Water R E 3 Flood Runoff 2015April25 Lecture-2

8/17/2019 Water R E 3 Flood Runoff 2015April25 Lecture-2

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Flood Runof Flood Runof April 25, 2015


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Runof is water that ows across theland surace ater a storm event.

The eoraphical area that contri!utes to

the ow o a river is called a watershed(basin).

The river !asins o "avao #it$ are theollowin%

River Basin Drainage Area,km2

1. "avao River 1&5&.&'2. Talomo River 215.&(

). *asan River +5).

+. *ipadas River 1'&.'

5. -i!ulan River 2(2.1)

'. atina River &(.&

&. Tuana$ River &5&.+&

(. /unawan River 252.1)

TTA* )'5.2-ource%


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Flood Runof Flood Runof

Flood runof results rom the ollowing:1.-hortduration hihl$ intense rainall

2.*onduration low intensit$ rainall

).-nowmelt

+.Failure o dam or levee s$stem

5.#om!inations o the a!ove conditions

The !est inormation on ood manitudes that areliel$ to occur in the uture is o!tained rom o!served

ow records, what has occurred in the past.


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#hoice o ethod o Flood#hoice o ethod o Flood

3stimation3stimation

The choice o ood estimation is oten made on alarel$ su!4ective and intuitive !asis.

For desin at a site where o!served ood data areavaila!le, a choice must !e made !etween someorm o ood re!uen"# anal#sis or one o the

methods !ased on design rainall.a$ Flood re!uen"# anal$sis ives a direct estimate o

the ood o selected eceedance pro!a!ilit$ and usedow data.

b$Design rainall are less varia!le over time, are

availa!le at more locations and have reater spatialconsistenc$ in the surroundin reion !ecause areenerall$ loner than ow records.


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Rainall 6 RunofRainall 6 Runof

RelationshipsRelationships1. Flood runof has oten !een considered to consist osurace runof produced at round surace when the

rainall intensit$ eceeds the in7ltration capacit$, nownas %ortonian overland ow.

2$ &aturated 'verland Flow occurs when, on part o thedrainae !asin, the surace hori8on o the soil !ecomes

saturated as a result o either the !uild up o a saturated8one a!ove a soil hori8on o lower h$draulic conductivit$or the rise o shallow water ta!le to the surace.

$ hroughow is water that in7ltrates into the soil andpercolates rapidl$, larel$ throuh macropores such as

cracs and root and animal holes, and then moveslaterall$ in a temporaril$ saturated 8one, oten a!ove ala$er o low h$draulic conductivit$.


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*nter"e+tion 6 a

phenomenon when rainall isintercepted !$ veetation!eore it reaches the round.

De+ression or storage 6 is

water that is retained ininnumera!le depressions onthe !asin surace.

'verland ow the amount

o rainall in ecess o thein7ltrated 9uantit$ ows overthe round surace ollowinthe land slope

*nterow -


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%#drogra+h is the plot o the stream ow at aparticular location as a unction o time.

%#etogra+h is the raphical plot o the rainall plottedaainst time.

verland and =nterowverland and =nterow


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"istinct #ases o"istinct #ases o

>$etoraph>$etoraph1. Recorded Rainall or simulatin an actualh$droraph. The h$etoraph ma$ !e assumed to !e

the same at all points over the !asin, or i severalaues are availa!le to provide de7nition, diferenth$etoraphs ma$ !e used or diferent su!areas.

2. "esin Rainall with averae intensit$ o!tained rompu!lished intensit$durationre9uenc$ :="F;inormation or the reion. The desin storm ma$ !eassumed to have a constant intensit$ or a more

realistic varia!le intensit$ pattern over its duration.


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"esin Flood 3stimation"esin Flood 3stimation•

A ood is commonl$ considered to !e an unusuall$ hihstae o a river.

• For a river in its natural state, occurrence o a ood usuall$7lls up the stream up to its !ans and oten spills over to thead4oinin ood plains.

• For h$draulic structures planned within the river, due

consideration should !e iven to the desin o the structureso as to prevent it rom collapsin and causin urtherdamae !$ the orce o water released rom !ehind thestructure.

• =t must !e remem!ered that proper selection o desin oodvalue is o reat importance.

• ?hile a hiher value would result in an increase in the costo h$draulic structures, an underestimated value is liel$ toplace the structure and population involved at some ris.


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"esin Flood 3stimation"esin Flood 3stimation

The "esin Flood or a h$draulic structure ma$ also!e de7ned in a num!er o wa$s, lie%

1. The maimum ood that an$ structure can sael$pass.

2. The ood considered or the desin o a structurecorrespondin to a maimum tolera!le ris.

). The ood which a pro4ect :involvin a h$draulicstructure; can sustain without an$ su!stantialdamae, either to the o!4ects which it protects or to

its own structures.

+. The larest ood that ma$ !e selected or desin assaet$ evaluation o a structure.


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3stimation o a Flood o3stimation o a Flood o

-elected


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3stimation o a Flood o -elected3stimation o a Flood o -elected


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.$ Rational /ethod3stimation o pea ows is re9uired or

h$draulic structures. The commonl$used method is the Rational ethodiven the ormula@

where% B pea ows, m)Csec# B runof coeDcient

= B rainall intensit$ :in mmCh; orthe re9uired return period,correspondin to a duration e9ual tothe time o concentration o thewatershed

Runof #oeDcient Ta!le

-ource% A-#3 :1&';


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ime o 0on"entration is the time taen !$ surace runofto travel rom the remotest point in the watershed to thepoint where pea ow is to !e estimated. A ew relativel$simple and useul methods are%

a.Eirpich ethod :-/R 1&;

where%

tc B time o concentration :hour;

* B lenth o the lonest watercourse :m;> B diference in elevation !etween the upper

end o the watershed and the location at which owis to !e estimated :m;

Note: This method results in relatively low estimates of t c (Prakash 19!"

385.0)87.0

(3

H

Lt c =


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ime o 0on"entration

!.-oil #onservation -ervice :-#-; #urve Gum!er ethod:-"A 1&2, 1(5;

where%

t* B !asin la :hour;

* B h$draulic lenth o watershed :m;

#G B curve num!er o the watershed-a B potential maimum retention :cm;

-p B averae land slope o the watershed

:H;

( )

p

a f

LS

S Lt

1900

54.2347.1 7.08.0 +=

c L t t 6.0=

4.252540

−

= CN S

a


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ime o 0on"entration

c.-n$derIs ethod :#how 1'+;

where%

*ca B lenth o the lonest watercourse rom

the location where ow is to !e

estimated to the centroid o thewatershed :m;

#t B a coeDcient or !$ usin,

where%- B !asin slope :mCm;

Note: %dvisable to use calibrated values of & t

( ) 3.0

7517.0 cat L LLC t =

S C t

6.0=


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1am+le 3roblem:

1.3stimate the time o concentration or a )'.)' m2 watershed in -outhern indanao usin :a; Eirpich ethod,

:!; -#- ethod and :c; -n$derIs ethod with the ollowinrelevant watershed parameters%

?atershed *enth B 11.'& m/asin -lope B )).(&H

> B 2,0+1 m#G B '0#t B 0.50

2. 3stimate the


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ReerencesReerences

aidment, ". , >and!oo o >$drolo$ and!oo

o ethods and "esin

Eharapur, ?ater Resources 3nineerin


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17D17D

Water R E 3 Flood Runoff 2015April25 Lecture-2

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