003-river and chawsnnel training works design.pdf

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-