Water R E 3 Flood Runoff 2015April25 Lecture-2
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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
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