flood and drought management backup.doc

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DEBRETABOR UNIVERSITYFACULTY OF TECHNOLOGY

DEPARTMENT OF HYDRAULIC AND WATER RESOURCEENGINEERING

CourseCode-HEng-!"#

MODULE COORDINATOR$- Mr% S&'d M%( B%S)*

O)+o,er ./!0


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i

T&,1e o2 Con+en+s page

CHAPTER-ONE............................................................................................................... .1

Overview of Hydro-Climatic Hazards ............................................................................... 1

1.1 !trod"ctio!..........................................................................................................................1

Types of Hazards ....................................................................................................................... ..1

1.# $ocial% Eco!omic a!d E!viro!me!tal mpacts ......................................................................&

1.' (e!efits a!d Costs of Early )ar!i!g a!d Prepared!ess.........................................................*

1.'.1 +actors T,at etermi!e t,e (e!efits of Early )ar!i!g $ystems.....................................*

1.'.# isaster Prepared!ess..........................................................................................................

CHAPTER-T)O ............................................................................................................ ..1/

Origi! a!d Nat"re of Hydro-Climatic Hazards ................................................................ 11

#.1 0eteorological regimes........................................................................................................11

#.# Hydrological Regimes..........................................................................................................1#

#.' P,ysical C,aracteristics of +loods .......................................................................................1'

#.* 0eas"reme!t Tec,!i"es ....................................................................................................1'

#.2 +re"e!cy of +loodi!g ........................................................................................................1&

CHAPTER-THREE ..........................................................................................................13

0a!ageme!t a!d 0itigatio! of Hazards4isasters ...........................................................13

'.1 Hazard 0aps% Ris5 !formatio! a!d Ed"catio!...................................................................16

'.# Possi7le +lood Ris5 Red"ctio! 0eas"res............................................................................#/

'.' $trategic Approac,8 $tr"ct"ral 0eas"res ............................................................................#'

'.& Early )ar!i!g ....................................................................................................................#'

'.&.1 T,e +lood )ar!i!g Process.........................................................................................#&

'.*+lood co!trol )or5s............................................................................................................#3

'.*.1O79ectives8 ...................................................................................................................#


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ii

'.*.# Classificatio! ................................................................................................................#

'.*.' 0et,ods ......................................................................................................................#6

'.&.# T,e Nat"re of +lood Ris5 ............................................................................................'2

'.&.' Assessi!g +lood Ris5 ...................................................................................................'

'.&.& +lood Ris5 0odeli!g ...................................................................................................&1

'.* Emerge!cy Respo!se .........................................................................................................

'.2 +lood Emerge!cy Pla!!i!g.................................................................................................&*

'.3 isaster 0a!ageme!t.........................................................................................................&2

'. Evac"atio! .........................................................................................................................&2

'.6 !formatio! $ystem............................................................................................................&3

'.6.1 0eteorological a!d Hydrological +orecasti!g :0eteorological a!d Hydrological

Networ5s; ............................................................................................................................&3

'.6.# !str"me!tatio! Networ5s............................................................................................&

'.6.' !tegrated +lood 0a!ageme!t .....................................................................................&6

'.1/ etectio!..........................................................................................................................*/

'.11 Alert $ystems< Target =ro"ps...........................................................................................*/

CHAPTER-+O>R .......................................................................................................... *#

!stit"tio!al +ramewor5 .................................................................................................. *#

&.1 Orga!izatio! a!d $tr"ct"re of a! Early )ar!i!g $ystem8 Role of Comm"!ity% Natio!al%

Regio!al a!d !ter!atio!al Age!cies ........................................................................................*#

&.1.1 Orga!izatio!al ss"es...................................................................................................*#

&.1.# Role of Comm"!ity.....................................................................................................*'

&.# !stit"tio!al $tr"ct"re.........................................................................................................**

&.' !stit"tio!al Respo!si7ilities for isaster 0a!ageme!t a!d Respo!se 0ec,a!isms ............**

&.& =aps i! !stit"tio!al Capacity i! !tegrated Early )ar!i!g a!d Respo!se $ystems .............*3

CHAPTER-+?E............................................................................................................ *

ro"g,t ........................................................................................................................... *6


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*.1 Ca"ses of ro"g,t..............................................................................................................*6

*.# =e!eral C,aracteristics.......................................................................................................2/

*.' 0eas"ri!g ro"g,t ............................................................................................................2/

*.& Possi7le Ris5 Red"ctio! 0eas"res8 ..................................................................................21

*.* ro"g,t $everity ................................................................................................................2'

*.2 ro"g,t 0a!ageme!t.........................................................................................................2'

*.3 ro"g,t Early )ar!i!g $ystem..........................................................................................22

CHAPTER-$@ ............................................................................................................... 2

Case $t"dies.................................................................................................................... ...2

2./ !trod"ctio!............................................................................................................... ..2

Case st"dies mea!s .....................................................................................................................2

T,e process of developi!g a case st"dy8 .....................................................................................2

2.1+lood i! Et,iopia.....................................................................................................................26

2.1.1 +lood i! ire awa ..................................................................................................... ...3/

2.1.# 0assive +las, +lood i! ire awa .............................................................................. ...3#

2.1.' +lood i! Afar Regio! ................................................................................................... ...3'

Early History ............................................................................................................................3'

2.# Awas, River +loods................................................................................................................3&

2.#.1 (ac5gro"!d ......................................................................................................................3&

2.#.# =e!eral $it"atio!..............................................................................................................32

2.#.' =e!eral Ca"ses of +loodi!g .........................................................................................33

2.#.* Pro7lem of ismissed Temporary Commercial +arm Employees >!solved .................3

2.' Awas, River floods.................................................................................................................36

2.& ro"g,t i! Et,iopia...............................................................................................................#

2.&.1!trod"ctio! ...........................................................................................................................#

2.2 Respo!se to t,e +ami!e............................................................................................................'


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2.2.1 ive Aid .........................................................................................................................*

2.3 ),y a!ot,er ro"g,t is !ot occ"rred i! Et,iopiaB...............................................................*

2. $AC Early )ar!i!g $ystem +or +ood $ec"rity...............................................................6

Refere!ce.6#


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v

ist of +ig"res

+ig'. 18 +lood plai! ..........................................................................................................................#/

+ig'. #ll"stratio! of t,e compo!e!ts of a flood war!i!g% forecasti!g a!d emerge!cy respo!se

system ................................................................................................................................................#*

+ig'. '+loods cases a damage i! some area of t,e world .............................................................. ....'

+ig'. &EDamples of river a!d coastal flood defe!ces a!d a flood gate for was, la!d drai!age .........&&

+ig2. 10ap locatio! of ire-awa.....................................................................................................3/

+ig2. #0ap of Et,iopia s,owi!g t,e Afar Regio!..............................................................................3&

+ig2. '+lood i! Et,iopia .....................................................................................................................1


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vi

ist of Ta7les

Ta7le1. 18 ?ario"s Types of Hazards.&

Ta7le'. 18 Typical compo!e!ts i! t,e flood war!i!g% forecasti!g a!d emerge!cy respo!se ..........#2


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vii

In+rodu)+'on o2 +3e Modu1e

Hydro-climatic disasters are respo!si7le for t,e serio"s disr"ptio! of t,e f"!ctio!i!g of a

society or comm"!ity a!d wide spread ,"ma!% material or e!viro!me!tal losses. T,ese

disasters a!d t,e comm"!ities eDpose to t,em may 7e eDpected to clim7 wit, i!creased

climate varia7ility as a res"lt of climate c,a!ge. Tragically% t,e spa! of atte!tio! give! to

,ydro-climatic disasters is ofte! s,ort% pro7a7ly 7eca"se t,e disaster Fve!ts co!ti!"e o!ly

for a s,ort w,ile% a!d as t,e 0emory of disaster eve!ts fades% so does t,e rege!cy for

isaster ris5 rFd"ctio! stratFgies.

O,4e)+'5e$

Have t,e mai! o79ective% t,is co"rse "sed to 5!ow t,e 7asic a!d s"stai!a7le preve!tio! for

flood %dro"g,t a!d ot,er e!viro!me!tal ,azards 7y "si!g differe!t mec,a!isms li5eco!str"ctio! of Hydra"lic str"ct"res to preve!t t,e flood ,azards : early war!i!g for all

disasters ;.

Re6u're7en+ o2 +3e )ourse

At t,e e!d of t,is co"rse st"de!ts s,o"ld 7e "!dersta!d t,e ca"se a!d ma!ageme!ts of

e!viro!me!tal ,azards a!d ot,er related co!cepts.

Pre-re6u's'+es8- E!gi!eeri!g Hydrology

T&rge+ grou8


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FLOOD AND DROUGHT MANAGEMENT

Prepared by Said M. ( B.SC) DTU

CHAPTER-ONE

O5er5'e: o2 H;dro-C1'7&+') H&


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releases to t,e air or i!to rivers a!d water 7odies. =eological ,azards a!d ,ydro-

meteorological ,azards :eDcept dro"g,ts;.

#. $low-o!set :or Kcreepi!gL;8 !creme!tal 7"t lo!g-term a!d c"m"lative e!viro!me!tal

c,a!ges t,at "s"ally receive little atte!tio! i! t,eir early p,ases 7"t w,ic,% over time% may

ca"se serio"s crises. T,ese wo"ld i!cl"de s"c, iss"es as8 air a!d water "ality% soil

poll"tio!% acid rai!% climate c,a!ge% desertificatio! processes :i!cl"di!g soil erosio! a!d

la!d degradatio!;% dro"g,ts.

Nat"ral disasters ca"sed 7y weat,er i!cl"de tropical cyclo!es :,"rrica!es;% tor!adoes%

floods% ,eat waves% a!d dro"g,ts. T,e ca"ses of floods a!d flood ,azards are a compleD

miDt"re of meteorological% ,ydrological a!d ,"ma! factors. t m"st 7e emp,asized t,at

,"ma! eDpos"re to flood ,azards is largely t,e res"lt of people wor5i!g a!d livi!g i! areas

t,at are !at"rally M al7eit rarely M s"79ect to floodi!g.

• ! ge!eral ,ydro-climatic ,azards ca! 7e divided i! to t,ree parts

. 0eteorological Hazards8 Cyclo!es% +loods% Tor!adoes% Heat% ,"rrica!es% dro"g,t

. =eological Hazards8 Ts"!amis% ?olca!oes% Eart,"a5es

. Tec,!ological Hazard8 A ,azard origi!ati!g from tec,!ological or i!d"strial co!ditio!s%

i!cl"di!g accide!ts% da!gero"s proced"res% i!frastr"ct"re fail"res or specific ,"ma!

activities% t,at may ca"se loss of life% i!9"ry% ill!ess or ot,er ,ealt, impacts% property

damage% loss of liveli,oods a!d services% social a!d eco!omic disr"ptio!% or e!viro!me!tal

damage.

; 0eteorological Hazards

A. +loods are a very commo! type of !at"ral disaster% a!d t,ey ca"se s"7sta!tial amo"!ts of

property damage% a!d s"7sta!tial losses of life. ),ile t,ere seems to 7e limited pote!tial

for a flood war!i!g system to prod"ce sig!ifica!t 7e!efits wit, respect to property damage

from floods% t,e large !"m7ers of lives t,at are 7ei!g saved 7y flood war!i!g systems is

"ite li5ely large% a!d co"ld 7e larger wit, f"rt,er ed"catio! of t,e p"7lic.

(. ro"g,ts are differe!t from ot,er !at"ral disasters i! several respects. ! t,e developed

world t,ey rarely claim lives% si!ce wealt,y co"!tries ,ave t,e fi!a!cial a7ility to replace

food prod"ctio! lost d"e to dro"g,ts. ! years past% t,ey ,ave 7ee! amo!g t,e most deadly


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of ,azards i! less developed co"!tries% t,o"g, t,is may 7e less tr"e !ow a!d i!to t,e f"t"re

as Cs ,ave 7ecome wealt,ier a!d i!ter!atio!al aid respo!ses ,ave developed a!d

7ecome more effective. ! terms of property damage% ,owever% dro"g,ts are amo!g t,e

most costly of !at"ral ,azards. ! additio!% t,ey are diffic"lt to predict% si!ce t,ey are

i!,ere!tly lo!g-term weat,er eve!ts lasti!g mo!t,s or years% a!d predicti!g weat,er over

t,ese 5i!ds of time spa!s is ,arder t,a! predicti!g weat,er over t,e !eDt few to several

days.

C. Tropical cyclo!es% or ,"rrica!es% as t,ey are called i! t,e >!ited $tates% are large storms

w,ic, form over warm ocea! waters i! t,e tropics or s"7-tropics. T,ey te!d to occ"r

seaso!ally for eDample% Atla!tic ,"rrica!es affecti!g t,e >!ited $tates are co!ce!trated i!

late s"mmer a!d early fall. H"rrica!e damage occ"rs as a res"lt of powerf"l wi!ds%

floodi!g :from coastal storm s"rge a!d ,eavy rai!fall;% a!d tor!adoes t,at may 7e spaw!ed

7y t,e ,"rrica!e.

. A tor!ado is a col"m! of rapidly rotati!g air t,at develops wit,i! a t,"!derstorm.

Compared to ,"rrica!es% w,ic, ca! 7e ,"!dreds of miles wide% tor!adoes are m"c,

smaller% wit, a widt, ra!gi!g from less t,a! 1// meters to a7o"t a mile.

E. Heat waves are a relatively "!derappreciated !at"ral disaster. A K,eat waveL may 7e

defi!ed as a period of time w,e! temperat"re a!d ,"midity co!ditio!s c,a!ge relatively

"ic5ly i! ways t,at prese!t a ris5 to ,"ma! ,ealt,. Heat waves !ever ca"ses property

damage% a!d t,ey may create o!ly mi!or discomfort for ,ealt,y a!d relatively well-to-do

people w,o live% wor5% a!d drive i! air-co!ditio!ed spaces.

; =eological Hazards

A. Eart,"a5es are moveme!ts of t,e gro"!d t,at are 7elieved to occ"r as a res"lt of t,e

eart,s tecto!ic plates s"dde!ly slidi!g wit, respect to eac, ot,er.

(. Ts"!amis are ,"ge ocea! waves% pote!tially eDceedi!g '/ meters i! ,eig,t w,e! t,ey

reac, la!d. T,ey are most ofte! ge!erated 7y eart,"a5es% t,o"g, !ot all eart,"a5escreate ts"!amis.

Nat"ral ,azards are ,azards w,ic, are ca"sed 7eca"se of !at"ral p,e!ome!a :,azards wit,

meteorological% geological or eve! 7iological origi!;. EDamples of !at"ral ,azards are

cyclo!es% ts"!amis% eart,"a5e a!d volca!ic er"ptio! w,ic, are eDcl"sively of !at"ral


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origi!. a!dslides% floods% dro"g,t% fires are socio-!at"ral ,azards si!ce t,eir ca"ses are

7ot, !at"ral a!d ma!made. +or eDample floodi!g may 7e ca"sed 7eca"se of ,eavy rai!s%

la!dslide or 7loc5i!g of drai!s wit, ,"ma! waste.

0a!made ,azards are ,azards w,ic, are d"e to ,"ma! !eglige!ce. 0a!made ,azards are

associated wit, i!d"stries or e!ergy ge!eratio! facilities a!d i!cl"de eDplosio!s% lea5age of

toDic waste% poll"tio!% dam fail"re% wars or civil strife etc. T,e list of ,azards is very lo!g.

0a!y occ"r fre"e!tly w,ile ot,ers ta5e place occasio!ally. However% o! t,e 7asis of t,eir

ge!esis% t,ey ca! 7e categorized as follows8

Ta7le1. 18 ?ario"s Types of Hazards

Activates1.1. ),at is t,e differe!ce 7etwee! Tor!ados a!d Ts"!amisB

1.# $ocial% Eco!omic a!d E!viro!me!tal mpacts

EDtreme floods ,ave serio"s social a!d eco!omic impacts. T,e most importa!t

co!se"e!ce of flood is t,e loss of life a!d property. $tr"ct"res li5e ,o"ses% 7ridges< roads

etc. get damaged 7y t,e g"s,i!g water% la!dslides triggered o! acco"!t of water getti!g

sat"rated% 7oats a!d fis,i!g !ets get damaged. T,ere is ,"ge loss to life a!d livestoc5

ca"sed 7y drow!i!g. ac5 of proper dri!5i!g water facilities% co!tami!atio! of water

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Prepared by Said M.(B.SC) $ DTU

:well% gro"!d water% piped water s"pply; leads to o"t7rea5 of epidemics% diarr,ea% viral

i!fectio!% malaria a!d ma!y ot,er i!fectio"s diseases.

+loodi!g also leads to a large area of agric"lt"ral la!d getti!g i!"!dated as a res"lt t,ere is

a ,"ge crop loss. T,is res"lts i! s,ortage of food% a!d a!imal fodder. +loods may also

affect t,e soil c,aracteristics. T,e la!d may 7e re!dered i!fertile d"e to erosio! of top layer

or may t"r! sali!e if sea water floods t,e area.

>r7a! floods ,ave large impacts partic"larly i! terms of eco!omic losses 7ot, direct a!d

i!direct. +lood ris5s are a f"!ctio! of eDpos"re of t,e people a!d t,e eco!omic activities

alo!g wit, t,e v"l!era7ility of social a!d eco!omic fa7ric. As s"c, t,e impact of s"c,

floods o! t,e lives a!d liveli,oods of people% a f"!ctio! of t,eir v"l!era7ility% !eeds to 7e

"!derstood.

1.' (e!efits a!d Costs of Early )ar!i!g a!d Prepared!ess

1.'.1 +actors T,at etermi!e t,e (e!efits of Early )ar!i!g $ystems

)ar!i!g systems provide i!formatio! a7o"t possi7le f"t"re !at"ral ,azards% or !at"ral

disasters% w,ic, may t,reate! i!9"ry or loss of life a!d damage to property.

T,ere are siD factors t,at determi!e t,e gross 7e!efits of a war!i!g system.

T,e first two of t,ese relate to t,e !at"re of t,e !at"ral ,azard itself8

:1; +re"e!cy is t,e !at"ral ,azard commo! or rareB

:#; $everity w,at is t,e mag!it"de of t,e ris5 to life or t,e damage to property t,at t,e

,azard co"ld ca"seB

f a partic"lar 5i!d of !at"ral ,azard occ"rs relatively fre"e!tly% a!d if t,e war!i!g system

wor5s% t,ere will simply 7e more opport"!ities for t,at system to prod"ce its 7e!efits. A!d%

if t,e typical severity of t,at 5i!d of !at"ral ,azard is greater% t,e 7e!efits of a s"ccessf"l

war!i!g a!d respo!se are li5ely :7"t !ot certai!; to 7e greater. A special c,alle!ge arises

w,e! a 5i!d of ,azard is eDtremely severe 7"t very i!fre"e!t% e.g. ts"!amis. ! t,esesit"atio!s% it may 7e diffic"lt to s"stai! s"pport for a via7le war!i!g system% a!d to s"stai!

t,e p"7lics readi!ess to respo!d to a war!i!g% over t,e lo!g periods 7etwee! rec"rre!ces

of t,ese eve!ts.

+o"r additio!al factors 9oi!tly determi!e t,e most appropriate respo!se w,e! a disaster

war!i!g is iss"ed8


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:'; ead-time give! w,e! t,e war!i!g is iss"ed% w,at respo!ses are possi7leB

ead-time 7etwee! a war!i!g a!d t,e act"al occ"rre!ce of a disaster esse!tially determi!es

t,e ra!ge of respo!ses t,at o!e co"ld ta5e more lead-time ge!erally mea!s t,at t,ere is a

wider ra!ge of possi7le respo!ses to a disaster war!i!g.

:&; Acc"racy is t,e war!i!g correctB

f t,e war!i!g is !ot very acc"rate% little or !o respo!se may 7e appropriate. O! t,e ot,er

,a!d% if t,e war!i!g is ,ig,ly acc"rate% it will 7e ratio!al for people to ma5e sig!ifica!t

a!d possi7ly costly c,a!ges i! 7e,avior.

:*; Respo!se Costs w,at are t,e costs of possi7le respo!ses to t,e war!i!gB

T,e possi7le respo!ses to a disaster war!i!g will ,ave differe!t costs. Relatively low cost

respo!ses are more li5ely to ma5e se!se t,a! relatively ,ig, cost respo!ses. Hig, cost

respo!ses will ma5e se!se o!ly w,e! t,e pote!tial disaster is severe% t,e war!i!g is

acc"rate% a!d t,e respo!se ma5es a real differe!ce.

A! eDample of a low cost respo!se is movi!g to t,e so"t,west cor!er of a 7aseme!t w,e!

t,ere is a tor!ado war!i!g. (eca"se lead-times for tor!ado war!i!gs are 9"st mi!"tes% t,is

is a7o"t t,e o!ly respo!se actio! t,at ma5es se!se% 7"t it is !ot costly. A! eDample of a

,ig, cost respo!se is large scale coastal evac"atio! i! adva!ce of a possi7le ,"rrica!e

la!dfall. epe!di!g o! t,e pop"latio! de!sity of t,e t,reate!ed coastli!e% t,e cost of

evac"atio! co"ld 7e i! t,e ra!ge of 1/ millio! per mile of coast evac"ated.

:2; oss Red"ctio!8 ,ow m"c, are t,e eDpected costs of t,e disaster red"ced% give! t,e

li5ely p"7lic respo!se to t,e war!i!gB

T,e loss red"ctio! depe!ds o! t,e i!tri!sic effective!ess of possi7le actio!s t,at may 7e

ta5e! i! adva!ce of t,e !at"ral disaster% as well as t,e a!ticipated degree or eDte!t of

p"7lic respo!se to t,e war!i!g.

Ofte! t,e most diffic"lt iss"e i! assessi!g t,e li5ely 7e!efits from a! early war!i!g system

is predicti!g t,e act"al p"7lic respo!se t,at will 7e fort,comi!g w,e! a disaster war!i!g isiss"ed. deally% o!e wo"ld ,ope t,at t,e p"7lic respo!se to a disaster war!i!g wo"ld 7e a

ratio!al eDpected cost mi!imizi!g respo!se t,at ta5es i!to acco"!t forecast lead-time%

forecast acc"racy% respo!se costs% a!d loss red"ctio!s. However% s"c, ratio!ality is

diffic"lt to ac,ieve. Nat"ral disasters typically t,reate! large !"m7ers of people w,o are


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Prepared by Said M.(B.SC) & DTU

ordi!ary people !ot trai!ed i! t,e process of ma5i!g optimal decisio!s "!der "!certai!ty%

a!d w,o are almost certai!ly "!aware of t,e systematic 7iases t,at plag"e decisio!-ma5i!g

a7o"t "!certai! eve!ts% eve! for people wit, a sop,isticated "!dersta!di!g of s"c,

decisio!-ma5i!g. ! fact% it is ofte! a ma9or c,alle!ge% 7ot, i! pla!!i!g a! act"al respo!se

to a war!i!g% a!d i! estimati!g t,e 7e!efits of a war!i!g% to determi!e w,at respo!se is

optimal% a!d ,ow closely t,e act"al respo!se ca! 7e eDpected to approDimate t,e optimal

o!e.

),e! t,e people at ris5 from a forecasted ,azard are !ot "p to t,e tas5 of ma5i!g t,eir

ow! good decisio!s :e.g. w,et,er to evac"ate a coastal area t,reate!ed 7y a ,"rrica!e;% t,e

s"ccess of a war!i!g system will depe!d importa!tly o! ,ow p"7lic a"t,orities ma!age t,e

respo!se to t,e t,reate!ed disaster.

! some co"!tries w,ere t,ere is a ,ig, degree of state co!trol over citize!s% a!d it may 7e

fairly easy to comma!d a! optimal respo!se from t,e people t,reate!ed 7y a !at"ral

,azard. 0ore ofte!% ,owever% t,e p"7lic respo!se will 7e vol"!tary% a!d t,e tools availa7le

to p"7lic a"t,orities will 7e limited to comm"!icatio! a!d pers"asio! as mea!s to i!d"ce

people to ma5e a ratio!al respo!se to a !at"ral disaster war!i!g. +ail"re to ac,ieve a

ratio!al p"7lic respo!se to a disaster war!i!g ca! mea! t,at t,e ,oped-for 7e!efits of t,e

war!i!g system will 7e s"7sta!tially or w,olly "!realized.

+i!ally% t,e !et eco!omic 7e!efit of a! early war!i!g system for !at"ral disasters also

depe!ds o! a seve!t, factor.

:3; Early )ar!i!g $ystem Cost

O7vio"sly% if t,is cost is low relative to t,e gross 7e!efits of t,e war!i!g system% t,e !et

7e!efits of t,e system will 7e large% a!d co!versely.

0a!y of t,e !at"ral ,azards t,at pose serio"s t,reats are meteorological% e.g. cyclo!es%

floods% ,eat% tor!adoes. +orecasts for ,azards of t,is !at"re are prod"ced 9oi!tly 7y t,esame weat,er forecasti!g system t,at also prod"ces everyday weat,er forecasts of ge!eral

"se to t,e pop"latio!< t,at system may !eed some "pgrades to 7e a7le to forecast a

partic"lar ,azard% 7"t a s"7sta!tial fractio! of t,e system compo!e!ts !eeded for s"c,

forecasti!g are already i! place. T,is ,as two implicatio!s. +irst% t,ere is am7ig"ity a7o"t


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Prepared by Said M. (B.SC) ' DTU

,ow m"c, of t,e cost of a weat,er forecasti!g system s,o"ld 7e attri7"ted to forecasti!g a

prod"cts% a!d t,e cost of prod"ci!g forecasts for all s"c, ,azards is !early t,e same as t,e cost of

prod"ci!g a forecast for a!y si!gle o!e of t,em. T,is mea!s t,at it will ma5e eco!omic se!se to

prod"ce forecasts for all t,e meteorological ,azards partic"lar ,azard% e.g. cyclo!es. $eco!d%forecasts for meteorological ,azards are 9oi!t if it ma5es se!se to prod"ce a forecast for a!y

o!e of t,em.

0a!y factors determi!e t,e 7e!efits of early war!i!gs systems for t,e vario"s !at"ral

,azards. To develop a s"79ective scori!g system% we first o7serve t,at% i! order for t,e

7e!efits of a! early war!i!g system to 7e ,ig,% a !at"ral ,azard m"st sim"lta!eo"sly

satisfy several criteria8 it m"st 7e fre"e!t% severe% predicta7le wit, reaso!a7le lead-time

a!d acc"racy% a!d t,ere m"st eDist cost-effective respo!ses to war!i!gs of a! impe!di!g

occ"rre!ce. f a!y o!e of t,ese criteria is !ot met% t,e pote!tial 7e!efits from a war!i!g

system may 7e small or eve! zero. +or eDample% if t,e ,azard is !ot predicta7le% it does !ot

matter ,ow fre"e!t% severe% or cost-effective respo!ses to war!i!gs mig,t 7e --- if o!e

ca!!ot predict t,e ,azard% t,e 7e!efits of a! early war!i!g system will 7e zero.

Activity 1.#. How ca! we adopt early war!i!gB

1.'.# D's&s+er Pre8&redness

!volves forecasti!g a!d ta5i!g preca"tio!ary meas"res prior to a! immi!e!t t,reat w,e!

adva!ce war!i!gs are possi7le. Prepared!ess pla!!i!g improves t,e respo!se to t,e effects

of a disaster 7y orga!izi!g t,e delivery of timely a!d effective resc"e% relief a!d assista!ce.

Prepared!ess i!volves t,e developme!t a!d reg"lar testi!g of war!i!g systems :li!5ed to

forecasti!g systems; a!d pla!s for evac"atio! or ot,er meas"res to 7e ta5e! d"ri!g a

disaster alert period to mi!imize pote!tial loss of life a!d p,ysical damage. t also i!volves

t,e ed"catio! a!d trai!i!g of officials a!d t,e pop"latio! at ris5% t,e trai!i!g of

!terve!tio! teams% a!d t,e Esta7lis,me!t of policies% sta!dards% orga!izatio!al

Arra!geme!ts a!d operatio!al pla!s to 7e applied followi!g a disaster.


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Prepared by Said M. (B.SC) DTU

isaster prepared!ess mi!imizes t,e adverse effects of a ,azard t,ro"g, effective

preca"tio!ary actio!s% re,a7ilitatio! a!d recovery to e!s"re t,e timely% appropriate a!d

effective orga!izatio! a!d delivery of relief a!d assista!ce followi!g disaster.A

coordi!ated disaster prepared!ess a!d respo!se system is a! esse!tial co!ditio! of a!y

disaster prepared!ess pla!. T,ere is !o sta!dard way of e!s"ri!g effective coordi!atio!.

Eac, desig! will depe!d "po! t,e traditio!s a!d gover!me!tal str"ct"re of t,e co"!try

"!der review. However% a pla! will rapidly deteriorate "!less t,ere is K,orizo!tal

coordi!atio!L at ce!tral gover!me!t a!d s"7-!atio!al levels amo!g mi!istries a!d

specialized age!cies a!d Kvertical coordi!atio!L 7etwee! ce!tral a!d local a"t,orities.


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Prepared by Said M. (B.SC)

DTU

Un'+ su77&r;

Overview of ,ydro-climatic ,azards :floods% dro"g,t% t,"!derstorms% wi!d g"sts% tidal

waves% TCs% ,"rrica!es% tor!ados% frost a!d ,eat waves< El Ni!o eve!ts;.

A ,azard is a da!gero"s p,e!ome!o!% s"7sta!ce% ,"ma! activity or co!ditio! t,at may

ca"se t,e loss of life% i!9"ry or ot,er ,ealt, impacts% property damage% loss of liveli,oods

a!d services% social a!d eco!omic disr"ptio!% or e!viro!me!tal damage.

)ar!i!g systems provide i!formatio! a7o"t possi7le f"t"re !at"ral ,azards% or !at"ral

disasters% w,ic, may t,reate! i!9"ry or loss of life a!d damage to property.

T,ere are siD factors t,at determi!e t,e gross 7e!efits of a war!i!g system.

T,e first two of t,ese relate to t,e !at"re of t,e !at"ral ,azard itself8

EDercise

1. ),at are HazardsB

#. 0e!tio! t,e types of HazardsB

'. etermi!e t,e (e!efit of early war!i!g systemB

&. ),at are t,e factors t,at are "sed to determi!e t,e early war!i!gB

*. isc"s a7o"t !at"ral ,azardsB

2. Have yo" ever see! wi!d ,azardsB

3. Ca! we solve all t,e ,azardsB

. ),e! a dam fail"re ca"sed floodi!g a!d it may affect t,e dow!stream of t,e % ca! we say

!at"ral ,azards B

6. ),at is t,e differe!ce 7etwee! Heat wave a!d Tor!adosB

1/. ),at does it mea! lead timeB


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CHAPTER-TWO

Or'g'n &nd N&+ure o2 H;dro-C1'7&+') H&


20/100


Prepared by Said M. (B.SC) ! DTU

T,ere is a ge!eral correlatio! amo!g storm d"ratio!% storm areal eDte!t% t,e size of t,e

waters,ed associated wit, t,e flood% t,e d"ratio! of floodi!g% a!d t,e time from t,e

7egi!!i!g of t,e storm to t,e flood pea5.

Assessme!t of meteorological ,azards s,o"ld i!varia7ly go t,ro"g, t,e followi!g steps.

:a; +orecasti!g a!d war!i!g !eeds

M ocatio! of area at ris5

M Type of forecast or war!i!g

M Re"ired lead time

M Re"ired acc"racy<

:7; ata collectio!

M N"m7er of satellite pict"res availa7le

M Availa7ility of ot,er types of data s"c, as sy!optic% aircraft-reco!!aissa!ce flig,t

reports

M 0a!!er a!d timeli!ess of receptio!<

:c; Processi!g a!d a!alysis of data

:d; Tra!smissio! of data

M Availa7ility a!d types of tra!smissio! system

M Relia7ility

M Timeli!ess<

:e; Acc"racy of forecast a!d war!i!g

#.# Hydrological Regimes

$everal ,ydrological processes ca! lead to floodi!g% a!d several factors ca! affect t,e flood

pote!tial of a partic"lar rai!storm or s!owmelt eve!t. $ome of factors t,at affect t,e

vol"me of r"!off i!cl"de8

:a; $oil moist"re levels prior to t,e storm<

:7; evel of s,allow gro"!dwater prior to t,e storm<

:c; $"rface i!filtratio! rate8 affected 7y vegetatio!< soil teDt"re% de!sity a!d str"ct"re< soil

moist"re< gro"!d litter< a!d t,e prese!ce of froze! soil< a!d

:d; T,e prese!ce of impervio"s cover a!d w,et,er r"!off from t,e impervio"s cover

directly drai!s i!to t,e stream or sewer !etwor5


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Prepared by Said M.(B.SC) "

DTU

.%# P3;s')&1 C3&r&)+er's+')s o2 F1oods

T,e followi!g c,aracteristics are importa!t i! terms of t,e p,ysical ,azard posed 7y a

partic"lar flood8

:a; T,e dept, of water a!d its spatial varia7ility<

:7; T,e areal eDte!t of i!"!datio!% a!d i! partic"lar t,e area t,at is !ot !ormally covered

wit, water<

:c; T,e water velocity a!d its spatial varia7ility<

:d; "ratio! of floodi!g<

:e; $"dde!!ess of o!set of floodi!g< a!d

:f; Capacity for erosio! a!d sedime!tatio!.

T,e importa!ce of water velocity s,o"ld !ot 7e "!derestimated% as ,ig, velocity water ca! 7e eDtremely da!gero"s a!d destr"ctive. ! t,e case of a flood flowi!g i!to a reservoir% t,e

flood vol"me a!d possi7ly ,ydrograp, s,ape s,o"ld 7e added to t,e list of importa!t

c,aracteristics.

Activity #.# .0e!tio! ot,er types of p,ysical c,aracteristics of floodB

#.* 0eas"reme!t Tec,!i"es

! order to "!dersta!d t,e c,aracteristics a!d limitatio!s of flood data% it is ,elpf"l to

"!dersta!d meas"reme!t tec,!i"es :)0O% 16/;. $tream flow rates ca! 7e meas"red

directly :isc,arge 0eas"reme!t; or !directly :stage 0eas"reme!t or slope-area

meas"reme!t;.irect meas"reme!ts ca! 7e ta5e! 7y loweri!g a device i!to t,e water t,at

meas"res water dept, a!d velocity. T,ese are 0eas"red repeatedly alo!g a i!e

perpe!dic"lar to t,e directio! of flow.

+or a!y reaso!a7ly sized river a 7ridge% ca7leway or 7oat is !ecessary for disc,arge

meas"reme!t. isc,arge :m'4s; t,ro"g, eac, cross-sectio! is calc"lated as t,e prod"ct of

t,e velocity a!d t,e cross-sectio!al flow area.0ost ga"gi!g statio!s are located s"c, t,at

t,ere is a "!i"e or approDimately "!i"e relatio! 7etwee! flow rate% velocity a!d stage.


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T,e flow-rate meas"reme!ts may% t,erefore% 7e plotted agai!st stage meas"reme!ts to

prod"ce a rati!g c"rve. O!ce t,e rati!g c"rve is esta7lis,ed% co!ti!"o"s or periodic stage

meas"reme!ts made eit,er a"tomatically or ma!"ally% ca! 7e co!verted to estimates of

disc,arge. (eca"se meas"reme!ts of disc,arge d"ri!g floods are diffic"lt to ma5e w,e!

t,e water levels a!d flow velocities are ,ig,% it is commo! to ,ave records of o!ly stage

meas"reme!t d"ri!g ma9or floods.

#.2 Fre6uen); o2 F1ood'ng

+lood fre"e!cies ca! 7e determi!ed for a!y give! stream if data is availa7le for disc,arge

of t,e stream over a! eDte!ded period of time. $"c, data allows statistical a!alysis to

determi!e ,ow ofte! a give! disc,arge or stage of a river is eDpected. +rom t,is a!alysis a

rec"rre!ce i!terval ca! 7e determi!ed a!d a pro7a7ility calc"lated for t,e li5eli,ood of a

give! disc,arge i! t,e stream for a!y year. T,e data !eeded to perform t,is a!alysis are

t,e yearly maDim"m disc,arge of a stream from o!e ga"gi!g statio! over a lo!g e!o"g,

period of time.

v ! order to determi!e t,e rec"rre!ce i!terval% t,e yearly disc,arge val"es are first

ra!5ed. Eac, disc,arge is associated wit, a ra!5% m% wit, m 1 give! to t,e maDim"m

disc,arge over t,e years of record% m # give! to t,e seco!d ,ig,est disc,arge% m '

give! to t,e t,ird ,ig,est disc,arge% etc.

v T,e smallest disc,arge will receive a ra!5 e"al to t,e !"m7er of years over w,ic,

t,ere is a record% !. T,"s% t,e disc,arge wit, t,e smallest val"e will ,ave m !.

v T,e !"m7er of years of record% !% a!d t,e ra!5 for eac, pea5 disc,arge are t,e! "sed to

calc"late rec"rre!ce i!terval% R 7y t,e followi!g e"atio!% called t,e )ei7"ll e"atio!8


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Prepared by Said M. (B.SC) $ DTU

R :!Q1;4m

• T,e pro7a7ility% Pe% of a certai! disc,arge ca! 7e calc"lated "si!g t,e i!verse of t,e

)ei7"ll e"atio!8

Pe m4:!Q1;

• T,e val"e% Pe% is called t,e a!!"al eDceede!ce pro7a7ility.

+or eDample% a disc,arge e"al to t,at of a 1/-year flood wo"ld ,ave a! a!!"al

eDceede!ce pro7a7ility of 141/ /.1 or 1/I. T,is wo"ld say t,at i! a!y give! year%

t,e pro7a7ility t,at a flood wit, a disc,arge e"al to or greater t,a! t,at of a 1/ year

flood wo"ld 7e /.1 or 1/I. $imilarly% t,e pro7a7ility of a flood wit, disc,arge

eDceedi!g t,e 1// year flood i! a!y give! year wo"ld 7e 141// /./1% or 1I.


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Prepared by Said M. (B.SC) % DTU

Un'+ su77&r;

+loods ca! 7e s"c, devastati!g disasters t,at a!yo!e ca! 7e affected at almost a!y time.

As we ,ave see!% w,e! water falls o! t,e s"rface of t,e Eart,% it ,as to go somew,ere. !

order to red"ce t,e ris5 d"e to floods% t,ree mai! approac,es are ta5e! to flood predictio!.

$tatistical st"dies ca! 7e "!derta5e! to attempt to determi!e t,e pro7a7ility a!d fre"e!cy

of ,ig, disc,arges of streams t,at ca"se floodi!g.

$everal ,ydrological processes ca! lead to floodi!g% a!d several factors ca! affect t,e flood

pote!tial of a partic"lar rai!storm or s!owmelt eve!t.

+loods ca! 7e modeled a!d maps ca! 7e made to determi!e t,e eDte!t of possi7le floodi!g

w,e! it occ"rs i! t,e f"t"re. A!d% si!ce t,e mai! ca"ses of floodi!g are a7!ormal amo"!ts

of rai!fall a!d s"dde! t,awi!g of s!ow or ice% storms a!d s!ow levels ca! 7e mo!itored to provide s,ort-term flood predictio!.

EDercise

1. isc"s a!d 0e!tio! t,e meteorological ca"ses of floodB

#. ),at does mea! flood fre"e!cy a!d ,ow ca! we estimateB

'. isc"s t,e p,ysical c,aracteristics of floodB

&. ll"strate a!d disc"s t,e assessme!t of meteorological ,azardsB

*. How ca! we protect climatic HazardsB2. 0e!tio! t,e p,ysical properties of floodsB

3. ),at is t,e differe!ce 7etwee! forecasti!g a!d war!i!g of ,azardsB


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Prepared by Said M. (B.SC) & DTU

CHAPTER-THREE

M&n&ge7en+ &nd M'+'g&+'on o2 H&


26/100


Prepared by Said M. (B.SC) ' DTU

A; (efore a disaster :pre-disaster;

Activities ta5e! to red"ce ,"ma! a!d property losses ca"sed 7y a pote!tial ,azard% for

eDample carryi!g o"t aware!ess campaig!s% stre!gt,e!i!g t,e eDisti!g wea5 str"ct"res%

Preparatio! of t,e disaster ma!ageme!t pla!s at ,o"se,old a!d comm"!ity level etc. $"c,

ris5 red"ctio! meas"res ta5e! "!der t,is stage are termed as mitigatio! a!d prepared!ess

activities.

(; "ri!g a disaster :disaster occ"rre!ce;

!itiatives ta5e! to e!s"re t,at t,e !eeds a!d provisio!s of victims are met a!d s"fferi!g is

mi!imized. Activities ta5e! "!der t,is stage are called emerge!cy respo!se activities.

C; After a disaster :post-disaster;

!itiatives ta5e! i! respo!se to a disaster wit, a p"rpose to ac,ieve early recovery a!d

re,a7ilitatio! of affected comm"!ities% immediately after a disaster stri5es. T,ese are

called as respo!se a!d recovery activities.

0itigatio! of flood ,azards ca! 7e attempted i! two mai! ways8 A! e!gi!eeri!g approac,%

to co!trol floodi!g% a!d a reg"latory approac, desig!ed to decrease v"l!era7ility to

floodi!g.

0itigatio! em7races meas"res ta5e! to red"ce 7ot, t,e effect of t,e ,azard a!d t,e

v"l!era7le co!ditio!s to it i! order to red"ce t,e scale of a f"t"re disaster. T,ereforemitigatio! activities ca! 7e foc"sed o! t,e ,azard itself or t,e eleme!ts eDposed to t,e

t,reat. EDamples of mitigatio! meas"res w,ic, are ,azard specific i!cl"de water

ma!ageme!t i! dro"g,t pro!e areas% relocati!g people away from t,e ,azard pro!e areas

a!d 7y stre!gt,e!i!g str"ct"res to red"ce damage w,e! a ,azard occ"rs.

! additio! to t,ese p,ysical meas"res% mitigatio! s,o"ld also aim at red"ci!g t,e

eco!omic a!d social v"l!era7ilities of pote!tial disasters.


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'.1 Hazard 0aps% Ris5 !formatio! a!d Ed"catio!

+ood ,azard mappi!g is "sed to determi!e t,e areas s"scepti7le to floodi!g w,e!

disc,arge of a stream eDceeds t,e 7a!5-f"ll stage. >si!g ,istorical data o! river stages a!d

disc,arge of previo"s floods% alo!g wit, topograp,ic data% maps ca! 7e co!str"cted to

s,ow areas eDpected to 7e covered wit, floodwaters for vario"s disc,arges or stages.

Comm"!icatio! is critical d"ri!g a! emerge!cy a!d !eeds to 7e addressed t,oro"g,ly

wit,i! t,e disaster-respo!se pla!. $"ccessf"l disaster respo!se pla!s re"ires i!formatio!

a!d mo7ilisatio! respo!se teams% provide g"ida!ce% i!str"ctio!s to t,e affected people% a!d

comm"!icate wit, t,e appropriate a"t,orities a!d eDter!al sta5e,olders

C,alle!ges i!cl"de reac,i!g people i! differe!t locatio!s "ic5ly a!d sim"lta!eo"sly<

providi!g t,e rig,t message :i! terms of co!te!t% le!gt,% a!d format;< mo!itori!g deliverya!d respo!se< a!d e!s"ri!g t,at t,e process is i!itiated a!d s"spe!ded at t,e rig,t times.

0appi!g is a ce!tral tool i! comm"!icati!g ,azard ide!tificatio! a!d assessme!t. 0aps

ca! acc"rately record t,e locatio!% pro7a7le severity a!d li5eli,ood of occ"rre!ce of

,azards a!d display t,is i!formatio! clearly a!d co!ve!ie!tly. t may 7e 7ased o! a ra!ge

of data so"rces e.g. eDisti!g maps% remote se!si!g% s"rveyi!g.

Additio!al i!formatio! from p,otograp,y% field s"rveys a!d ot,er so"rces ca! 7e overlaid

o!to 7ase maps. =eograp,ical !formatio! $ystems :=$; are ma5i!g t,is m"c, easier.

Comm"!ity ,azard mappi!g eDercises ca! also 7e "!derta5e!.

Comm"!ities are ofte! 5!owledgea7le a7o"t t,e locatio! a!d !at"re of local ,azards a!d

t,eir ca"sal factors. $"c, i!formatio! is partic"larly val"a7le i! ide!tifyi!g a!d appraisi!g

localised ,azards 7"t comm"!ity level o"tp"ts ca! also feed i!to Comm"!ity ,azard

mappi!g eDercises ca! also 7e "!derta5e!. Comm"!ities are ofte! 5!owledgea7le a7o"t

t,e locatio! a!d !at"re of local ,azards a!d t,eir ca"sal factors. $"c, i!formatio! is

partic"larly val"a7le i! ide!tifyi!g a!d appraisi!g localised ,azards 7"t comm"!ity level

o"tp"ts ca! also feed i!to ,ig,er level mappi!g a!d pla!!i!g. 0aps are a good medi"m for

comm"!icati!g ,azard i!formatio! to decisio!-ma5ers a!d !o!-specialists% 7"t t,e! t,e

partic"lar formats a!d sym7ols s,o"ld 7e 5ept familiar.


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Prepared by Said M.(B.SC) ! DTU

+ig'. 18 +lood plai!

! co!str"cti!g s"c, maps aerial p,otograp,s a!d satellite images of prior floods are

st"died to ,elp to determi!e t,e areas t,at wo"ld 7e covered. T,e ill"stratio! a7ove s,ows

a possi7le ,azard map 7ased o! estimated disc,arges or river stages for a ,ypot,etical 1/-

year flood% */-year flood% a!d 1//-year flood.

'.# Possi7le +lood Ris5 Red"ctio! 0eas"res

• 0appi!g of t,e flood pro!e areas

t is t,e primary step i!volved i! red"ci!g t,e ris5 of t,e regio!. Historical records give t,e

i!dicatio! of t,e flood i!"!datio! areas a!d t,e period of occ"rre!ce a!d t,e eDte!t of t,e

coverage. )ar!i!g ca! 7e iss"ed loo5i!g i!to t,e earlier mar5ed ,eig,ts of t,e water levels

i! case of pote!tial t,reat. ! t,e coastal areas t,e tide levels a!d t,e la!d c,aracteristics

will determi!e t,e s"7merge!ce areas. +lood ,azard mappi!g will give t,e proper

i!dicatio! of water flow d"ri!g floods.

• a!d "se co!trol

t will red"ce da!ger of life a!d property w,e! waters i!"!date t,e floodplai!s a!d t,e

coastal areas. T,e !"m7er of cas"alties is related to t,e pop"latio! i! t,e area at ris5. !

areas w,ere people already ,ave 7"ilt t,eir settleme!ts% meas"res s,o"ld 7e ta5e! to


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relocate to 7etter sites so as to red"ce v"l!era7ility. No ma9or developme!t s,o"ld 7e

permitted i! t,e areas w,ic, are s"79ected to ,ig, floodi!g. mporta!t facilities li5e

,ospitals% sc,ools s,o"ld 7e 7"ilt i! safe areas. ! "r7a! areas% water ,oldi!g areas ca! 7e

created li5e po!ds% la5es or low-lyi!g areas.

• Co!str"ctio! of e!gi!eered str"ct"res

! t,e flood plai!s a!d stre!gt,e!i!g of str"ct"res to wit,sta!d flood forces a!d seepage.

T,e 7"ildi!gs s,o"ld 7e co!str"cted o! a! elevated area. f !ecessary 7"ild o! stilts or

platform.

• +lood Co!trol

Aims to red"ce flood damage% T,is ca! 7e do!e 7y decreasi!g t,e amo"!t of r"!off wit,

t,e ,elp of reforestatio! :to i!crease a7sorptio! co"ld 7e a mitigatio! strategy i! certai!

areas;%protectio! of vegetatio!% cleari!g of de7ris from streams a!d ot,er water ,oldi!g

areas% co!servatio! of po!ds a!d la5es etc. +lood iversio! i!cl"des levees% em7a!5me!ts%

dams a!d c,a!!el improveme!t. ams ca! store water a!d ca! release water at a

ma!agea7le rate. ("t fail"re of dams i! eart,"a5es a!d operatio! of releasi!g t,e water

ca! ca"se floods i! t,e lower areas.

+lood Proofi!g red"ces t,e ris5 of damage. 0eas"res i!cl"de "se of sa!d 7ags to 5eep

flood water away% 7loc5i!g or seali!g of doors a!d wi!dows of ,o"ses etc. Ho"ses may 7e

elevated 7y 7"ildi!g o! raised la!d. ("ildi!gs s,o"ld 7e co!str"cted away from water

7odies.

• +lood 0a!ageme!t

+lood ma!ageme!t ca! aim at

:i; A red"ced ris5 of floods<

:ii; A red"ced v"l!era7ility to floods<

:iii; mproved prepared!ess<

:iv; $treamli!ed emerge!cy ma!ageme!t o!ce a flood damage ,as occ"rred< a!d:v; mproved 5!owledge :a7o"t ca"se-effect relatio!s,ips% drivi!g forces a!d ma!ageme!t

Optio!s;<

=ood flood ma!ageme!t wo"ld ta5e its starti!g poi!t i! a s"ita7le 5!owledge a7o"t

:i; T,e flood ris5 :so t,at ,ig,-ris5 areas are deli!eated;< a!d


30/100


Prepared by Said M. (B.SC) !! DTU

:ii; T,e flood v"l!era7ility :so t,at t,e most importa!t pote!tial co!se"e!ces are

de!tified;.

iffere!t met,ods of flood protectio! str"ct"ral as well as !o!str"ct"ral ,ave 7ee! adopted

i! differe!t states depe!di!g "po! t,e !at"re of t,e pro7lem a!d local co!ditio!s.

$tr"ct"ral meas"res i!cl"de storage reservoirs% flood em7a!5me!ts% drai!age c,a!!els% a!t

erosio! wor5s% c,a!!el improveme!t wor5s% dete!tio! 7asi!s etc. a!d !o!-str"ct"ral

meas"res i!cl"de flood forecasti!g% flood plai! zo!i!g% flood proofi!g% disaster

Prepared!ess etc.

?ario"s possi7le flood ,azard mitigatio! optio!s to ma!age "r7a! flood ris5s are

s"mmarized 7elow8

• Red"ci!g local floods 7y i!d"ci!g i!filtratio! t,ro"g,8

- Preservatio! of "!sealed areas%

- Preservatio! of !at"ral po!ds%

- i!d"ci!g gro"!dwater rec,arge a!d gree!i!g of "!sealed areas%

- i!trod"ci!g permea7le pavi!gs%

- Provisio! of i!filtratio! tre!c,es% soa5 ways etc.

• Retai!i!g4 tra!sferri!g local floods8

- 0i!or a!d ma9or "r7a! drai!age system :storm water c,a!!els% g"tters% c"lverts% p"mps

etc.;

- preve!ti!g cloggi!g of drai!age facilities :clea!i!g% dredgi!g% solid waste collectio! etc.;

- ete!tio! a!d rete!tio! 7asi!s

- Rai!water ,arvesti!g

• Preve!ti!g storm water co!tami!atio!8

- $trict separatio! of sewage a!d storm water drai!age

- protect pote!tial co!tami!atio! so"rces :sewage pla!ts% la!dfills% patrol statio!s etc.;

agai!st floods

Activitiy8 isc"s t,e type of ,ydra"lic str"ct"res t,at are "sed for flood co!trol

met,odsB


31/100


Prepared by Said M. (B.SC) !" DTU

'.' $trategic Approac,8 $tr"ct"ral 0eas"res

Preve!tio! a!d prepared!ess approac,es are8

$tr"ct"ral meas"res8 !frastr"ct"re% e!viro!me!tal s,ield% retrofitti!g

No!str"ct"ral meas"res8 Poverty alleviatio!% empowerme!t of disadva!taged gro"ps%

capacity 7"ildi!g% ris5 tra!sfer% i!s"ra!ce

Amo!g t,ese !o!-str"ct"ral meas"res are8

• +loodplai! zo!i!g - aws ca! 7e passed t,at restrict co!str"ctio! a!d ,a7itatio! of

floodplai!s. !stead floodplai!s ca! 7e zo!ed for agric"lt"ral "se% recreatio!% or ot,er "ses

w,erei! lives a!d property are !ot e!da!gered w,e! flood waters re-occ"py t,e floodplai!.

• +loodplai! 7"ildi!g codes - $tr"ct"res t,at are allowed wit,i! t,e floodplai! co"ld 7e

restricted t,ose t,at ca! wit,sta!d t,e ,ig, velocity of flood waters a!d are ,ig, e!o"g,

off t,e gro"!d to red"ce ris5 of co!tact wit, water.

• +loodplai! 7"yo"t programs - ! areas t,at ,ave 7ee! rece!tly flooded% it may 7e more

cost effective for t,e gover!me!t% w,ic, "s"ally pays for flood damage eit,er t,ro"g,

s"7sidized flood i!s"ra!ce or direct disaster relief% to 7"y t,e rig,ts to t,e la!d rat,er t,a!

pay t,e cost of reco!str"ctio! a!d t,e! ,ave to pay agai! t,e !eDt time t,e river floods.

• 0ortgage limitatio!s - e!di!g i!stit"tio!s co"ld ref"se to give loa!s to 7"y or co!str"ct

dwelli!gs or 7"si!esses i! flood pro!e areas.

'.& Early )ar!i!g

Early )ar!i!g $ystems are Ofte! escri7ed i! terms of T,e etectio!+ )ar!i!g

issemi!atio!% respo!se% recovery a!d review stages. ! ma!y cases% a forecasti!g

compo!e!t will also 7e i!cl"ded% a!d prepared!ess is esse!tial for a! effective emerge!cy

respo!se. T,is str"ct"re is also adopted ,ere% alt,o"g, wit, o!ly a s,ort disc"ssio! of t,e

recovery p,ase% si!ce flood war!i!g a!d forecasti!g ,as a less importa!t role to play o!ce

flood levels start to recede% s"c, as estimati!g w,e! floodwaters will drai!% or if a!y

f"rt,er floodi!g is immi!e!t. (y co!trast% t,e war!i!g aspect is disc"ssed i! several

locatio!s% i!cl"di!g a c,apter o! t,e decisio! criteria "sed for iss"i!g flood war!i!gs


32/100


Prepared by Said M. (B.SC) !# DTU

:ofte! called t,res,olds; a!d sectio!s o! decisio! s"pport a!d decisio!-ma5i!g "!der

"!certai!ty.

Early war!i!g systems ,elp to red"ce eco!omic losses a!d mitigate t,e !"m7er of i!9"ries

or deat,s from a disaster% 7y providi!g i!formatio! t,at allows i!divid"als a!d

comm"!ities to protect t,eir live sa!d property. Early war!i!g i!formatio! empowers

people to ta5e actio! w,e! a disaster closes to ,appe!i!g. f well i!tegrated wit, ris5

assessme!t st"dies a!d comm"!icatio! a!d actio! pla!s% early war!i!g systems ca! lead to

s"7sta!tive 7e!efits.

T,ere are t,ree mai! sectio!s i! flood war!i!g systems8

, Part . +lood )ar!i!g- w,ic, disc"sses t,e topics of detectio!% t,res,olds a!d

dissemi!atio!

, Part . +lood +orecasti!g- w,ic, disc"sses ge!eral pri!ciples% specific types of

river a!d coastal forecasti!g models% a!d eDamples of specific applicatio!s

, Part . Emerge!cy Respo!se- w,ic, covers t,e topic of prepared!ess% respo!se

a!d review

'.&.1 T,e +lood )ar!i!g Process

+lood war!i!g systems provide a well-esta7lis,ed way to ,elp to red"ce ris5 to life% a!d to

allow comm"!ities a!d t,e emerge!cy services time to prepare for floodi!g a!d to protect

possessio!s a!d property. Actio!s may also 7e ta5e! to red"ce or preve!t floodi!g< for

eDample% 7y operati!g river co!trol str"ct"res% a!d flood fig,ti!g activities s"c, as

rei!forci!g flood defe!ses% a!d i!stalli!g temporary or demo"!ta7le 7arriers.

!formal flood war!i!g systems ,ave eDisted ever si!ce people started to live a!d wor5

!ear rivers a!d coastli!es. Heavy rai!fall% ,ig, river levels% "!"s"al sea states a!d ot,er

c"es% s"c, as t,e so"!d of r"!!i!g water% all provide "sef"l i!formatio! o! impe!di!g

floodi!g% wit, traditio!al met,ods for providi!g war!i!gs i!cl"di!g word of mo"t,%

messe!gers% a!d raisi!g flags a!d storm co!es.

T,ese approac,es still ,ave a val"a7le role to play% partic"larly w,ere floodi!g develops

rapidly% a!d comm"!ities m"st rely o! t,eir ow! reso"rces for t,e i!itial respo!se. A flood

war!i!g system ca! i!cl"de rai!fall a!d tidal detectio! systems% river a!d coastal flood


33/100


Prepared by Said M. (B.SC) !$ DTU

forecasti!g models% flood war!i!g dissemi!atio! systems% a!d emerge!cy respo!se

proced"res.

+ig '. # ll"stratio! of t,e compo!e!ts of a flood war!i!g% forecasti!g a!d emerge!cy

Respo!se system


34/100


Prepared by Said M. (B.SC) !% DTU

Ta7le'. 18 Typical compo!e!ts i! t,e flood war!i!g% forecasti!g a!d emerge!cy respo!se process

T,e resilie!ce of flood war!i!g systems to fail"re is also a! importa!t co!sideratio!% a!d

ris5 7ased tec,!i"es from ot,er tec,!ical sectors a!d types of emerge!cy are grad"ally 7ei!g i!trod"ced to ,elp to ide!tify pote!tial poi!ts of fail"re% a!d appropriate ris5

red"ctio! meas"res. T,ere is also m"c, de7ate a7o"t t,e effective!ess of flood war!i!gs.

Clearly% a war!i!g is s"ccessf"l if it i!itiates actio! w,ic, preve!ts floodi!g w,ic, mig,t

ot,erwise ,ave occ"rred i! t,e a7se!ce of t,at war!i!g< for eDample 7y triggeri!g t,e


35/100


Prepared by Said M. (B.SC) !& DTU

clos"re of a tidal 7arrier% or i!stallatio! of a temporary defe!ce. However% researc,

s"ggests t,at s"ccess wit, providi!g war!i!gs to t,e p"7lic is miDed% alt,o"g, i! some

co"!tries ,as improved mar5edly i! rece!t years t,ro"g, a com7i!atio! of "si!g flood

forecasti!g models to eDte!d t,e lead time a!d acc"racy of war!i!gs% a 7etter

"!dersta!di!g of ,ow to comm"!icate war!i!gs% a!d a! i!creased emp,asis o!

Comm"!ity participatio! a!d i!ter-age!cy colla7oratio!.

+or eDample% o!e recomme!datio! :Emerge!cy 0a!ageme!t A"stralia 1666; is t,at t,e

flood war!i!g tas5 ca! 7e 7oiled dow! to providi!g appropriate respo!ses to t,e followi!g

five "estio!s8

A partic"lar iss"e to co!sider is t,at of t,e re"ireme!ts for war!i!g lead time% w,ic, ca!

ra!ge from a few mi!"tes or less for people o! a steep slopi!g river 7a!5 to reac, ,ig,er

gro"!d% to ma!y ,o"rs or days for some sit"atio!s% s"c, as raisi!g temporary defe!ses%

evac"ati!g large !"m7ers of people% or drawi!g dow! a reservoir i! adva!ce of floodi!g.

$imilarly% t,e re"ireme!ts for acc"racy% a!d tolera!ce to false alarms% will vary 7etwee!

orga!izatio!s a!d comm"!ities% a!d ca! 7e i!fl"e!ced 7y ed"catio! a!d p"7lic aware!ess

eDercises.

'.*+lood co!trol )or5s

+loodi!g ca! 7e co!trol 7y co!str"cti!g Hydra"lic str"ct"re li5e flood co!trol dams%

levees% g"ide 7"!5s a!d ot,er ,ydra"lic str"ct"res

River trai!i!g i! its wider aspects covers all t,ose e!gi!eeri!g wor5s% w,ic, are

co!str"cted o! river so as to g"ide a!d co!fi!e t,e flow to t,e river c,a!!el a!d to co!trol

a!d reg"late t,e river 7ed co!fig"ratio! e!s"ri!g safe a!d effective disposal of floods a!d

sedime!t loads sta7ilizi!g a!d trai!i!g t,e river alo!g a certai! alig!me!t wit, a s"ita7le

water way is t,erefore t,e first a!d foremost aim of river trai!i!g.


36/100


Prepared by Said M. (B.SC) !' DTU

'.*.1O79ectives8

T,e River trai!i!g wor5s may serve t,e followi!g o79ectives or adva!tages8

:i; To preve!t t,e river from c,a!gi!g its co"rse a!d to avoid o"tfla!5i!g of str"ct"res

li5e 7ridges weirs a"ed"cts% etc.

:ii; To preve!t floodi!g of t,e s"rro"!di!g co"!tries 7y providi!g a safe passage for

t,e flood waters wit,o"t overtoppi!g t,e 7a!5s.

:iii;To protect t,e river 7a!5s 7y deflecti!g t,e river away from t,e attac5ed 7a!5s.

:iv;To e!s"re effective disposal of sedime!t load

:v; To provide mi!im"m water dept, re"ired for !avigatio!

'.*.# Classificatio!8

epe!di!g "p o! t,e p"rpose for w,ic, a river trai!i!g programme is "!derta5e! t,e

river trai!i!g wor5s may 7e classified i! to t,e followi!g t,ree categories8

:1; Hig, water trai!i!g or Trai!i!g for disc,arge

:#; ow water trai!i!g or Trai!i!g for dept,

:'; 0ea! water trai!i!g or Trai!i!g fro sedime!t.

:1; Hig, water trai!i!g or Trai!i!g for disc,arge. Hig, water trai!i!g is "!derta5e!

wit, t,e primary p"rpose of flood co!trol. t% t,erefore% aims at providi!gs"fficie!t river cross- sectio! for t,e safe passage of maDim"m flood a!d is

co!cer!ed wit, ma5i!g t,e ad9oi!i!g are flood proof 7y co!str"ctio! of dy5es or

levees etc

:#; ow water trai!i!g or Trai!i!g for dept,. ow water is "!derta5e! wit, t,e

primary p"rpose of providi!g s"fficie!t water dept, i! !aviga7le c,a!!els d"ri!g

low water periods it may 7e accomplis,ed 7y co!ce!trati!g a!d e!,a!ci!g t,e

flow i! t,e desired c,a!!el 7y t,e process of 7"!dli!g 7y co!tracti!g t,e widt,

of t,e c,a!!el wit, t,e ,elp of groy!es etc

:'; 0ea! water trai!i!g or for sedime!t. 0ea! water trai!i!g aims at efficie!t disposal

of s"spe!ded load a!d 7ed load a!d t,"s to preserve t,e c,a!!el i! good s,ape.

T,e maDim"m accretio! capacity of a river occ"rs i! t,e vici!ity of mea! water or

domi!a!t


37/100



disc,arge t,erefore t,e c,a!ge i! t,e river 7ed are a attempted i! accorda!ce wit,

t,at stage of flood flow t,e mea! water trai!i!g is t,e most importa!t type a!d

forms t,e 7asis o! w,ic, t,e former two are pla!!ed.

'.*.' 0et,ods

T,e c,ief aim of river trai!i!g is to ac,ieve "ltimate sta7ility of t,e river wit, t,e aid of

river trai!i!g meas"res. T,e sta7ility of a river does !ot mea! t,at c,a!ges li5e

sco"ri!g a!d silti!g of 7ed adva!ceme!t of delta i! to t,e sea% etc will !ot ta5e place. t

o!ly mea!s t,at t,e river attai!s e"ili7ri"m stage a!d !o sig!ifica!t c,a!ges occ"rs i!

its alig!me!t slope regime etc t,e regime may c,a!ge wit,i! a year 7"t s,ows little

variatio! from year to year eDcept t,at t,e river may mea!der wit,i! its 5,adirs :i.e. t,e

eDtreme li!es wit,i! w,ic, t,e river is ever 5!ow! to wa!der;.

t was disc"ssed earlier t,at a river ad9"sts its alig!me!t% perimeter% area% slope etc.< wit,

respect to t,e disc,arge a!d sedime!t load% eit,er 7y aggradi!g% mea!deri!g or 7y

degradi!g. t may 7e co!cl"ded t,at mea!deri!g type is t,e f"ll a!d fi!al developme!t of

a! all"vial river. T,e ot,er two types are t,e i!terim p,ases a!d are mai!tai!ed so lo!g as

t,e factors ca"si!g t,em remai! operative.

Aggradi!g rivers are% t,erefore% e"ally ame!a7le to river trai!i!g o! acco"!t of t,eir

i!sta7ility. River trai!i!g wor5 "!derta5e! o! aggradi!g or degradi!g rivers may%

t,erefore% fail to impose a!y sta7ility o! s"c, rivers. +or eDample% 7a!5 protectio! wor5s

"!derta5e! o! a! aggradi!g river may eit,er 7e destroyed 7y severe erosio! or get 7"ried

"!der sedime!t depositio!.

$oil co!servatio! meas"res are t,e most effective meas"res to 7e "!derta5e! for

co!trolli!g aggradi!g rivers% 7efore ta5i!g "p a!y river trai!i!g wor5s o! s"c, rivers.

$imilarly% t,e trai!i!g wor5s "!derta5e! o! degradi!g rivers may fail d"e to sco"r a!d

"!dermi!i!g of fo"!datio!s 7y 7ed sco"r.

T,e sco"ri!g te!de!cy of degradi!g rivers m"st 7e co!trolled 7y 7"ildi!g cross-7ars%

weirs% etc. 7efore attempti!g a!y ot,er river trai!i!g wor5s o! s"c, rivers.


38/100



T,e followi!g are t,e ge!erally adopted met,ods for trai!i!g rivers% i!cl"di!g 7ar

protectio!.

1. 0argi!al em7a!5me!ts or levees

#. ="ide 7a!5s

'. =roy!es or $p"rs.

&. Artificial c"t-offs

*. Pitc,i!g of 7a!5s a!d provisio! of la"!c,i!g apro!s

2. Pitc,ed isla!ds

3. 0iscella!eo"s met,ods% s"c, as $ills% (a!dalli!g% etc.

1. 0argi!al em7a!5me!ts or levees8 -

0argi!al em7a!5me!ts are ge!eral eart,e! em7a!5me!ts% r"!!i!g parallel to t,e river% at

some s"ita7le dista!ce from it. T,ey may 7e co!str"cted o! 7ot, sides of t,e river or o!ly

o!e side% for s"ita7le river le!gt,% w,ere t,e river is passi!g t,ro"g, tow!s or cities or

a!y ot,er places of importa!ce. T,ese em7a!5me!t-walls% retai! t,e flood water a!d t,"s%

preve!ti!g it from spreadi!g i!to t,e !ear7y la!ds a!d tow!s. A levee or a dy5e is mai!ly

"sed for flood protectio! 7y co!trolli!g t,e river a!d !ot 7y trai!i!g t,e river.

T,e alig!me!t of levees s,o"ld follow t,e !ormal mea!deri!g patter! of t,e river. T,e

retireme!t of t,e levees ,as to 7e gover!ed 7y tec,!ical as well as eco!omical a!d

political co!sideratio!s% 7eca"se t,e la!d falli!g wit,i! t,e levees is eit,er to 7e ac"ired

7y t,e gover!me!t or remai!s s"scepti7le to floods. T,e levees are ma!y times% pitc,ed

o! t,e "pstream side :i.e. water side;. a"!c,i!g apro! may also 7e provided% if t,e 7a!5

or levee is close to t,e mai! river c,a!!el.

esig! of levee $ectio!8 - evees are 9"st li5e eart,e! dams wit, t,e differe!ce t,at t,ey

are very lo!g% come i! operatio! disco!ti!"o"sly a!d for a s,ort time a!d ,ave limited

possi7ilities for selectio! of t,eir alig!me!t alo!g favora7le geological strata.

T,eir sectio!s s,o"ld 7e desig!ed i! s"c, a way as to 5eep t,e seepage gradie!t i!side t,e

7ody of t,e em7a!5me!t 7y at least o!e meter from 7elow t,e top s"rface of t,e

em7a!5me!t. T,e !ormal val"e of "s"ally adopted seepage gradie!t varies 7etwee! &8 1

to 28 1 :i.e. H8 ?; depe!di!g "p o! c,aracter of t,e soil w,ic, may - !ecessitate river side


39/100



slopes varyi!g 7etwee! #8 1 to *8 1% la!d side slopes 7etwee! #8 1 to 38 1% a!d top widt,

7etwee! #.* to 1/ m. T,e top level of t,e levee s,o"ld 7e decided 7y leavi!g a s"fficie!t

free7oard varyi!g 7etwee! /.' to 1.* m a7ove t,e ,ig, flood level.

T,e followi!g levee sectio!s for differe!t ,eig,t are ge!erally adopted8

Effects of levees o! river 7ed8 ! order to assess t,e effects of di5es or levees o! rivers% we

s,all% first of all disc"ss t,e ,ydra"lics of a! "! di5e river d"ri!g floods% a!d t,e! disc"ss

t,e ,ydra"lics of a dir5ed river.

"ri!g floods% water spills over t,e !at"ral 7a!5s of a river. T,e velocity of t,is water will

7e red"ced 7eca"se it spreads over t,e ad9oi!i!g la!d. "e to t,is red"ctio! i! velocity soil

gets deposited as t,e silt depositio! varies i!versely wit, t,e velocity. ! a! "! di5e

all"vial river% t,e flood water spreads over a vast area% so t,at t,e velocity 7ecomes so

small t,at almost t,e e!tire silt gets deposited o! t,e flood area a!d clear water flows 7ac5

i! to t,e mai! river as t,e flood recedes.

),e! t,e river is e!closed 7y di5es% t,e water spills over t,e !at"ral 7a!5s of t,e river a!d

will spread i! t,e area co!fi!ed 7etwee! t,e river a!d t,e di5es. "e to t,is% t,e velocity of

flow red"ces a!d silt gets deposited. ("t i! fact t,is red"ctio! i! velocity i! a di5ed river is

m"c, less t,e i! a! "! di5e river% 7eca"se t,e spread area is less i! a di5e river. He!ce itca! 7e co!cl"ded t,at t,e silt depositio! will 7e less i! a di5e river compared to t,at i! a!

"!di5e river.

At t,e same time% it ca! also 7e co!cl"ded t,at i! a di5e river% silt will 7e deposited o!ly i!

t,e co!fi!ed fiDed area :i.e. area e!closed 7etwee! di5es a!d river;. A!d t,erefore% t,e 7ed

level of t,e river as well as t,at of t,is e!closed area will i!crease% ,e!ce t,e la!d e!closed

7etwee! t,e di5es will appear to 7e ,ig,er t,a! t,e ad9oi!i!g la!d a!d t,is pro7a7ly leads

to t,e false commo! ma! 7elief t,at t,e di5es ca"se ill- effect of raisi!g t,e river-7ed

O! t,e ot,er ,a!d i! a! "!di5ed river% t,e level of t,e e!tire la!d will i!crease a!d t,ere

will !ot appear a!y apprecia7le risi!g of t,e 7ed. $i!ce t,e clear-c"t raisi!g of t,e 7ed is

!ot ge!erally visi7le o! "! di5e rivers% people get misg"ide a!d t,ey feel t,at t,ere was !o


40/100


Prepared by Said M. (B.SC) "! DTU

7ed rise i! "!di5ed rivers a!d 7ecome agai!st di5i!g stati!g di5es ca"se t,e ill- effects of

raisi!g t,e 7ed alt,o"g, it is !ot a correct stateme!t.

# ="ide (a!5s.

f a! e!gi!eeri!g str"ct"re s"c, as a weir or a 7arrage or a 7ridge etc is co!str"cted across

a river% t,e river widt, is red"ced a!d trai!ed i! s"c, a fas,io! as to e!s"re !ot o!ly a safe

a!d eDpeditio"s disposal of flood water 7"t also to e!s"re a perma!e!t reaso!a7le widt, of

t,e water way for t,e river flow. t ,as already 7ee! disc"ssed t,at t,e all"vial rivers do

s,ift t,eir co"rses. Now% if today% a str"ct"re s"c, as a 7ridge is co!str"cted across t,e

eDisti!g river widt, t,e ot,er day% t,e river may s,ift a!d t,ere may !ot 7e a!y river 7elow

t,e eDisti!g river widt,% day% a!d t,e river may 7e fo"!d to 7e flowi!g away from it%

!ecessitati!g t,e co!str"ctio! of a!ot,er str"ct"re.

("t it is "!wise a!d "!eco!omical to spa! t,e e!tire widt, of t,e river a!d to eDpose t,e

str"ct"re to vagaries of attac5 a!d deep sco"r. He!ce% a str"ct"re s"c, as a weir% or a

7arrage% or a 7ridge% etc is eDte!ded i! a smaller widt, of t,e river% a!d water is trai!ed to

flow almost aDially t,ro"g, t,is wit,o"t o"t-fla!5i!g t,e str"ct"re. T,e river is !ormally

trai!ed for t,is p"rpose wit, t,e ,elp of a pair of g"ide 7a!5s.

T,e g"ide 7a!5s are ge!erally provided i! pairs% symmetrical i! pla! a!d may eit,er 7e

5ept parallel or may diverge slig,tly "pstream of wor5s. $ymmetrical a!d parallel g"ide

7a!5s are "s"ally adopted% "!less% t,e local co!ditio!s warra!t ot,erwise.

(efore t,e water e!ters i! to t,e tro"g, formed 7etwee! t,ese two g"ide 7a!5s% t,e flow

may ,ave to 7e partially co!trolled a!d directed wit, t,e ,elp of margi!al 7"!ds or 7y

groy!es or 7ot,. T,e g"ide 7a!5s "s"ally co!sist of two $electio! of wor5s-site- it is

evide!t t,at a! e!gi!eeri!g str"ct"re% s"c, as a 7ridge or a weir s,o"ld 7e spa!!ed i! t,at

portio! of t,e river w,ere t,e dista!ce 7etwee! t,e 5,adir 7a!5s is mi!im"m. T,is red"cest,e eDte!t of possi7le em7ayme!t at t,e 7ac5 of t,e g"ide 7a!5s :disc"ssed at little later;

a!d permits s,orter g"ide 7a!5s. +"rt,er o! a mea!deri!g river t,e river sectio! at t,e 7e!d

is always wide a!d !o!- "!iform deep o! t,e co!cave 7a!5 a!d s,allow o! t,e co!veD.

),ile t,e tra!sitio! reac,es co!!ecti!g two ad9ace!t 7e!ds are !arrower a!d "!iform i!


41/100


Prepared by Said M. (B.SC) "" DTU

dept,. (ridges s,o"ld t,erefore% prefera7ly 7e 7"ilt i! t,ese tra!sitio! reac,es rat,er t,a!

o! t,e 7e!ds.

+"rt,er i! case of 7ridges% t,e river 7ed at t,e proposed 7ridge site s,o"ld co!sist of deep

strata of erodi7le la!d% so t,at after co!str"ctio! t,e river may 7e a7le to deepe! t,e 7ed to

gai! a! ade"ate waterway. f t,e 7ed co!sists of stiff clay etc t,e co!strictio! ca! 7e do!e

o!ly after d"e allowa!ce is made for affl"D% w,ic, may ma5e it costlier.

! case of weirs a!d 7arrages% t,e "s"al practice is to co!str"ct t,e weir or t,e 7arrage%

o"tside t,e mai! river c,a!!el i! a mi!or cree5 w,ic, is dry i! wi!ter% a!d t,e! to divert

t,e mai! river c,a!!el t,ro"g, it. At t,e weir site% t,e river widt, is co!stricted.

T,e mea!deri!g river "pstream ,as t,erefore% to 7e trai!ed to flow 7etwee! t,e two

a7"tme!ts of t,e weir% wit,o"t ca"si!g a!y damage.

Pri!ciple a!d +actors =over!i!g t,e esig! of g"ide ("!5s

:; Top level of g"ide 7a!5s8 T,e top level of g"ide 7a!5s is gover!ed 7y H+% Affl"D%

velocity ,ead% a!d free7oard. t ca! 7e o7tai!ed 7y addi!g all t,ese fo"r val"es

• Affl"D8

(y affl"D% we mea! t,e rise i! t,e ,ig, flood level of t,e river% "pstream of t,e weir :or t,e

7ridge i! case of !o!- erodi7le soil; or 7arrage as a res"lt of its co!str"ctio!. T,is rise i!

water level is maDim"m 9"st !ear t,e site of co!strictio! a!d red"ces as we go away from

it% "pstream. T,e affl"D eDte!ds for a lo!g dista!ce o! t,e "pstream:; $,ape of g"ide

7a!5s i! pla!8 As stated earlier% t,e g"ide 7a!5s are ge!erally provided i! pairs%

symmetrical i! pla! a!d may eit,er 7e 5ept parallel or may diverge slig,tly "pstream of t,e

wor5s. T,e divergi!g g"ide 7a!5s may 7e favored o! t,e gro"!d t,at t,ey cover larger

wit, of t,e 5,adirs a!d eDert a! attracti!g i!fl"e!ce o! t,e flow. ("t t,ey are !otrecomme!ded mai!ly 7eca"se for e"al 7a!5 le!gt,s t,ey provide relatively less

protectio! to t,e approac, em7a!5me!t "!der t,e worst possi7le em7ayme!t as s,ow! i!

t,e fig"re 7elow.


42/100


Prepared by Said M. (B.SC) "# DTU

; e!gt, of t,e g"ide 7a!5s8 $pri!g a!d =ales ,ave correlated t,e le!gt, of g"ide 7a!5

wit, t,e le!gt, of str"ct"re 7etwee! t,e a7"tme!ts :;. Accordi!g to spri!g% t,e le!gt, of

t,e "pstream part of t,e g"ide 7a!5 s,o"ld 7e e"al to 1.1 or eve! lo!ger if re"ired to

o7viate t,e possi7ility of t,e river c"rvi!g at t,e 7ac5 a!d c"tti!g i!to approac, 7a!5. T,ele!gt, of t,e g"ide 7a!5 o! t,e dow!stream side s,o"ld 7e 7etwee! /. 1 to /.#

Accordi!g to =ales% t,e "pstream le!gt, of t,e g"ide 7a!5 is 1.#* for flow disc,arges "p

to #/%/// comics% a!d 1.* for flood disc,arges more t,a! #/%/// c"mecs. T,e le!gt, of

t,e g"ide 7a!5 o! t,e dow!stream side s,o"ld 7e ta5e! e"al to /.#* . A co!verge!ce of

1 i! #/ i! t,e former case a!d 1 i! &/ i! t,e letter case also 7ee! s"ggested 7y ,im :?;

Radi"s of c"rved ,ead of t,e g"ide 7a!5s8

:a; >pstream c"rved portio!8 T,e "pstream c"rved portio! of a g"ide 7a!5 is called t,e

impreg!a7le ,ead. T,e radi"s of c"rvat"re of t,e impreg!a7le ,ead s,o"ld 7e s"fficie!t

e!o"g, so as !ot to ca"se i!te!se eddies d"e to t,e c"rved flow !ear it =reater t,e radi"s%

flatter t,e c"rve% a!d lesser is t,e pro7a7ility of formatio! of eddies. +or same river slope%

coarser t,e 7ed material% s,orter ca! 7e t,e radi"s depe!di!g o! t,e eDpected velocity. A

safe val"e for t,e radi"s :R; may 7e ta5e! e"al to

R /.&* .

:7; ow!stream c"rved portio!. O! t,e dow!stream% t,e river fa!s o"t so as to attai! its

!ormal widt,. T,e dow!stream portio! of t,e g"ide 7a!5 e!s"res t,e safety of approac,

em7a!5me!ts a!d preve!ts t,e river from attac5i!g t,em. T,is p"rpose ca! 7e well served

7y providi!g s,ort g"ide 7"!d wit, s,arp c"rved ,ead. A radi"s e"al to ,alf t,e radi"s at

t,e "pstream side may 7e provided% as s,ow! i! t,e a7ove fig"re.

:?; T,e s,a!5 portio! a!d sectio! of 7"!d8 T,e straig,t portio! of t,e weir% $pri!g

s"ggested a val"e of R e"al to 1/ to #*/ meters for g"ide 7"!d (C is called t,e s,a!5

portio!. T,e g"ide 7a!5s s,o"ld ,ave a mi!im"m top wit, of & meters so as to provide

s"fficie!t carriage-way. EDtra widt, may% ,owever 7e provided for stori!g pitc,i!g

materials% etc side slope may vary from 1.*. H.8 1? to #.* H ?% :7"t ge!erally 5ept as # H8


43/100


Prepared by Said M. (B.SC) "$ DTU

1 ?;% depe!di!g o! t,e co!str"ctio! materials a!d t,e ,eig,t of t,e 7"!d A free7oard of

1.# to 1.* m is ge!erally provided.

:?; $lope pitc,i!g8 T,e slopi!g water side of t,e e!tire g"ide 7"!d as well as t,e slopi!g

rear side of t,e c"rved portio!s are pitc,ed wit, o!e ma! sto!e : i.e. a sto!e w,ic, ca! 7e

lifted 7y o!e perso!- weig,i!g &/ to */ 5g; or co!crete 7loc5s. T,e pitc,i!g s,o"ld

eDte!d "p to 1 m% ,ig,er t,a! H+. T,e rear side of t,e s,a!5 portio! is !ot pitc,ed% 7"t is

ge!erally coated wit, /.' to /.2 eart,s for e!co"ragi!g vegetatio! growt,% so as to ma5e it

resista!t agai!st rai!% wi!d etc.

T,e t,ic5!ess of t,e pitc,i!g o! t,e river side may 7e calc"lated 7y t,e form"la

t /.//2 14'

%),ere% t is t,e t,ic5!ess of sto!e pitc,i!g i! meters is t,e disc,arge i!

s

m'

.

T,e t,ic5!ess of pitc,i!g s,o"ld 7e #*I more at t,e impreg!a7le ,ead t,a! for t,e rest of

t,e 7"!d.

:?; a"!c,i!g apro!8 ),e!ever a slopi!g face is protected 7y sto!e pitc,i!g agai!st

sco"r t,e pitc,i!g is eDte!ded 7eyo!d t,e toe o! t,e 7ed% called a"!c,i!g apro!.

'. $p"rs or =roy!es

=roy!es are t,e em7a!5me!t type str"ct"re co!str"cted tra!sverse to t,e river flow%

eDte!di!g from t,e 7a!5 i! to t,e river. T,at is w,y t,ey may also 7e called Tra!sverse

y5es. T,ey are co!str"cted i! order to protect t,e 7a!5 from w,ic, t,ey are eDte!ded 7y

deflecti!g t,e c"rre!t away from t,e 7a!5. As t,e water is "!a7le to ta5e a s,arp

em7ayme!t t,e 7a!5 gets protected for certai! dista!ce "pstream a!d dow!stream of t,e

groy!e is s"79ected to treme!do"s actio! of water a!d ,as to 7e ,eavily protected 7y

pitc,i!g etc. T,e actio! of eddies red"ces from t,e ,ead towards t,e 7a!5 a!d t,erefore%

t,e t,ic5!ess of slope pitc,i!g a!d apro! ca! 7e red"ced accordi!gly.

Activity8 isc"ss ot,er types of flood co!trol str"ct"resB


44/100


Prepared by Said M. (B.SC) "% DTU

'.&.# T,e Nat"re of +lood Ris5

+loodi!g is a t,reat to ma!y comm"!ities a!d 7"si!esses% a!d flood ris5 is i!creasi!g i!

some locatio!s d"e to developme!t o! floodplai!s% migratio! to "r7a! areas at ris5 from

floodi!g% a!d artificial i!fl"e!ces o! flow regimes< for eDample% "r7a! developme!t ca!

sometimes i!crease flood ris5 t,ro"g, c,a!ges to r"!off c,aracteristics a!d t,e drai!age

pat,s of floodwater. Climate c,a!ge may also 7e i!creasi!g t,e li5eli,ood of floodi!g i!

some places t,ro"g, c,a!ges i! t,e fre"e!cy a!d severity of storms% patter!s of s!owfall

a!d s!owmelt% a!d risi!g sea levels.

Compared to ot,er types of !at"ral disaster% floods acco"!t for approDimately #/&/I of

t,e eve!ts w,ic, are reported. +loods ca! also ca"se eDte!sive damage to property%

i!frastr"ct"re a!d crops% a!d ca! c"t across admi!istrative a!d !atio!al 7o"!daries.

T,e ca"ses of floodi!g are mai!ly atmosp,eric or geotec,!ical :as descri7ed i! t,e ta7le

7elow;.Atmosp,eric ,azards i!cl"de ,eavy rai!fall% ca"si!g rivers to flood% sometimes

li!5ed to s!owmelt a!d ice-9ams i! colder climates% a!d coastal a!d est"ari!e floodi!g d"e

to s"rge% wave a!d wi!d effects% most !ota7ly i! tropical cyclo!es% ,"rrica!es a!d

typ,oo!s. =eotec,!ical factors s"c, as la!dslides% de7ris flows a!d eart,"a5es ca! also

lead to raised river levels ca"si!g i!la!d floodi!g% a!d Ts"!ami waves res"lti!g i! coastal

floodi!g. $eco!dary effects may i!cl"de overtoppi!g or 7reac,es of river a!d sea defe!ce

str"ct"res% de7ris 7loc5ages at 7ridges a!d ot,er str"ct"res% s"rc,argi!g of drai!age

!etwor5s i! "r7a! areas% a!d dam fail"re or overtoppi!g.

EDamples of floodi!g mec,a!isms


45/100


Prepared by Said M. (B.SC) "& DTU

"e to t,e s,ort time availa7le for people to react% fast developi!g floods prese!t a

partic"lar ris5 to life% i!cl"di!g flas, floods% dam or defe!ce 7reac,es% a!d some ice-9am

a!d local s"rge a!d wave overtoppi!g eve!ts .

Tropical cyclo!es% ,"rrica!es a!d typ,oo!s are all forms of tropical storm% wit, t,e term

tropical cyclo!e "sed i! t,e !dia! Ocea!% ,"rrica!e i! t,e Atla!tic a!d Easter! Pacific

Ocea!s% a!d typ,oo! i! t,e )ester! Pacific. +ro!tal depressio!s are most commo! i! mid-

latit"des% a!d ca! ca"se prolo!ged rai!fall% as ca! mo!soo!s w,ic, are drive! 7y seaso!al

variatio!s i! temperat"re 7etwee! sea a!d la!d masses.

T,"!derstorms ca! occ"r at most latit"des% a!d ca! ca"se i!te!se rai!fall for periods of

typically "p to a few ,o"rs. $!ow a!d ice related pro7lems affect ma!y ,ig, latit"deregio!s o! all co!ti!e!ts% a!d ,ig, mo"!tai! ra!ges elsew,ere. am a!d defe!ce ris5s are

possi7le a!yw,ere t,at reservoirs or polders ,ave 7ee! co!str"cted% or dams 7"ilt across

la5es% as are 7reac,es i! river or coastal flood defe!ces :ofte! 5!ow! as levees or di5es;.


46/100


Prepared by Said M. (B.SC) "' DTU

'. &. ' Assessi!g +lood Ris5

+lood ris5 is ofte! eDpressed as t,e com7i!atio! of two factors< pro7a7ility :or ,azard; a!d

co!se"e!ce :or impact;. T,e pro7a7ility eDpresses t,e li5eli,ood of damagi!g flood

levels or flows 7ei!g reac,ed% w,ilst t,e co!se"e!ce ca! 7e eDpressed i! terms of

i!dicators s"c, as t,e !"m7ers of properties affected% loss of life% or eco!omic damages.


47/100



Estimates for t,e !"m7ers of people at ris5 from floodi!g% a!d affected i! i!divid"al

eve!ts% are of co"rse s"79ect to ma!y "!certai!ties% i!cl"di!g t,e degree to w,ic, eve!ts

are reported% t,e Approac, ta5e! to flood ris5 assessme!ts a!d% for i!ter!atio!al

compariso!s% differe!ces i! t,e datasets a!d recordi!g met,ods w,ic, are "sed. However%

some st"dies :e.g. Par5er #///< $mit, #//&; s"ggest t,at t,e perce!tages of people at ris5

from floodi!g ra!ge from 'I to *I of t,e pop"latio! i! t,e >S a!d +ra!ce% to a7o"t 1#I

i! t,e >$A% */I i! t,e Net,erla!ds% a!d 3//I i! ?iet!am a!d (a!glades,. Estimates

are also complicated 7y tra!sie!t pop"latio!s% w,ic, ca! i!cl"de to"rists% ,i5ers%

temporary wor5ers% 7"si!ess travelers% a!d t,e ,omeless.

T,e li!5 7etwee! flood ris5 a!d social% political a!d eco!omic factors% partic"larly ris5 to

life% is well doc"me!ted% a!d ca! arise from iss"es s"c, as a lac5 of p"7lic aware!ess of

floodi!g iss"es% or co!trols o! floodplai! developme!t% limited f"!ds availa7le for flood

co!trol a!d protectio! :e.g. river a!d sea defe!ses;% low resilie!ce of 7"ildi!gs to floodi!g

:e.g. temporary compared to perma!e!t settleme!ts;% a!d a lac5 of i!vestme!t i! flood

war!i!g% forecasti!g a!d emerge!cy respo!se systems. ),ere t,ese factors are sig!ifica!t%

t,e !"m7ers of people affected 7y a flood eve!t ca! 7e m"c, ,ig,er t,a! e"ivale!t eve!ts

i! locatio!s wit,o"t t,ese pro7lems.

0eas"res of v"l!era7ility to floodi!g are also i!creasi!gly co!sidered i! flood ris5 st"dies8

for eDample% com7i!i!g t,e followi!g factors :e.g. )ade et al. #//*;8

+lood ,azard :dept,% velocity% de7ris;

Area ?"l!era7ility :effective!ess of flood war!i!g% speed of o!set of floodi!g% a!d type

of ("ildi!gs e.g. low rise4,ig, rise;

People ?"l!era7ility :a7ility to e!s"re ow! safety a!d t,at of depe!de!ts e.g. t,e elderly%

i!firm% c,ildre!; Of co"rse% v"l!era7ility to floodi!g ca! depe!d o! a wide ra!ge of

p,ysical% e!viro!me!tal% social% eco!omic% political% c"lt"ral a!d i!stit"tio!al factors% a!dca! vary widely 7etwee! i!divid"als% ,o"se,olds a!d comm"!ities< for eDample% t,e le!gt,

of time t,at people ,ave lived i! t,e floodplai! :or if t,ey are visiti!g t,e area e.g.

to"rists;% rece!t eDperie!ce of floodi!g% a!d local i!stit"tio!al capacity to respo!d to

floodi!g.


48/100



),e! desig!i!g a flood war!i!g sc,eme% a starti!g poi!t is ofte! to ma5e a! assessme!t of

t,e locatio!s a!d !"m7ers of people a!d properties at ris5 from floodi!g. ?"l!era7ility

st"dies ca! also ,ig,lig,t w,ere to target effort i! p"7lic aware!ess campaig!s% developi!g

flood emerge!cy pla!s% a!d i! emerge!cy respo!se. 0et,ods for assessi!g ris5 i!cl"de

i!terviews wit, people w,o 5!ow t,e area well% eDami!atio! of ,istorical flood records

:tras, mar5 s"rveys% aerial a!d ot,er p,otograp,s% !ewspaper reports% satellite images etc.;%

a!d ,ydrody!amic a!d ot,er modeli!g tec,!i"es.

Ris5 is t,e pro7a7ility of somet,i!g ,appe!i!g i! t,e f"t"re% w,ic, ,as a !egative

co!se"e!ce. t is a predictio! of s"fferi!g ,arm or loss or of meeti!g da!ger. Alt,o"g,

disaster ris5 is sometimes ta5e! as sy!o!ymo"s wit, ,azard% it ,as a! additio!al

implicatio! of li5eli,ood of a partic"lar ,azard to occ"r a!d ca"se damage or loss to a

v"l!era7le comm"!ity or gro"p. isaster Ris5 :or recipe for disaster; ,as 7ee! prese!ted

7y )ard% 1666 as follows8

isaster Ris5 Hazard D ?"l!era7ility

0a!agea7ility

0a!agea7ility ,ere sta!ds for t,e degree to w,ic, a comm"!ity ca! i!terve!e a!d ma!age

a ,azard i! order to red"ce its pote!tial impact. T,is implies t,at 7ased o! peopleUs

perceptio! of t,eir disaster ris5% t,ey are a7le to ma5e decisio!s to adapt to% modify or

ig!ore t,e ris5. 0a!agea7ility is sy!o!ymo"s to Capacity so we ca! s"7stit"te to ,ave t,e

followi!g disaster ris5 form"la8

isaster Ris5 Hazard D ?"l!era7ility

Capacity

ro"!d s"rvey a!d remote se!si!g tec,!i"es ca! also provide detailed maps of floodi!g

eDte!t% alt,o"g, !ot !ecessarily for t,e pea5 of t,e flood% a!d satellite o7servatio!s are

i!creasi!gly 7ei!g "sed to mo!itor flood eDte!ts "si!g 7ot, optical a!d microwave

fre"e!cies% a!d to 7"ild "p data7ases of flood eDte!t i!formatio!. 0odels provide a moreformal way of assessi!g flood ris5% a!d ca! ra!ge from simple correlatio! a!d ot,er

met,ods for si!gle locatio!s% t,ro"g, to detailed ,ydra"lic models for river a!d coastal

processes.


49/100



'.&.& F1ood R's= Mode1'ng

T,e !atio!al flood ris5 mappi!g programmed i! ma!y co"!tries "se a ra!ge of modeli!g

tec,!i"es to estimate flood dept,s% velocities a!d eDte!ts. +or rivers% for eDample% act"al

or sy!t,etic rai!fall eve!ts ca! 7e fed i!to a !etwor5 of rai!fall r"!off models represe!ti!g

ma9or s"7-catc,me!ts% w,ose o"tp"ts provide t,e i!p"ts to a model for t,e river !etwor5

a!d sig!ifica!t feat"res s"c, as floodplai!s a!d reservoirs.

! areas pro!e to floodi!g% t,e model detail may i!cl"de all sig!ifica!t co!trols o! river

levels a!d flows% s"c, as 7ridges% c"lverts% gates% defe!ces a!d ot,er feat"res% as well as t,e

mai! details of t,e floodplai!% "si!g co!str"ctio! a!d topograp,ic i!formatio! o7

flood and drought management backup.doc

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