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003-river and chawsnnel training works design.pdf

Date post: 30-Sep-2015
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River and Channel Training Works River training works are constructed for a variety of reasons, but the fundamental objective is to restrict horizontal move- ment of the river channel. Training works can also be used to improve navigation by restricting the width of the channel. Training works are installed where local infrastructure or agricultural l.and needs to be protected from the damage which would result from erosion of the river banks. Training works are also provided at bridges and other structures to improve flow conditions and prevent outflanking. River training can be achieved by river bank protection (revetment), groynes (spurs), or a combination of the two. Gabions are ideal for both applications since they have the considerable advantages of flexibility and permeability. River Bank Protection River bank protection is achieved by placing a revetment which prevents erosion of the bank. In general the bank protection is aligned roughly parallel to the flow so as to minimise the hydraulic loading and to avoid adverse ef- fects on the river regime. The design of any river training works should be carried out with a full understanding of the fluvial processes involved. The three main aspects to be considered are: . the velocity ofthe flow; . the likely maximum depth of scour in the river channel; . the overall extent and geometry of the works. Flow Velocity - Design flow velocities may be obtained from measurements on site or from hydraulic models ofthe river (either physical or numerical). In the absence of such data, the Manning equation can be used to estimate average velocity in the channel: where Manning's Equation 1 V = - R 2/3 S II2 n R = hydraulic radius = cross-sectional area wettedperimeter S =longitudinal channelslope n = Manning's roughness coefficient -10-
Transcript
  • RiverandChannelTrainingWorks

    Rivertrainingworksareconstructedfor

    avarietyof reasons,butthefundamental

    objectiveis to restricthorizontalmove-

    mentoftheriverchannel.Trainingworks

    canalsobeusedto improvenavigation

    byrestrictingthewidthof thechannel.

    Trainingworksareinstalledwherelocal

    infrastructureor agriculturall.andneeds

    tobeprotectedfromthedamagewhich

    wouldresultfromerosionof theriver

    banks.Trainingworksarealsoprovided

    atbridgesandotherstructurestoimproveflowconditionsandpreventoutflanking.

    Rivertrainingcanbeachievedbyriverbankprotection(revetment),groynes(spurs),oracombinationof thetwo.

    Gabionsareidealforbothapplicationssincetheyhavetheconsiderableadvantagesof flexibilityandpermeability.

    RiverBankProtection

    Riverbankprotectionisachievedbyplacingarevetment

    whichpreventserosionof thebank.In generalthebank

    protectionisalignedroughlyparalleltotheflowsoasto

    minimisethehydraulicloadingandtoavoidadverseef-

    fectsontheriverregime.

    Thedesignofanyrivertrainingworksshouldbecarried

    outwithafullunderstandingofthefluvialprocesses

    involved.Thethreemainaspectstobeconsideredare:

    . thevelocityoftheflow;

    . thelikelymaximumdepthofscourintheriverchannel;

    . theoverallextentandgeometryoftheworks.

    FlowVelocity- Designflowvelocitiesmaybeobtainedfrommeasurementsonsiteor fromhydraulicmodelsofthe

    river(eitherphysicalornumerical).Intheabsenceofsuchdata,theManningequationcanbeusedtoestimateaverage

    velocityinthechannel:

    where

    Manning'sEquation1

    V=- R 2/3S II2n

    R =hydraulicradius=cross-sectionalareawettedperimeter

    S=longitudinalchannelslopen =Manning'sroughnesscoefficient

    -10-

  • Scour-Failureof arevetmentis oftentheresultof inadequateprotectionagainstscouratthetoeof therevetment.

    Rigidrevetmentshavetheconsiderabledisadvantagethattheyareeasilyundenninedbyscour.Gabionsontheother

    handcandefonninacontrolledmannerattheonsetof scourandmaintaintheprotectionof thechanneledge.

    Gabionmattressscourapronslaidatthetoeof theriverbankshouldextendintotherivetbetween1.5and2.0times

    theanticipatedmaximumscourdepth.Thisensuresthatthegabionmattresstakesupthescouredprofileandcontin-

    uestoprotectthebankagainsterosion.

    Estimatinglikelydepthof scouris difficulttodowithanyaccuracy,andthereis nosubstitutefor localknowledge

    or fieldmeasurements.However,it shouldbenotedthatlargescourholesfonnedin floodscanbe filledin by

    sedimentdepositionduringthefloodrecession,soaccuratephysicaldataarehardtocomeby.Thedesignershould

    ensurethatfonnulaeusedforscourdepthestimatesareappropriateforthebedmaterialconsidered.

    Groynes

    Groynes(orspurs)deflectorguideflowawayfromthevulnerableriverbanks.Theycanthereforebeusedbothfor

    bankprotectionandtohelpmaintainanavigablechannel.

    Groynescannotbespacedtoofarapartotherwisetheriverwill attackthebankinbetween.Furthennore,groynesby

    theirnatureobstructtheflowandwill generateturbulenceandconsequentialscour.Groynesthereforerequirecareful

    detailingtoensurethattheyarenotoutflankedorundenninedin floods.

    Groynedesign

    Mal) lobepro~ecllSd

    Chenneledgeprolecllon u/.of IIrsl groyne

    Flow direclion

    Croyne slepped lo reducelurbulence. and lIow ob.lrucllon

    SECI'(ON

    f-I~"I

    , 1..1-------IIIIII1-------.-L T-

    I

    l-'~It

    S =3 to 4 X L

    where

    0.03y1.33y ~L ~O.25BS< 2n

    y =meandepthof flow atthedesigndischarge(m)n=Manning'sroughnesscoefficient.

    FlowdlreclionPLAN

    -11-


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