en normal precipitation of t'e surroun%ing gages

is wit'in 13 B of t'e missing gage

01/17/11 10:10 PM 58

2 ?ormal Ratio met'o%



2 ?ormal Ratio met'o% A'en normal precipitation of t'e surroun%ing gages

is more t'an 13 B of t'e missing gage

∑=

=

+++=

n

i i

xii

i

x x x x

nN

N P P

N

N P

N

N P

N

N

n P

1

2

2

1

1

(((1

0 5nverse &istance (Cua%rant)met'o%

∑=

= N

i

i

ii

D

Da

1

2

2

1

1

.alculate t'e weig'ts of t'e surroun%ing gages

"ase% on t'e %istances from t'e gage missing t'e

rainfall %ata

i is the normal precipitation (a%erage %alue of a particular date month or

&ear o%er a specied long period

01/17/11 10:10 PM 59



96ample

Station Annual precipitation(cm)

Montl! precipitation(cm)

A 114 11"5

# 95 9"0

$ 122 12"4

% 102 ??

Dse: (1) rit'metic verage #et'o% an% (2) ?ormal

Ratio #et'o%

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5ntensity+&uration+/re$uency

5&/ curves

Various returnperio%s

%urations

Dse% for %rainage%esign

Dse% for Eoo%plain%esigns

62

.'aracteristics of 5&/



.'aracteristics of 5&/

.urves

5&/ curves %o not representtime 'istories of real stormsintensities are averages overin%icate% %urations

single curve represents

%ata from several stormevents li-ely from %iFerent

years

&uration is not t'e %uration

of an actual storm (typicallyrepresents a s'orter perio%

wit'in a longer storm)

5t is t'eoretically incorrect

to o"tain a storm event01/17/11 10:10 PM 63

&evelopment of 5&/ .urves



&evelopment of 5&/ .urves

Dsing a long+term rainfall recor% ( 23 – 24 years) foreac' speciGe% %uration (common values 14 min 03 ;3

123 up to 2 'ours) t'e following steps are use%

T'e annual ma6imum (or e6cee%ences) rainfall %ept'sare e6tracte% from t'e perio% of recor% T'is results inone %ept' value for eac' year of recor%

fre$uency analysis is con%ucte% on t'e annual series(or partial %uration series): T'e precipitation values arearrange% in %escen%ing or%er an% t'e return perio% foreac' value is o"taine% using t'e formula: T(nI1)*m

T'e intensity an% %uration points are plotte% an%smoot'e% for selecte% fre$uencies

5&/ curves provi%e t'e average intensity (%ept') for aspeciGe% %uration an% fre$uency an% serve as t'e most

common source for synt'etic %esign storms01/17/11 10:10 PM 64

5&/ curves are %evelope% "ase% on statistical analysis ofrainfall recor%s



rainfall recor%s

T'e intensities are ran-e% in %escen%ing or%er an% assigne%a ran- m

T'e return perio% (T) are calculate% accor%ing to a plotting+position formula suc' Aei"ull w'ere:

m ran- of %atan num"er of o"servationsm

+n=T(year!

1

/or eac' %uration series of intensities an% return perio%sare plotte%

01/17/11 10:10 PM 65

96ample –&ar irport &ata96ample –&ar irport &ata



p p O2AR 1I?: 30?: 1 ER 4 ER 3 ER F ER 14 ER 4B ER

1GII 41#IG 30#BP 33#QP 3B#0B 3B#0B BF#GG Q3#G1 G3#Q3

1GIF 40#34 30#BP B3#GB I0#4G I3#3B IP#1Q IP#1Q IP#1Q

1GIQ 41#PB 30#BP 3F#P3 I1#0I IQ#1I QB#G3 PI#0G G4#40

1GIP 10#10 1F#4F 41#0P 44#F1 4Q#FG B3#1P I0#IB I0#II

1GIG 1I#4B 4P#Q0 34#00 3F#P3 3Q#IG 3P#3I 3P#3I B0#FB

1GF0 4I#B0 BF#QB I4#0Q I4#0Q I4#0Q I3#3B I3#3B I3#3B

1GF1 4B#13 3F#0Q BB#40 IF#FB F4#QB PQ#F3 PP#1B PP#1B

1GF4 41#IG 44#3I 4P#Q0 33#I3 3B#4G BP#4F BG#QP I1#0I

1GF3 30#BP B0#FB I3#3B QF#BI QQ#Q4 PB#PB PB#PB PB#PB

1GFB 41#PB 4P#1G 3Q#PI 3P#3I 3G#F4 BG#04 I0#II II#II

1GFI 4F#FQ 3Q#3B BG#QP I1#31 I4#IP I4#P3 F4#QB FF#II

1GFF 1G#0I 4Q#GB 4P#GF 4P#GF 30#BP 33#04 33#04 33#04

1GFQ 44#PF 3F#10 BQ#4B BP#4F I0#4G I0#4G I0#4G I0#4G

1GFP 4B#13 B0#FB IP#B4 P1#4P P1#I3 GB#43 GF#QQ GF#QQ1GFG 4B#3P 34#00 BP#I1 BG#I3 I4#34 FF#P0 FQ#P4 Q1#PP

1GQ0 4B#13 3B#4G 3F#34 3P#3I BI#41 BP#4F BP#4F BP#4F

1GQ1 40#P3 44#3I 44#FB 44#F1 4Q#1P 4Q#FG 4Q#GB 3B#4G

1GQ4 44#PF 31#I0 B1#G1 I3#3B F0#GF Q1#F3 QF#40 QQ#44

1GQ3 1Q#QP 3I#IF B0#FB B0#FB B4#4F I3#3B IB#10 FG#PI

1GQB 44#I0 3F#00 I4#I0 I4#I0 II#I0 IQ#I0 IP#I0 IP#I0

1GQI 4I#00 I0#00 103#00 111#00 113#00 113#00 113#00 113#00

1GQF 1G#00 4I#00 I0#00 IG#00 IG#00 F0#00 F0#00 F0#001GQQ 1I#00 1G#00 30#00 3Q#00 3G#00 I4#00 I4#Q0 I4#Q0

1GQP 4B#I0 33#I0 I1#00 I4#00 I4#10 I4#I0 I4#I0 Q1#00

1GQG B1#00 I3#00 FB#00 FF#00 FF#00 FF#00 FF#00 FF#00

1GP0 4G#00 B1#00 I4#00 IQ#I0 FI#00 P0#00 P0#00 P0#00

1GP1 33#00 B3#I0 BB#00 BB#30 BB#30 BB#I0 BI#00 II#I0

1GP4 40#00 31#00 3I#00 B3#00 BF#00 IB#I0 QB#G0 QQ#00

1GP3 40#40 4B#00 30#40 3G#00 B4#00 I1#I0 IG#00 FQ#40

1GPB 4I#I0 34#Q0 I0#00 I1#G0 I3#00 I3#00 I3#00 I3#001GPI 1I#00 1F#P0 4Q#00 3P#I0 BI#G0 BF#I0 BQ#00 IP#0001/17/11 10:10 PM 66



&ata &ar irport&ata &ar irport

15 MIN 30 MIN 1 HR 2 HRS 3 HRS 6 HRS 12 HRS 24 HRS

2 YRS 92.08 65.04 42.78 23.66 16.62 9.50 4.99 3.90

5 YRS 117.48 84.18 57.02 31.61 21.92 12.77 7.49

10 YRS 134.32 96.84 66.44 36.87 25.42 14.93 7.91 4.08

25 YRS 155.60 112.84 78.35 43.52 29.85 17.67 9.37 4.80

50 YRS 171.40 124.72 87.19 48.45 33.13 19.70 10.46 5.33

100 YRS 187.08 136.50 95.96 53.34 36.39 21.71 11.54 5.86

01/17/11 10:10 PM 67



5&/ .DRV95&/ .DRV9

01/17/11 10:10 PM 68



PPJ5.T5@?S PPJ5.T5@?S

:/* Cur%es:/* Cur%es

01/17/11 10:10 PM 69

RT5@?J #9TH@&



9mpirical met'o% for small waters'e%s (less t'en 2333acres)

/or small ungage% waters'e%s

C . 5

where5

J pea. runo< rate cfsC J runo< coe<icient non-dimensional: J rainfall intensit& in*'r

A J area acres

C 32<8 . 5

:mperial s&stem

?etric s&stemwhere5

J pea. runo< rate m0*sC J runo< coe<icient non-dimensional: J rainfall intensit& mm*'r

A J area -m201/17/11 10:10 PM 70



01/17/11 10:10 PM 71

T'e KrationaleK of t'is met'o% is: (1) Dnits agree: 1 cfs 1



T'e rationale of t'is met'o% is: (1) Dnits agree: 1 cfs 1in*'r 6 1 acre an% (2) . (a %imensionless $uantity) variesfrom 3 to 1 an% can "e t'oug't of as t'e percent of rainfall

t'at "ecomes runoF #

ssumptions for the rational formula are related to the intensityterm and to $uantifying . (t'e runoF coeFicient)5

Rainfall occurs uniforml& o%er the entire watershed#

Rainfall occurs with a uniform intensit& for a duration equal to thetime of concentration for the watershed#

'he runo< coe<icient C is dependent upon ph&sicalcharacteristics of the watershed e.g. soil t&pe#

:t is assumed that when the duration of a storm equals the time ofconcentration all parts of watershed are contri,uting

simultaneousl& to the discharge at the outlet##

01/17/11 10:10 PM 72

ea nesses o t e at ona et o 5• 2stimation of tc# 2speciall& critical on small watershed where



tc is short and changes in design intensities can occur quic.l&#

• Re;ects onl& the pea. and gi%es no indication of the %olume

or the time distri,ution of the runo<#

• Lumps man& watershed %aria,les into one runo< coe<icient#

• Lends little insight into our understanding of runo< processes

- eware of cases where watershed conditions %ar& greatl&across the watershed#

• 'his method is a great o%ersimplication of a complicatedprocessH howe%er the method is considered su<icientl&accurate for runo< estimation in the design of relati%el&ineDpensi%e structures where the consequences of failure arelimited#

• Application of rational method is normall& limited towatersheds of less than 4000 acres#

01/17/11 10:10 PM 73

RunoF .oeFicient K.K:• ecause most watersheds contain more than one soil t&pe



• ecause most watersheds contain more than one soil t&pewith multiple land uses and slopes it is necessar& to determinethe runo< coe<icient that represents this total %aria,ilit&#• A%erage coe<icients for composite areas ma& ,e calculated onan area weighted ,asis using5

where Ci is the coe<icient applica,le to the area Ai# :n areaswhere large parts are laid out in t&pical repeating patternssuch as su,-di%isions the weighting factors and weighted C can ,e determined ,& considering a single t&pical la&out#

∑∑

i

ii

A

AC =C

'&pical %alues for C:/owntown areas 5 0#Q0-0#GIeigh,orhood areas 5 0#I0 – 0#Q0Lawns 5 4 9 slopes – 0#0I – 0#10Lawns 5 Q 9 slopes – 0#1I – 0#40

01/17/11 10:10 PM 74

.oncentration time LtcK:• 'he time needed for a water particle to tra%el from the most



C$ 3140

• 'he time needed for a water particle to tra%el from the mosth&draulicall& distant part of the watershed to the outlet#•*or the rational method it is the time at which the entirewatershed will contri,ute to the runo< at the outlet

• 'he storm duration is assumed equal to tc

( ) 380300;:(0 " #t $ ∆=

where5L J length of main channel (ft) J / 2L2A':> /:**2R2C2 (min)tc J concentration time (min)

Sirpich method – for small drainage ,asins

"∆

?orgali – Linsle& method – for small ur,an drainage areas

( )3(04(0

6(0

54(0

% &

n#t $ =

where5L J length of ;ow (ft): J rainfall intensit& (in*'r)

n J ?anning coe<icient (dimensionless)J

01/17/11 10:10 PM 75

a n a ntens ty :•Chosen ,ased on the concentration time tc and the return



period 'r

• Assume stead& intensit& for the entire duration of the rain –

over%esignM

• Return )eriod* +r ,)g! (%#* Singh-

• <an $e cacuate% a#o $a#e% on the = curve# %rawn "or the re'ion "or which

the cacuation i# &a%e:

( ) e$ d t

' &

+=

where5

: J design rainfall intensit& (in*'r)tc J time of concentration (min)

, d e J parameters(%ar&ing with location andreturn period)

01/17/11 10:10 PM 76

• Proce%ure for use:



Proce%ure for use:i) elect design return period# (2D#'r J 10 &ears)

ii) /etermine time of concentration for the watershed#iii) /etermine design intensit& for T r Treturn periodU J selection

fordesign and duration J tc#

i%) /etermine weighted runo< coe<icient# %) /etermine watershed area# %i) Calculate pea. ;ow#

01/17/11 10:10 PM 77

Hy%raulic S'apes

Manning’s Eq ation sed to



Manning’s Equation used to

estimate flow rates

Q = k/n A R 2/3 S 1/2

Where Q = flow rate

n = roughness

A = cross sect A

R = A / P

S = Sloe

! = "#$% imerial

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