Angel Part | Flood

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  2.4.1 Defning the design ood  The Design Flood or a hydraulic structure may also be defned in a number o ways, like! The ma imum ood that any structure can saely #ass.! The ood considered or the design o a structure corres#onding to ama imum tolerable risk.! The ood which a #ro$ect %in&ol&ing a hydraulic structure' can sustainwithout any substantial damage, either to the ob$ects which it #rotects orto its own structures.! The largest ood that may be selected or design as saety e&aluation o astructure.Design Flood is also known as the (now Design Flood %(DF'. (t is the oodado#ted or design #ur#ose, and could be! The entire ood hydrogra#h, that is, the #ossible &alues o discharge as aunction o time.! The #eak discharge o the ood hydrogra#h.. 2.4.2 )hoice o design ood  The *ureau o (ndian standard guidelines (+ 4-- %art (/' recommends that the(now Design Flood %(DF' o a structure, de#ending on its im#ortance or riskin&ol&ed, may be chosen rom either one o the ollowing! Probable Maximum Flood (PMF):  This is the ood resulting rom the most se&ere combination o criticalmeteorological and hydrological conditions that rare reasonably #ossiblein the region. The 0F is com#uted by using the robable 0a imum+torm %0+' which is an estimate o the #hysical u##er limit to stormrainall o&er the catchment. This is obtained rom the studies o all thestorms that ha&e occurred o&er the region and ma imiing them or themost critical atmos#heric conditions.! Standard Project Flood (SPF):  This is the ood resulting rom the most se&er combination o meteorological and hydrological conditions considered reasonablycharacteristic o the region. The +F is com#uted rom the +tandardro$ect +torm %++' o&er the watershed considered and may be taken asthe largest storm obser&ed in the region o the watershed. (t is notma imied or the most critical atmos#heric conditions but it may betrans#osed rom an ad$acent region to the watershed under consideration. Version 2 CE IIT, Kharagpur  ! Flood of a specific return period:  This ood is estimated by reuency analysis o the annual ood &alues o adeuate length. +ometimes when the ood data is inadeuate,reuency analysis recorded storm data is made and the storm o a#articular reuency a##lied to the unit hydrogra#h to deri&e the design  ood. This ood usually has a return #eriod greater than the storm. The (DF3s or dierent ty#es o structures constructed across ri&ers are dierent.+ome o the structures which are o im#ortance to water resources engineeringare! +torage Dams.! *arrages and 5eirs! Di&ersion 5orks and )oer dams! )ross drainage works6 brie descri#tion o the structures and their corres#onding (DF3s are discussed+ubseuently 2.4.7 Design ood or storage dams Dams are im#ortant hydraulic structures which are constructed to ser&e a &arietyo #ur#ose, more o which shall be discussed in detail in lesson 7.2. 0ost damsha&e a ca#acity to store substantial amount o water in the reser&oir, and a#ortion o the inow ood gets stored and the e cess o&erows through thes#illways. 6ccording to *ureau o (ndian +tandard guidelines (+ 11227819:,;<uidelines or f ing s#illway ca#acity=, the (DF to be considered or dierentreuirements 2.4.3.1 !F for t e safet# of t e dam (t is the ood or which, when used with standard s#ecifcations, the #erormanceo the dam should be sae against o&erto##ing, structural ailures, and thes#illway and its energy dissi#ation arrangement, i #ro&ided or a lower ood,should unction reasonable well.! For large dams %defned as those with gross storage greater than >? millionm 7 or hydraulic head greater than 7? m', (DF should be based on 0F.! For intermediate dams %gross storage between 1? and >? million m 7 orhydraulic head between 12 m and 7? m', (DF should be based on +F.! For small dams %gross storage between ?. to 1? million m 7 or hydraulic headbetween -. m to 12 m', (DF may be taken as 1?? years return #eriod ood. Version 2 CE IIT, Kharagpur  Floods o larger or smaller magnitude may be used i the haard in&ol&ed in thee&entuality o a ailure is #articularly high or low. The rele&ant #arameters to beconsidered in $udging the haard in addition to the sie would be! Distance to and location o the human habitations on the downstream aterconsidering the likely uture de&elo#ments@ and  ! 0a imum hydraulic ca#acity o the downstream channel at a le&el whichcatastro#hic damage is not e #ected. 2.4.3.2 !F for efficient operation of ener$# dissipation s#stem (t is a ood which may be lower than the (DF or the saety o the dam. 5henthis ood is used with standard s#ecifcations or other actors aecting the#erormance, the energy dissi#ation arrangements are e #ected to work mosteAciently. 2.4.3.3 !Ffor c ec%in$ extent of upstream submer$ence  This de#ends u#on local conditions, ty#e o #ro#erty and eects o thesubmergence or &ery im#ortant structures u#stream like #ower house, mines,etc. Be&els corres#onding to +F or 0F may be used to determinesubmergence eects. For other structures consideration o smaller design oodsand corres#onding le&els attained may suAce. (n general, a 2 C year ood orland acuisition and ? C year ood or built u# #ro#erty acuisition may beado#ted. 2.4.3.4 !F for c ec%in$ extent of do&nstream dama$e in t e 'alle#  This de#ends on local conditions, the ty#e o #ro#erty and eects o itssubmergence. For &ery im#ortant acilities like #owerhouse, outowscorres#onding to the inow design ood or saety o the dam, with all s#illwaygates o#erati&e or o that order may be rele&ant. ormally damage due to#hysical ooding may not be allowed under this condition, but disru#tion o o#eration may be allowed. 2.4.4 Design ood or barrages and weirs 5eirs and barrages, which are di&ersion structures, ha&e usually small storageca#acities, and the risk o loss o lie and #ro#erty would rarely be enhanced byailure o the structure. 6#art rom damageEloss o structure the ailure wouldcause disru#tion o irrigation and communications that are de#endent on thebarrage. 6ccording to the bureau o (ndian +tandard guidelines (+ >9>>%art8(' 819:9, ;ydraulic design o barrages and weirs8guidelines or allu&ial reaches=,the ollowing are recommended! +F or ?? year return #eriod ood or designing free board  ! ? year return #eriod ood or designing o items other than ree board. 2.4. Design ood or di&ersion works and coer dam 5hene&er a hydraulic structure like a dam or a barrage is constructed acrossa  ri&er, a tem#orary structure called a coer dam is built frst or obstructing theri&er ow and the water di&erted though a di&ersion channel or tunnel. The*ureau o (ndian +tandards in its guideline (+ 1??:4 %art (' 819:2, ;)riteria ordesign o di&ersion works 8 art ( )oer Dams= recommends the ollowing;The coer dam being a tem#orary structure is normally designed or aood with reuency less than that or the design o the main structure. The choice o a #articular reuency shall be made on #ractical $udgmentkee#ing in &iew the construction #eriod and the stage o construction o the main structure and its im#ortance. 6ccordingly, the design ood ischosen.For seasonal coerdams %those which are constructed e&ery year andwashed out during the ood season', and the initial construction stages o the main structure, a ood reuency o 2? years or more can be ado#ted.For coer dams to be retained or more than one season and or thead&anced construction stage o the main structure, a ood o 1?? yearsreuency may be ado#ted=. 2.4.> Flood or cross drainage works )ross drainage works are normally encountered in irrigation canal networksystem. <enerally canals ow under gra&ity and oten are reuired to cross localstreams and ri&ers. This is done by either con&eying the canal water o&er thestream by o&erhead aueducts or by #assing below the stream though si#honaueducts. These structures are called cross drainage works and according tothe *ureau o (ndian +tandard guidelines (+--:4 %art (' C 1997, ;)ode o #ractice or design o cross C drainage works= the ollowing is recommended.;Design ood or drainage channel to be ado#ted or cross drainage worksshould de#end u#on the sie o the canal, sie o the drainage channel andlocation o the cross drainage. 6 &ery long canal, crossing drainage channels inthe initial reach, damage to which is likely to aect the canal su##lies o&er alarge area and or a long #eriod, should be gi&en #ro#er im#ortance.)ross drainage structures are di&ided into our categories de#ending u#on thecanal discharge and drainage discharge. Design ood to be ado#ted or theseour categories o cross drainage structures is gi&en as in the ollowing table
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