Padma Multipurpose Bridge Project

Criterion Alternative 1Revetment along bankScour alongside RTW(The risk of deep scour alongside theRTW, which increases the risk to theoverall stability)Hydraulic PerformanceStreamlined flow at bridge(The primary purpose of the RTW is toguide and control the approach flow so itwill pass smoothly under the bridge)Response to critical flow(The RTW should not createunacceptable velocities, turbulences, orscour under extreme scour conditionswhen subject to critical approach flowconditions)Outflanking possibility(The sensitivity of the RTW to outflankingflow and the distance of the outflankingfrom the main bridge)Downstream impact(The RTW should not induce significantlong-term changes to the downstreammorphology of the River)Upstream impactLow scour potential due to consistentguidance of flowThe sour behaviour along the river bankdepends on a number of factors andtype of RTW structure. Based oncomparison of existing data inBangladesh, the scour during one floodseason associated with revetments ishalf the depth of scour along theprotruding structures.Guide and control the flow largelyparallel to bankRevetments are associated with lowturbulences and the scour rates anddepths are generally half of the hardpoints.ModerateOutflanking at upstream end is distantfrom the bridge but future upstreamextension required to protect roadLessParallel flow along existing longprotected bank causes minimal impactAlternative 2Guide bund & hard points onbankHigher scour potential due tomultiple upstream flowterminations prone to outflanking.In addition, guide bund will beperiodically subject to river attackand exposed to additional localscour in comparison withrevetment option.Guide the flow to a certain extent,but generate more turbulent flownear RTWShort structures are associatedwith locally very turbulent flowand double the scour rates anddepths of revetmentsHighOutflanking risk at three locationsand closer to the bridge andfuture upstream extensionrequired to protect existing roadMoreLimited length of upstreamstructures is associated withadditional erosion downstream ofthe workAlternative 2 modifiedGuide bund on bank, hard points oncharHigher exposure than A1 and A2 butbetter than A2 due to betterupstream hard point despite morefrequent and longer attackEffectively control and streamline theflow through the bridge opening forall potential future situations.Short structures are associated withlocally very turbulent flow and doublethe scour rates and depths ofrevetmentsLessOutflanking is set back far from roadand critical structures, protectsportions of existing road outsideprojectLessFlow straightened out by upstreamstructures minimizes downstreamimpactsContinuous revetment along the existing Protection along the existing bank Protection along the existing bank is6-24